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ESP: PubMed Auto Bibliography 03 Mar 2021 at 01:32 Created:

Reynolds Number

It is well known that relative size greatly affects *how*
organisms interact with the world. Less well known, at least among
biologists, is that at sufficiently small sizes, mechanical
interaction with the environment becomes difficult and then virtually
impossible. In fluid dynamics, an important dimensionless parameter is
the Reynolds Number (abbreviated *Re*), which is the ratio of
inertial to viscous forces affecting the movement of objects in a
fluid medium (or the movement of a fluid in a pipe). Since Re is
determined mainly by the size of the object (pipe) and the properties
(density and viscosity) of the fluid, organisms of different sizes
exhibit significantly different Re values when moving through air or
water. A fish, swimming at a high ratio of inertial to viscous forces,
gives a flick of its tail and then glides for several body lengths. A
bacterium, "swimming" in an environment dominated by viscosity,
possesses virtually no inertia. When the bacterium stops moving its
flagellum, the bacterium "coasts" for about a half of a microsecond,
coming to a stop in a distance less than a tenth the diameter of a
hydrogen atom. Similarly, the movement of molecules (nutrients toward,
wastes away) in the vicinity of a bacterium is dominated by diffusion.
Effective stirring — the generation of bulk flow through
mechanical means — is impossible at very low *Re*. An
understanding of the constraints imposed by life at low Reynolds
numbers is essentially for understanding the prokaryotic biosphere.

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RevDate: 2021-02-28

**Experimental investigation of oscillating flow characteristics at the exit of a stacked mesh grid regenerator.**

*The Journal of the Acoustical Society of America*, **149(2):**807.

The aim of this study is to investigate the oscillating flow velocity field at the exit of different stacked mesh grid regenerators using Particle Image Velocimetry measurements. Twelve different experimental cases are discussed, yielding oscillating flow fields at the exit of four kinds of regenerators for different acoustic levels. The regenerators are classified according to the mesh wire size to viscous penetration depth ratio and according to the method of stacking the mesh grids. Based on the analysis of the vorticity fields at the exit of the regenerator, three groups of flow patterns are identified. This classification is correctly verified by using the Reynolds number (based on the acoustic amplitude and wire diameter) and the Strouhal number (based on the acoustic displacement amplitude and wire diameter). The characteristics of the fluctuating velocity components are investigated for these various flow patterns. The critical Reynolds number, past which the flow is highly dissipative, is determined. The dissipation timescale is investigated and the quasi-steady approximation is found to be valid for the analysis of the oscillating flow at the exit of the regenerator mesh.

Additional Links: PMID-33639789

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@article {pmid33639789,

year = {2021},

author = {Ramadan, IA and Bailliet, H and ValiÃ¨re, JC},

title = {Experimental investigation of oscillating flow characteristics at the exit of a stacked mesh grid regenerator.},

journal = {The Journal of the Acoustical Society of America},

volume = {149},

number = {2},

pages = {807},

doi = {10.1121/10.0003375},

pmid = {33639789},

issn = {1520-8524},

abstract = {The aim of this study is to investigate the oscillating flow velocity field at the exit of different stacked mesh grid regenerators using Particle Image Velocimetry measurements. Twelve different experimental cases are discussed, yielding oscillating flow fields at the exit of four kinds of regenerators for different acoustic levels. The regenerators are classified according to the mesh wire size to viscous penetration depth ratio and according to the method of stacking the mesh grids. Based on the analysis of the vorticity fields at the exit of the regenerator, three groups of flow patterns are identified. This classification is correctly verified by using the Reynolds number (based on the acoustic amplitude and wire diameter) and the Strouhal number (based on the acoustic displacement amplitude and wire diameter). The characteristics of the fluctuating velocity components are investigated for these various flow patterns. The critical Reynolds number, past which the flow is highly dissipative, is determined. The dissipation timescale is investigated and the quasi-steady approximation is found to be valid for the analysis of the oscillating flow at the exit of the regenerator mesh.},

}

RevDate: 2021-02-26

**Catastrophic Phase Inversion in High-Reynolds-Number Turbulent Taylor-Couette Flow.**

*Physical review letters*, **126(6):**064501.

Emulsions are omnipresent in the food industry, health care, and chemical synthesis. In this Letter the dynamics of metastable oil-water emulsions in highly turbulent (10^{11}â‰¤Taâ‰¤3Ã—10^{13}) Taylor-Couette flow, far from equilibrium, is investigated. By varying the oil-in-water void fraction, catastrophic phase inversion between oil-in-water and water-in-oil emulsions can be triggered, changing the morphology, including droplet sizes, and rheological properties of the mixture, dramatically. The manifestation of these different states is exemplified by combining global torque measurements and local in situ laser induced fluorescence microscopy imaging. Despite the turbulent state of the flow and the dynamic equilibrium of the oil-water mixture, the global torque response of the system is found to be as if the fluid were Newtonian, and the effective viscosity of the mixture was found to be several times bigger or smaller than either of its constituents.

Additional Links: PMID-33635696

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@article {pmid33635696,

year = {2021},

author = {Bakhuis, D and Ezeta, R and Bullee, PA and Marin, A and Lohse, D and Sun, C and Huisman, SG},

title = {Catastrophic Phase Inversion in High-Reynolds-Number Turbulent Taylor-Couette Flow.},

journal = {Physical review letters},

volume = {126},

number = {6},

pages = {064501},

doi = {10.1103/PhysRevLett.126.064501},

pmid = {33635696},

issn = {1079-7114},

abstract = {Emulsions are omnipresent in the food industry, health care, and chemical synthesis. In this Letter the dynamics of metastable oil-water emulsions in highly turbulent (10^{11}â

‰¤Taâ‰¤3Ã—10^{13})

Taylor-Couette flow, far from equilibrium, is investigated. By varying the oil-in-water void fraction, catastrophic phase inversion between oil-in-water and water-in-oil emulsions can be triggered, changing the morphology, including droplet sizes, and rheological properties of the mixture, dramatically. The manifestation of these different states is exemplified by combining global torque measurements and local in situ laser induced fluorescence microscopy imaging. Despite the turbulent state of the flow and the dynamic equilibrium of the oil-water mixture, the global torque response of the system is found to be as if the fluid were Newtonian, and the effective viscosity of the mixture was found to be several times bigger or smaller than either of its constituents.},

}

RevDate: 2021-02-26

**Performance analysis of multi-gap V-roughness with staggered elements of solar air heater based on artificial neural network and experimental investigations.**

*Environmental science and pollution research international* [Epub ahead of print].

Among all renewable energy sources, solar power is one of the major sources which contributes for pollution control and protection of environment. For a number of decades, technologies for utilizing the solar power have been the area of research and development. In the current research, thermal performance parameters of multi-gap V-roughness with staggered elements of a solar air heater (SAH) are experimentally investigated. The artificial neural network (ANN) is also utilized for predicting the thermal performance parameters of SAH. Experiments were executed in a rectangular channel with one roughened side at the top exposed to a uniform heat flux. A significant rise in thermal efficiency performance was reported under a predefined range of Reynolds number (Re) from 3000 to 14000 with an optimized value of relative roughness pitch ratio (P/e) and relative staggered rib length (w/g) as 12 and 1, respectively. The maximum thermal efficiency was attained in the range from 42.15 to 87.02% under considered Reynolds numbers for optimum value of P/e as 12 and w/g as 1. A multilayered perceptron (MLP) feed-forward ANN trained by the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm was utilized to predict the thermal efficiency (Î·th), friction (f), and Nusselt number (Nu). The thermal performance parameters such as P/e, w/g, Re, and temperature at the inlet, outlet, and plate were the critical input parameters/signals used in the ANN method. The optimum ANN arrangement/structure to predict the Nu, f, and Î·th demonstrate higher accurateness in assessing the performance characteristics of SAH by attaining the root mean squared error (RMSE) in prediction and the Pearson coefficient of association (R2) of 1.591 and 0.994; 0.0012 and 0.851; and 0.025 and 0.981, respectively. The prediction profile plots of the ANN demonstrate the influence of various input parameters on the thermal performance parameters.

Additional Links: PMID-33635462

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Citation:

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@article {pmid33635462,

year = {2021},

author = {Jain, PK and Lanjewar, A and Jain, R and Rana, KB},

title = {Performance analysis of multi-gap V-roughness with staggered elements of solar air heater based on artificial neural network and experimental investigations.},

journal = {Environmental science and pollution research international},

volume = {},

number = {},

pages = {},

pmid = {33635462},

issn = {1614-7499},

abstract = {Among all renewable energy sources, solar power is one of the major sources which contributes for pollution control and protection of environment. For a number of decades, technologies for utilizing the solar power have been the area of research and development. In the current research, thermal performance parameters of multi-gap V-roughness with staggered elements of a solar air heater (SAH) are experimentally investigated. The artificial neural network (ANN) is also utilized for predicting the thermal performance parameters of SAH. Experiments were executed in a rectangular channel with one roughened side at the top exposed to a uniform heat flux. A significant rise in thermal efficiency performance was reported under a predefined range of Reynolds number (Re) from 3000 to 14000 with an optimized value of relative roughness pitch ratio (P/e) and relative staggered rib length (w/g) as 12 and 1, respectively. The maximum thermal efficiency was attained in the range from 42.15 to 87.02% under considered Reynolds numbers for optimum value of P/e as 12 and w/g as 1. A multilayered perceptron (MLP) feed-forward ANN trained by the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm was utilized to predict the thermal efficiency (Î·th), friction (f), and Nusselt number (Nu). The thermal performance parameters such as P/e, w/g, Re, and temperature at the inlet, outlet, and plate were the critical input parameters/signals used in the ANN method. The optimum ANN arrangement/structure to predict the Nu, f, and Î·th demonstrate higher accurateness in assessing the performance characteristics of SAH by attaining the root mean squared error (RMSE) in prediction and the Pearson coefficient of association (R2) of 1.591 and 0.994; 0.0012 and 0.851; and 0.025 and 0.981, respectively. The prediction profile plots of the ANN demonstrate the influence of various input parameters on the thermal performance parameters.},

}

RevDate: 2021-02-26

**On the ridge of instability in ferrofluidic Couette flow via alternating magnetic field.**

*Scientific reports*, **11(1):**4705.

There is a huge number of natural and industrial flows, which are subjected to time-dependent boundary conditions. The flow of a magnetic fluid under the influence of temporal modulations is such an example. Here, we perform numerical simulations of ferrofluidic Couette flow subject to time-periodic modulation (with frequency [Formula: see text]) in a spatially homogeneous magnetic field and report how such a modulation can lead to a significant Reynolds number Re enhancement. Consider a modified Niklas approximation we explain the relation between modulation amplitude, driving frequency and stabilization effect. From this, we describe the system response around the primary instability to be sensitive/critical by an alternating field. We detected that such an alternating field provides an easy and in particular accurate controllable key parameter to trigger the system to change from subcritical to supercritical and vice versa. Our findings provide a framework to study other types of magnetic flows driven by time-dependent forcing.

Additional Links: PMID-33633249

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@article {pmid33633249,

year = {2021},

author = {Altmeyer, S},

title = {On the ridge of instability in ferrofluidic Couette flow via alternating magnetic field.},

journal = {Scientific reports},

volume = {11},

number = {1},

pages = {4705},

pmid = {33633249},

issn = {2045-2322},

abstract = {There is a huge number of natural and industrial flows, which are subjected to time-dependent boundary conditions. The flow of a magnetic fluid under the influence of temporal modulations is such an example. Here, we perform numerical simulations of ferrofluidic Couette flow subject to time-periodic modulation (with frequency [Formula: see text]) in a spatially homogeneous magnetic field and report how such a modulation can lead to a significant Reynolds number Re enhancement. Consider a modified Niklas approximation we explain the relation between modulation amplitude, driving frequency and stabilization effect. From this, we describe the system response around the primary instability to be sensitive/critical by an alternating field. We detected that such an alternating field provides an easy and in particular accurate controllable key parameter to trigger the system to change from subcritical to supercritical and vice versa. Our findings provide a framework to study other types of magnetic flows driven by time-dependent forcing.},

}

RevDate: 2021-02-24

**Visualizing the Flow Patterns in an Expanding and Contracting Pulmonary Alveolated Duct Based On Micro-Computed Tomography Images.**

*Journal of biomechanical engineering* pii:1100568 [Epub ahead of print].

We visualized the flow patterns in an alveolated duct model with breathing-like expanding and contracting wall motions using particle image velocimetry, then we investigated the effect of acinar deformation on the flow patterns. We reconstructed a compliant, scaled-up model of an alveolated duct from synchrotron micro-computed tomography images of a mammalian lung. The alveolated duct did not include any bifurcation, and its entire surface was covered with alveoli. We embedded the alveolated duct in a sealed container that was filled with fluid. We oscillated the fluid in the duct and container simultaneously and independently to control the flow and duct volume. We examined the flow patterns in alveoli, with the Reynolds number (Re) at 0.03 or 0.22 and the acinar volume change at 0%, 20%, or 80%. At the same Re, the heterogeneous deformation induced different inspiration and expiration flow patterns, and the recirculating regions in alveoli changed during respiratory cycle. During a larger acinar deformation at Re = 0.03, the flow patterns tended to change from recirculating flow to radial flow during inspiration and vice versa during expiration. Additionally, the alveolar geometric characteristics, particularly the angle between the alveolar duct and mouth, affected this differences in flow patterns. At Re = 0.22, recirculating flow patterns tended to form during inspiration and expiration, regardless of the magnitude of the acinar deformation. Our in vitro experiments suggest the alveolated flows with non-self-similar and heterogeneous wall motions may promote particle mixing and deposition.

Additional Links: PMID-33625506

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PubMed:

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@article {pmid33625506,

year = {2021},

author = {Sera, T and Kamiya, N and Fukushima, T and Tanaka, G},

title = {Visualizing the Flow Patterns in an Expanding and Contracting Pulmonary Alveolated Duct Based On Micro-Computed Tomography Images.},

journal = {Journal of biomechanical engineering},

volume = {},

number = {},

pages = {},

doi = {10.1115/1.4050285},

pmid = {33625506},

issn = {1528-8951},

abstract = {We visualized the flow patterns in an alveolated duct model with breathing-like expanding and contracting wall motions using particle image velocimetry, then we investigated the effect of acinar deformation on the flow patterns. We reconstructed a compliant, scaled-up model of an alveolated duct from synchrotron micro-computed tomography images of a mammalian lung. The alveolated duct did not include any bifurcation, and its entire surface was covered with alveoli. We embedded the alveolated duct in a sealed container that was filled with fluid. We oscillated the fluid in the duct and container simultaneously and independently to control the flow and duct volume. We examined the flow patterns in alveoli, with the Reynolds number (Re) at 0.03 or 0.22 and the acinar volume change at 0%, 20%, or 80%. At the same Re, the heterogeneous deformation induced different inspiration and expiration flow patterns, and the recirculating regions in alveoli changed during respiratory cycle. During a larger acinar deformation at Re = 0.03, the flow patterns tended to change from recirculating flow to radial flow during inspiration and vice versa during expiration. Additionally, the alveolar geometric characteristics, particularly the angle between the alveolar duct and mouth, affected this differences in flow patterns. At Re = 0.22, recirculating flow patterns tended to form during inspiration and expiration, regardless of the magnitude of the acinar deformation. Our in vitro experiments suggest the alveolated flows with non-self-similar and heterogeneous wall motions may promote particle mixing and deposition.},

}

RevDate: 2021-02-24

**The effect of external flow on the feeding currents of sessile microorganisms.**

*Journal of the Royal Society, Interface*, **18(175):**20200953.

Microscopic sessile suspension feeders live attached to surfaces and, by consuming bacteria-sized prey and by being consumed, they form an important part of aquatic ecosystems. Their environmental impact is mediated by their feeding rate, which depends on a self-generated feeding current. The feeding rate has been hypothesized to be limited by recirculating eddies that cause the organisms to feed from water that is depleted of food particles. However, those results considered organisms in still water, while ambient flow is often present in their natural habitats. We show, using a point-force model, that even very slow ambient flow, with speed several orders of magnitude less than that of the self-generated feeding current, is sufficient to disrupt the eddies around perpendicular suspension feeders, providing a constant supply of food-rich water. However, the feeding rate decreases in external flow at a range of non-perpendicular orientations due to the formation of recirculation structures not seen in still water. We quantify the feeding flow and observe such recirculation experimentally for the suspension feeder Vorticella convallaria in external flows typical of streams and rivers.

Additional Links: PMID-33622143

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PubMed:

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@article {pmid33622143,

year = {2021},

author = {Pepper, RE and Riley, EE and Baron, M and Hurot, T and Nielsen, LT and Koehl, MAR and KiÃ¸rboe, T and Andersen, A},

title = {The effect of external flow on the feeding currents of sessile microorganisms.},

journal = {Journal of the Royal Society, Interface},

volume = {18},

number = {175},

pages = {20200953},

doi = {10.1098/rsif.2020.0953},

pmid = {33622143},

issn = {1742-5662},

abstract = {Microscopic sessile suspension feeders live attached to surfaces and, by consuming bacteria-sized prey and by being consumed, they form an important part of aquatic ecosystems. Their environmental impact is mediated by their feeding rate, which depends on a self-generated feeding current. The feeding rate has been hypothesized to be limited by recirculating eddies that cause the organisms to feed from water that is depleted of food particles. However, those results considered organisms in still water, while ambient flow is often present in their natural habitats. We show, using a point-force model, that even very slow ambient flow, with speed several orders of magnitude less than that of the self-generated feeding current, is sufficient to disrupt the eddies around perpendicular suspension feeders, providing a constant supply of food-rich water. However, the feeding rate decreases in external flow at a range of non-perpendicular orientations due to the formation of recirculation structures not seen in still water. We quantify the feeding flow and observe such recirculation experimentally for the suspension feeder Vorticella convallaria in external flows typical of streams and rivers.},

}

RevDate: 2021-02-24

**Double diffusive convection and Hall effect in creeping flow of viscous nanofluid through a convergent microchannel: a biotechnological applications.**

*Computer methods in biomechanics and biomedical engineering* [Epub ahead of print].

Current analysis presents the mathematical modeling for peristaltic transport of nanofluid with applications of double-diffusive convection and Hall features. The flow has been induced by a convergent channel due to peristaltic propulsion. These rheological equations are transformed from fixed to wave frames by using a linear mathematical relation between these two frames. The dimensionless variables are used to transform these rheological equations into nondimensional forms. The flow analysis is carried out under two distinct scientific biological assumptions, one is known as long wavelength and the second one is low Reynolds number. The analytical solutions of these rheological equations are obtained with the help of a rigorous analytical method known as integration in the term of stream function. The physical effects of magnetic and Hall devices, respectively, on the flow features are also considered in the present analysis. The physical influences of dominant hydro-mechanical parameters on the axial velocity, pressure gradient, trapping, volumetric fraction of nanofluid, heat and mass transfer phenomena are studied. The complex scenario of biomimetic propulsions are considered in boundary walls to boost the proficiency of peristaltic micropumps.

Additional Links: PMID-33622097

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@article {pmid33622097,

year = {2021},

author = {Ge-JiLe, H and Javid, K and Khan, SU and Raza, M and Khan, MI and Qayyum, S},

title = {Double diffusive convection and Hall effect in creeping flow of viscous nanofluid through a convergent microchannel: a biotechnological applications.},

journal = {Computer methods in biomechanics and biomedical engineering},

volume = {},

number = {},

pages = {1-18},

doi = {10.1080/10255842.2021.1888373},

pmid = {33622097},

issn = {1476-8259},

abstract = {Current analysis presents the mathematical modeling for peristaltic transport of nanofluid with applications of double-diffusive convection and Hall features. The flow has been induced by a convergent channel due to peristaltic propulsion. These rheological equations are transformed from fixed to wave frames by using a linear mathematical relation between these two frames. The dimensionless variables are used to transform these rheological equations into nondimensional forms. The flow analysis is carried out under two distinct scientific biological assumptions, one is known as long wavelength and the second one is low Reynolds number. The analytical solutions of these rheological equations are obtained with the help of a rigorous analytical method known as integration in the term of stream function. The physical effects of magnetic and Hall devices, respectively, on the flow features are also considered in the present analysis. The physical influences of dominant hydro-mechanical parameters on the axial velocity, pressure gradient, trapping, volumetric fraction of nanofluid, heat and mass transfer phenomena are studied. The complex scenario of biomimetic propulsions are considered in boundary walls to boost the proficiency of peristaltic micropumps.},

}

RevDate: 2021-02-19

**Odd-viscosity-induced instability of a falling thin film with an external electric field.**

*Physical review. E*, **103(1-1):**013104.

The influence of odd viscosity of Newtonian fluid on the instability of thin film flowing along an inclined plane under a normal electric field is studied. By odd viscosity, we mean apart from the well-known coefficient of shear viscosity, a classical liquid with broken time-reversal symmetry is endowed with a second viscosity coefficient in biological, colloidal, and granular systems. Under the long wave approximation, a nonlinear evolution equation of the free surface is derived by the method of systematic asymptotic expansion. The effects of the odd viscosity and external electric field are considered in this evolution equation and an analytical expression of critical Reynolds number is obtained. It is interesting to find that, by linear stability analysis, the critical Reynolds number increases with odd viscosity and decreases with external strength of electric field. In other words, odd viscosity has a stable effect and electric field has a destabilized effect on flowing of thin film. In addition, through nonlinear analysis, we obtain a Ginsburg-Landau equation and find that the film has not only the supercritical stability zone and the subcritical instability zone but also the unconditional stability zone and the explosive zone. The variations of each zone with related parameters, such as the strength of electric field, odd viscosity, and Reynolds number, etc., are investigated. The results are conducive to the further development of related experiments.

Additional Links: PMID-33601572

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@article {pmid33601572,

year = {2021},

author = {Bao, G and Jian, Y},

title = {Odd-viscosity-induced instability of a falling thin film with an external electric field.},

journal = {Physical review. E},

volume = {103},

number = {1-1},

pages = {013104},

doi = {10.1103/PhysRevE.103.013104},

pmid = {33601572},

issn = {2470-0053},

abstract = {The influence of odd viscosity of Newtonian fluid on the instability of thin film flowing along an inclined plane under a normal electric field is studied. By odd viscosity, we mean apart from the well-known coefficient of shear viscosity, a classical liquid with broken time-reversal symmetry is endowed with a second viscosity coefficient in biological, colloidal, and granular systems. Under the long wave approximation, a nonlinear evolution equation of the free surface is derived by the method of systematic asymptotic expansion. The effects of the odd viscosity and external electric field are considered in this evolution equation and an analytical expression of critical Reynolds number is obtained. It is interesting to find that, by linear stability analysis, the critical Reynolds number increases with odd viscosity and decreases with external strength of electric field. In other words, odd viscosity has a stable effect and electric field has a destabilized effect on flowing of thin film. In addition, through nonlinear analysis, we obtain a Ginsburg-Landau equation and find that the film has not only the supercritical stability zone and the subcritical instability zone but also the unconditional stability zone and the explosive zone. The variations of each zone with related parameters, such as the strength of electric field, odd viscosity, and Reynolds number, etc., are investigated. The results are conducive to the further development of related experiments.},

}

RevDate: 2021-02-19

**Molecular viscosity and diffusivity effects in transitional and shock-driven mixing flows.**

*Physical review. E*, **103(1-1):**013106.

This paper investigates the importance of molecular viscosity and diffusivity for the prediction of transitional and shock-driven mixing flows featuring high and low Reynolds and Mach number regions. Two representative problems are computed with implicit large-eddy simulations using the inviscid Euler equations (EE) and viscous Navier-Stokes equations (NSE): the Taylor-Green vortex at Reynolds number Re=3000 and initial Mach number Ma=0.28, and an air-SF_{6}-air gas curtain subjected to two shock waves at Ma=1.2. The primary focus is on differences between NSE and EE predictions due to viscous effects. The outcome of the paper illustrates the advantages of utilizing NSE. In contrast to the EE, where the effective viscosity decreases upon grid refinement, NSE predictions can be assessed for simulations of flows with transition to turbulence at prescribed constant Re. The NSE can achieve better agreement between solutions and reference data, and the results converge upon grid refinement. On the other hand, the EE predictions do not converge with grid refinement, and can only exhibit similarities with the NSE results at coarse grid resolutions. We also investigate the effect of viscous effects on the dynamics of the coherent and turbulent fields, as well as on the mechanisms contributing to the production and diffusion of vorticity. The results show that nominally inviscid calculations can exhibit significantly varying flow dynamics driven by changing effective resolution-dependent Reynolds number, and highlight the role of viscous processes affecting the vorticity field. These tendencies become more pronounced upon grid refinement. The discussion of the results concludes with the assessment of the computational cost of inviscid and viscous computations.

Additional Links: PMID-33601565

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PubMed:

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@article {pmid33601565,

year = {2021},

author = {Pereira, FS and Grinstein, FF and Israel, DM and Rauenzahn, R},

title = {Molecular viscosity and diffusivity effects in transitional and shock-driven mixing flows.},

journal = {Physical review. E},

volume = {103},

number = {1-1},

pages = {013106},

doi = {10.1103/PhysRevE.103.013106},

pmid = {33601565},

issn = {2470-0053},

abstract = {This paper investigates the importance of molecular viscosity and diffusivity for the prediction of transitional and shock-driven mixing flows featuring high and low Reynolds and Mach number regions. Two representative problems are computed with implicit large-eddy simulations using the inviscid Euler equations (EE) and viscous Navier-Stokes equations (NSE): the Taylor-Green vortex at Reynolds number Re=3000 and initial Mach number Ma=0.28, and an air-SF_{6}-

air gas curtain subjected to two shock waves at Ma=1.2. The primary focus is on differences between NSE and EE predictions due to viscous effects. The outcome of the paper illustrates the advantages of utilizing NSE. In contrast to the EE, where the effective viscosity decreases upon grid refinement, NSE predictions can be assessed for simulations of flows with transition to turbulence at prescribed constant Re. The NSE can achieve better agreement between solutions and reference data, and the results converge upon grid refinement. On the other hand, the EE predictions do not converge with grid refinement, and can only exhibit similarities with the NSE results at coarse grid resolutions. We also investigate the effect of viscous effects on the dynamics of the coherent and turbulent fields, as well as on the mechanisms contributing to the production and diffusion of vorticity. The results show that nominally inviscid calculations can exhibit significantly varying flow dynamics driven by changing effective resolution-dependent Reynolds number, and highlight the role of viscous processes affecting the vorticity field. These tendencies become more pronounced upon grid refinement. The discussion of the results concludes with the assessment of the computational cost of inviscid and viscous computations.},

}

RevDate: 2021-02-19

**Influence of numerical resolution on the dynamics of finite-size particles with the lattice Boltzmann method.**

*Physical review. E*, **103(1-1):**013303.

We investigate and compare the accuracy and efficiency of different numerical approaches to model the dynamics of finite-size particles using the lattice Boltzmann method (LBM). This includes the standard bounce-back (BB) and the equilibrium interpolation (EI) schemes. To accurately compare the different implementations, we first introduce a boundary condition to approximate the flow properties of an unbounded fluid in a finite simulation domain, taking into account the perturbation induced by a moving particle. We show that this boundary treatment is efficient in suppressing detrimental effects on the dynamics of spherical and ellipsoidal particles arising from the finite size of the simulation domain. We then investigate the performances of the BB and EI schemes in modeling the dynamics of a spherical particle settling under Stokes conditions, which can now be reproduced with great accuracy thanks to the treatment of the exterior boundary. We find that the EI scheme outperforms the BB scheme in providing a better accuracy scaling with respect to the resolution of the settling particle, while suppressing finite-size effects due to the particle discretization on the lattice grid. Additionally, in order to further increase the capability of the algorithm in modeling particles of sizes comparable to the lattice spacing, we propose an improvement to the EI scheme, the complete equilibrium interpolation (CEI). This approach allows us to accurately capture the boundaries of the particle also when located between two fluid nodes. We evaluate the CEI performance in solving the dynamics of an under-resolved particle under analogous Stokes conditions and also for the case of a rotating ellipsoid in a shear flow. Finally, we show that EI and CEI are able to recover the correct flow solutions also at small, but finite, Reynolds number. Adopting the CEI scheme it is not only possible to detect particles with zero lattice occupation, but also to increase up to one order of magnitude the accuracy of the dynamics of particles with a size comparable to the lattice spacing with respect to the BB and the EI schemes.

Additional Links: PMID-33601495

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@article {pmid33601495,

year = {2021},

author = {Livi, C and Di Staso, G and Clercx, HJH and Toschi, F},

title = {Influence of numerical resolution on the dynamics of finite-size particles with the lattice Boltzmann method.},

journal = {Physical review. E},

volume = {103},

number = {1-1},

pages = {013303},

doi = {10.1103/PhysRevE.103.013303},

pmid = {33601495},

issn = {2470-0053},

abstract = {We investigate and compare the accuracy and efficiency of different numerical approaches to model the dynamics of finite-size particles using the lattice Boltzmann method (LBM). This includes the standard bounce-back (BB) and the equilibrium interpolation (EI) schemes. To accurately compare the different implementations, we first introduce a boundary condition to approximate the flow properties of an unbounded fluid in a finite simulation domain, taking into account the perturbation induced by a moving particle. We show that this boundary treatment is efficient in suppressing detrimental effects on the dynamics of spherical and ellipsoidal particles arising from the finite size of the simulation domain. We then investigate the performances of the BB and EI schemes in modeling the dynamics of a spherical particle settling under Stokes conditions, which can now be reproduced with great accuracy thanks to the treatment of the exterior boundary. We find that the EI scheme outperforms the BB scheme in providing a better accuracy scaling with respect to the resolution of the settling particle, while suppressing finite-size effects due to the particle discretization on the lattice grid. Additionally, in order to further increase the capability of the algorithm in modeling particles of sizes comparable to the lattice spacing, we propose an improvement to the EI scheme, the complete equilibrium interpolation (CEI). This approach allows us to accurately capture the boundaries of the particle also when located between two fluid nodes. We evaluate the CEI performance in solving the dynamics of an under-resolved particle under analogous Stokes conditions and also for the case of a rotating ellipsoid in a shear flow. Finally, we show that EI and CEI are able to recover the correct flow solutions also at small, but finite, Reynolds number. Adopting the CEI scheme it is not only possible to detect particles with zero lattice occupation, but also to increase up to one order of magnitude the accuracy of the dynamics of particles with a size comparable to the lattice spacing with respect to the BB and the EI schemes.},

}

RevDate: 2021-02-16

**Investigation of red blood cell partitioning in an in vitro microvascular bifurcation.**

*Artificial organs* [Epub ahead of print].

There is a long history of research examining red blood cell (RBC) partitioning in microvasculature bifurcations. These studies commonly report results describing partitioning that exists as either regular partitioning, which occurs when the RBC flux ratio is greater than the bulk fluid flow rate ratio, or reverse partitioning when the RBC flux ratio is less than or equal to that of the bulk fluid flowrate. This paper presents a study of RBC partitioning in a single bifurcating microchannel with dimensions of 6 to 16 Î¼m, investigating the effects of hematocrit, channel width, daughter channel flowrate ratio, and bifurcation angle. The erythrocyte flux ratio, N*, manifests itself as either regular or reverse partitioning, and time-dependent partitioning is much more dynamic, occurring as both regular and reverse partitioning. We report a significant reduction in the well-known sigmoidal variation of the erythrocyte flux ratio (N*) versus the volumetric flowrate ratio (Q*), partitioning behavior with increasing hematocrit in microchannels when the channel dimensions are comparable to cell size. RBCs "lingering" or jamming at the bifurcation were also observed and quantified in vitro. Results from trajectory analyses suggest that the RBC position in the feeder channel strongly affects both partitioning and lingering frequency of RBCs, with both being significantly reduced when RBCs flow on streamlines near the edge of the channel as opposed to the center of the channel. Furthermore, our experiments suggest that even at low Reynolds number, partitioning is affected by the bifurcation angle by increasing cell-cell interactions. The presented results provide further insight into RBC partitioning as well as perfusion throughout the microvasculature.

Additional Links: PMID-33590890

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@article {pmid33590890,

year = {2021},

author = {Pskowski, A and Bagchi, P and Zahn, JD},

title = {Investigation of red blood cell partitioning in an in vitro microvascular bifurcation.},

journal = {Artificial organs},

volume = {},

number = {},

pages = {},

doi = {10.1111/aor.13941},

pmid = {33590890},

issn = {1525-1594},

abstract = {There is a long history of research examining red blood cell (RBC) partitioning in microvasculature bifurcations. These studies commonly report results describing partitioning that exists as either regular partitioning, which occurs when the RBC flux ratio is greater than the bulk fluid flow rate ratio, or reverse partitioning when the RBC flux ratio is less than or equal to that of the bulk fluid flowrate. This paper presents a study of RBC partitioning in a single bifurcating microchannel with dimensions of 6 to 16 Î¼m, investigating the effects of hematocrit, channel width, daughter channel flowrate ratio, and bifurcation angle. The erythrocyte flux ratio, N*, manifests itself as either regular or reverse partitioning, and time-dependent partitioning is much more dynamic, occurring as both regular and reverse partitioning. We report a significant reduction in the well-known sigmoidal variation of the erythrocyte flux ratio (N*) versus the volumetric flowrate ratio (Q*), partitioning behavior with increasing hematocrit in microchannels when the channel dimensions are comparable to cell size. RBCs "lingering" or jamming at the bifurcation were also observed and quantified in vitro. Results from trajectory analyses suggest that the RBC position in the feeder channel strongly affects both partitioning and lingering frequency of RBCs, with both being significantly reduced when RBCs flow on streamlines near the edge of the channel as opposed to the center of the channel. Furthermore, our experiments suggest that even at low Reynolds number, partitioning is affected by the bifurcation angle by increasing cell-cell interactions. The presented results provide further insight into RBC partitioning as well as perfusion throughout the microvasculature.},

}

RevDate: 2021-02-13

**From diffusive mass transfer in Stokes flow to low Reynolds number Marangoni boats.**

*The European physical journal. E, Soft matter*, **44(1):**4.

We present a theory for the self-propulsion of symmetric, half-spherical Marangoni boats (soap or camphor boats) at low Reynolds numbers. Propulsion is generated by release (diffusive emission or dissolution) of water-soluble surfactant molecules, which modulate the air-water interfacial tension. Propulsion either requires asymmetric release or spontaneous symmetry breaking by coupling to advection for a perfectly symmetrical swimmer. We study the diffusion-advection problem for a sphere in Stokes flow analytically and numerically both for constant concentration and constant flux boundary conditions. We derive novel results for concentration profiles under constant flux boundary conditions and for the Nusselt number (the dimensionless ratio of total emitted flux and diffusive flux). Based on these results, we analyze the Marangoni boat for small Marangoni propulsion (low Peclet number) and show that two swimming regimes exist, a diffusive regime at low velocities and an advection-dominated regime at high swimmer velocities. We describe both the limit of large Marangoni propulsion (high Peclet number) and the effects from evaporation by approximative analytical theories. The swimming velocity is determined by force balance, and we obtain a general expression for the Marangoni forces, which comprises both direct Marangoni forces from the surface tension gradient along the air-water-swimmer contact line and Marangoni flow forces. We unravel whether the Marangoni flow contribution is exerting a forward or backward force during propulsion. Our main result is the relation between Peclet number and swimming velocity. Spontaneous symmetry breaking and, thus, swimming occur for a perfectly symmetrical swimmer above a critical Peclet number, which becomes small for large system sizes. We find a supercritical swimming bifurcation for a symmetric swimmer and an avoided bifurcation in the presence of an asymmetry.

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@article {pmid33580288,

year = {2021},

author = {Ender, H and Kierfeld, J},

title = {From diffusive mass transfer in Stokes flow to low Reynolds number Marangoni boats.},

journal = {The European physical journal. E, Soft matter},

volume = {44},

number = {1},

pages = {4},

pmid = {33580288},

issn = {1292-895X},

abstract = {We present a theory for the self-propulsion of symmetric, half-spherical Marangoni boats (soap or camphor boats) at low Reynolds numbers. Propulsion is generated by release (diffusive emission or dissolution) of water-soluble surfactant molecules, which modulate the air-water interfacial tension. Propulsion either requires asymmetric release or spontaneous symmetry breaking by coupling to advection for a perfectly symmetrical swimmer. We study the diffusion-advection problem for a sphere in Stokes flow analytically and numerically both for constant concentration and constant flux boundary conditions. We derive novel results for concentration profiles under constant flux boundary conditions and for the Nusselt number (the dimensionless ratio of total emitted flux and diffusive flux). Based on these results, we analyze the Marangoni boat for small Marangoni propulsion (low Peclet number) and show that two swimming regimes exist, a diffusive regime at low velocities and an advection-dominated regime at high swimmer velocities. We describe both the limit of large Marangoni propulsion (high Peclet number) and the effects from evaporation by approximative analytical theories. The swimming velocity is determined by force balance, and we obtain a general expression for the Marangoni forces, which comprises both direct Marangoni forces from the surface tension gradient along the air-water-swimmer contact line and Marangoni flow forces. We unravel whether the Marangoni flow contribution is exerting a forward or backward force during propulsion. Our main result is the relation between Peclet number and swimming velocity. Spontaneous symmetry breaking and, thus, swimming occur for a perfectly symmetrical swimmer above a critical Peclet number, which becomes small for large system sizes. We find a supercritical swimming bifurcation for a symmetric swimmer and an avoided bifurcation in the presence of an asymmetry.},

}

RevDate: 2021-02-12

**Lattice Boltzmann Solver for Multiphase Flows: Application to High Weber and Reynolds Numbers.**

*Entropy (Basel, Switzerland)*, **23(2):** pii:e23020166.

The lattice Boltzmann method, now widely used for a variety of applications, has also been extended to model multiphase flows through different formulations. While already applied to many different configurations in low Weber and Reynolds number regimes, applications to higher Weber/Reynolds numbers or larger density/viscosity ratios are still the topic of active research. In this study, through a combination of a decoupled phase-field formulation-the conservative Allen-Cahn equation-and a cumulant-based collision operator for a low-Mach pressure-based flow solver, we present an algorithm that can be used for higher Reynolds/Weber numbers. The algorithm was validated through a variety of test cases, starting with the Rayleigh-Taylor instability in both 2D and 3D, followed by the impact of a droplet on a liquid sheet. In all simulations, the solver correctly captured the flow dynamics andmatched reference results very well. As the final test case, the solver was used to model droplet splashing on a thin liquid sheet in 3D with a density ratio of 1000 and kinematic viscosity ratio of 15, matching the water/air system at We = 8000 and Re = 1000. Results showed that the solver correctly captured the fingering instabilities at the crown rim and their subsequent breakup, in agreement with experimental and numerical observations reported in the literature.

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@article {pmid33573067,

year = {2021},

author = {Hosseini, SA and Safari, H and Thevenin, D},

title = {Lattice Boltzmann Solver for Multiphase Flows: Application to High Weber and Reynolds Numbers.},

journal = {Entropy (Basel, Switzerland)},

volume = {23},

number = {2},

pages = {},

doi = {10.3390/e23020166},

pmid = {33573067},

issn = {1099-4300},

support = {TRR287-422037413//Deutsche Forschungsgemeinschaft/ ; },

abstract = {The lattice Boltzmann method, now widely used for a variety of applications, has also been extended to model multiphase flows through different formulations. While already applied to many different configurations in low Weber and Reynolds number regimes, applications to higher Weber/Reynolds numbers or larger density/viscosity ratios are still the topic of active research. In this study, through a combination of a decoupled phase-field formulation-the conservative Allen-Cahn equation-and a cumulant-based collision operator for a low-Mach pressure-based flow solver, we present an algorithm that can be used for higher Reynolds/Weber numbers. The algorithm was validated through a variety of test cases, starting with the Rayleigh-Taylor instability in both 2D and 3D, followed by the impact of a droplet on a liquid sheet. In all simulations, the solver correctly captured the flow dynamics andmatched reference results very well. As the final test case, the solver was used to model droplet splashing on a thin liquid sheet in 3D with a density ratio of 1000 and kinematic viscosity ratio of 15, matching the water/air system at We = 8000 and Re = 1000. Results showed that the solver correctly captured the fingering instabilities at the crown rim and their subsequent breakup, in agreement with experimental and numerical observations reported in the literature.},

}

RevDate: 2021-02-11

**Implications of changing synchronization in propulsive performance of side-by-side pitching foils.**

*Bioinspiration & biomimetics* [Epub ahead of print].

The unsteady hydrodynamics of side-by-side pitching foils are studied numerically at Reynolds number of 4000 with altering phase differences in the middle of an oscillation cycle. This represents a change in synchronization of oscillating foils, inspired by experimental observations on group swimming of Red nose tetra fish. The hybrid oscillation cases are based on an initially out-of-phase pitching that switch to in-phase at the 20th cycle of oscillation. Various sequential combinations of out-of-phase and in-phase pitching are also examined in terms of foil propulsive performance. It is observed that out-of-phase pitching foils initially produce zero total side-force. However, they start producing negative total side-force after 13 oscillation cycles. Contrarily for the in-phase oscillation cases, the initially positive total side-force reverted to zero over time. In hybrid oscillation cases, the negative total side-force produced during the initial out-of-phase oscillations abruptly adjusted to zero following a change of synchronization that led to in-phase oscillations, which is inspired from a particular swimming behavior in fish. Based on three hybrid modes, defined on the onset of mid-cycle switch to in-phase oscillations, it was apparent that the benefit of synchronization, or there lack of, greatly depended on the timing of the change in synchronization. Thus, mid-swimming change of synchronization in side-by-side systems inspired by fish schools compensates for their non-zero total side-force production to maintain their lateral position. Such changes do not translate to significant gains in neither thrust generation nor efficiency.

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@article {pmid33571986,

year = {2021},

author = {Gungor, A and Hemmati, A},

title = {Implications of changing synchronization in propulsive performance of side-by-side pitching foils.},

journal = {Bioinspiration & biomimetics},

volume = {},

number = {},

pages = {},

doi = {10.1088/1748-3190/abe54b},

pmid = {33571986},

issn = {1748-3190},

abstract = {The unsteady hydrodynamics of side-by-side pitching foils are studied numerically at Reynolds number of 4000 with altering phase differences in the middle of an oscillation cycle. This represents a change in synchronization of oscillating foils, inspired by experimental observations on group swimming of Red nose tetra fish. The hybrid oscillation cases are based on an initially out-of-phase pitching that switch to in-phase at the 20th cycle of oscillation. Various sequential combinations of out-of-phase and in-phase pitching are also examined in terms of foil propulsive performance. It is observed that out-of-phase pitching foils initially produce zero total side-force. However, they start producing negative total side-force after 13 oscillation cycles. Contrarily for the in-phase oscillation cases, the initially positive total side-force reverted to zero over time. In hybrid oscillation cases, the negative total side-force produced during the initial out-of-phase oscillations abruptly adjusted to zero following a change of synchronization that led to in-phase oscillations, which is inspired from a particular swimming behavior in fish. Based on three hybrid modes, defined on the onset of mid-cycle switch to in-phase oscillations, it was apparent that the benefit of synchronization, or there lack of, greatly depended on the timing of the change in synchronization. Thus, mid-swimming change of synchronization in side-by-side systems inspired by fish schools compensates for their non-zero total side-force production to maintain their lateral position. Such changes do not translate to significant gains in neither thrust generation nor efficiency.},

}

RevDate: 2021-02-09

**Snaps of a tiny amphipod push the boundary of ultrafast, repeatable movement.**

*Current biology : CB*, **31(3):**R116-R117.

Surprisingly, the fastest motions are not produced by large animals or robots. Rather, small organisms or structures, including cnidarian stinging cells, fungal shooting spores, and mandible strikes of ants, termites, and spiders, hold the world acceleration records.1-5 These diverse systems share common features: they rapidly convert potential energy - stored in deformed material or fluid - into kinetic energy when a latch is released.4-6 However, the fastest of these are not repeatable, because mechanical components are broken or ejected.5,6 Furthermore, some of these systems must overcome the added challenge of moving in water, where high density and viscosity constrain acceleration at small sizes. Here we report the kinematics of repeatable, ultrafast snaps by tiny marine amphipods (Dulichiella cf. appendiculata). Males use their enlarged major claw, which can exceed 30% of body mass, to snap a 1 mm-long dactyl with a diameter equivalent to a human hair (184 Î¼m). The claw snaps closed extremely rapidly, averaging 93 Î¼s, 17 m s-1, and 2.4 x 105 m s-2. These snaps are among the smallest and fastest of any documented repeatable movement, and are sufficiently fast to operate in the inertial hydrodynamic regime (Reynolds number (Re) >10,000). They generate audible pops and rapid water jets, which occasionally yield cavitation, and may be used for defense. These amphipod snaps push the boundaries of acceleration and size for repeatable movements, particularly in water, and exemplify how new biomechanical insights can arise from unassuming animals. VIDEO ABSTRACT.

Additional Links: PMID-33561405

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@article {pmid33561405,

year = {2021},

author = {Longo, SJ and Ray, W and Farley, GM and Harrison, J and Jorge, J and Kaji, T and Palmer, AR and Patek, SN},

title = {Snaps of a tiny amphipod push the boundary of ultrafast, repeatable movement.},

journal = {Current biology : CB},

volume = {31},

number = {3},

pages = {R116-R117},

doi = {10.1016/j.cub.2020.12.025},

pmid = {33561405},

issn = {1879-0445},

abstract = {Surprisingly, the fastest motions are not produced by large animals or robots. Rather, small organisms or structures, including cnidarian stinging cells, fungal shooting spores, and mandible strikes of ants, termites, and spiders, hold the world acceleration records.1-5 These diverse systems share common features: they rapidly convert potential energy - stored in deformed material or fluid - into kinetic energy when a latch is released.4-6 However, the fastest of these are not repeatable, because mechanical components are broken or ejected.5,6 Furthermore, some of these systems must overcome the added challenge of moving in water, where high density and viscosity constrain acceleration at small sizes. Here we report the kinematics of repeatable, ultrafast snaps by tiny marine amphipods (Dulichiella cf. appendiculata). Males use their enlarged major claw, which can exceed 30% of body mass, to snap a 1 mm-long dactyl with a diameter equivalent to a human hair (184 Î¼m). The claw snaps closed extremely rapidly, averaging 93 Î¼s, 17 m s-1, and 2.4 x 105 m s-2. These snaps are among the smallest and fastest of any documented repeatable movement, and are sufficiently fast to operate in the inertial hydrodynamic regime (Reynolds number (Re) >10,000). They generate audible pops and rapid water jets, which occasionally yield cavitation, and may be used for defense. These amphipod snaps push the boundaries of acceleration and size for repeatable movements, particularly in water, and exemplify how new biomechanical insights can arise from unassuming animals. VIDEO ABSTRACT.},

}

RevDate: 2021-02-05

**Homogeneous nucleation in a Poiseuille flow.**

*Physical chemistry chemical physics : PCCP* [Epub ahead of print].

Nucleation in a dynamical environment plays an important role in the synthesis and manufacturing of quantum dots and nanocrystals. In this work, we investigate the effects of fluid flow (low Reynolds number flow) on the homogeneous nucleation in a circular microchannel in the framework of the classical nucleation theory. The contributions of the configuration entropy from the momentum-phase space and the kinetic energy and strain energy of a microcluster are incorporated in the calculation of the change of the Gibbs free energy from a flow state without a microcluster to a flow state with a microcluster. An analytical equation is derived for the determination of the critical nucleus size. Using this analytical equation, an analytical solution of the critical nucleus size for the formation of a critical liquid nucleus is found. For the formation of a critical solid nucleus, the contributions from both the kinetic energy and the strain energy are generally negligible. We perform numerical analysis of the homogeneous nucleation of a sucrose microcluster in a representative volume element of an aqueous solution, which flows through a circular microchannel. The numerical results reveal the decrease of the critical nucleus size and the corresponding work of formation of a critical nucleus with the increase of the distance to axisymmetric axis for the same numbers of solvent atoms and solute atoms/particles.

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@article {pmid33544100,

year = {2021},

author = {Yang, F},

title = {Homogeneous nucleation in a Poiseuille flow.},

journal = {Physical chemistry chemical physics : PCCP},

volume = {},

number = {},

pages = {},

doi = {10.1039/d0cp06132h},

pmid = {33544100},

issn = {1463-9084},

abstract = {Nucleation in a dynamical environment plays an important role in the synthesis and manufacturing of quantum dots and nanocrystals. In this work, we investigate the effects of fluid flow (low Reynolds number flow) on the homogeneous nucleation in a circular microchannel in the framework of the classical nucleation theory. The contributions of the configuration entropy from the momentum-phase space and the kinetic energy and strain energy of a microcluster are incorporated in the calculation of the change of the Gibbs free energy from a flow state without a microcluster to a flow state with a microcluster. An analytical equation is derived for the determination of the critical nucleus size. Using this analytical equation, an analytical solution of the critical nucleus size for the formation of a critical liquid nucleus is found. For the formation of a critical solid nucleus, the contributions from both the kinetic energy and the strain energy are generally negligible. We perform numerical analysis of the homogeneous nucleation of a sucrose microcluster in a representative volume element of an aqueous solution, which flows through a circular microchannel. The numerical results reveal the decrease of the critical nucleus size and the corresponding work of formation of a critical nucleus with the increase of the distance to axisymmetric axis for the same numbers of solvent atoms and solute atoms/particles.},

}

RevDate: 2021-02-05

**Small-Scale Isotropy and Ramp-Cliff Structures in Scalar Turbulence.**

*Physical review letters*, **126(3):**034504.

Passive scalars advected by three-dimensional Navier-Stokes turbulence exhibit a fundamental anomaly in odd-order moments because of the characteristic ramp-cliff structures, violating small-scale isotropy. We use data from direct numerical simulations with grid resolution of up to 8192^{3} at high PÃ©clet numbers to understand this anomaly as the scalar diffusivity, D, diminishes, or as the Schmidt number, Sc=Î½/D, increases; here Î½ is the kinematic viscosity of the fluid. The microscale Reynolds number varies from 140 to 650 and Sc varies from 1 to 512. A simple model for the ramp-cliff structures is developed and shown to characterize the scalar derivative statistics very well. It accurately captures how the small-scale isotropy is restored in the large-Sc limit, and additionally suggests a possible correction to the Batchelor length scale as the relevant smallest scale in the scalar field.

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@article {pmid33543985,

year = {2021},

author = {Buaria, D and Clay, MP and Sreenivasan, KR and Yeung, PK},

title = {Small-Scale Isotropy and Ramp-Cliff Structures in Scalar Turbulence.},

journal = {Physical review letters},

volume = {126},

number = {3},

pages = {034504},

doi = {10.1103/PhysRevLett.126.034504},

pmid = {33543985},

issn = {1079-7114},

abstract = {Passive scalars advected by three-dimensional Navier-Stokes turbulence exhibit a fundamental anomaly in odd-order moments because of the characteristic ramp-cliff structures, violating small-scale isotropy. We use data from direct numerical simulations with grid resolution of up to 8192^{3}

at high PÃ©clet numbers to understand this anomaly as the scalar diffusivity, D, diminishes, or as the Schmidt number, Sc=Î½/D, increases; here Î½ is the kinematic viscosity of the fluid. The microscale Reynolds number varies from 140 to 650 and Sc varies from 1 to 512. A simple model for the ramp-cliff structures is developed and shown to characterize the scalar derivative statistics very well. It accurately captures how the small-scale isotropy is restored in the large-Sc limit, and additionally suggests a possible correction to the Batchelor length scale as the relevant smallest scale in the scalar field.},

}

RevDate: 2021-02-05

**Minimum Dissipation Theorem for Microswimmers.**

*Physical review letters*, **126(3):**034503.

We derive a theorem for the lower bound on the energy dissipation rate by a rigid surface-driven active microswimmer of arbitrary shape in a fluid at a low Reynolds number. We show that, for any swimmer, the minimum dissipation at a given velocity can be expressed in terms of the resistance tensors of two passive bodies of the same shape with a no-slip and perfect-slip boundary. To achieve the absolute minimum dissipation, the optimal swimmer needs a surface velocity profile that corresponds to the flow around the perfect-slip body, and a propulsive force density that corresponds to the no-slip body. Using this theorem, we propose an alternative definition of the energetic efficiency of microswimmers that, unlike the commonly used Lighthill efficiency, can never exceed unity. We validate the theory by calculating the efficiency limits of spheroidal swimmers.

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@article {pmid33543965,

year = {2021},

author = {Nasouri, B and Vilfan, A and Golestanian, R},

title = {Minimum Dissipation Theorem for Microswimmers.},

journal = {Physical review letters},

volume = {126},

number = {3},

pages = {034503},

doi = {10.1103/PhysRevLett.126.034503},

pmid = {33543965},

issn = {1079-7114},

abstract = {We derive a theorem for the lower bound on the energy dissipation rate by a rigid surface-driven active microswimmer of arbitrary shape in a fluid at a low Reynolds number. We show that, for any swimmer, the minimum dissipation at a given velocity can be expressed in terms of the resistance tensors of two passive bodies of the same shape with a no-slip and perfect-slip boundary. To achieve the absolute minimum dissipation, the optimal swimmer needs a surface velocity profile that corresponds to the flow around the perfect-slip body, and a propulsive force density that corresponds to the no-slip body. Using this theorem, we propose an alternative definition of the energetic efficiency of microswimmers that, unlike the commonly used Lighthill efficiency, can never exceed unity. We validate the theory by calculating the efficiency limits of spheroidal swimmers.},

}

RevDate: 2021-02-04

**Application of Proper Orthogonal Decomposition to Study Coherent Flow Structures in a Saccular Aneurysm.**

*Journal of biomechanical engineering* pii:1097187 [Epub ahead of print].

Aneurysms are localized expansions of weakened blood vessels that can be debilitating or fatal upon rupture. Previous studies have shown that flow in an aneurysm exhibits complex flow structures that are correlated with its inflow conditions. Therefore, the objective of this study was to demonstrate the application of Proper Orthogonal Decomposition (POD) to study the impact of different inflow conditions on energetic flow structures and their temporal behavior in an aneurysm. To achieve this objective, experiments were performed on an idealized rigid sidewall aneurysm model. A piston pump system was used for precise inflow control, i.e., peak Reynolds number ($Re_p$) and Womersley number ($\alpha$) were varied from 50-270 and 2-5, respectively. The velocity flow field measurements at the mid-plane location of the idealized aneurysm model were performed using Particle Image Velocimetry (PIV). The results demonstrate the efficacy of POD in decomposing complex data, and POD was able to capture the energetic flow structures unique to each studied inflow condition. Furthermore, the time-varying coefficient results highlighted the interplay between the coefficients and their corresponding POD modes, which in turn helped explain how POD modes impact certain flow features. The low-order reconstruction results were able to capture the flow evolution and provide information on complex flow in an aneurysm. The POD and low-order reconstruction results also indicated that vortex formation, evolution, and convection varied with an increase in $\alpha$, while vortex strength and formation of secondary structures were correlated with an increase in $Re_p$.

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@article {pmid33537715,

year = {2021},

author = {Yu, P and Durgesh, V and Xing, T and Budwig, R},

title = {Application of Proper Orthogonal Decomposition to Study Coherent Flow Structures in a Saccular Aneurysm.},

journal = {Journal of biomechanical engineering},

volume = {},

number = {},

pages = {},

doi = {10.1115/1.4050032},

pmid = {33537715},

issn = {1528-8951},

abstract = {Aneurysms are localized expansions of weakened blood vessels that can be debilitating or fatal upon rupture. Previous studies have shown that flow in an aneurysm exhibits complex flow structures that are correlated with its inflow conditions. Therefore, the objective of this study was to demonstrate the application of Proper Orthogonal Decomposition (POD) to study the impact of different inflow conditions on energetic flow structures and their temporal behavior in an aneurysm. To achieve this objective, experiments were performed on an idealized rigid sidewall aneurysm model. A piston pump system was used for precise inflow control, i.e., peak Reynolds number ($Re_p$) and Womersley number ($\alpha$) were varied from 50-270 and 2-5, respectively. The velocity flow field measurements at the mid-plane location of the idealized aneurysm model were performed using Particle Image Velocimetry (PIV). The results demonstrate the efficacy of POD in decomposing complex data, and POD was able to capture the energetic flow structures unique to each studied inflow condition. Furthermore, the time-varying coefficient results highlighted the interplay between the coefficients and their corresponding POD modes, which in turn helped explain how POD modes impact certain flow features. The low-order reconstruction results were able to capture the flow evolution and provide information on complex flow in an aneurysm. The POD and low-order reconstruction results also indicated that vortex formation, evolution, and convection varied with an increase in $\alpha$, while vortex strength and formation of secondary structures were correlated with an increase in $Re_p$.},

}

RevDate: 2021-02-03

**Thermal Management and Modeling of Forced Convection and Entropy Generation in a Vented Cavity by Simultaneous Use of a Curved Porous Layer and Magnetic Field.**

*Entropy (Basel, Switzerland)*, **23(2):** pii:e23020152.

The effects of using a partly curved porous layer on the thermal management and entropy generation features are studied in a ventilated cavity filled with hybrid nanofluid under the effects of inclined magnetic field by using finite volume method. This study is performed for the range of pertinent parameters of Reynolds number (100â‰¤Reâ‰¤1000), magnetic field strength (0â‰¤Haâ‰¤80), permeability of porous region (10-4â‰¤Daâ‰¤5Ã—10-2), porous layer height (0.15Hâ‰¤tpâ‰¤0.45H), porous layer position (0.25Hâ‰¤ypâ‰¤0.45H), and curvature size (0â‰¤bâ‰¤0.3H). The magnetic field reduces the vortex size, while the average Nusselt number of hot walls increases for Ha number above 20 and highest enhancement is 47% for left vertical wall. The variation in the average Nu with permeability of the layer is about 12.5% and 21% for left and right vertical walls, respectively, while these amounts are 12.5% and 32.5% when the location of the porous layer changes. The entropy generation increases with Hartmann number above 20, while there is 22% increase in the entropy generation for the case at the highest magnetic field. The porous layer height reduced the entropy generation for domain above it and it give the highest contribution to the overall entropy generation. When location of the curved porous layer is varied, the highest variation of entropy generation is attained for the domain below it while the lowest value is obtained at yp=0.3H. When the size of elliptic curvature is varied, the overall entropy generation decreases from b=0 to b=0.2H by about 10% and then increases by 5% from b=0.2H to b=0.3H.

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@article {pmid33530578,

year = {2021},

author = {Selimefendigil, F and Ã–ztop, HF},

title = {Thermal Management and Modeling of Forced Convection and Entropy Generation in a Vented Cavity by Simultaneous Use of a Curved Porous Layer and Magnetic Field.},

journal = {Entropy (Basel, Switzerland)},

volume = {23},

number = {2},

pages = {},

doi = {10.3390/e23020152},

pmid = {33530578},

issn = {1099-4300},

abstract = {The effects of using a partly curved porous layer on the thermal management and entropy generation features are studied in a ventilated cavity filled with hybrid nanofluid under the effects of inclined magnetic field by using finite volume method. This study is performed for the range of pertinent parameters of Reynolds number (100â‰¤Reâ‰¤1000), magnetic field strength (0â‰¤Haâ‰¤80), permeability of porous region (10-4â‰¤Daâ‰¤5Ã—10-2), porous layer height (0.15Hâ‰¤tpâ‰¤0.45H), porous layer position (0.25Hâ‰¤ypâ‰¤0.45H), and curvature size (0â‰¤bâ‰¤0.3H). The magnetic field reduces the vortex size, while the average Nusselt number of hot walls increases for Ha number above 20 and highest enhancement is 47% for left vertical wall. The variation in the average Nu with permeability of the layer is about 12.5% and 21% for left and right vertical walls, respectively, while these amounts are 12.5% and 32.5% when the location of the porous layer changes. The entropy generation increases with Hartmann number above 20, while there is 22% increase in the entropy generation for the case at the highest magnetic field. The porous layer height reduced the entropy generation for domain above it and it give the highest contribution to the overall entropy generation. When location of the curved porous layer is varied, the highest variation of entropy generation is attained for the domain below it while the lowest value is obtained at yp=0.3H. When the size of elliptic curvature is varied, the overall entropy generation decreases from b=0 to b=0.2H by about 10% and then increases by 5% from b=0.2H to b=0.3H.},

}

RevDate: 2021-01-29

**Concentration profiles of ions and particles under hydrodynamic focusing in Y-shaped square microchannel.**

*Scientific reports*, **11(1):**2585.

Three-dimensional ion and particle concentrations under hydrodynamic focusing in a Y-shaped square microchannel are numerically simulated to clarify the decrease of the ion concentration along the flow direction within the focused particle stream. The simulation model is theoretically governed by the laminar flow and advection-diffusion equations. The governing equations are solved by the finite volume method. The ion and particle concentration distributions at five cross sections after the confluence of the branch channels are analyzed in 30 cases in which the sheath to sample flow rate ratio Qsh/Qsam and the Reynolds number Re are varied as parameters. The results show that the decrease of the cross-sectional average ion concentration along the flow direction within the particle stream [Formula: see text] is described by the diffusion length during the residence time with a characteristic velocity scale. In addition, the deformation of the particle stream due to inertial effects is described by a scaled Reynolds number that is a function of the flow rate ratio. The simulated particle stream thicknesses are validated by theory and a simple experiment. This paper reveals the relationship between the ion and particle concentrations and the dimensionless parameters for hydrodynamic focusing in the Y-shaped square microchannel under typical conditions.

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@article {pmid33510410,

year = {2021},

author = {Sato, N and Kawashima, D and Takei, M},

title = {Concentration profiles of ions and particles under hydrodynamic focusing in Y-shaped square microchannel.},

journal = {Scientific reports},

volume = {11},

number = {1},

pages = {2585},

pmid = {33510410},

issn = {2045-2322},

abstract = {Three-dimensional ion and particle concentrations under hydrodynamic focusing in a Y-shaped square microchannel are numerically simulated to clarify the decrease of the ion concentration along the flow direction within the focused particle stream. The simulation model is theoretically governed by the laminar flow and advection-diffusion equations. The governing equations are solved by the finite volume method. The ion and particle concentration distributions at five cross sections after the confluence of the branch channels are analyzed in 30 cases in which the sheath to sample flow rate ratio Qsh/Qsam and the Reynolds number Re are varied as parameters. The results show that the decrease of the cross-sectional average ion concentration along the flow direction within the particle stream [Formula: see text] is described by the diffusion length during the residence time with a characteristic velocity scale. In addition, the deformation of the particle stream due to inertial effects is described by a scaled Reynolds number that is a function of the flow rate ratio. The simulated particle stream thicknesses are validated by theory and a simple experiment. This paper reveals the relationship between the ion and particle concentrations and the dimensionless parameters for hydrodynamic focusing in the Y-shaped square microchannel under typical conditions.},

}

RevDate: 2021-01-29

**A lower-than-expected saltation threshold at Martian pressure and below.**

*Proceedings of the National Academy of Sciences of the United States of America*, **118(5):**.

Aeolian sediment transport is observed to occur on Mars as well as other extraterrestrial environments, generating ripples and dunes as on Earth. The search for terrestrial analogs of planetary bedforms, as well as environmental simulation experiments able to reproduce their formation in planetary conditions, are powerful ways to question our understanding of geomorphological processes toward unusual environmental conditions. Here, we perform sediment transport laboratory experiments in a closed-circuit wind tunnel placed in a vacuum chamber and operated at extremely low pressures to show that Martian conditions belong to a previously unexplored saltation regime. The threshold wind speed required to initiate saltation is only quantitatively predicted by state-of-the art models up to a density ratio between grain and air of [Formula: see text] but unexpectedly falls to much lower values for higher density ratios. In contrast, impact ripples, whose emergence is continuously observed on the granular bed over the whole pressure range investigated, display a characteristic wavelength and propagation velocity essentially independent of pressure. A comparison of these findings with existing models suggests that sediment transport at low Reynolds number but high grain-to-fluid density ratio may be dominated by collective effects associated with grain inertia in the granular collisional layer.

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@article {pmid33509927,

year = {2021},

author = {Andreotti, B and Claudin, P and Iversen, JJ and Merrison, JP and Rasmussen, KR},

title = {A lower-than-expected saltation threshold at Martian pressure and below.},

journal = {Proceedings of the National Academy of Sciences of the United States of America},

volume = {118},

number = {5},

pages = {},

doi = {10.1073/pnas.2012386118},

pmid = {33509927},

issn = {1091-6490},

abstract = {Aeolian sediment transport is observed to occur on Mars as well as other extraterrestrial environments, generating ripples and dunes as on Earth. The search for terrestrial analogs of planetary bedforms, as well as environmental simulation experiments able to reproduce their formation in planetary conditions, are powerful ways to question our understanding of geomorphological processes toward unusual environmental conditions. Here, we perform sediment transport laboratory experiments in a closed-circuit wind tunnel placed in a vacuum chamber and operated at extremely low pressures to show that Martian conditions belong to a previously unexplored saltation regime. The threshold wind speed required to initiate saltation is only quantitatively predicted by state-of-the art models up to a density ratio between grain and air of [Formula: see text] but unexpectedly falls to much lower values for higher density ratios. In contrast, impact ripples, whose emergence is continuously observed on the granular bed over the whole pressure range investigated, display a characteristic wavelength and propagation velocity essentially independent of pressure. A comparison of these findings with existing models suggests that sediment transport at low Reynolds number but high grain-to-fluid density ratio may be dominated by collective effects associated with grain inertia in the granular collisional layer.},

}

RevDate: 2021-01-27

**Steric scattering of rod-like swimmers in low Reynolds number environments.**

*Soft matter* [Epub ahead of print].

Microbes form integral components of all natural ecosystems. In most cases, the surrounding micro-environment has physical variations that affect the movements of micro-swimmers, including solid objects of varying size, shape and density. As swimmers move through viscous environments, a combination of hydrodynamic and steric forces are known to significantly alter their trajectories in a way that depends on surface curvature. In this work, our goal was to clarify the role of steric forces when rod-like swimmers interact with solid objects comparable to cell size. We imaged hundreds-of-thousands of scattering interactions between swimming bacteria and micro-fabricated pillars with radii from âˆ¼1 to âˆ¼10 cell lengths. Scattering interactions were parameterized by the angle of the cell upon contact with the pillar, and primarily produced forward-scattering events that fell into distinct chiral distributions for scattering angle - no hydrodynamic trapping was observed. The chirality of a scattering event was a stochastic variable whose probability smoothly and symmetrically depended on the contact angle. Neglecting hydrodynamics, we developed a model that only considers contact forces and torques for a rear-pushed thin-rod scattering from a cylinder - the model predictions were in good agreement with measured data. Our results suggest that alteration of bacterial trajectories is subject to distinct mechanisms when interacting with objects of different size; primarily steric for objects below âˆ¼10 cell lengths and requiring incorporation of hydrodynamics at larger scales. These results contribute to a mechanistic framework in which to examine (and potentially engineer) microbial movements through natural and synthetic environments that present complex steric structure.

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@article {pmid33503087,

year = {2021},

author = {Hoeger, K and Ursell, T},

title = {Steric scattering of rod-like swimmers in low Reynolds number environments.},

journal = {Soft matter},

volume = {},

number = {},

pages = {},

doi = {10.1039/d0sm01551b},

pmid = {33503087},

issn = {1744-6848},

abstract = {Microbes form integral components of all natural ecosystems. In most cases, the surrounding micro-environment has physical variations that affect the movements of micro-swimmers, including solid objects of varying size, shape and density. As swimmers move through viscous environments, a combination of hydrodynamic and steric forces are known to significantly alter their trajectories in a way that depends on surface curvature. In this work, our goal was to clarify the role of steric forces when rod-like swimmers interact with solid objects comparable to cell size. We imaged hundreds-of-thousands of scattering interactions between swimming bacteria and micro-fabricated pillars with radii from âˆ¼1 to âˆ¼10 cell lengths. Scattering interactions were parameterized by the angle of the cell upon contact with the pillar, and primarily produced forward-scattering events that fell into distinct chiral distributions for scattering angle - no hydrodynamic trapping was observed. The chirality of a scattering event was a stochastic variable whose probability smoothly and symmetrically depended on the contact angle. Neglecting hydrodynamics, we developed a model that only considers contact forces and torques for a rear-pushed thin-rod scattering from a cylinder - the model predictions were in good agreement with measured data. Our results suggest that alteration of bacterial trajectories is subject to distinct mechanisms when interacting with objects of different size; primarily steric for objects below âˆ¼10 cell lengths and requiring incorporation of hydrodynamics at larger scales. These results contribute to a mechanistic framework in which to examine (and potentially engineer) microbial movements through natural and synthetic environments that present complex steric structure.},

}

RevDate: 2021-01-26

**Homogeneous and Multiphase Analysis of Nanofluids Containing Nonspherical MWCNT and GNP Nanoparticles Considering the Influence of Interfacial Layering.**

*Nanomaterials (Basel, Switzerland)*, **11(2):** pii:nano11020277.

The practical implication of nanofluids is essentially dependent on their accurate modelling, particularly in comparison with the high cost of experimental investigations, yet the accuracy of different computational approaches to simulate nanofluids remains controversial to this day. Therefore, the present study is aimed at analysing the homogenous, multiphase Eulerian-Eulerian (volume of fluid, mixture, Eulerian) and Lagrangian-Eulerian approximation of nanofluids containing nonspherical nanoparticles. The heat transfer and pressure drop characteristics of the multiwalled carbon nanotubes (MWCNT)-based and multiwalled carbon nanotubes/graphene nanoplatelets (MWCNT/GNP)-based nanofluids are computed by incorporating the influence of several physical mechanisms, including interfacial nanolayering. The accuracy of tested computational approaches is evaluated by considering particle concentration and Reynolds number ranges of 0.075-0.25 wt% and 200-470, respectively. The results demonstrate that for all nanofluid combinations and operational conditions, the Lagrangian-Eulerian approximation provides the most accurate convective heat transfer coefficient values with a maximum deviation of 5.34% for 0.25 wt% of MWCNT-water nanofluid at the largest Reynolds number, while single-phase and Eulerian-Eulerian multiphase models accurately estimate the thermal fields of the diluted nanofluids at low Reynolds numbers, but overestimate the results for denser nanofluids at high Reynolds numbers.

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@article {pmid33494348,

year = {2021},

author = {Ambreen, T and Saleem, A and Park, CW},

title = {Homogeneous and Multiphase Analysis of Nanofluids Containing Nonspherical MWCNT and GNP Nanoparticles Considering the Influence of Interfacial Layering.},

journal = {Nanomaterials (Basel, Switzerland)},

volume = {11},

number = {2},

pages = {},

doi = {10.3390/nano11020277},

pmid = {33494348},

issn = {2079-4991},

abstract = {The practical implication of nanofluids is essentially dependent on their accurate modelling, particularly in comparison with the high cost of experimental investigations, yet the accuracy of different computational approaches to simulate nanofluids remains controversial to this day. Therefore, the present study is aimed at analysing the homogenous, multiphase Eulerian-Eulerian (volume of fluid, mixture, Eulerian) and Lagrangian-Eulerian approximation of nanofluids containing nonspherical nanoparticles. The heat transfer and pressure drop characteristics of the multiwalled carbon nanotubes (MWCNT)-based and multiwalled carbon nanotubes/graphene nanoplatelets (MWCNT/GNP)-based nanofluids are computed by incorporating the influence of several physical mechanisms, including interfacial nanolayering. The accuracy of tested computational approaches is evaluated by considering particle concentration and Reynolds number ranges of 0.075-0.25 wt% and 200-470, respectively. The results demonstrate that for all nanofluid combinations and operational conditions, the Lagrangian-Eulerian approximation provides the most accurate convective heat transfer coefficient values with a maximum deviation of 5.34% for 0.25 wt% of MWCNT-water nanofluid at the largest Reynolds number, while single-phase and Eulerian-Eulerian multiphase models accurately estimate the thermal fields of the diluted nanofluids at low Reynolds numbers, but overestimate the results for denser nanofluids at high Reynolds numbers.},

}

RevDate: 2021-01-25

**Numerical study on the energy cascade of pulsatile Newtonian and power-law flow models in an ICA bifurcation.**

*PloS one*, **16(1):**e0245775 pii:PONE-D-20-01106.

The complex physics and biology underlying intracranial hemodynamics are yet to be fully revealed. A fully resolved direct numerical simulation (DNS) study has been performed to identify the intrinsic flow dynamics in an idealized carotid bifurcation model. To shed the light on the significance of considering blood shear-thinning properties, the power-law model is compared to the commonly used Newtonian viscosity hypothesis. We scrutinize the kinetic energy cascade (KEC) rates in the Fourier domain and the vortex structure of both fluid models and examine the impact of the power-law viscosity model. The flow intrinsically contains coherent structures which has frequencies corresponding to the boundary frequency, which could be associated with the regulation of endothelial cells. From the proposed comparative study, it is found that KEC rates and the vortex-identification are significantly influenced by the shear-thinning blood properties. Conclusively, from the obtained results, it is found that neglecting the non-Newtonian behavior could lead to underestimation of the hemodynamic parameters at low Reynolds number and overestimation of the hemodynamic parameters by increasing the Reynolds number. In addition, we provide physical insight and discussion onto the hemodynamics associated with endothelial dysfunction which plays significant role in the pathogenesis of intracranial aneurysms.

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@article {pmid33493237,

year = {2021},

author = {Mahrous, SA and Sidik, NAC and Saqr, KM},

title = {Numerical study on the energy cascade of pulsatile Newtonian and power-law flow models in an ICA bifurcation.},

journal = {PloS one},

volume = {16},

number = {1},

pages = {e0245775},

doi = {10.1371/journal.pone.0245775},

pmid = {33493237},

issn = {1932-6203},

abstract = {The complex physics and biology underlying intracranial hemodynamics are yet to be fully revealed. A fully resolved direct numerical simulation (DNS) study has been performed to identify the intrinsic flow dynamics in an idealized carotid bifurcation model. To shed the light on the significance of considering blood shear-thinning properties, the power-law model is compared to the commonly used Newtonian viscosity hypothesis. We scrutinize the kinetic energy cascade (KEC) rates in the Fourier domain and the vortex structure of both fluid models and examine the impact of the power-law viscosity model. The flow intrinsically contains coherent structures which has frequencies corresponding to the boundary frequency, which could be associated with the regulation of endothelial cells. From the proposed comparative study, it is found that KEC rates and the vortex-identification are significantly influenced by the shear-thinning blood properties. Conclusively, from the obtained results, it is found that neglecting the non-Newtonian behavior could lead to underestimation of the hemodynamic parameters at low Reynolds number and overestimation of the hemodynamic parameters by increasing the Reynolds number. In addition, we provide physical insight and discussion onto the hemodynamics associated with endothelial dysfunction which plays significant role in the pathogenesis of intracranial aneurysms.},

}

RevDate: 2021-01-25

**Finite element analysis of mixed convection flow in a trapezoidal cavity with non-uniform temperature.**

*Heliyon*, **7(1):**e05933 pii:S2405-8440(21)00038-4.

A two dimensional flow analysis in a cavity shaped isosceles trapezium is carried out. Non-parallel sides of a trapezium are adiabatic. A varying sinusoidal temperature is applied to the lower wall while the upper wall is at constant temperature. Upper wall of the cavity moves with a velocity Î· 0 in the positive x-direction. Also, B 0 is constant magnetic field of strength aligned in the same x-direction and Newtonian fluid is considered. The values of magnetic field parameter used are H a = 0 , 50 , the Richardson number is R i = 0.1 , 1 , 10 , R e = 100 is Reynolds number used for the analysis, the amplitude of sinusoidal temperature is m = 0.25 , 0.5 , 1 . The impacts of different leading parameters are analyzed by plotting streamlines for flow fields and isotherm contours for temperature of the flow dynamics. The graphs that signify the variation of average Nusselt number and local Nusselt number are sketched for both lower and upper walls of the cavity. Result indicated that with constant temperature the top wall of the boundary layer thickness decreases as Richardson number Ri increases and for bottom wall with variable temperature. The Nusselt number gets higher with an increment in the amplitude of the oscillation of temperature function. Furthermore, the study revealed that the average Nusselt number gets reduced as the intensity of magnetic field is enhanced. The variation in transit of heat at the bottom wall is similar but the maximum value of heat transfer at the bottom wall shows a variation from 3.8 to 20 when H a = 0 and from 3 to 18 when H a = 50 . The accuracy of the present numerical algorithms is also established.

Additional Links: PMID-33490686

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@article {pmid33490686,

year = {2021},

author = {Ibrahim, W and Hirpho, M},

title = {Finite element analysis of mixed convection flow in a trapezoidal cavity with non-uniform temperature.},

journal = {Heliyon},

volume = {7},

number = {1},

pages = {e05933},

doi = {10.1016/j.heliyon.2021.e05933},

pmid = {33490686},

issn = {2405-8440},

abstract = {A two dimensional flow analysis in a cavity shaped isosceles trapezium is carried out. Non-parallel sides of a trapezium are adiabatic. A varying sinusoidal temperature is applied to the lower wall while the upper wall is at constant temperature. Upper wall of the cavity moves with a velocity Î· 0 in the positive x-direction. Also, B 0 is constant magnetic field of strength aligned in the same x-direction and Newtonian fluid is considered. The values of magnetic field parameter used are H a = 0 , 50 , the Richardson number is R i = 0.1 , 1 , 10 , R e = 100 is Reynolds number used for the analysis, the amplitude of sinusoidal temperature is m = 0.25 , 0.5 , 1 . The impacts of different leading parameters are analyzed by plotting streamlines for flow fields and isotherm contours for temperature of the flow dynamics. The graphs that signify the variation of average Nusselt number and local Nusselt number are sketched for both lower and upper walls of the cavity. Result indicated that with constant temperature the top wall of the boundary layer thickness decreases as Richardson number Ri increases and for bottom wall with variable temperature. The Nusselt number gets higher with an increment in the amplitude of the oscillation of temperature function. Furthermore, the study revealed that the average Nusselt number gets reduced as the intensity of magnetic field is enhanced. The variation in transit of heat at the bottom wall is similar but the maximum value of heat transfer at the bottom wall shows a variation from 3.8 to 20 when H a = 0 and from 3 to 18 when H a = 50 . The accuracy of the present numerical algorithms is also established.},

}

RevDate: 2021-01-22

**Numerical Investigation of Roughness Effects on Transition on Spherical Capsules.**

*Journal of spacecraft and rockets*, **56(2):**388-404.

To address the hitherto unknown mechanism of boundary-layer transition on blunt reentry capsules, the role of roughness-induced disturbance growth on a spherical-section forebody is assessed via optimal transient growth theory and direct numerical simulations (DNS). Optimal transient-growth studies have been performed for the blunt capsule experiments at Mach 5.9 in the Hypersonic Ludwieg tube Braunschweig (HLB) of the Technische UniversitÃ¤t Braunschweig, which included measurements behind a patch of controlled, distributed micron-sized surface roughness. Transient-growth results for the HLB capsule indicate similar trends as the corresponding numerical data for a Mach 6 experiment in the Actively Controlled Expansion (ACE) facility of the Texas A&M University (TAMU) at a lower Reynolds number. Both configurations indicate a similar dependence on surface temperature ratio, and more important, rather low values of maximum energy gain. DNS are performed for the conditions of the HLB experiment to understand the generation of stationary disturbances by the roughness patch and the accompanying evolution of unsteady perturbations. However, no evidence of either modal or nonmodal disturbance growth in the wake of the roughness patch is found in the DNS data; thus, the physical mechanism underlying the observed onset of transition still remains unknown.

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@article {pmid33479548,

year = {2019},

author = {Hein, S and Theiss, A and Di Giovanni, A and Stemmer, C and Schilden, T and SchrÃ¶der, W and Paredes, P and Choudhari, MM and Li, F and Reshotko, E},

title = {Numerical Investigation of Roughness Effects on Transition on Spherical Capsules.},

journal = {Journal of spacecraft and rockets},

volume = {56},

number = {2},

pages = {388-404},

doi = {10.2514/1.A34247},

pmid = {33479548},

issn = {0022-4650},

abstract = {To address the hitherto unknown mechanism of boundary-layer transition on blunt reentry capsules, the role of roughness-induced disturbance growth on a spherical-section forebody is assessed via optimal transient growth theory and direct numerical simulations (DNS). Optimal transient-growth studies have been performed for the blunt capsule experiments at Mach 5.9 in the Hypersonic Ludwieg tube Braunschweig (HLB) of the Technische UniversitÃ¤t Braunschweig, which included measurements behind a patch of controlled, distributed micron-sized surface roughness. Transient-growth results for the HLB capsule indicate similar trends as the corresponding numerical data for a Mach 6 experiment in the Actively Controlled Expansion (ACE) facility of the Texas A&M University (TAMU) at a lower Reynolds number. Both configurations indicate a similar dependence on surface temperature ratio, and more important, rather low values of maximum energy gain. DNS are performed for the conditions of the HLB experiment to understand the generation of stationary disturbances by the roughness patch and the accompanying evolution of unsteady perturbations. However, no evidence of either modal or nonmodal disturbance growth in the wake of the roughness patch is found in the DNS data; thus, the physical mechanism underlying the observed onset of transition still remains unknown.},

}

RevDate: 2021-01-22

**Inertial Microfluidics-Based Separation of Microalgae Using a Contraction-Expansion Array Microchannel.**

*Micromachines*, **12(1):** pii:mi12010097.

Microalgae separation technology is essential for both executing laboratory-based fundamental studies and ensuring the quality of the final algal products. However, the conventional microalgae separation technology of micropipetting requires highly skilled operators and several months of repeated separation to obtain a microalgal single strain. This study therefore aimed at utilizing microfluidic cell sorting technology for the simple and effective separation of microalgae. Microalgae are characterized by their various morphologies with a wide range of sizes. In this study, a contraction-expansion array microchannel, which utilizes these unique properties of microalgae, was specifically employed for the size-based separation of microalgae. At Reynolds number of 9, two model algal cells, Chlorella vulgaris (C. vulgaris) and Haematococcus pluvialis (H. pluvialis), were successfully separated without showing any sign of cell damage, yielding a purity of 97.9% for C. vulgaris and 94.9% for H. pluvialis. The result supported that the inertia-based separation technology could be a powerful alternative to the labor-intensive and time-consuming conventional microalgae separation technologies.

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@article {pmid33477950,

year = {2021},

author = {Kim, GY and Son, J and Han, JI and Park, JK},

title = {Inertial Microfluidics-Based Separation of Microalgae Using a Contraction-Expansion Array Microchannel.},

journal = {Micromachines},

volume = {12},

number = {1},

pages = {},

doi = {10.3390/mi12010097},

pmid = {33477950},

issn = {2072-666X},

support = {NRF-2011-0031348//National Research Foundation of Korea/ ; NRF-2011-0031348//National Research Foundation of Korea/ ; NRF-2019R1A2B5B03070494//National Research Foundation of Korea/ ; },

abstract = {Microalgae separation technology is essential for both executing laboratory-based fundamental studies and ensuring the quality of the final algal products. However, the conventional microalgae separation technology of micropipetting requires highly skilled operators and several months of repeated separation to obtain a microalgal single strain. This study therefore aimed at utilizing microfluidic cell sorting technology for the simple and effective separation of microalgae. Microalgae are characterized by their various morphologies with a wide range of sizes. In this study, a contraction-expansion array microchannel, which utilizes these unique properties of microalgae, was specifically employed for the size-based separation of microalgae. At Reynolds number of 9, two model algal cells, Chlorella vulgaris (C. vulgaris) and Haematococcus pluvialis (H. pluvialis), were successfully separated without showing any sign of cell damage, yielding a purity of 97.9% for C. vulgaris and 94.9% for H. pluvialis. The result supported that the inertia-based separation technology could be a powerful alternative to the labor-intensive and time-consuming conventional microalgae separation technologies.},

}

RevDate: 2021-01-20

**Input-output inspired method for permissible perturbation amplitude of transitional wall-bounded shear flows.**

*Physical review. E*, **102(6-1):**063108.

The precise set of parameters governing transition to turbulence in wall-bounded shear flows remains an open question; many theoretical bounds have been obtained, but there is not yet a consensus between these bounds and experimental or simulation results. In this work, we focus on a method to provide a provable Reynolds-number-dependent bound on the amplitude of perturbations a flow can sustain while maintaining the laminar state. Our analysis relies on an input-output approach that partitions the dynamics into a feedback interconnection of the linear and nonlinear dynamics (i.e., a LurÃ© system that represents the nonlinearity as static feedback). We then construct quadratic constraints of the nonlinear term that is restricted by system physics to be energy-conserving (lossless) and to have bounded input-output energy. Computing the region of attraction of the laminar state (set of safe perturbations) and permissible perturbation amplitude are then reformulated as linear matrix inequalities, which allows more computationally efficient solutions than prevailing nonlinear approaches based on the sum of squares programming. The proposed framework can also be used for energy method computations and linear stability analysis. We apply our approach to low-dimensional nonlinear shear flow models for a range of Reynolds numbers. The results from our analytically derived bounds are consistent with the bounds identified through exhaustive simulations. However, they have the added benefit of being achieved at a much lower computational cost and providing a provable guarantee that a certain level of perturbation is permissible.

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@article {pmid33465973,

year = {2020},

author = {Liu, C and Gayme, DF},

title = {Input-output inspired method for permissible perturbation amplitude of transitional wall-bounded shear flows.},

journal = {Physical review. E},

volume = {102},

number = {6-1},

pages = {063108},

doi = {10.1103/PhysRevE.102.063108},

pmid = {33465973},

issn = {2470-0053},

abstract = {The precise set of parameters governing transition to turbulence in wall-bounded shear flows remains an open question; many theoretical bounds have been obtained, but there is not yet a consensus between these bounds and experimental or simulation results. In this work, we focus on a method to provide a provable Reynolds-number-dependent bound on the amplitude of perturbations a flow can sustain while maintaining the laminar state. Our analysis relies on an input-output approach that partitions the dynamics into a feedback interconnection of the linear and nonlinear dynamics (i.e., a LurÃ© system that represents the nonlinearity as static feedback). We then construct quadratic constraints of the nonlinear term that is restricted by system physics to be energy-conserving (lossless) and to have bounded input-output energy. Computing the region of attraction of the laminar state (set of safe perturbations) and permissible perturbation amplitude are then reformulated as linear matrix inequalities, which allows more computationally efficient solutions than prevailing nonlinear approaches based on the sum of squares programming. The proposed framework can also be used for energy method computations and linear stability analysis. We apply our approach to low-dimensional nonlinear shear flow models for a range of Reynolds numbers. The results from our analytically derived bounds are consistent with the bounds identified through exhaustive simulations. However, they have the added benefit of being achieved at a much lower computational cost and providing a provable guarantee that a certain level of perturbation is permissible.},

}

RevDate: 2021-01-19

**Electro-Osmotic Propulsion of Jeffrey Fluid in a Ciliated Channel Under the Effect of Nonlinear Radiation and Heat Source/Sink.**

*Journal of biomechanical engineering* pii:1096600 [Epub ahead of print].

Mathematical modelling of mechanical system in microfluidics is an emerging area of interest in micro scale engineering. Since microfluidic devices use the hair like structure of artificial cilia for pumping, mixing and sensing in different fields, therefore; electro osmotic cilia driven flow help to generate the fluid velocity for the Newtonian and viscoelastic fluid. Due to the deployment of artificial ciliated walls, the present research reports the combined effect of an electro osmotic flow and convective heat transfer on Jeffrey viscoelastic electrolytic fluid flow in a two-dimensional ciliated vertical channel. Heat generation/absorption and nonlinear radiation effects are included in the present mathematical model. After applying Debye-Huckel approximation and small Reynolds number approximation to momentum and energy equation, the system of nonlinear partial differential equation is reduced into non-homogenous boundary value problem. The problem determines the velocity, pressure and temperature profiles by the application of semi-analytical technique known as Homotopy Perturbation Method (HPM) with the help of software Mathematica. The graphical results of the study suggest that HPM is a reliable methodology for thermo physical electro-osmotic rheological transport in micro channels.

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@article {pmid33462593,

year = {2021},

author = {Shaheen, S and Beg, OA and Gul, F and Maqbool, K},

title = {Electro-Osmotic Propulsion of Jeffrey Fluid in a Ciliated Channel Under the Effect of Nonlinear Radiation and Heat Source/Sink.},

journal = {Journal of biomechanical engineering},

volume = {},

number = {},

pages = {},

doi = {10.1115/1.4049810},

pmid = {33462593},

issn = {1528-8951},

abstract = {Mathematical modelling of mechanical system in microfluidics is an emerging area of interest in micro scale engineering. Since microfluidic devices use the hair like structure of artificial cilia for pumping, mixing and sensing in different fields, therefore; electro osmotic cilia driven flow help to generate the fluid velocity for the Newtonian and viscoelastic fluid. Due to the deployment of artificial ciliated walls, the present research reports the combined effect of an electro osmotic flow and convective heat transfer on Jeffrey viscoelastic electrolytic fluid flow in a two-dimensional ciliated vertical channel. Heat generation/absorption and nonlinear radiation effects are included in the present mathematical model. After applying Debye-Huckel approximation and small Reynolds number approximation to momentum and energy equation, the system of nonlinear partial differential equation is reduced into non-homogenous boundary value problem. The problem determines the velocity, pressure and temperature profiles by the application of semi-analytical technique known as Homotopy Perturbation Method (HPM) with the help of software Mathematica. The graphical results of the study suggest that HPM is a reliable methodology for thermo physical electro-osmotic rheological transport in micro channels.},

}

RevDate: 2021-01-15

**Peristaltic activity for electro-kinetic complex driven cilia transportation through a non-uniform channel.**

*Computer methods and programs in biomedicine*, **200:**105926 pii:S0169-2607(20)31759-4 [Epub ahead of print].

MOTIVATIONS: Now-a-days in medical science, the transport study of biological fluids through non-uniform vessels are going to increase due to their close relation to the reality. Motivated through such type of complex transportation, the current study is presented of cilia hydro-dynamics of an aqueous electrolytic viscous fluid through a non-uniform channel under an applied axial electric field. Mathematical Formulations: Because of the complexity shape and nature of flow channel, we have used curvilinear coordinates in the derivation of continuity and momentum equationsin a fixed frame of reference. A linear transformation is used to renovate the flow system of equations from fixed (laboratory) to moving (wave) frame. For further simplification, the dimensionless variables are introduced to make the flow system of equations into the dimensionless form and at last convert these equations in term of stream function by using the mathematical terminologies of streamlines. The whole analysis is performed under (low Reynolds number) creeping phenomena and long wavelength approximation, respectively. Additionally, small ionic Peclet number and Debye-Huckel linearization are used to simplify the Nernst-Planck and Poisson-Boltzmann equations. The BVP4C technique is used to obtain the numerical solution for velocity distribution, pressure gradient, pressure rise and stream function through MATLAB.

MAIN OUTCOMES: The amplitude of velocity distribution is increased (decreased) at larger values of non-uniform parameter (cilia length). The non-uniform parameter played a vital role not only in the enhancement of circulation at the upper half of the channel but also the length of bolus increased. Results of straight channel are gained for larger value of the dimensionless radius of curvature parameter as well as cilia length.

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@article {pmid33450503,

year = {2021},

author = {Javid, K and Riaz, M and Chu, YM and Ijaz Khan, M and Ullah Khan, S and Kadry, S},

title = {Peristaltic activity for electro-kinetic complex driven cilia transportation through a non-uniform channel.},

journal = {Computer methods and programs in biomedicine},

volume = {200},

number = {},

pages = {105926},

doi = {10.1016/j.cmpb.2020.105926},

pmid = {33450503},

issn = {1872-7565},

abstract = {MOTIVATIONS: Now-a-days in medical science, the transport study of biological fluids through non-uniform vessels are going to increase due to their close relation to the reality. Motivated through such type of complex transportation, the current study is presented of cilia hydro-dynamics of an aqueous electrolytic viscous fluid through a non-uniform channel under an applied axial electric field. Mathematical Formulations: Because of the complexity shape and nature of flow channel, we have used curvilinear coordinates in the derivation of continuity and momentum equationsin a fixed frame of reference. A linear transformation is used to renovate the flow system of equations from fixed (laboratory) to moving (wave) frame. For further simplification, the dimensionless variables are introduced to make the flow system of equations into the dimensionless form and at last convert these equations in term of stream function by using the mathematical terminologies of streamlines. The whole analysis is performed under (low Reynolds number) creeping phenomena and long wavelength approximation, respectively. Additionally, small ionic Peclet number and Debye-Huckel linearization are used to simplify the Nernst-Planck and Poisson-Boltzmann equations. The BVP4C technique is used to obtain the numerical solution for velocity distribution, pressure gradient, pressure rise and stream function through MATLAB.

MAIN OUTCOMES: The amplitude of velocity distribution is increased (decreased) at larger values of non-uniform parameter (cilia length). The non-uniform parameter played a vital role not only in the enhancement of circulation at the upper half of the channel but also the length of bolus increased. Results of straight channel are gained for larger value of the dimensionless radius of curvature parameter as well as cilia length.},

}

RevDate: 2021-01-15

**Regime Map and Triple Point in Selective Withdrawal.**

*Physical review letters*, **125(26):**264502.

Entrainment in selective withdrawal occurs when both the top and bottom phases are withdrawn through a capillary tube oriented perpendicular to a flat gravitationally separated liquid-liquid interface. The tube introduces two distinct features to the conditions for fluid entrainment. First, the ratio of the two phases being withdrawn is affected by the region of influence of the flow upstream of the tube's orifice. Second, a minimum withdrawal flow rate must be reached for entrainment regardless of the distance between the interface and the tube. We show that these phenomena can be understood based on the Reynolds number that governs the external flow field around the capillary tube and the capillary number that regulates the effect of the viscosity and capillarity.

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@article {pmid33449773,

year = {2020},

author = {Pan, Z and Nunes, JK and Stone, HA},

title = {Regime Map and Triple Point in Selective Withdrawal.},

journal = {Physical review letters},

volume = {125},

number = {26},

pages = {264502},

doi = {10.1103/PhysRevLett.125.264502},

pmid = {33449773},

issn = {1079-7114},

abstract = {Entrainment in selective withdrawal occurs when both the top and bottom phases are withdrawn through a capillary tube oriented perpendicular to a flat gravitationally separated liquid-liquid interface. The tube introduces two distinct features to the conditions for fluid entrainment. First, the ratio of the two phases being withdrawn is affected by the region of influence of the flow upstream of the tube's orifice. Second, a minimum withdrawal flow rate must be reached for entrainment regardless of the distance between the interface and the tube. We show that these phenomena can be understood based on the Reynolds number that governs the external flow field around the capillary tube and the capillary number that regulates the effect of the viscosity and capillarity.},

}

RevDate: 2021-01-13

**Mathematical model to verify the role of magnetic field on blood flow and its impact on thermal behavior of biological tissue for tumor treatment.**

*Biomedical physics & engineering express*, **6(1):**015032.

The numerical computation has been performed to study the effects of static magnetic field on thermal behavior of tumor surrounded by living biological tissues and blood vessels. A small rectangular shaped tumor enclosing the blood vessel surrounded by healthy tissue is considered. The model consists of two-layer composite system in which the microvessel for blood flow is considered as a fluid layer and the living biological tissue including tumor as a solid layer. The wave bioheat transfer equation in the tissue layer together with energy transport equation for blood flow layer has been used in the cylindrical polar coordinates. The analytical expression for blood velocity in the presence of magnetic field has been used from Gold's solution. The computational work has been performed by employing the Crank-Nicolson finite difference method. A comparison has been made to validate our numerical results with the previous solution by setting some parameters. The temperature profiles have been plotted at different locations of the axial tissue length for various values of the Hartmann number, Prandtl number, Womersley number and Reynolds number. It is observed that the application of magnetic field increases heat transfer rate within tumor tissues which in turn attribute to an enhancement of temperature about 316 K or above for hyperthermic treatment in cancer therapy.

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@article {pmid33438620,

year = {2020},

author = {Shit, GC and Bera, A},

title = {Mathematical model to verify the role of magnetic field on blood flow and its impact on thermal behavior of biological tissue for tumor treatment.},

journal = {Biomedical physics & engineering express},

volume = {6},

number = {1},

pages = {015032},

doi = {10.1088/2057-1976/ab6e22},

pmid = {33438620},

issn = {2057-1976},

abstract = {The numerical computation has been performed to study the effects of static magnetic field on thermal behavior of tumor surrounded by living biological tissues and blood vessels. A small rectangular shaped tumor enclosing the blood vessel surrounded by healthy tissue is considered. The model consists of two-layer composite system in which the microvessel for blood flow is considered as a fluid layer and the living biological tissue including tumor as a solid layer. The wave bioheat transfer equation in the tissue layer together with energy transport equation for blood flow layer has been used in the cylindrical polar coordinates. The analytical expression for blood velocity in the presence of magnetic field has been used from Gold's solution. The computational work has been performed by employing the Crank-Nicolson finite difference method. A comparison has been made to validate our numerical results with the previous solution by setting some parameters. The temperature profiles have been plotted at different locations of the axial tissue length for various values of the Hartmann number, Prandtl number, Womersley number and Reynolds number. It is observed that the application of magnetic field increases heat transfer rate within tumor tissues which in turn attribute to an enhancement of temperature about 316 K or above for hyperthermic treatment in cancer therapy.},

}

RevDate: 2021-01-12

**Wind Tunnel Testing of Plasma Actuator with Two Mesh Electrodes to Boundary Layer Control at High Angle of Attack.**

*Sensors (Basel, Switzerland)*, **21(2):** pii:s21020363.

The manuscript presents experimental research carried out on the wing model with the SD 7003 profile. A plasma actuator with DBD (Dielectric Barrier Discharge) discharges was placed on the wing surface to control boundary layer. The experimental tests were carried out in the AeroLab wind tunnel where the forces acting on the wing during the tests were measured. The conducted experimental research concerns the analysis of the phenomena that take place on the surface of the wing with the DBD plasma actuator turned off and on. The plasma actuator used during the experimental tests has a different structure compared to the classic plasma actuator. The commonly tested plasma actuator uses solid/impermeable electrodes, while in the research, the plasma actuator uses a new type of electrodes, two mesh electrodes separated by an impermeable Kapton dielectric. The experimental research was carried out for the angle of attack Î± = 15Â° and several air velocities V = 5-15 m/s with a step of 5 m/s for the Reynolds number Re = 87,500-262,500. The critical angle of attack at which the SD 7003 profile has the maximum lift coefficient is about 11Â°; during the experimental research, the angle was 15Â°. Despite the high angle of attack, it was possible to increase the lift coefficient. The use of a plasma actuator with two mesh electrodes allowed to increase the lift by 5%, even at a high angle of attack. During experimental research used high voltage power supply for powering the DBD plasma actuator in the voltage range from 7.5 to 15 kV.

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@article {pmid33430317,

year = {2021},

author = {Gnapowski, E and Pytka, J and JÃ³zwik, J and Laskowski, J and MichaÅ‚owska, J},

title = {Wind Tunnel Testing of Plasma Actuator with Two Mesh Electrodes to Boundary Layer Control at High Angle of Attack.},

journal = {Sensors (Basel, Switzerland)},

volume = {21},

number = {2},

pages = {},

doi = {10.3390/s21020363},

pmid = {33430317},

issn = {1424-8220},

abstract = {The manuscript presents experimental research carried out on the wing model with the SD 7003 profile. A plasma actuator with DBD (Dielectric Barrier Discharge) discharges was placed on the wing surface to control boundary layer. The experimental tests were carried out in the AeroLab wind tunnel where the forces acting on the wing during the tests were measured. The conducted experimental research concerns the analysis of the phenomena that take place on the surface of the wing with the DBD plasma actuator turned off and on. The plasma actuator used during the experimental tests has a different structure compared to the classic plasma actuator. The commonly tested plasma actuator uses solid/impermeable electrodes, while in the research, the plasma actuator uses a new type of electrodes, two mesh electrodes separated by an impermeable Kapton dielectric. The experimental research was carried out for the angle of attack Î± = 15Â° and several air velocities V = 5-15 m/s with a step of 5 m/s for the Reynolds number Re = 87,500-262,500. The critical angle of attack at which the SD 7003 profile has the maximum lift coefficient is about 11Â°; during the experimental research, the angle was 15Â°. Despite the high angle of attack, it was possible to increase the lift coefficient. The use of a plasma actuator with two mesh electrodes allowed to increase the lift by 5%, even at a high angle of attack. During experimental research used high voltage power supply for powering the DBD plasma actuator in the voltage range from 7.5 to 15 kV.},

}

RevDate: 2021-01-08

**Quantitative Experimental Observation of Weak Inertial-Wave Turbulence.**

*Physical review letters*, **125(25):**254502.

We report the quantitative experimental observation of the weak inertial-wave turbulence regime of rotating turbulence. We produce a statistically steady homogeneous turbulent flow that consists of nonlinearly interacting inertial waves, using rough top and bottom boundaries to prevent the emergence of a geostrophic flow. As the forcing amplitude increases, the temporal spectrum evolves from a discrete set of peaks to a continuous spectrum. Maps of the bicoherence of the velocity field confirm such a gradual transition between discrete wave interactions at weak forcing amplitude and the regime described by weak turbulence theory (WTT) for stronger forcing. In the former regime, the bicoherence maps display a near-zero background level, together with sharp localized peaks associated with discrete resonances. By contrast, in the latter regime, the bicoherence is a smooth function that takes values of the order of the Rossby number in line with the infinite-domain and random-phase assumptions of WTT. The spatial spectra then display a power-law behavior, both the spectral exponent and the spectral level being accurately predicted by WTT at high Reynolds number and low Rossby number.

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@article {pmid33416336,

year = {2020},

author = {Monsalve, E and Brunet, M and Gallet, B and Cortet, PP},

title = {Quantitative Experimental Observation of Weak Inertial-Wave Turbulence.},

journal = {Physical review letters},

volume = {125},

number = {25},

pages = {254502},

doi = {10.1103/PhysRevLett.125.254502},

pmid = {33416336},

issn = {1079-7114},

abstract = {We report the quantitative experimental observation of the weak inertial-wave turbulence regime of rotating turbulence. We produce a statistically steady homogeneous turbulent flow that consists of nonlinearly interacting inertial waves, using rough top and bottom boundaries to prevent the emergence of a geostrophic flow. As the forcing amplitude increases, the temporal spectrum evolves from a discrete set of peaks to a continuous spectrum. Maps of the bicoherence of the velocity field confirm such a gradual transition between discrete wave interactions at weak forcing amplitude and the regime described by weak turbulence theory (WTT) for stronger forcing. In the former regime, the bicoherence maps display a near-zero background level, together with sharp localized peaks associated with discrete resonances. By contrast, in the latter regime, the bicoherence is a smooth function that takes values of the order of the Rossby number in line with the infinite-domain and random-phase assumptions of WTT. The spatial spectra then display a power-law behavior, both the spectral exponent and the spectral level being accurately predicted by WTT at high Reynolds number and low Rossby number.},

}

RevDate: 2021-01-06

**A Primer on Microfluidics: From Basic Principles to Microfabrication.**

*Advances in biochemical engineering/biotechnology* [Epub ahead of print].

Microfluidic systems enable manipulating fluids in different functional units which are integrated on a microchip. This chapter describes the basics of microfluidics, where physical effects have a different impact compared to macroscopic systems. Furthermore, an overwiew is given on the microfabrication of these systems. The focus lies on clean-room fabrication methods based on photolithography and soft lithography. Finally, an outlook on advanced maskless micro- and nanofabrication methods is given. Special attention is paid to laser structuring processes.

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@article {pmid33404675,

year = {2021},

author = {Klein, AK and Dietzel, A},

title = {A Primer on Microfluidics: From Basic Principles to Microfabrication.},

journal = {Advances in biochemical engineering/biotechnology},

volume = {},

number = {},

pages = {},

pmid = {33404675},

issn = {0724-6145},

abstract = {Microfluidic systems enable manipulating fluids in different functional units which are integrated on a microchip. This chapter describes the basics of microfluidics, where physical effects have a different impact compared to macroscopic systems. Furthermore, an overwiew is given on the microfabrication of these systems. The focus lies on clean-room fabrication methods based on photolithography and soft lithography. Finally, an outlook on advanced maskless micro- and nanofabrication methods is given. Special attention is paid to laser structuring processes.},

}

RevDate: 2021-01-05

**Magnetically actuated intelligent hydrogel-based child-parent microrobots for targeted drug delivery.**

*Journal of materials chemistry. B* [Epub ahead of print].

Small intestine-targeted drug delivery by oral administration has aroused the growing interest of researchers. In this work, the child-parent microrobot (CPM) as a vehicle protects the child microrobots (CMs) under a gastric acid environment and releases them in the small intestinal environment. The intelligent hydrogel-based CPMs with sphere, mushroom, red blood cell, and teardrop shapes are fabricated by an extrusion-dripping method. The CPMs package uniform CMs, which are fabricated by designed microfluidic (MF) devices. The fabrication mechanism and tunability of CMs and CPMs with different sizes and shapes are analyzed, modeled, and simulated. The shape of CPM can affect its drug release efficiency and kinetic characteristics. A vision-feedback magnetic driving system (VMDS) actuates and navigates CPM along the predefined path to the destination and continuously releases drug in the simulated intestinal fluid (SIF, a low Reynolds number (Re) regime) using a new motion control method with the tracking-learning-detection (TLD) algorithm. The newly designed CPM combines the advantages of powerful propulsion, good biocompatibility, and remarkable drug loading and release capacity at the intestinal level, which is expected to be competent for oral administration of small intestine-targeted therapy in the future.

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@article {pmid33398321,

year = {2021},

author = {Chen, W and Wen, Y and Fan, X and Sun, M and Tian, C and Yang, M and Xie, H},

title = {Magnetically actuated intelligent hydrogel-based child-parent microrobots for targeted drug delivery.},

journal = {Journal of materials chemistry. B},

volume = {},

number = {},

pages = {},

doi = {10.1039/d0tb02384a},

pmid = {33398321},

issn = {2050-7518},

abstract = {Small intestine-targeted drug delivery by oral administration has aroused the growing interest of researchers. In this work, the child-parent microrobot (CPM) as a vehicle protects the child microrobots (CMs) under a gastric acid environment and releases them in the small intestinal environment. The intelligent hydrogel-based CPMs with sphere, mushroom, red blood cell, and teardrop shapes are fabricated by an extrusion-dripping method. The CPMs package uniform CMs, which are fabricated by designed microfluidic (MF) devices. The fabrication mechanism and tunability of CMs and CPMs with different sizes and shapes are analyzed, modeled, and simulated. The shape of CPM can affect its drug release efficiency and kinetic characteristics. A vision-feedback magnetic driving system (VMDS) actuates and navigates CPM along the predefined path to the destination and continuously releases drug in the simulated intestinal fluid (SIF, a low Reynolds number (Re) regime) using a new motion control method with the tracking-learning-detection (TLD) algorithm. The newly designed CPM combines the advantages of powerful propulsion, good biocompatibility, and remarkable drug loading and release capacity at the intestinal level, which is expected to be competent for oral administration of small intestine-targeted therapy in the future.},

}

RevDate: 2021-01-05

**Second-Order Phase Transition in Counter-Rotating Taylor-Couette Flow Experiment.**

*Entropy (Basel, Switzerland)*, **23(1):** pii:e23010058.

In many basic shear flows, such as pipe, Couette, and channel flow, turbulence does not arise from an instability of the laminar state, and both dynamical states co-exist. With decreasing flow speed (i.e., decreasing Reynolds number) the fraction of fluid in laminar motion increases while turbulence recedes and eventually the entire flow relaminarizes. The first step towards understanding the nature of this transition is to determine if the phase change is of either first or second order. In the former case, the turbulent fraction would drop discontinuously to zero as the Reynolds number decreases while in the latter the process would be continuous. For Couette flow, the flow between two parallel plates, earlier studies suggest a discontinuous scenario. In the present study we realize a Couette flow between two concentric cylinders which allows studies to be carried out in large aspect ratios and for extensive observation times. The presented measurements show that the transition in this circular Couette geometry is continuous suggesting that former studies were limited by finite size effects. A further characterization of this transition, in particular its relation to the directed percolation universality class, requires even larger system sizes than presently available.

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@article {pmid33396499,

year = {2020},

author = {Avila, K and Hof, B},

title = {Second-Order Phase Transition in Counter-Rotating Taylor-Couette Flow Experiment.},

journal = {Entropy (Basel, Switzerland)},

volume = {23},

number = {1},

pages = {},

doi = {10.3390/e23010058},

pmid = {33396499},

issn = {1099-4300},

support = {ZF04B /2019/FB04 Avila_Kerstin//Central Reserach Development Fund of the University of Bremen/ ; },

abstract = {In many basic shear flows, such as pipe, Couette, and channel flow, turbulence does not arise from an instability of the laminar state, and both dynamical states co-exist. With decreasing flow speed (i.e., decreasing Reynolds number) the fraction of fluid in laminar motion increases while turbulence recedes and eventually the entire flow relaminarizes. The first step towards understanding the nature of this transition is to determine if the phase change is of either first or second order. In the former case, the turbulent fraction would drop discontinuously to zero as the Reynolds number decreases while in the latter the process would be continuous. For Couette flow, the flow between two parallel plates, earlier studies suggest a discontinuous scenario. In the present study we realize a Couette flow between two concentric cylinders which allows studies to be carried out in large aspect ratios and for extensive observation times. The presented measurements show that the transition in this circular Couette geometry is continuous suggesting that former studies were limited by finite size effects. A further characterization of this transition, in particular its relation to the directed percolation universality class, requires even larger system sizes than presently available.},

}

RevDate: 2020-12-28

**Extreme dissipation and intermittency in turbulence at very high Reynolds numbers.**

*Proceedings. Mathematical, physical, and engineering sciences*, **476(2243):**20200591.

Extreme dissipation events in turbulent flows are rare, but they can be orders of magnitude stronger than the mean dissipation rate. Despite its importance in many small-scale physical processes, there is presently no accurate theory or model for predicting the extrema as a function of the Reynolds number. Here, we introduce a new model for the dissipation probability density function (PDF) based on the concept of significant shear layers, which are thin regions of elevated local mean dissipation. At very high Reynolds numbers, these significant shear layers develop layered substructures. The flow domain is divided into the different layer regions and a background region, each with their own PDF of dissipation. The volume-weighted regional PDFs are combined to obtain the overall PDF, which is subsequently used to determine the dissipation variance and maximum. The model yields Reynolds number scalings for the dissipation maximum and variance, which are in agreement with the available data. Moreover, the power law scaling exponent is found to increase gradually with the Reynolds numbers, which is also consistent with the data. The increasing exponent is shown to have profound implications for turbulence at atmospheric and astrophysical Reynolds numbers. The present results strongly suggest that intermittent significant shear layer structures are key to understanding and quantifying the dissipation extremes, and, more generally, extreme velocity gradients.

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@article {pmid33362423,

year = {2020},

author = {Elsinga, GE and Ishihara, T and Hunt, JCR},

title = {Extreme dissipation and intermittency in turbulence at very high Reynolds numbers.},

journal = {Proceedings. Mathematical, physical, and engineering sciences},

volume = {476},

number = {2243},

pages = {20200591},

doi = {10.1098/rspa.2020.0591},

pmid = {33362423},

issn = {1364-5021},

abstract = {Extreme dissipation events in turbulent flows are rare, but they can be orders of magnitude stronger than the mean dissipation rate. Despite its importance in many small-scale physical processes, there is presently no accurate theory or model for predicting the extrema as a function of the Reynolds number. Here, we introduce a new model for the dissipation probability density function (PDF) based on the concept of significant shear layers, which are thin regions of elevated local mean dissipation. At very high Reynolds numbers, these significant shear layers develop layered substructures. The flow domain is divided into the different layer regions and a background region, each with their own PDF of dissipation. The volume-weighted regional PDFs are combined to obtain the overall PDF, which is subsequently used to determine the dissipation variance and maximum. The model yields Reynolds number scalings for the dissipation maximum and variance, which are in agreement with the available data. Moreover, the power law scaling exponent is found to increase gradually with the Reynolds numbers, which is also consistent with the data. The increasing exponent is shown to have profound implications for turbulence at atmospheric and astrophysical Reynolds numbers. The present results strongly suggest that intermittent significant shear layer structures are key to understanding and quantifying the dissipation extremes, and, more generally, extreme velocity gradients.},

}

RevDate: 2020-12-28

**Tomographic imaging using multi-simultaneous measurements (TIMes) for flame emission reconstructions.**

*Optics express*, **29(1):**244-255.

The method of tomographic imaging using multi-simultaneous measurements (TIMes) for flame emission reconstructions is presented. Measurements of the peak natural CH* chemiluminescence in the flame and luminescence from different vaporised alkali metal salts that were seeded in a multi-annulus burner were used. An array of 29 CCD cameras around the Cambridge-Sandia burner was deployed, with 3 sets of cameras each measuring a different colour channel using bandpass optical filters. The three-dimensional instantaneous and time-averaged fields of the individual measured channels were reconstructed and superimposed for two new operating conditions, with differing cold flow Reynolds numbers. The contour of the reconstructed flame front followed the interface between the burnt side of the flame, where the alkali salt luminescence appears, and the cold gas region. The increased mixing between different reconstructed channels in the downstream direction that is promoted by the higher levels of turbulence in the larger Reynolds number case was clearly demonstrated. The TIMes method enabled combustion zones originating from different streams and the flame front to be distinguished and their overlap regions to be identified, in the entire volume.

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@article {pmid33362112,

year = {2021},

author = {Foo, CT and Unterberger, A and Menser, J and Mohri, K},

title = {Tomographic imaging using multi-simultaneous measurements (TIMes) for flame emission reconstructions.},

journal = {Optics express},

volume = {29},

number = {1},

pages = {244-255},

doi = {10.1364/OE.412048},

pmid = {33362112},

issn = {1094-4087},

abstract = {The method of tomographic imaging using multi-simultaneous measurements (TIMes) for flame emission reconstructions is presented. Measurements of the peak natural CH* chemiluminescence in the flame and luminescence from different vaporised alkali metal salts that were seeded in a multi-annulus burner were used. An array of 29 CCD cameras around the Cambridge-Sandia burner was deployed, with 3 sets of cameras each measuring a different colour channel using bandpass optical filters. The three-dimensional instantaneous and time-averaged fields of the individual measured channels were reconstructed and superimposed for two new operating conditions, with differing cold flow Reynolds numbers. The contour of the reconstructed flame front followed the interface between the burnt side of the flame, where the alkali salt luminescence appears, and the cold gas region. The increased mixing between different reconstructed channels in the downstream direction that is promoted by the higher levels of turbulence in the larger Reynolds number case was clearly demonstrated. The TIMes method enabled combustion zones originating from different streams and the flame front to be distinguished and their overlap regions to be identified, in the entire volume.},

}

RevDate: 2020-12-28

**Vortex dynamics and transport phenomena in stenotic aortic models using Echo-PIV.**

*Physics in medicine and biology* [Epub ahead of print].

Atherosclerosis is the most fatal cardiovascular disease. As disease progresses, stenoses grow inside the arteries blocking their lumen and altering blood flow. Analysing flow dynamics can provide a deeper insight on the stenosis evolution. In this work we combined Eulerian and Lagrangian descriptors to analyse blood flow dynamics and fluid transport in stenotic aortic models with morphology, mechanical and optical properties close to those of real arteries. To this end, vorticity, particle residence time (PRT), particle's final position (FP) and finite time Lyapunov's exponents (FTLE) were computed from the experimental fluid velocity fields acquired using ultrasonic particle imaging velocimetry (Echo-PIV). For the experiments, CT-images were used to create morphological realistic models of the descending aorta with 0%, 35% and 50% occlusion degree with same mechanical properties as real arteries. Each model was connected to a circuit with a pulsatile programmable pump which mimics physiological flow and pressure conditions. The pulsatile frequency was set to â‰ˆ 0.9 Hz (55 bpm) and the upstream peak Reynolds number (Re) was changed from 1100 to 2000. Flow in the post-stenotic region was composed of two main structures: a high velocity jet over the stenosis throat and a recirculation region behind the stenosis where vortex form and shed. We characterized vortex kinematics showing that vortex propagation velocity increases withRe. Moreover, from the FTLE field we identified Lagrangian Coherent Structures (i.e. material barriers) that dictate transport behind the stenosis. The size and strength of those barriers increased withReand the occlusion degree. Finally, from the PRT and FP maps, we showed that independently ofRe, the same amount of fluid remains on the stenosis over more than a pulsatile period.

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@article {pmid33361564,

year = {2020},

author = {Brum, J and Bernal, M and Barrere, N and Negreira, C and Cabeza, C},

title = {Vortex dynamics and transport phenomena in stenotic aortic models using Echo-PIV.},

journal = {Physics in medicine and biology},

volume = {},

number = {},

pages = {},

doi = {10.1088/1361-6560/abd670},

pmid = {33361564},

issn = {1361-6560},

abstract = {Atherosclerosis is the most fatal cardiovascular disease. As disease progresses, stenoses grow inside the arteries blocking their lumen and altering blood flow. Analysing flow dynamics can provide a deeper insight on the stenosis evolution. In this work we combined Eulerian and Lagrangian descriptors to analyse blood flow dynamics and fluid transport in stenotic aortic models with morphology, mechanical and optical properties close to those of real arteries. To this end, vorticity, particle residence time (PRT), particle's final position (FP) and finite time Lyapunov's exponents (FTLE) were computed from the experimental fluid velocity fields acquired using ultrasonic particle imaging velocimetry (Echo-PIV). For the experiments, CT-images were used to create morphological realistic models of the descending aorta with 0%, 35% and 50% occlusion degree with same mechanical properties as real arteries. Each model was connected to a circuit with a pulsatile programmable pump which mimics physiological flow and pressure conditions. The pulsatile frequency was set to â‰ˆ 0.9 Hz (55 bpm) and the upstream peak Reynolds number (Re) was changed from 1100 to 2000. Flow in the post-stenotic region was composed of two main structures: a high velocity jet over the stenosis throat and a recirculation region behind the stenosis where vortex form and shed. We characterized vortex kinematics showing that vortex propagation velocity increases withRe. Moreover, from the FTLE field we identified Lagrangian Coherent Structures (i.e. material barriers) that dictate transport behind the stenosis. The size and strength of those barriers increased withReand the occlusion degree. Finally, from the PRT and FP maps, we showed that independently ofRe, the same amount of fluid remains on the stenosis over more than a pulsatile period.},

}

RevDate: 2020-12-23

**Numerical Study of T-Shaped Micromixers with Vortex-Inducing Obstacles in the Inlet Channels.**

*Micromachines*, **11(12):** pii:mi11121122.

To enhance fluid mixing, a new approach for inlet flow modification by adding vortex-inducing obstacles (VIOs) in the inlet channels of a T-shaped micromixer is proposed and investigated in this work. We use a commercial computational fluid dynamics code to calculate the pressure and the velocity vectors and, to reduce the numerical diffusion in high-Peclet-number flows, we employ the particle-tracking simulation with an approximation diffusion model to calculate the concentration distribution in the micromixers. The effects of geometric parameters, including the distance between the obstacles and the angle of attack of the obstacles, on the mixing performance of micromixers are studied. From the results, we can observe the following trends: (i) the stretched contact surface between different fluids caused by antisymmetric VIOs happens for the cases with the Reynolds number (Re) greater than or equal to 27 and the enhancement of mixing increases with the increase of Reynolds number gradually, and (ii) the onset of the engulfment flow happens at Reâ‰ˆ125 in the T-shaped mixer with symmetric VIOs or at Reâ‰ˆ140 in the standard planar T-shaped mixer and results in a sudden increase of the degree of mixing. The results indicate that the early initiation of transversal convection by either symmetric or antisymmetric VIOs can enhance fluid mixing at a relatively lower Re.

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@article {pmid33352968,

year = {2020},

author = {Wu, CY and Lai, BH},

title = {Numerical Study of T-Shaped Micromixers with Vortex-Inducing Obstacles in the Inlet Channels.},

journal = {Micromachines},

volume = {11},

number = {12},

pages = {},

doi = {10.3390/mi11121122},

pmid = {33352968},

issn = {2072-666X},

support = {NSC 101 - 2221 - E - 006 - 108 - MY3//Ministry of Science and Technology of the Republic of China on Taiwan/ ; },

abstract = {To enhance fluid mixing, a new approach for inlet flow modification by adding vortex-inducing obstacles (VIOs) in the inlet channels of a T-shaped micromixer is proposed and investigated in this work. We use a commercial computational fluid dynamics code to calculate the pressure and the velocity vectors and, to reduce the numerical diffusion in high-Peclet-number flows, we employ the particle-tracking simulation with an approximation diffusion model to calculate the concentration distribution in the micromixers. The effects of geometric parameters, including the distance between the obstacles and the angle of attack of the obstacles, on the mixing performance of micromixers are studied. From the results, we can observe the following trends: (i) the stretched contact surface between different fluids caused by antisymmetric VIOs happens for the cases with the Reynolds number (Re) greater than or equal to 27 and the enhancement of mixing increases with the increase of Reynolds number gradually, and (ii) the onset of the engulfment flow happens at Reâ‰ˆ125 in the T-shaped mixer with symmetric VIOs or at Reâ‰ˆ140 in the standard planar T-shaped mixer and results in a sudden increase of the degree of mixing. The results indicate that the early initiation of transversal convection by either symmetric or antisymmetric VIOs can enhance fluid mixing at a relatively lower Re.},

}

RevDate: 2020-12-22

**Hydrodynamics of sponge pumps and evolution of the sponge body plan.**

*eLife*, **9:** pii:61012.

Sponges are suspension feeders that filter vast amounts of water. Pumping is carried out by flagellated chambers that are connected to an inhalant and exhalant canal system. In 'leucon' sponges with relatively high-pressure resistance due to a complex and narrow canal system, pumping and filtering are only possible owing to the presence of a gasket-like structure (forming a canopy above the collar filters). Here, we combine numerical and experimental work and demonstrate how sponges that lack such sealing elements are able to efficiently pump and force the flagella-driven flow through their collar filter, thanks to the formation of a 'hydrodynamic gasket' above the collar. Our findings link the architecture of flagellated chambers to that of the canal system, and lend support to the current view that the sponge aquiferous system evolved from an open-type filtration system, and that the first metazoans were filter feeders.

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@article {pmid33252039,

year = {2020},

author = {Asadzadeh, SS and KiÃ¸rboe, T and Larsen, PS and Leys, SP and Yahel, G and Walther, JH},

title = {Hydrodynamics of sponge pumps and evolution of the sponge body plan.},

journal = {eLife},

volume = {9},

number = {},

pages = {},

doi = {10.7554/eLife.61012},

pmid = {33252039},

issn = {2050-084X},

support = {7014-00033B//Danish council for Independent Research/ ; 9278//Villum Fonden/ ; 2016-05446//NSERC Discovery grant/ ; 2016-05446//NSERC/ ; 7014-00033B//Danish Council for Independent Research Natural Sciences/ ; },

abstract = {Sponges are suspension feeders that filter vast amounts of water. Pumping is carried out by flagellated chambers that are connected to an inhalant and exhalant canal system. In 'leucon' sponges with relatively high-pressure resistance due to a complex and narrow canal system, pumping and filtering are only possible owing to the presence of a gasket-like structure (forming a canopy above the collar filters). Here, we combine numerical and experimental work and demonstrate how sponges that lack such sealing elements are able to efficiently pump and force the flagella-driven flow through their collar filter, thanks to the formation of a 'hydrodynamic gasket' above the collar. Our findings link the architecture of flagellated chambers to that of the canal system, and lend support to the current view that the sponge aquiferous system evolved from an open-type filtration system, and that the first metazoans were filter feeders.},

}

RevDate: 2020-12-18

**Propulsion Mechanism of Flexible Microbead Swimmers in the Low Reynolds Number Regime.**

*Micromachines*, **11(12):** pii:mi11121107.

A propulsion mechanism for a flexible microswimmer constructed from superparamagnetic microbeads with different diameters and subjected to an oscillating field was studied experimentally and theoretically herein. Various types of artificial swimmers with different bending patterns were fabricated to determine the flexibility and an effective waveform for a planar beating flagellum. Waveform evolutions for various swimmer configurations were studied to determine the flexible mechanism of the swimmers. A one-armed microswimmer can propel itself only if the friction of its wavelike body is anisotropic. A swimmer with a larger head and a stronger magnetic dipole moment with a flexible tail allows the bending wave to propagate from the head toward the tail to generate forward thrust. The oscillating head and tail do not simultaneously generate positive thrust all the time within a period of oscillation. To increase the propulsion for a bending swimmer, this study proposes a novel configuration for a microbead swimmer that ensures better swimming efficiency. The ratio of the oscillation amplitude of the head to the length of the swimmer (from 0.26 to 0.28) produces a faster swimmer. On the other hand, the swimmer is propelled more effectively if the ratio of the oscillation amplitude of the tail to the length of the swimmer is from 0.29 to 0.33. This study determined the optimal configuration for a flexible microbead swimmer that generates the greatest propulsion in a low Reynolds number environment.

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@article {pmid33333847,

year = {2020},

author = {Li, YH and Chen, SC},

title = {Propulsion Mechanism of Flexible Microbead Swimmers in the Low Reynolds Number Regime.},

journal = {Micromachines},

volume = {11},

number = {12},

pages = {},

doi = {10.3390/mi11121107},

pmid = {33333847},

issn = {2072-666X},

support = {MOST 107-2218-E-606-003-MY2 and MOST 109-2221-E-606-003//Ministry of Science and Technology, Taiwan/ ; },

abstract = {A propulsion mechanism for a flexible microswimmer constructed from superparamagnetic microbeads with different diameters and subjected to an oscillating field was studied experimentally and theoretically herein. Various types of artificial swimmers with different bending patterns were fabricated to determine the flexibility and an effective waveform for a planar beating flagellum. Waveform evolutions for various swimmer configurations were studied to determine the flexible mechanism of the swimmers. A one-armed microswimmer can propel itself only if the friction of its wavelike body is anisotropic. A swimmer with a larger head and a stronger magnetic dipole moment with a flexible tail allows the bending wave to propagate from the head toward the tail to generate forward thrust. The oscillating head and tail do not simultaneously generate positive thrust all the time within a period of oscillation. To increase the propulsion for a bending swimmer, this study proposes a novel configuration for a microbead swimmer that ensures better swimming efficiency. The ratio of the oscillation amplitude of the head to the length of the swimmer (from 0.26 to 0.28) produces a faster swimmer. On the other hand, the swimmer is propelled more effectively if the ratio of the oscillation amplitude of the tail to the length of the swimmer is from 0.29 to 0.33. This study determined the optimal configuration for a flexible microbead swimmer that generates the greatest propulsion in a low Reynolds number environment.},

}

RevDate: 2020-12-17

**Programmable topotaxis of magnetic rollers in time-varying fields.**

*Soft matter* [Epub ahead of print].

We describe how spatially uniform, time-periodic magnetic fields can be designed to power and direct the migration of ferromagnetic spheres up (or down) local gradients in the topography of a solid substrate. Our results are based on a dynamical model that considers the time-varying magnetic torques on the particle and its motion through the fluid at low Reynolds number. We use both analytical theory and numerical simulation to design magnetic fields that maximize the migration velocity up (or down) an inclined plane. We show how "topotaxis" of spherical particles relies on differences in the hydrodynamic resistance to rotation about axes parallel and perpendicular to the plane. Importantly, the designed fields can drive multiple independent particles to move simultaneously in different directions as determined by gradients in their respective environments. Experiments on ferromagnetic spheres provide evidence for topotactic motions up inclined substrates. The ability to program the autonomous navigation of driven particles within anisotropic environments is relevant to the design of colloidal robots.

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@article {pmid33331388,

year = {2020},

author = {Dou, Y and Tzelios, PM and Livitz, D and Bishop, KJM},

title = {Programmable topotaxis of magnetic rollers in time-varying fields.},

journal = {Soft matter},

volume = {},

number = {},

pages = {},

doi = {10.1039/d0sm01443e},

pmid = {33331388},

issn = {1744-6848},

abstract = {We describe how spatially uniform, time-periodic magnetic fields can be designed to power and direct the migration of ferromagnetic spheres up (or down) local gradients in the topography of a solid substrate. Our results are based on a dynamical model that considers the time-varying magnetic torques on the particle and its motion through the fluid at low Reynolds number. We use both analytical theory and numerical simulation to design magnetic fields that maximize the migration velocity up (or down) an inclined plane. We show how "topotaxis" of spherical particles relies on differences in the hydrodynamic resistance to rotation about axes parallel and perpendicular to the plane. Importantly, the designed fields can drive multiple independent particles to move simultaneously in different directions as determined by gradients in their respective environments. Experiments on ferromagnetic spheres provide evidence for topotactic motions up inclined substrates. The ability to program the autonomous navigation of driven particles within anisotropic environments is relevant to the design of colloidal robots.},

}

RevDate: 2020-12-17

**Diffusion coefficients for rigid macromolecules with irregular shapes that allow rotational-translational coupling.**

*Biopolymers*, **20(2):**303-326.

We consider six-dimensional diffusion and frictional tensors for a rigid macromolecule immersed in a viscous fluid at low Reynolds number. Our treatment allows for screwlike properties which couple rotational and translational movements. We show that the center of diffusion of a screwlike body can be distinct from its hydrodynamic center of reaction. Symmetry conditions which ensure coincidence are examined. The center of diffusion is found to be the point of a body with the slowest diffusive movements, while rotations about the center of reaction encounter the least average resistance. The macroscopic translational diffusion coefficient is evaluated from a perturbation analysis of the six-dimensional diffusion equation. We show that methodologies which ignore translational-rotational coupling will necessarily underestimate the diffusion rate of screwlike particles. A procedural framework is presented to calculate diffusion coefficients of complicated bodies. As an example we treat a long bent rod.

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@article {pmid33327683,

year = {1981},

author = {Wegener, WA},

title = {Diffusion coefficients for rigid macromolecules with irregular shapes that allow rotational-translational coupling.},

journal = {Biopolymers},

volume = {20},

number = {2},

pages = {303-326},

doi = {10.1002/bip.1981.360200205},

pmid = {33327683},

issn = {1097-0282},

abstract = {We consider six-dimensional diffusion and frictional tensors for a rigid macromolecule immersed in a viscous fluid at low Reynolds number. Our treatment allows for screwlike properties which couple rotational and translational movements. We show that the center of diffusion of a screwlike body can be distinct from its hydrodynamic center of reaction. Symmetry conditions which ensure coincidence are examined. The center of diffusion is found to be the point of a body with the slowest diffusive movements, while rotations about the center of reaction encounter the least average resistance. The macroscopic translational diffusion coefficient is evaluated from a perturbation analysis of the six-dimensional diffusion equation. We show that methodologies which ignore translational-rotational coupling will necessarily underestimate the diffusion rate of screwlike particles. A procedural framework is presented to calculate diffusion coefficients of complicated bodies. As an example we treat a long bent rod.},

}

RevDate: 2020-12-17

**Colloid particles in microfluidic inertial hydrodynamic ratchet at moderate Reynolds number.**

*Physical review. E*, **102(5-1):**052601.

The movement of spherical Brownian particle carried by an alternating fluid flow in a tube of periodically variable diameter is investigated. On the basis of our previous results [Phys. Rev. E 99, 012604 (2019)10.1103/PhysRevE.99.012604] on the hydrodynamics of the problem, we look at the competition of hydrodynamics and diffusion. We use the method of Fick-Jacobs mapping on an effective one-dimensional problem. We calculate the ratchet current and show that is is strictly related to finite size of the particles. The ratchet current grows quadratically with particle radius. We also show that the dominant contribution to the ratchet current is due to inertial hydrodynamic effects. This means that Reynolds number must be at least of order one. We discuss the possible use for separation of particles by size and perspectives of optimization of the tube shape.

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@article {pmid33327115,

year = {2020},

author = {Slanina, F},

title = {Colloid particles in microfluidic inertial hydrodynamic ratchet at moderate Reynolds number.},

journal = {Physical review. E},

volume = {102},

number = {5-1},

pages = {052601},

doi = {10.1103/PhysRevE.102.052601},

pmid = {33327115},

issn = {2470-0053},

abstract = {The movement of spherical Brownian particle carried by an alternating fluid flow in a tube of periodically variable diameter is investigated. On the basis of our previous results [Phys. Rev. E 99, 012604 (2019)10.1103/PhysRevE.99.012604] on the hydrodynamics of the problem, we look at the competition of hydrodynamics and diffusion. We use the method of Fick-Jacobs mapping on an effective one-dimensional problem. We calculate the ratchet current and show that is is strictly related to finite size of the particles. The ratchet current grows quadratically with particle radius. We also show that the dominant contribution to the ratchet current is due to inertial hydrodynamic effects. This means that Reynolds number must be at least of order one. We discuss the possible use for separation of particles by size and perspectives of optimization of the tube shape.},

}

RevDate: 2020-12-16

**H2 production by photofermentation in an innovative plate-type photobioreactor with meandering channels.**

*Biotechnology and bioengineering* [Epub ahead of print].

Hydrogen production by Rhodobacter capsulatus is an anaerobic, photobiological process requiring specific mixing conditions. In this study, an innovative design of photobioreactor is proposed. The design is based on a plate-type photobioreactor with an interconnected meandering channel to allow culture mixing and H2 degassing. The culture flow was characterized as a quasi plug-flow with radial mixing caused by a turbulent-like regime achieved at low Reynolds number. The dissipated volumetric power was decreased ten-fold while maintaining PBR performances (production and yields) when compared with a magnetically stirred tank reactor. To increase hydrogen production flow rate, several bacterial concentrations were tested by increasing the glutamate concentration using fed-batch cultures. The maximum hydrogen production flow rate (157.7 Â± 9.3 mL H2 /L/h) achieved is one of the highest values so far reported for H2 production by R. capsulatus. These first results are encouraging for future scale-up of the plate-type reactor. This article is protected by copyright. All rights reserved.

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@article {pmid33325030,

year = {2020},

author = {Turon, V and Ollivier, S and Cwicklinski, G and Willison, JC and Anxionnaz-Minvielle, Z},

title = {H2 production by photofermentation in an innovative plate-type photobioreactor with meandering channels.},

journal = {Biotechnology and bioengineering},

volume = {},

number = {},

pages = {},

doi = {10.1002/bit.27656},

pmid = {33325030},

issn = {1097-0290},

abstract = {Hydrogen production by Rhodobacter capsulatus is an anaerobic, photobiological process requiring specific mixing conditions. In this study, an innovative design of photobioreactor is proposed. The design is based on a plate-type photobioreactor with an interconnected meandering channel to allow culture mixing and H2 degassing. The culture flow was characterized as a quasi plug-flow with radial mixing caused by a turbulent-like regime achieved at low Reynolds number. The dissipated volumetric power was decreased ten-fold while maintaining PBR performances (production and yields) when compared with a magnetically stirred tank reactor. To increase hydrogen production flow rate, several bacterial concentrations were tested by increasing the glutamate concentration using fed-batch cultures. The maximum hydrogen production flow rate (157.7 Â± 9.3 mL H2 /L/h) achieved is one of the highest values so far reported for H2 production by R. capsulatus. These first results are encouraging for future scale-up of the plate-type reactor. This article is protected by copyright. All rights reserved.},

}

RevDate: 2020-12-16

**Intermittency, Moments, and Friction Coefficient during the Subcritical Transition of Channel Flow.**

*Entropy (Basel, Switzerland)*, **22(12):** pii:e22121399.

The intermittent distribution of localized turbulent structures is a key feature of the subcritical transitions in channel flows, which are studied in this paper with a wind channel and theoretical modeling. Entrance disturbances are introduced by small beads, and localized turbulent patches can be triggered at low Reynolds numbers (Re). High turbulence intensity represents strong ability of perturbation spread, and a maximum turbulence intensity is found for every test case as Re â‰¥ 950, where the turbulence fraction increases abruptly with Re. Skewness can reflect the velocity defects of localized turbulent patches and is revealed to become negative when Re is as low as about 660. It is shown that the third-order moments of the midplane streamwise velocities have minima, while the corresponding forth-order moments have maxima during the transition. These kinematic extremes and different variation scenarios of the friction coefficient during the transition are explained with an intermittent structure model, where the robust localized turbulent structure is simplified as a turbulence unit, a structure whose statistical properties are only weak functions of the Reynolds number.

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@article {pmid33322374,

year = {2020},

author = {Liu, J and Xiao, Y and Li, M and Tao, J and Xu, S},

title = {Intermittency, Moments, and Friction Coefficient during the Subcritical Transition of Channel Flow.},

journal = {Entropy (Basel, Switzerland)},

volume = {22},

number = {12},

pages = {},

doi = {10.3390/e22121399},

pmid = {33322374},

issn = {1099-4300},

support = {91752203, 11772173, 11490553//National Natural Science Foundation of China/ ; },

abstract = {The intermittent distribution of localized turbulent structures is a key feature of the subcritical transitions in channel flows, which are studied in this paper with a wind channel and theoretical modeling. Entrance disturbances are introduced by small beads, and localized turbulent patches can be triggered at low Reynolds numbers (Re). High turbulence intensity represents strong ability of perturbation spread, and a maximum turbulence intensity is found for every test case as Re â‰¥ 950, where the turbulence fraction increases abruptly with Re. Skewness can reflect the velocity defects of localized turbulent patches and is revealed to become negative when Re is as low as about 660. It is shown that the third-order moments of the midplane streamwise velocities have minima, while the corresponding forth-order moments have maxima during the transition. These kinematic extremes and different variation scenarios of the friction coefficient during the transition are explained with an intermittent structure model, where the robust localized turbulent structure is simplified as a turbulence unit, a structure whose statistical properties are only weak functions of the Reynolds number.},

}

RevDate: 2020-12-08

**Intestines of non-uniform stiffness mold the corners of wombat feces.**

*Soft matter* [Epub ahead of print].

The bare-nosed wombat (Vombatus ursinus) is a fossorial, herbivorous, Australian marsupial, renowned for its cubic feces. However, the ability of the wombat's soft intestine to sculpt flat faces and sharp corners in feces is poorly understood. In this combined experimental and numerical study, we show one mechanism for the formation of corners in a highly damped environment. Wombat dissections show that cubes are formed within the last 17 percent of the intestine. Using histology and tensile testing, we discover that the cross-section of the intestine exhibits regions with a two-fold increase in thickness and a four-fold increase in stiffness, which we hypothesize facilitates the formation of corners by contractions of the intestine. Using a mathematical model, we simulate a series of azimuthal contractions of a damped elastic ring composed of alternating stiff and soft regions. Increased stiffness ratio and higher Reynolds number yield shapes that are more square. The corners arise from faster contraction in the stiff regions and relatively slower movement in the center of the soft regions. These results may have applications in manufacturing, clinical pathology, and digestive health.

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@article {pmid33289747,

year = {2020},

author = {Yang, PJ and Lee, AB and Chan, M and Kowalski, M and Qiu, K and Waid, C and Cervantes, G and Magondu, B and Biagioni, M and Vogelnest, L and Martin, A and Edwards, A and Carver, S and Hu, DL},

title = {Intestines of non-uniform stiffness mold the corners of wombat feces.},

journal = {Soft matter},

volume = {},

number = {},

pages = {},

doi = {10.1039/d0sm01230k},

pmid = {33289747},

issn = {1744-6848},

abstract = {The bare-nosed wombat (Vombatus ursinus) is a fossorial, herbivorous, Australian marsupial, renowned for its cubic feces. However, the ability of the wombat's soft intestine to sculpt flat faces and sharp corners in feces is poorly understood. In this combined experimental and numerical study, we show one mechanism for the formation of corners in a highly damped environment. Wombat dissections show that cubes are formed within the last 17 percent of the intestine. Using histology and tensile testing, we discover that the cross-section of the intestine exhibits regions with a two-fold increase in thickness and a four-fold increase in stiffness, which we hypothesize facilitates the formation of corners by contractions of the intestine. Using a mathematical model, we simulate a series of azimuthal contractions of a damped elastic ring composed of alternating stiff and soft regions. Increased stiffness ratio and higher Reynolds number yield shapes that are more square. The corners arise from faster contraction in the stiff regions and relatively slower movement in the center of the soft regions. These results may have applications in manufacturing, clinical pathology, and digestive health.},

}

RevDate: 2020-12-08

**Low- and High-Drag Intermittencies in Turbulent Channel Flows.**

*Entropy (Basel, Switzerland)*, **22(10):** pii:e22101126.

Recent direct numerical simulations (DNS) and experiments in turbulent channel flow have found intermittent low- and high-drag events in Newtonian fluid flows, at ReÏ„=uÏ„h/Î½ between 70 and 100, where uÏ„, h and Î½ are the friction velocity, channel half-height and kinematic viscosity, respectively. These intervals of low-drag and high-drag have been termed "hibernating" and "hyperactive", respectively, and in this paper, a further investigation of these intermittent events is conducted using experimental and numerical techniques. For experiments, simultaneous measurements of wall shear stress and velocity are carried out in a channel flow facility using hot-film anemometry (HFA) and laser Doppler velocimetry (LDV), respectively, for ReÏ„ between 70 and 250. For numerical simulations, DNS of a channel flow is performed in an extended domain at ReÏ„ = 70 and 85. These intermittent events are selected by carrying out conditional sampling of the wall shear stress data based on a combined threshold magnitude and time-duration criteria. The use of three different scalings (so-called outer, inner and mixed) for the time-duration criterion for the conditional events is explored. It is found that if the time-duration criterion is kept constant in inner units, the frequency of occurrence of these conditional events remain insensitive to Reynolds number. There exists an exponential distribution of frequency of occurrence of the conditional events with respect to their duration, implying a potentially memoryless process. An explanation for the presence of a spike (or dip) in the ensemble-averaged wall shear stress data before and after the low-drag (or high-drag) events is investigated. During the low-drag events, the conditionally-averaged streamwise velocities get closer to Virk's maximum drag reduction (MDR) asymptote, near the wall, for all Reynolds numbers studied. Reynolds shear stress (RSS) characteristics during these conditional events are investigated for ReÏ„ = 70 and 85. Except very close to the wall, the conditionally-averaged RSS is higher than the time-averaged value during the low-drag events.

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@article {pmid33286895,

year = {2020},

author = {Agrawal, R and Ng, HC and Davis, EA and Park, JS and Graham, MD and Dennis, DJC and Poole, RJ},

title = {Low- and High-Drag Intermittencies in Turbulent Channel Flows.},

journal = {Entropy (Basel, Switzerland)},

volume = {22},

number = {10},

pages = {},

doi = {10.3390/e22101126},

pmid = {33286895},

issn = {1099-4300},

support = {FA9550-16-1-0076//Air Force Office of Scientific Research/ ; FA9550-18-1-0174//Air Force Office of Scientific Research/ ; OIA-1832976//National Science Foundation/ ; },

abstract = {Recent direct numerical simulations (DNS) and experiments in turbulent channel flow have found intermittent low- and high-drag events in Newtonian fluid flows, at ReÏ„=uÏ„h/Î½ between 70 and 100, where uÏ„, h and Î½ are the friction velocity, channel half-height and kinematic viscosity, respectively. These intervals of low-drag and high-drag have been termed "hibernating" and "hyperactive", respectively, and in this paper, a further investigation of these intermittent events is conducted using experimental and numerical techniques. For experiments, simultaneous measurements of wall shear stress and velocity are carried out in a channel flow facility using hot-film anemometry (HFA) and laser Doppler velocimetry (LDV), respectively, for ReÏ„ between 70 and 250. For numerical simulations, DNS of a channel flow is performed in an extended domain at ReÏ„ = 70 and 85. These intermittent events are selected by carrying out conditional sampling of the wall shear stress data based on a combined threshold magnitude and time-duration criteria. The use of three different scalings (so-called outer, inner and mixed) for the time-duration criterion for the conditional events is explored. It is found that if the time-duration criterion is kept constant in inner units, the frequency of occurrence of these conditional events remain insensitive to Reynolds number. There exists an exponential distribution of frequency of occurrence of the conditional events with respect to their duration, implying a potentially memoryless process. An explanation for the presence of a spike (or dip) in the ensemble-averaged wall shear stress data before and after the low-drag (or high-drag) events is investigated. During the low-drag events, the conditionally-averaged streamwise velocities get closer to Virk's maximum drag reduction (MDR) asymptote, near the wall, for all Reynolds numbers studied. Reynolds shear stress (RSS) characteristics during these conditional events are investigated for ReÏ„ = 70 and 85. Except very close to the wall, the conditionally-averaged RSS is higher than the time-averaged value during the low-drag events.},

}

RevDate: 2020-12-08

**Flow Statistics in the Transitional Regime of Plane Channel Flow.**

*Entropy (Basel, Switzerland)*, **22(9):** pii:e22091001.

The transitional regime of plane channel flow is investigated above the transitional point below which turbulence is not sustained, using direct numerical simulation in large domains. Statistics of laminar-turbulent spatio-temporal intermittency are reported. The geometry of the pattern is first characterized, including statistics for the angles of the laminar-turbulent stripes observed in this regime, with a comparison to experiments. High-order statistics of the local and instantaneous bulk velocity, wall shear stress and turbulent kinetic energy are then provided. The distributions of the two former quantities have non-trivial shapes, characterized by a large kurtosis and/or skewness. Interestingly, we observe a strong linear correlation between their kurtosis and their skewness squared, which is usually reported at much higher Reynolds number in the fully turbulent regime.

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@article {pmid33286770,

year = {2020},

author = {Kashyap, PV and Duguet, Y and Dauchot, O},

title = {Flow Statistics in the Transitional Regime of Plane Channel Flow.},

journal = {Entropy (Basel, Switzerland)},

volume = {22},

number = {9},

pages = {},

doi = {10.3390/e22091001},

pmid = {33286770},

issn = {1099-4300},

abstract = {The transitional regime of plane channel flow is investigated above the transitional point below which turbulence is not sustained, using direct numerical simulation in large domains. Statistics of laminar-turbulent spatio-temporal intermittency are reported. The geometry of the pattern is first characterized, including statistics for the angles of the laminar-turbulent stripes observed in this regime, with a comparison to experiments. High-order statistics of the local and instantaneous bulk velocity, wall shear stress and turbulent kinetic energy are then provided. The distributions of the two former quantities have non-trivial shapes, characterized by a large kurtosis and/or skewness. Interestingly, we observe a strong linear correlation between their kurtosis and their skewness squared, which is usually reported at much higher Reynolds number in the fully turbulent regime.},

}

RevDate: 2020-12-08

**Lognormality in Turbulence Energy Spectra.**

*Entropy (Basel, Switzerland)*, **22(6):** pii:e22060669.

The maximum entropy principle states that the energy distribution will tend toward a state of maximum entropy under the physical constraints, such as the zero energy at the boundaries and a fixed total energy content. For the turbulence energy spectra, a distribution function that maximizes entropy with these physical constraints is a lognormal function due to its asymmetrical descent to zero energy at the boundary lengths scales. This distribution function agrees quite well with the experimental data over a wide range of energy and length scales. For turbulent flows, this approach is effective since the energy and length scales are determined primarily by the Reynolds number. The total turbulence kinetic energy will set the height of the distribution, while the ratio of length scales will determine the width. This makes it possible to reconstruct the power spectra using the Reynolds number as a parameter.

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@article {pmid33286441,

year = {2020},

author = {Lee, T},

title = {Lognormality in Turbulence Energy Spectra.},

journal = {Entropy (Basel, Switzerland)},

volume = {22},

number = {6},

pages = {},

doi = {10.3390/e22060669},

pmid = {33286441},

issn = {1099-4300},

abstract = {The maximum entropy principle states that the energy distribution will tend toward a state of maximum entropy under the physical constraints, such as the zero energy at the boundaries and a fixed total energy content. For the turbulence energy spectra, a distribution function that maximizes entropy with these physical constraints is a lognormal function due to its asymmetrical descent to zero energy at the boundary lengths scales. This distribution function agrees quite well with the experimental data over a wide range of energy and length scales. For turbulent flows, this approach is effective since the energy and length scales are determined primarily by the Reynolds number. The total turbulence kinetic energy will set the height of the distribution, while the ratio of length scales will determine the width. This makes it possible to reconstruct the power spectra using the Reynolds number as a parameter.},

}

RevDate: 2020-12-08

**Constructal Design of Elliptical Cylinders with Heat Generating for Entropy Generation Minimization.**

*Entropy (Basel, Switzerland)*, **22(6):** pii:e22060651.

A heat dissipation model of discrete elliptical cylinders with heat generation on a thermal conduction pedestal cooled by forced convection is established. Constructal design is conducted numerically by taking the distributions of thermal conductivity and heat generating intensity as design variables, the dimensionless entropy generation rate (DEGR) as performance indicator. The optimal designs for discrete elliptical cylinders with heat generating are obtained respectively, i.e., there are optimal distributions of heat generating intensity with its fixed total amount of heat sources, and there are optimal distributions of thermal conductivity with its fixed total amount of heat sources. These optimums for minimum DEGRs are different at different Reynolds numbers of airflow. The heat generating intensity can be decreased one by one appropriately in the fluid flow direction to achieve the best effect. When the Reynolds number of airflow is smaller, the thermal conductivity of heat source can be increased one by one appropriately in the fluid flow direction to achieve the best effect; when the Reynolds number of airflow is larger, the thermal conductivity of each heat source should be equalized to achieve the best effect. The results can give thermal design guidelines for the practical heat generating devices with different materials and heat generating intensities.

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@article {pmid33286423,

year = {2020},

author = {Wang, R and Xie, Z and Yin, Y and Chen, L},

title = {Constructal Design of Elliptical Cylinders with Heat Generating for Entropy Generation Minimization.},

journal = {Entropy (Basel, Switzerland)},

volume = {22},

number = {6},

pages = {},

doi = {10.3390/e22060651},

pmid = {33286423},

issn = {1099-4300},

support = {51579244//National Natural Science Foundation of China/ ; },

abstract = {A heat dissipation model of discrete elliptical cylinders with heat generation on a thermal conduction pedestal cooled by forced convection is established. Constructal design is conducted numerically by taking the distributions of thermal conductivity and heat generating intensity as design variables, the dimensionless entropy generation rate (DEGR) as performance indicator. The optimal designs for discrete elliptical cylinders with heat generating are obtained respectively, i.e., there are optimal distributions of heat generating intensity with its fixed total amount of heat sources, and there are optimal distributions of thermal conductivity with its fixed total amount of heat sources. These optimums for minimum DEGRs are different at different Reynolds numbers of airflow. The heat generating intensity can be decreased one by one appropriately in the fluid flow direction to achieve the best effect. When the Reynolds number of airflow is smaller, the thermal conductivity of heat source can be increased one by one appropriately in the fluid flow direction to achieve the best effect; when the Reynolds number of airflow is larger, the thermal conductivity of each heat source should be equalized to achieve the best effect. The results can give thermal design guidelines for the practical heat generating devices with different materials and heat generating intensities.},

}

RevDate: 2020-12-08

**Effect of Prandtl Number on Mixed Convective Heat Transfer from a Porous Cylinder in the Steady Flow Regime.**

*Entropy (Basel, Switzerland)*, **22(2):** pii:e22020184.

The effect of the Prandtl number (Pr) on the flow and heat transfer from a porous circular cylinder with internal heat generation in the mixed convection regime is numerically investigated. The steady flow regime is considered over the ranges of the Reynolds number (Re), Darcy number (Da), and Richardson number (Ri), varying from 5 to 40, 10-6 to 10-2, and 0 to 2, respectively. The wake structure, the temperature distribution, and the heat transfer rate are discussed. Besides precipitating the growth of the recirculating wake, the Prandtl number is found to have a significant impact on the thermal characteristics. The concave isotherms, resembling a saddle-shaped structure, occur behind the cylinder at larger Pr, resulting in swells of the isotherms pairing off at the lateral sides. These swells are found to have a negative effect on heat transfer owing to a relatively smaller temperature gradient there. Then, the heat transfer rate in terms of the local Nusselt number (Nu) and enhancement ratio (Er) is calculated, which is closely related to Pr, Re, Da, and Ri. The local minimum heat transfer rate along the cylinder surface is found at the position where the swells of the isotherms form.

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@article {pmid33285959,

year = {2020},

author = {Yu, S and Tang, T and Li, J and Yu, P},

title = {Effect of Prandtl Number on Mixed Convective Heat Transfer from a Porous Cylinder in the Steady Flow Regime.},

journal = {Entropy (Basel, Switzerland)},

volume = {22},

number = {2},

pages = {},

doi = {10.3390/e22020184},

pmid = {33285959},

issn = {1099-4300},

support = {11672124//the National Natural Science Foundation of China/ ; ZDSYS201802081843517//Shenzhen Key Laboratory of Complex Aerospace Flows/ ; KQTD2016022620054656//the Shenzhen Peacock Plan/ ; },

abstract = {The effect of the Prandtl number (Pr) on the flow and heat transfer from a porous circular cylinder with internal heat generation in the mixed convection regime is numerically investigated. The steady flow regime is considered over the ranges of the Reynolds number (Re), Darcy number (Da), and Richardson number (Ri), varying from 5 to 40, 10-6 to 10-2, and 0 to 2, respectively. The wake structure, the temperature distribution, and the heat transfer rate are discussed. Besides precipitating the growth of the recirculating wake, the Prandtl number is found to have a significant impact on the thermal characteristics. The concave isotherms, resembling a saddle-shaped structure, occur behind the cylinder at larger Pr, resulting in swells of the isotherms pairing off at the lateral sides. These swells are found to have a negative effect on heat transfer owing to a relatively smaller temperature gradient there. Then, the heat transfer rate in terms of the local Nusselt number (Nu) and enhancement ratio (Er) is calculated, which is closely related to Pr, Re, Da, and Ri. The local minimum heat transfer rate along the cylinder surface is found at the position where the swells of the isotherms form.},

}

RevDate: 2020-12-08

**On Heat Transfer Performance of Cooling Systems Using Nanofluid for Electric Motor Applications.**

*Entropy (Basel, Switzerland)*, **22(1):** pii:e22010099.

This paper studies the fluid flow and heat transfer characteristics of nanofluids as advance coolants for the cooling system of electric motors. Investigations are carried out using numerical analysis for a cooling system with spiral channels. To solve the governing equations, computational fluid dynamics and 3D fluid motion analysis are used. The base fluid is water with a laminar flow. The fluid Reynolds number and turn-number of spiral channels are evaluation parameters. The effect of nanoparticles volume fraction in the base fluid on the heat transfer performance of the cooling system is studied. Increasing the volume fraction of nanoparticles leads to improving the heat transfer performance of the cooling system. On the other hand, a high-volume fraction of the nanofluid increases the pressure drop of the coolant fluid and increases the required pumping power. This paper aims at finding a trade-off between effective parameters by studying both fluid flow and heat transfer characteristics of the nanofluid.

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@article {pmid33285875,

year = {2020},

author = {Deriszadeh, A and de Monte, F},

title = {On Heat Transfer Performance of Cooling Systems Using Nanofluid for Electric Motor Applications.},

journal = {Entropy (Basel, Switzerland)},

volume = {22},

number = {1},

pages = {},

doi = {10.3390/e22010099},

pmid = {33285875},

issn = {1099-4300},

abstract = {This paper studies the fluid flow and heat transfer characteristics of nanofluids as advance coolants for the cooling system of electric motors. Investigations are carried out using numerical analysis for a cooling system with spiral channels. To solve the governing equations, computational fluid dynamics and 3D fluid motion analysis are used. The base fluid is water with a laminar flow. The fluid Reynolds number and turn-number of spiral channels are evaluation parameters. The effect of nanoparticles volume fraction in the base fluid on the heat transfer performance of the cooling system is studied. Increasing the volume fraction of nanoparticles leads to improving the heat transfer performance of the cooling system. On the other hand, a high-volume fraction of the nanofluid increases the pressure drop of the coolant fluid and increases the required pumping power. This paper aims at finding a trade-off between effective parameters by studying both fluid flow and heat transfer characteristics of the nanofluid.},

}

RevDate: 2020-12-08

**Entropy Generation and Consequences of Binary Chemical Reaction on MHD Darcy-Forchheimer Williamson Nanofluid Flow Over Non-Linearly Stretching Surface.**

*Entropy (Basel, Switzerland)*, **22(1):** pii:e22010018.

The current article aims to present a numerical analysis of MHD Williamson nanofluid flow maintained to flow through porous medium bounded by a non-linearly stretching flat surface. The second law of thermodynamics was applied to analyze the fluid flow, heat and mass transport as well as the aspects of entropy generation using Buongiorno model. Thermophoresis and Brownian diffusion is considered which appears due to the concentration and random motion of nanoparticles in base fluid, respectively. Uniform magnetic effect is induced but the assumption of tiny magnetic Reynolds number results in zero magnetic induction. The governing equations (PDEs) are transformed into ordinary differential equations (ODEs) using appropriately adjusted transformations. The numerical method is used for solving the so-formulated highly nonlinear problem. The graphical presentation of results highlights that the heat flux receives enhancement for augmented Brownian diffusion. The Bejan number is found to be increasing with a larger Weissenberg number. The tabulated results for skin-friction, Nusselt number and Sherwood number are given. A decent agreement is noted in the results when compared with previously published literature on Williamson nanofluids.

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@article {pmid33285793,

year = {2019},

author = {Rasool, G and Zhang, T and Chamkha, AJ and Shafiq, A and Tlili, I and Shahzadi, G},

title = {Entropy Generation and Consequences of Binary Chemical Reaction on MHD Darcy-Forchheimer Williamson Nanofluid Flow Over Non-Linearly Stretching Surface.},

journal = {Entropy (Basel, Switzerland)},

volume = {22},

number = {1},

pages = {},

doi = {10.3390/e22010018},

pmid = {33285793},

issn = {1099-4300},

support = {11771389 and 11621101//The National Science Foundation of China/ ; },

abstract = {The current article aims to present a numerical analysis of MHD Williamson nanofluid flow maintained to flow through porous medium bounded by a non-linearly stretching flat surface. The second law of thermodynamics was applied to analyze the fluid flow, heat and mass transport as well as the aspects of entropy generation using Buongiorno model. Thermophoresis and Brownian diffusion is considered which appears due to the concentration and random motion of nanoparticles in base fluid, respectively. Uniform magnetic effect is induced but the assumption of tiny magnetic Reynolds number results in zero magnetic induction. The governing equations (PDEs) are transformed into ordinary differential equations (ODEs) using appropriately adjusted transformations. The numerical method is used for solving the so-formulated highly nonlinear problem. The graphical presentation of results highlights that the heat flux receives enhancement for augmented Brownian diffusion. The Bejan number is found to be increasing with a larger Weissenberg number. The tabulated results for skin-friction, Nusselt number and Sherwood number are given. A decent agreement is noted in the results when compared with previously published literature on Williamson nanofluids.},

}

RevDate: 2020-12-08

**Numerical Simulation of Swirling Impinging Jet Issuing from a Threaded Hole under Inclined Condition.**

*Entropy (Basel, Switzerland)*, **22(1):** pii:e22010015.

There are some inclined jet holes in the cooling structure of the leading edge region of gas turbine blades. In order to improve the cooling effect of traditional round holes, this paper proposes to replace the round holes with threaded holes, and studies the complex flow and heat transfer performance of the swirling impinging jet (SIJ) issuing from the 45Â° threaded holes in the inclined condition by numerical simulation. The influencing factors include jet inclination angle Î± (45Â°-90Â°), jet-to-plate distance (H/d = 2, 4, 6), and Reynolds number (6000-24,000). The results show that the inclination angle and jet-to-plate distance have a great influence on the size, shape, and position of vortices in the jet space, while the Reynolds number has little effect on the vortices. In the inclined state, the impinging cooling effect of the swirling impinging jet is better than that of the circular impinging jet (CIJ), both heat transfer coefficients will degrade significantly when the inclination angle is 45Â°. When the inclination angle is greater than 45Â°, compared with the round hole, the enhanced heat transfer region for the swirling jet is in the region of r/d < 3, while both of the Nusselt numbers in the wall jet region are weak, with a value of just 20. At the same time, with the increasing of the inclination angle (Î± > 45Â°), the average Nusselt number on target surface holds a constant value. Under the inclined conditions, the heat transfer coefficient on the target surface for the swirling jet is increased totally with the increasing of the Re, but when the Re is larger than 18,000, the rate of enhanced heat transfer gradually weakens.

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@article {pmid33285790,

year = {2019},

author = {Xu, L and Xiong, Y and Xi, L and Gao, J and Li, Y and Zhao, Z},

title = {Numerical Simulation of Swirling Impinging Jet Issuing from a Threaded Hole under Inclined Condition.},

journal = {Entropy (Basel, Switzerland)},

volume = {22},

number = {1},

pages = {},

doi = {10.3390/e22010015},

pmid = {33285790},

issn = {1099-4300},

support = {51876157//National Natural Science Foundation of China/ ; 2018A030313183//Guangdong Natural Science Foundation/ ; xjj2018044//special fund for basic scientific research business expenses of Central Universities/ ; },

abstract = {There are some inclined jet holes in the cooling structure of the leading edge region of gas turbine blades. In order to improve the cooling effect of traditional round holes, this paper proposes to replace the round holes with threaded holes, and studies the complex flow and heat transfer performance of the swirling impinging jet (SIJ) issuing from the 45Â° threaded holes in the inclined condition by numerical simulation. The influencing factors include jet inclination angle Î± (45Â°-90Â°), jet-to-plate distance (H/d = 2, 4, 6), and Reynolds number (6000-24,000). The results show that the inclination angle and jet-to-plate distance have a great influence on the size, shape, and position of vortices in the jet space, while the Reynolds number has little effect on the vortices. In the inclined state, the impinging cooling effect of the swirling impinging jet is better than that of the circular impinging jet (CIJ), both heat transfer coefficients will degrade significantly when the inclination angle is 45Â°. When the inclination angle is greater than 45Â°, compared with the round hole, the enhanced heat transfer region for the swirling jet is in the region of r/d < 3, while both of the Nusselt numbers in the wall jet region are weak, with a value of just 20. At the same time, with the increasing of the inclination angle (Î± > 45Â°), the average Nusselt number on target surface holds a constant value. Under the inclined conditions, the heat transfer coefficient on the target surface for the swirling jet is increased totally with the increasing of the Re, but when the Re is larger than 18,000, the rate of enhanced heat transfer gradually weakens.},

}

RevDate: 2020-12-07

**Distribution and degradation trend of micropollutants in a surface flow treatment wetland revealed by 3D numerical modelling combined with LC-MS/MS.**

*Water research*, **190:**116672 pii:S0043-1354(20)31207-0 [Epub ahead of print].

Conventional wastewater treatment plants are not designed to treat micropollutants; thus, for 20 years, several complementary treatment systems, such as surface flow wetlands have been used to address this issue. Previous studies demonstrate that higher residence time and low global velocities promote nutrient removal rates or micropollutant photodegradation. Nevertheless, these studies were restricted to the system limits (inlet/outlet). Therefore, detailed knowledge of water flow is crucial for identifying areas that promote degradation and optimise surface flow wetlands. The present study combines 3D water flow numerical modelling and liquid chromatography coupled with high-resolution mass spectrometry (LC-HRMS/MS). Using this numerical model, validated by tracer experimental data, several velocity areas were distinguished in the wetland. Four areas were selected to investigate the waterflow influence and led to the following results: on the one hand, the number and concentration of micropollutants are independent of the waterflow, which could be due to several assumptions, such as the chronic exposure associated with a low Reynolds number; on the other hand, the potential degradation products (metabolites) were also assessed in the sludge to investigate the micropollutant biodegradation processes occurring in the wetland; micropollutant metabolites or degradation products were detected in higher proportions (both number and concentration) in lower flow rate areas. The relation to higher levels of plant and microorganism metabolites suggests higher biological activity that promotes degradation.

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@article {pmid33285453,

year = {2020},

author = {Maurer, L and Villette, C and Reiminger, N and Jurado, X and Laurent, J and Nuel, M and MosÃ©, R and Wanko, A and Heintz, D},

title = {Distribution and degradation trend of micropollutants in a surface flow treatment wetland revealed by 3D numerical modelling combined with LC-MS/MS.},

journal = {Water research},

volume = {190},

number = {},

pages = {116672},

doi = {10.1016/j.watres.2020.116672},

pmid = {33285453},

issn = {1879-2448},

abstract = {Conventional wastewater treatment plants are not designed to treat micropollutants; thus, for 20 years, several complementary treatment systems, such as surface flow wetlands have been used to address this issue. Previous studies demonstrate that higher residence time and low global velocities promote nutrient removal rates or micropollutant photodegradation. Nevertheless, these studies were restricted to the system limits (inlet/outlet). Therefore, detailed knowledge of water flow is crucial for identifying areas that promote degradation and optimise surface flow wetlands. The present study combines 3D water flow numerical modelling and liquid chromatography coupled with high-resolution mass spectrometry (LC-HRMS/MS). Using this numerical model, validated by tracer experimental data, several velocity areas were distinguished in the wetland. Four areas were selected to investigate the waterflow influence and led to the following results: on the one hand, the number and concentration of micropollutants are independent of the waterflow, which could be due to several assumptions, such as the chronic exposure associated with a low Reynolds number; on the other hand, the potential degradation products (metabolites) were also assessed in the sludge to investigate the micropollutant biodegradation processes occurring in the wetland; micropollutant metabolites or degradation products were detected in higher proportions (both number and concentration) in lower flow rate areas. The relation to higher levels of plant and microorganism metabolites suggests higher biological activity that promotes degradation.},

}

RevDate: 2020-12-03

**Metachronal patterns in artificial cilia for low Reynolds number fluid propulsion.**

*Science advances*, **6(49):** pii:6/49/eabd2508.

Cilia are hair-like organelles, present in arrays that collectively beat to generate flow. Given their small size and consequent low Reynolds numbers, asymmetric motions are necessary to create a net flow. Here, we developed an array of six soft robotic cilia, which are individually addressable, to both mimic nature's symmetry-breaking mechanisms and control asymmetries to study their influence on fluid propulsion. Our experimental tests are corroborated with fluid dynamics simulations, where we find a good agreement between both and show how the kymographs of the flow are related to the phase shift of the metachronal waves. Compared to synchronous beating, we report a 50% increase of net flow speed when cilia move in an antiplectic wave with phase shift of -Ï€/3 and a decrease for symplectic waves. Furthermore, we observe the formation of traveling vortices in the direction of the wave when metachrony is applied.

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@article {pmid33268359,

year = {2020},

author = {Milana, E and Zhang, R and Vetrano, MR and Peerlinck, S and De Volder, M and Onck, PR and Reynaerts, D and Gorissen, B},

title = {Metachronal patterns in artificial cilia for low Reynolds number fluid propulsion.},

journal = {Science advances},

volume = {6},

number = {49},

pages = {},

doi = {10.1126/sciadv.abd2508},

pmid = {33268359},

issn = {2375-2548},

abstract = {Cilia are hair-like organelles, present in arrays that collectively beat to generate flow. Given their small size and consequent low Reynolds numbers, asymmetric motions are necessary to create a net flow. Here, we developed an array of six soft robotic cilia, which are individually addressable, to both mimic nature's symmetry-breaking mechanisms and control asymmetries to study their influence on fluid propulsion. Our experimental tests are corroborated with fluid dynamics simulations, where we find a good agreement between both and show how the kymographs of the flow are related to the phase shift of the metachronal waves. Compared to synchronous beating, we report a 50% increase of net flow speed when cilia move in an antiplectic wave with phase shift of -Ï€/3 and a decrease for symplectic waves. Furthermore, we observe the formation of traveling vortices in the direction of the wave when metachrony is applied.},

}

RevDate: 2020-12-03

**Fluid Flow and Entropy Generation Analysis of Al2O3-Water Nanofluid in Microchannel Plate Fin Heat Sinks.**

*Entropy (Basel, Switzerland)*, **21(8):** pii:e21080739.

The flow in channels of microdevices is usually in the developing regime. Three-dimensional laminar flow characteristics of a nanofluid in microchannel plate fin heat sinks are investigated numerically in this paper. Deionized water and Al2O3-water nanofluid are employed as the cooling fluid in our work. The effects of the Reynolds number (100 < Re < 1000), channel aspect ratio (0 < Îµ < 1), and nanoparticle volume fraction (0.5% < Î¦ < 5%) on pressure drop and entropy generation in microchannel plate fin heat sinks are examined in detail. Herein, the general expression of the entropy generation rate considering entrance effects is developed. The results revealed that the frictional entropy generation and pressure drop increase as nanoparticle volume fraction and Reynolds number increase, while decrease as the channel aspect ratio increases. When the nanoparticle volume fraction increases from 0 to 3% at Re = 500, the pressure drop of microchannel plate fin heat sinks with Îµ = 0.5 increases by 9%. It is demonstrated that the effect of the entrance region is crucial for evaluating the performance of microchannel plate fin heat sinks. The study may shed some light on the design and optimization of microchannel heat sinks.

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@article {pmid33267453,

year = {2019},

author = {Ma, H and Duan, Z and Su, L and Ning, X and Bai, J and Lv, X},

title = {Fluid Flow and Entropy Generation Analysis of Al2O3-Water Nanofluid in Microchannel Plate Fin Heat Sinks.},

journal = {Entropy (Basel, Switzerland)},

volume = {21},

number = {8},

pages = {},

doi = {10.3390/e21080739},

pmid = {33267453},

issn = {1099-4300},

support = {2019YJS155//Fundamental Research Funds for the Central Universities/ ; },

abstract = {The flow in channels of microdevices is usually in the developing regime. Three-dimensional laminar flow characteristics of a nanofluid in microchannel plate fin heat sinks are investigated numerically in this paper. Deionized water and Al2O3-water nanofluid are employed as the cooling fluid in our work. The effects of the Reynolds number (100 < Re < 1000), channel aspect ratio (0 < Îµ < 1), and nanoparticle volume fraction (0.5% < Î¦ < 5%) on pressure drop and entropy generation in microchannel plate fin heat sinks are examined in detail. Herein, the general expression of the entropy generation rate considering entrance effects is developed. The results revealed that the frictional entropy generation and pressure drop increase as nanoparticle volume fraction and Reynolds number increase, while decrease as the channel aspect ratio increases. When the nanoparticle volume fraction increases from 0 to 3% at Re = 500, the pressure drop of microchannel plate fin heat sinks with Îµ = 0.5 increases by 9%. It is demonstrated that the effect of the entrance region is crucial for evaluating the performance of microchannel plate fin heat sinks. The study may shed some light on the design and optimization of microchannel heat sinks.},

}

RevDate: 2020-12-03

**Maximum Entropy Method for Solving the Turbulent Channel Flow Problem.**

*Entropy (Basel, Switzerland)*, **21(7):** pii:e21070675.

There are two components in this work that allow for solutions of the turbulent channel flow problem: One is the Galilean-transformed Navier-Stokes equation which gives a theoretical expression for the Reynolds stress (u'v'); and the second the maximum entropy principle which provides the spatial distribution of turbulent kinetic energy. The first concept transforms the momentum balance for a control volume moving at the local mean velocity, breaking the momentum exchange down to its basic components, u'v', u'2, pressure and viscous forces. The Reynolds stress gradient budget confirms this alternative interpretation of the turbulence momentum balance, as validated with DNS data. The second concept of maximum entropy principle states that turbulent kinetic energy in fully-developed flows will distribute itself until the maximum entropy is attained while conforming to the physical constraints. By equating the maximum entropy state with maximum allowable (viscous) dissipation at a given Reynolds number, along with other constraints, we arrive at function forms (inner and outer) for the turbulent kinetic energy. This allows us to compute the Reynolds stress, then integrate it to obtain the velocity profiles in channel flows. The results agree well with direct numerical simulation (DNS) data at ReÏ„ = 400 and 1000.

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@article {pmid33267389,

year = {2019},

author = {Lee, TW},

title = {Maximum Entropy Method for Solving the Turbulent Channel Flow Problem.},

journal = {Entropy (Basel, Switzerland)},

volume = {21},

number = {7},

pages = {},

doi = {10.3390/e21070675},

pmid = {33267389},

issn = {1099-4300},

abstract = {There are two components in this work that allow for solutions of the turbulent channel flow problem: One is the Galilean-transformed Navier-Stokes equation which gives a theoretical expression for the Reynolds stress (u'v'); and the second the maximum entropy principle which provides the spatial distribution of turbulent kinetic energy. The first concept transforms the momentum balance for a control volume moving at the local mean velocity, breaking the momentum exchange down to its basic components, u'v', u'2, pressure and viscous forces. The Reynolds stress gradient budget confirms this alternative interpretation of the turbulence momentum balance, as validated with DNS data. The second concept of maximum entropy principle states that turbulent kinetic energy in fully-developed flows will distribute itself until the maximum entropy is attained while conforming to the physical constraints. By equating the maximum entropy state with maximum allowable (viscous) dissipation at a given Reynolds number, along with other constraints, we arrive at function forms (inner and outer) for the turbulent kinetic energy. This allows us to compute the Reynolds stress, then integrate it to obtain the velocity profiles in channel flows. The results agree well with direct numerical simulation (DNS) data at ReÏ„ = 400 and 1000.},

}

RevDate: 2020-12-03

**MHD Casson Fluid Flow over a Stretching Sheet with Entropy Generation Analysis and Hall Influence.**

*Entropy (Basel, Switzerland)*, **21(6):** pii:e21060592.

The impacts of entropy generation and Hall current on MHD Casson fluid over a stretching surface with velocity slip factor have been numerically analyzed. Numerical work for the governing equations is established by using a shooting method with a fourth-order Runge-Kutta integration scheme. The outcomes show that the entropy generation is enhanced with a magnetic parameter, Reynolds number and group parameter. Further, the reverse behavior is observed with the Hall parameter, Eckert number, Casson parameter and slip factor. Also, it is viewed that Bejan number reduces with a group parameter.

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@article {pmid33267306,

year = {2019},

author = {Abd El-Aziz, M and Afify, AA},

title = {MHD Casson Fluid Flow over a Stretching Sheet with Entropy Generation Analysis and Hall Influence.},

journal = {Entropy (Basel, Switzerland)},

volume = {21},

number = {6},

pages = {},

doi = {10.3390/e21060592},

pmid = {33267306},

issn = {1099-4300},

abstract = {The impacts of entropy generation and Hall current on MHD Casson fluid over a stretching surface with velocity slip factor have been numerically analyzed. Numerical work for the governing equations is established by using a shooting method with a fourth-order Runge-Kutta integration scheme. The outcomes show that the entropy generation is enhanced with a magnetic parameter, Reynolds number and group parameter. Further, the reverse behavior is observed with the Hall parameter, Eckert number, Casson parameter and slip factor. Also, it is viewed that Bejan number reduces with a group parameter.},

}

RevDate: 2020-12-03

**Statistical Lyapunov Theory Based on Bifurcation Analysis of Energy Cascade in Isotropic Homogeneous Turbulence: A Physical-Mathematical Review.**

*Entropy (Basel, Switzerland)*, **21(5):** pii:e21050520.

This work presents a review of previous articles dealing with an original turbulence theory proposed by the author and provides new theoretical insights into some related issues. The new theoretical procedures and methodological approaches confirm and corroborate the previous results. These articles study the regime of homogeneous isotropic turbulence for incompressible fluids and propose theoretical approaches based on a specific Lyapunov theory for determining the closures of the von KÃ¡rmÃ¡n-Howarth and Corrsin equations and the statistics of velocity and temperature difference. While numerous works are present in the literature which concern the closures of the autocorrelation equations in the Fourier domain (i.e., Lin equation closure), few articles deal with the closures of the autocorrelation equations in the physical space. These latter, being based on the eddy-viscosity concept, describe diffusive closure models. On the other hand, the proposed Lyapunov theory leads to nondiffusive closures based on the property that, in turbulence, contiguous fluid particles trajectories continuously diverge. Therefore, the main motivation of this review is to present a theoretical formulation which does not adopt the eddy-viscosity paradigm and summarizes the results of the previous works. Next, this analysis assumes that the current fluid placements, together with velocity and temperature fields, are fluid state variables. This leads to the closures of the autocorrelation equations and helps to interpret the mechanism of energy cascade as due to the continuous divergence of the contiguous trajectories. Furthermore, novel theoretical issues are here presented among which we can mention the following ones. The bifurcation rate of the velocity gradient, calculated along fluid particles trajectories, is shown to be much larger than the corresponding maximal Lyapunov exponent. On that basis, an interpretation of the energy cascade phenomenon is given and the statistics of finite time Lyapunov exponent of the velocity gradient is shown to be represented by normal distribution functions. Next, the self-similarity produced by the proposed closures is analyzed and a proper bifurcation analysis of the closed von KÃ¡rmÃ¡n-Howarth equation is performed. This latter investigates the route from developed turbulence toward the non-chaotic regimes, leading to an estimate of the critical Taylor scale Reynolds number. A proper statistical decomposition based on extended distribution functions and on the Navier-Stokes equations is presented, which leads to the statistics of velocity and temperature difference.

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@article {pmid33267233,

year = {2019},

author = {de Divitiis, N},

title = {Statistical Lyapunov Theory Based on Bifurcation Analysis of Energy Cascade in Isotropic Homogeneous Turbulence: A Physical-Mathematical Review.},

journal = {Entropy (Basel, Switzerland)},

volume = {21},

number = {5},

pages = {},

doi = {10.3390/e21050520},

pmid = {33267233},

issn = {1099-4300},

abstract = {This work presents a review of previous articles dealing with an original turbulence theory proposed by the author and provides new theoretical insights into some related issues. The new theoretical procedures and methodological approaches confirm and corroborate the previous results. These articles study the regime of homogeneous isotropic turbulence for incompressible fluids and propose theoretical approaches based on a specific Lyapunov theory for determining the closures of the von KÃ¡rmÃ¡n-Howarth and Corrsin equations and the statistics of velocity and temperature difference. While numerous works are present in the literature which concern the closures of the autocorrelation equations in the Fourier domain (i.e., Lin equation closure), few articles deal with the closures of the autocorrelation equations in the physical space. These latter, being based on the eddy-viscosity concept, describe diffusive closure models. On the other hand, the proposed Lyapunov theory leads to nondiffusive closures based on the property that, in turbulence, contiguous fluid particles trajectories continuously diverge. Therefore, the main motivation of this review is to present a theoretical formulation which does not adopt the eddy-viscosity paradigm and summarizes the results of the previous works. Next, this analysis assumes that the current fluid placements, together with velocity and temperature fields, are fluid state variables. This leads to the closures of the autocorrelation equations and helps to interpret the mechanism of energy cascade as due to the continuous divergence of the contiguous trajectories. Furthermore, novel theoretical issues are here presented among which we can mention the following ones. The bifurcation rate of the velocity gradient, calculated along fluid particles trajectories, is shown to be much larger than the corresponding maximal Lyapunov exponent. On that basis, an interpretation of the energy cascade phenomenon is given and the statistics of finite time Lyapunov exponent of the velocity gradient is shown to be represented by normal distribution functions. Next, the self-similarity produced by the proposed closures is analyzed and a proper bifurcation analysis of the closed von KÃ¡rmÃ¡n-Howarth equation is performed. This latter investigates the route from developed turbulence toward the non-chaotic regimes, leading to an estimate of the critical Taylor scale Reynolds number. A proper statistical decomposition based on extended distribution functions and on the Navier-Stokes equations is presented, which leads to the statistics of velocity and temperature difference.},

}

RevDate: 2020-12-03

**Numerical Simulation of Entropy Generation for Power-Law Liquid Flow over a Permeable Exponential Stretched Surface with Variable Heat Source and Heat Flux.**

*Entropy (Basel, Switzerland)*, **21(5):** pii:e21050484.

This novel work explored the second law analysis and heat transfer in a magneto non-Newtonian power-law fluid model with the presence of an internal non-uniform heat source/sink. In this investigation, the motion of the studied fluid was induced by an exponentially stretching surface. The rheological behavior of the fluid model, including the shear thinning and shear thickening properties, are also considered as special case studies. The physical problem developed meaningfully with the imposed heat flux and the porosity of the stretched surface. Extensive numerical simulations were carried out for the present boundary layer flow, in order to study the influence of each control parameter on the boundary layer flow and heat transfer characteristics via various tabular and graphical illustrations. By employing the Shooting Runge-Kutta-Fehlberg Method (SRKFM), the resulting nonlinear ordinary differential equations were solved accurately. Based on this numerical procedure, the velocity and temperature fields are displayed graphically. By applying the second law of thermodynamics, and characterizing the entropy generation and Bejan number, the present physical problem was examined and discussed thoroughly in different situations. The attained results showed that the entropy generation can be improved significantly by raising the magnetic field strength and the group parameter. From an energetic point of view, it was found that the Reynolds number boosts the entropy generation of the fluidic medium and reduces the Bejan number. Also, it was observed that an amplification of the power-law index diminished the entropy generation near the stretched surface. As main results, it was proven that the heat transfer rate can be reduced with both the internal heat source intensity and the magnetic field strength.

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@article {pmid33267198,

year = {2019},

author = {Abd El-Aziz, M and Saleem, S},

title = {Numerical Simulation of Entropy Generation for Power-Law Liquid Flow over a Permeable Exponential Stretched Surface with Variable Heat Source and Heat Flux.},

journal = {Entropy (Basel, Switzerland)},

volume = {21},

number = {5},

pages = {},

doi = {10.3390/e21050484},

pmid = {33267198},

issn = {1099-4300},

support = {G.R.P-2/16/40//King Khalid University/ ; },

abstract = {This novel work explored the second law analysis and heat transfer in a magneto non-Newtonian power-law fluid model with the presence of an internal non-uniform heat source/sink. In this investigation, the motion of the studied fluid was induced by an exponentially stretching surface. The rheological behavior of the fluid model, including the shear thinning and shear thickening properties, are also considered as special case studies. The physical problem developed meaningfully with the imposed heat flux and the porosity of the stretched surface. Extensive numerical simulations were carried out for the present boundary layer flow, in order to study the influence of each control parameter on the boundary layer flow and heat transfer characteristics via various tabular and graphical illustrations. By employing the Shooting Runge-Kutta-Fehlberg Method (SRKFM), the resulting nonlinear ordinary differential equations were solved accurately. Based on this numerical procedure, the velocity and temperature fields are displayed graphically. By applying the second law of thermodynamics, and characterizing the entropy generation and Bejan number, the present physical problem was examined and discussed thoroughly in different situations. The attained results showed that the entropy generation can be improved significantly by raising the magnetic field strength and the group parameter. From an energetic point of view, it was found that the Reynolds number boosts the entropy generation of the fluidic medium and reduces the Bejan number. Also, it was observed that an amplification of the power-law index diminished the entropy generation near the stretched surface. As main results, it was proven that the heat transfer rate can be reduced with both the internal heat source intensity and the magnetic field strength.},

}

RevDate: 2020-12-03

**Heat Transfer and Entropy Generation Abilities of MWCNTs/GNPs Hybrid Nanofluids in Microtubes.**

*Entropy (Basel, Switzerland)*, **21(5):** pii:e21050480.

Massive improvements in the thermophysical properties of nanofluids over conventional fluids have led to the rapid evolution of using multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) in the field of heat transfer. In this study, the heat transfer and entropy generation abilities of MWCNTs/GNPs hybrid nanofluids were explored. Experiments on forced convective flow through a brass microtube with 300 Âµm inner diameter and 0.27 m in length were performed under uniform heat flux. MWCNTs/GNPs hybrid nanofluids were developed by adding 0.035 wt.% GNPs to MWCNTs water-based nanofluids with mass fractions of 0.075-0.125 wt.%. The range of the Reynolds number in this experiment was maintained at Re = 200-500. Results showed that the conventional approach for predicting the heat transfer coefficient was applicable for microtubes. The heat transfer coefficient increased markedly with the use of MWCNTs and MWCNTs/GNPs nanofluids, with increased pressure dropping by 12.4%. Results further showed a reduction by 37.5% in the total entropy generation rate in microtubes for hybrid nanofluids. Overall, MWCNTs/GNPs hybrid nanofluids can be used as alternative fluids in cooling systems for thermal applications.

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@article {pmid33267194,

year = {2019},

author = {Hussien, AA and Abdullah, MZ and Yusop, NM and Al-Kouz, W and Mahmoudi, E and Mehrali, M},

title = {Heat Transfer and Entropy Generation Abilities of MWCNTs/GNPs Hybrid Nanofluids in Microtubes.},

journal = {Entropy (Basel, Switzerland)},

volume = {21},

number = {5},

pages = {},

doi = {10.3390/e21050480},

pmid = {33267194},

issn = {1099-4300},

abstract = {Massive improvements in the thermophysical properties of nanofluids over conventional fluids have led to the rapid evolution of using multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) in the field of heat transfer. In this study, the heat transfer and entropy generation abilities of MWCNTs/GNPs hybrid nanofluids were explored. Experiments on forced convective flow through a brass microtube with 300 Âµm inner diameter and 0.27 m in length were performed under uniform heat flux. MWCNTs/GNPs hybrid nanofluids were developed by adding 0.035 wt.% GNPs to MWCNTs water-based nanofluids with mass fractions of 0.075-0.125 wt.%. The range of the Reynolds number in this experiment was maintained at Re = 200-500. Results showed that the conventional approach for predicting the heat transfer coefficient was applicable for microtubes. The heat transfer coefficient increased markedly with the use of MWCNTs and MWCNTs/GNPs nanofluids, with increased pressure dropping by 12.4%. Results further showed a reduction by 37.5% in the total entropy generation rate in microtubes for hybrid nanofluids. Overall, MWCNTs/GNPs hybrid nanofluids can be used as alternative fluids in cooling systems for thermal applications.},

}

RevDate: 2020-12-03

**About Universality and Thermodynamics of Turbulence.**

*Entropy (Basel, Switzerland)*, **21(3):** pii:e21030326.

This paper investigates the universality of the Eulerian velocity structure functions using velocity fields obtained from the stereoscopic particle image velocimetry (SPIV) technique in experiments and direct numerical simulations (DNS) of the Navier-Stokes equations. It shows that the numerical and experimental velocity structure functions up to order 9 follow a log-universality (Castaing et al. Phys. D Nonlinear Phenom. 1993); this leads to a collapse on a universal curve, when units including a logarithmic dependence on the Reynolds number are used. This paper then investigates the meaning and consequences of such log-universality, and shows that it is connected with the properties of a "multifractal free energy", based on an analogy between multifractal and thermodynamics. It shows that in such a framework, the existence of a fluctuating dissipation scale is associated with a phase transition describing the relaminarisation of rough velocity fields with different HÃ¶lder exponents. Such a phase transition has been already observed using the Lagrangian velocity structure functions, but was so far believed to be out of reach for the Eulerian data.

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@article {pmid33267040,

year = {2019},

author = {Geneste, D and Faller, H and Nguyen, F and Shukla, V and Laval, JP and Daviaud, F and Saw, EW and Dubrulle, B},

title = {About Universality and Thermodynamics of Turbulence.},

journal = {Entropy (Basel, Switzerland)},

volume = {21},

number = {3},

pages = {},

doi = {10.3390/e21030326},

pmid = {33267040},

issn = {1099-4300},

support = {ANR-16-CE06-0006-01//Agence Nationale de la Recherche/ ; 2A30096//Grand Ã‰quipement National De Calcul Intensif/ ; CFR//Commissariat Ã l'Ã‰nergie Atomique et aux Ã‰nergies Alternatives/ ; },

abstract = {This paper investigates the universality of the Eulerian velocity structure functions using velocity fields obtained from the stereoscopic particle image velocimetry (SPIV) technique in experiments and direct numerical simulations (DNS) of the Navier-Stokes equations. It shows that the numerical and experimental velocity structure functions up to order 9 follow a log-universality (Castaing et al. Phys. D Nonlinear Phenom. 1993); this leads to a collapse on a universal curve, when units including a logarithmic dependence on the Reynolds number are used. This paper then investigates the meaning and consequences of such log-universality, and shows that it is connected with the properties of a "multifractal free energy", based on an analogy between multifractal and thermodynamics. It shows that in such a framework, the existence of a fluctuating dissipation scale is associated with a phase transition describing the relaminarisation of rough velocity fields with different HÃ¶lder exponents. Such a phase transition has been already observed using the Lagrangian velocity structure functions, but was so far believed to be out of reach for the Eulerian data.},

}

RevDate: 2020-12-03

**Entropy Generation and Heat Transfer Performance in Microchannel Cooling.**

*Entropy (Basel, Switzerland)*, **21(2):** pii:e21020191.

Owing to its relatively high heat transfer performance and simple configurations, liquid cooling remains the preferred choice for electronic cooling and other applications. In this cooling approach, channel design plays an important role in dictating the cooling performance of the heat sink. Most cooling channel studies evaluate the performance in view of the first thermodynamics aspect. This study is conducted to investigate flow behaviour and heat transfer performance of an incompressible fluid in a cooling channel with oblique fins with regards to first law and second law of thermodynamics. The effect of oblique fin angle and inlet Reynolds number are investigated. In addition, the performance of the cooling channels for different heat fluxes is evaluated. The results indicate that the oblique fin channel with 20Â° angle yields the highest figure of merit, especially at higher Re (250-1000). The entropy generation is found to be lowest for an oblique fin channel with 90Â° angle, which is about twice than that of a conventional parallel channel. Increasing Re decreases the entropy generation, while increasing heat flux increases the entropy generation.

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@article {pmid33266906,

year = {2019},

author = {Kurnia, JC and Lim, DC and Chen, L and Jiang, L and Sasmito, AP},

title = {Entropy Generation and Heat Transfer Performance in Microchannel Cooling.},

journal = {Entropy (Basel, Switzerland)},

volume = {21},

number = {2},

pages = {},

doi = {10.3390/e21020191},

pmid = {33266906},

issn = {1099-4300},

support = {015LCO-026//Yayasan Universiti Teknologi PETRONAS/ ; },

abstract = {Owing to its relatively high heat transfer performance and simple configurations, liquid cooling remains the preferred choice for electronic cooling and other applications. In this cooling approach, channel design plays an important role in dictating the cooling performance of the heat sink. Most cooling channel studies evaluate the performance in view of the first thermodynamics aspect. This study is conducted to investigate flow behaviour and heat transfer performance of an incompressible fluid in a cooling channel with oblique fins with regards to first law and second law of thermodynamics. The effect of oblique fin angle and inlet Reynolds number are investigated. In addition, the performance of the cooling channels for different heat fluxes is evaluated. The results indicate that the oblique fin channel with 20Â° angle yields the highest figure of merit, especially at higher Re (250-1000). The entropy generation is found to be lowest for an oblique fin channel with 90Â° angle, which is about twice than that of a conventional parallel channel. Increasing Re decreases the entropy generation, while increasing heat flux increases the entropy generation.},

}

RevDate: 2020-12-03

**Entropy Generation Analysis and Thermodynamic Optimization of Jet Impingement Cooling Using Large Eddy Simulation.**

*Entropy (Basel, Switzerland)*, **21(2):** pii:e21020129.

In this work, entropy generation analysis is applied to characterize and optimize a turbulent impinging jet on a heated solid surface. In particular, the influence of plate inclinations and Reynolds numbers on the turbulent heat and fluid flow properties and its impact on the thermodynamic performance of such flow arrangements are numerically investigated. For this purpose, novel model equations are derived in the frame of Large Eddy Simulation (LES) that allows calculation of local entropy generation rates in a post-processing phase including the effect of unresolved subgrid-scale irreversibilities. From this LES-based study, distinctive features of heat and flow dynamics of the impinging fluid are detected and optimal operating designs for jet impingement cooling are identified. It turned out that (1) the location of the stagnation point and that of the maximal Nusselt number differ in the case of plate inclination; (2) predominantly the impinged wall acts as a strong source of irreversibility; and (3) a flow arrangement with a jet impinging normally on the heated surface allows the most efficient use of energy which is associated with lowest exergy lost. Furthermore, it is found that increasing the Reynolds number intensifies the heat transfer and upgrades the second law efficiency of such thermal systems. Thereby, the thermal efficiency enhancement can overwhelm the frictional exergy loss.

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@article {pmid33266845,

year = {2019},

author = {Ries, F and Li, Y and Nishad, K and Janicka, J and Sadiki, A},

title = {Entropy Generation Analysis and Thermodynamic Optimization of Jet Impingement Cooling Using Large Eddy Simulation.},

journal = {Entropy (Basel, Switzerland)},

volume = {21},

number = {2},

pages = {},

doi = {10.3390/e21020129},

pmid = {33266845},

issn = {1099-4300},

support = {SFB/TRR150//Deutsche Forschungsgemeinschaft/ ; },

abstract = {In this work, entropy generation analysis is applied to characterize and optimize a turbulent impinging jet on a heated solid surface. In particular, the influence of plate inclinations and Reynolds numbers on the turbulent heat and fluid flow properties and its impact on the thermodynamic performance of such flow arrangements are numerically investigated. For this purpose, novel model equations are derived in the frame of Large Eddy Simulation (LES) that allows calculation of local entropy generation rates in a post-processing phase including the effect of unresolved subgrid-scale irreversibilities. From this LES-based study, distinctive features of heat and flow dynamics of the impinging fluid are detected and optimal operating designs for jet impingement cooling are identified. It turned out that (1) the location of the stagnation point and that of the maximal Nusselt number differ in the case of plate inclination; (2) predominantly the impinged wall acts as a strong source of irreversibility; and (3) a flow arrangement with a jet impinging normally on the heated surface allows the most efficient use of energy which is associated with lowest exergy lost. Furthermore, it is found that increasing the Reynolds number intensifies the heat transfer and upgrades the second law efficiency of such thermal systems. Thereby, the thermal efficiency enhancement can overwhelm the frictional exergy loss.},

}

RevDate: 2020-12-03

**Modelling the Hindered Settling Velocity of a Falling Particle in a Particle-Fluid Mixture by the Tsallis Entropy Theory.**

*Entropy (Basel, Switzerland)*, **21(1):** pii:e21010055.

The settling velocity of a sediment particle is an important parameter needed for modelling the vertical flux in rivers, estuaries, deltas and the marine environment. It has been observed that a particle settles more slowly in the presence of other particles in the fluid than in a clear fluid, and this phenomenon has been termed 'hindered settling'. The Richardson and Zaki equation has been a widely used expression for relating the hindered settling velocity of a particle with that in a clear fluid in terms of a concentration function and the power of the concentration function, and the power index is known as the exponent of reduction of the settling velocity. This study attempts to formulate the model for the exponent of reduction of the settling velocity by using the probability method based on the Tsallis entropy theory. The derived expression is a function of the volumetric concentration of the suspended particle, the relative mass density of the particle and the particle's Reynolds number. This model is tested against experimental data collected from the literature and against five existing deterministic models, and this model shows good agreement with the experimental data and gives better prediction accuracy than the other deterministic models. The derived Tsallis entropy-based model is also compared with the existing Shannon entropy-based model for experimental data, and the Tsallis entropy-based model is comparable to the Shannon entropy-based model for predicting the hindered settling velocity of a falling particle in a particle-fluid mixture. This study shows the potential of using the Tsallis entropy together with the principle of maximum entropy to predict the hindered settling velocity of a falling particle in a particle-fluid mixture.

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@article {pmid33266771,

year = {2019},

author = {Zhu, Z and Wang, H and Peng, D and Dou, J},

title = {Modelling the Hindered Settling Velocity of a Falling Particle in a Particle-Fluid Mixture by the Tsallis Entropy Theory.},

journal = {Entropy (Basel, Switzerland)},

volume = {21},

number = {1},

pages = {},

doi = {10.3390/e21010055},

pmid = {33266771},

issn = {1099-4300},

support = {51509004//National Natural Science Foundation of China/ ; 2018KJ01//the Open Research Foundation of the Key Laboratory of the Pearl River Estuarine Dynamics and Associated Process Regulation, Ministry of Water Resources, China/ ; },

abstract = {The settling velocity of a sediment particle is an important parameter needed for modelling the vertical flux in rivers, estuaries, deltas and the marine environment. It has been observed that a particle settles more slowly in the presence of other particles in the fluid than in a clear fluid, and this phenomenon has been termed 'hindered settling'. The Richardson and Zaki equation has been a widely used expression for relating the hindered settling velocity of a particle with that in a clear fluid in terms of a concentration function and the power of the concentration function, and the power index is known as the exponent of reduction of the settling velocity. This study attempts to formulate the model for the exponent of reduction of the settling velocity by using the probability method based on the Tsallis entropy theory. The derived expression is a function of the volumetric concentration of the suspended particle, the relative mass density of the particle and the particle's Reynolds number. This model is tested against experimental data collected from the literature and against five existing deterministic models, and this model shows good agreement with the experimental data and gives better prediction accuracy than the other deterministic models. The derived Tsallis entropy-based model is also compared with the existing Shannon entropy-based model for experimental data, and the Tsallis entropy-based model is comparable to the Shannon entropy-based model for predicting the hindered settling velocity of a falling particle in a particle-fluid mixture. This study shows the potential of using the Tsallis entropy together with the principle of maximum entropy to predict the hindered settling velocity of a falling particle in a particle-fluid mixture.},

}

RevDate: 2020-12-03

**Wall-Normal Variation of Spanwise Streak Spacing in Turbulent Boundary Layer With Low-to-Moderate Reynolds Number.**

*Entropy (Basel, Switzerland)*, **21(1):** pii:e21010024.

Low-speed streaks in wall-bounded turbulence are the dominant structures in the near-wall turbulent self-sustaining cycle. Existing studies have well characterized their spanwise spacing in the buffer layer and below. Recent studies suggested the existence of these small-scale structures in the higher layer where large-scale structures usually receive more attention. The present study is thus devoted to extending the understanding of the streak spacing to the log layer. An analysis is taken on two-dimensional (2D) wall-parallel velocity fields in a smooth-wall turbulent boundary layer with R e Ï„ = 440âˆ¼2400, obtained via either 2D Particle Image Velocimetry (PIV) measurement taken here or public Direct Numerical Simulation (DNS). Morphological-based streak identification analysis yields a R e -independent log-normal distribution of the streak spacing till the upper bound of the log layer, based on which an empirical model is proposed to account for its wall-normal growth. The small-scale part of the spanwise spectra of the streamwise fluctuating velocity below y + = 100 is reasonably restored by a synthetic simulation that distributes elementary streak units based on the proposed empirical streak spacing model, which highlights the physical significance of streaks in shaping the small-scale part of the velocity spectra beyond the buffer layer.

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@article {pmid33266740,

year = {2018},

author = {Wang, W and Pan, C and Wang, J},

title = {Wall-Normal Variation of Spanwise Streak Spacing in Turbulent Boundary Layer With Low-to-Moderate Reynolds Number.},

journal = {Entropy (Basel, Switzerland)},

volume = {21},

number = {1},

pages = {},

doi = {10.3390/e21010024},

pmid = {33266740},

issn = {1099-4300},

support = {11672020,11490552//National Natural Science Foundation of China/ ; },

abstract = {Low-speed streaks in wall-bounded turbulence are the dominant structures in the near-wall turbulent self-sustaining cycle. Existing studies have well characterized their spanwise spacing in the buffer layer and below. Recent studies suggested the existence of these small-scale structures in the higher layer where large-scale structures usually receive more attention. The present study is thus devoted to extending the understanding of the streak spacing to the log layer. An analysis is taken on two-dimensional (2D) wall-parallel velocity fields in a smooth-wall turbulent boundary layer with R e Ï„ = 440âˆ¼2400, obtained via either 2D Particle Image Velocimetry (PIV) measurement taken here or public Direct Numerical Simulation (DNS). Morphological-based streak identification analysis yields a R e -independent log-normal distribution of the streak spacing till the upper bound of the log layer, based on which an empirical model is proposed to account for its wall-normal growth. The small-scale part of the spanwise spectra of the streamwise fluctuating velocity below y + = 100 is reasonably restored by a synthetic simulation that distributes elementary streak units based on the proposed empirical streak spacing model, which highlights the physical significance of streaks in shaping the small-scale part of the velocity spectra beyond the buffer layer.},

}

RevDate: 2020-12-03

**Optimal Design of Nanoparticle Enhanced Phan-Thien-Tanner Flow of a Viscoelastic Fluid in a Microchannel.**

*Entropy (Basel, Switzerland)*, **20(12):** pii:e20120895.

The excellent thermal characteristics of nanoparticles have increased their application in the field of heat transfer. In this paper, a thermophysical and geometrical parameter study is performed to minimize the total entropy generation of the viscoelastic flow of nanofluid. Entropy generation with respect to volume fraction (<0.04), the Reynolds number (20,000-100,000), and the diameter of the microchannel (20-20,000 Î¼m) with the circular cross-section under constant flux are calculated. As is shown, most of the entropy generation owes to heat transfer and by increasing the diameter of the channel, the Bejan number increases. The contribution of heat entropy generation in the microchannel is very poor and the major influence of entropy generation is attributable to friction. The maximum quantity of in-channel entropy generation happens in nanofluids with TiO2, CuO, Cu, and Ag nanoparticles, in turn, despite the fact in the microchannel this behavior is inverted, the minimum entropy generation occurs in nanofluids with CuO, Cu, Ag, and TiO2 nanoparticles, in turn. In the channel and microchannel for all nanofluids except water-TiO2, increasing the volume fraction of nanoparticles decreases entropy generation. In the channel and microchannel the total entropy generation increases by augmentation the Reynolds number.

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@article {pmid33266619,

year = {2018},

author = {Abdollahzadeh Jamalabadi, MY},

title = {Optimal Design of Nanoparticle Enhanced Phan-Thien-Tanner Flow of a Viscoelastic Fluid in a Microchannel.},

journal = {Entropy (Basel, Switzerland)},

volume = {20},

number = {12},

pages = {},

doi = {10.3390/e20120895},

pmid = {33266619},

issn = {1099-4300},

abstract = {The excellent thermal characteristics of nanoparticles have increased their application in the field of heat transfer. In this paper, a thermophysical and geometrical parameter study is performed to minimize the total entropy generation of the viscoelastic flow of nanofluid. Entropy generation with respect to volume fraction (<0.04), the Reynolds number (20,000-100,000), and the diameter of the microchannel (20-20,000 Î¼m) with the circular cross-section under constant flux are calculated. As is shown, most of the entropy generation owes to heat transfer and by increasing the diameter of the channel, the Bejan number increases. The contribution of heat entropy generation in the microchannel is very poor and the major influence of entropy generation is attributable to friction. The maximum quantity of in-channel entropy generation happens in nanofluids with TiO2, CuO, Cu, and Ag nanoparticles, in turn, despite the fact in the microchannel this behavior is inverted, the minimum entropy generation occurs in nanofluids with CuO, Cu, Ag, and TiO2 nanoparticles, in turn. In the channel and microchannel for all nanofluids except water-TiO2, increasing the volume fraction of nanoparticles decreases entropy generation. In the channel and microchannel the total entropy generation increases by augmentation the Reynolds number.},

}

RevDate: 2020-12-03

**Laminar-Turbulent Intermittency in Annular Couette-Poiseuille Flow: Whether a Puff Splits or Not.**

*Entropy (Basel, Switzerland)*, **22(12):** pii:e22121353.

Direct numerical simulations were carried out with an emphasis on the intermittency and localized turbulence structure occurring within the subcritical transitional regime of a concentric annular Couette-Poiseuille flow. In the annular system, the ratio of the inner to outer cylinder radius is an important geometrical parameter affecting the large-scale nature of the intermittency. We chose a low radius ratio of 0.1 and imposed a constant pressure gradient providing practically zero shear on the inner cylinder such that the base flow was approximated to that of a circular pipe flow. Localized turbulent puffs, that is, axial uni-directional intermittencies similar to those observed in the transitional circular pipe flow, were observed in the annular Couette-Poiseuille flow. Puff splitting events were clearly observed rather far from the global critical Reynolds number, near which given puffs survived without a splitting event throughout the observation period, which was as long as 104 outer time units. The characterization as a directed-percolation universal class was also discussed.

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@article {pmid33266252,

year = {2020},

author = {Morimatsu, H and Tsukahara, T},

title = {Laminar-Turbulent Intermittency in Annular Couette-Poiseuille Flow: Whether a Puff Splits or Not.},

journal = {Entropy (Basel, Switzerland)},

volume = {22},

number = {12},

pages = {},

doi = {10.3390/e22121353},

pmid = {33266252},

issn = {1099-4300},

support = {16H06066, 19H02071//Japan Society for the Promotion of Science/ ; },

abstract = {Direct numerical simulations were carried out with an emphasis on the intermittency and localized turbulence structure occurring within the subcritical transitional regime of a concentric annular Couette-Poiseuille flow. In the annular system, the ratio of the inner to outer cylinder radius is an important geometrical parameter affecting the large-scale nature of the intermittency. We chose a low radius ratio of 0.1 and imposed a constant pressure gradient providing practically zero shear on the inner cylinder such that the base flow was approximated to that of a circular pipe flow. Localized turbulent puffs, that is, axial uni-directional intermittencies similar to those observed in the transitional circular pipe flow, were observed in the annular Couette-Poiseuille flow. Puff splitting events were clearly observed rather far from the global critical Reynolds number, near which given puffs survived without a splitting event throughout the observation period, which was as long as 104 outer time units. The characterization as a directed-percolation universal class was also discussed.},

}

RevDate: 2020-12-03

**Heat Transfer and Flow Structures of Laminar Confined Slot Impingement Jet with Power-Law Non-Newtonian Fluid.**

*Entropy (Basel, Switzerland)*, **20(10):** pii:e20100800.

Heat transfer performances and flow structures of laminar impinging slot jets with power-law non-Newtonian fluids and corresponding typical industrial fluids (Carboxyl Methyl Cellulose (CMC) solutions and Xanthangum (XG) solutions) have been studied in this work. Investigations are performed for Reynolds number Re less than 200, power-law index n ranging from 0.5 to 1.5 and consistency index K varying from 0.001 to 0.5 to explore heat transfer and flow structure of shear-thinning fluid and shear-thickening fluid. Results indicate that with the increase of n, K for a given Re, wall Nusselt number increases mainly attributing to the increase of inlet velocity U. For a given inlet velocity, wall Nusselt number decreases with the increase of n and K, which mainly attributes to the increase of apparent viscosity and the reduction of momentum diffusion. For the same Re, U and Pr, wall Nusselt number decreases with the increase of n. Among the study of industrial power-law shear-thinning fluid, CMC solution with 100 ppm shows the best heat transfer performance at a given velocity. Moreover, new correlation of Nusselt number about industrial fluid is proposed. In general, for the heat transfer of laminar confined impinging jet, it is best to use the working fluid with low viscosity.

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@article {pmid33265887,

year = {2018},

author = {Qiang, Y and Wei, L and Luo, X and Jian, H and Wang, W and Li, F},

title = {Heat Transfer and Flow Structures of Laminar Confined Slot Impingement Jet with Power-Law Non-Newtonian Fluid.},

journal = {Entropy (Basel, Switzerland)},

volume = {20},

number = {10},

pages = {},

doi = {10.3390/e20100800},

pmid = {33265887},

issn = {1099-4300},

support = {51765033//National Natural Science Foundation of China/ ; },

abstract = {Heat transfer performances and flow structures of laminar impinging slot jets with power-law non-Newtonian fluids and corresponding typical industrial fluids (Carboxyl Methyl Cellulose (CMC) solutions and Xanthangum (XG) solutions) have been studied in this work. Investigations are performed for Reynolds number Re less than 200, power-law index n ranging from 0.5 to 1.5 and consistency index K varying from 0.001 to 0.5 to explore heat transfer and flow structure of shear-thinning fluid and shear-thickening fluid. Results indicate that with the increase of n, K for a given Re, wall Nusselt number increases mainly attributing to the increase of inlet velocity U. For a given inlet velocity, wall Nusselt number decreases with the increase of n and K, which mainly attributes to the increase of apparent viscosity and the reduction of momentum diffusion. For the same Re, U and Pr, wall Nusselt number decreases with the increase of n. Among the study of industrial power-law shear-thinning fluid, CMC solution with 100 ppm shows the best heat transfer performance at a given velocity. Moreover, new correlation of Nusselt number about industrial fluid is proposed. In general, for the heat transfer of laminar confined impinging jet, it is best to use the working fluid with low viscosity.},

}

RevDate: 2020-12-03

**Transition of Transient Channel Flow with High Reynolds Number Ratios.**

*Entropy (Basel, Switzerland)*, **20(5):** pii:e20050375.

Large-eddy simulations of turbulent channel flow subjected to a step-like acceleration have been performed to investigate the effect of high Reynolds number ratios on the transient behaviour of turbulence. It is shown that the response of the flow exhibits the same fundamental characteristics described in He & Seddighi (J. Fluid Mech., vol. 715, 2013, pp. 60-102 and vol. 764, 2015, pp. 395-427)-a three-stage response resembling that of the bypass transition of boundary layer flows. The features of transition are seen to become more striking as the Re-ratio increases-the elongated streaks become stronger and longer, and the initial turbulent spot sites at the onset of transition become increasingly sparse. The critical Reynolds number of transition and the transition period Reynolds number for those cases are shown to deviate from the trends of He & Seddighi (2015). The high Re-ratio cases show double peaks in the transient response of streamwise fluctuation profiles shortly after the onset of transition. Conditionally-averaged turbulent statistics based on a Î»_2-criterion are used to show that the two peaks in the fluctuation profiles are due to separate contributions of the active and inactive regions of turbulence generation. The peak closer to the wall is attributed to the generation of "new" turbulence in the active region, whereas the peak farther away from the wall is attributed to the elongated streaks in the inactive region. In the low Re-ratio cases, the peaks of these two regions are close to each other during the entire transient, resulting in a single peak in the domain-averaged profile.

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@article {pmid33265465,

year = {2018},

author = {Mathur, A and Seddighi, M and He, S},

title = {Transition of Transient Channel Flow with High Reynolds Number Ratios.},

journal = {Entropy (Basel, Switzerland)},

volume = {20},

number = {5},

pages = {},

doi = {10.3390/e20050375},

pmid = {33265465},

issn = {1099-4300},

abstract = {Large-eddy simulations of turbulent channel flow subjected to a step-like acceleration have been performed to investigate the effect of high Reynolds number ratios on the transient behaviour of turbulence. It is shown that the response of the flow exhibits the same fundamental characteristics described in He & Seddighi (J. Fluid Mech., vol. 715, 2013, pp. 60-102 and vol. 764, 2015, pp. 395-427)-a three-stage response resembling that of the bypass transition of boundary layer flows. The features of transition are seen to become more striking as the Re-ratio increases-the elongated streaks become stronger and longer, and the initial turbulent spot sites at the onset of transition become increasingly sparse. The critical Reynolds number of transition and the transition period Reynolds number for those cases are shown to deviate from the trends of He & Seddighi (2015). The high Re-ratio cases show double peaks in the transient response of streamwise fluctuation profiles shortly after the onset of transition. Conditionally-averaged turbulent statistics based on a Î»_2-criterion are used to show that the two peaks in the fluctuation profiles are due to separate contributions of the active and inactive regions of turbulence generation. The peak closer to the wall is attributed to the generation of "new" turbulence in the active region, whereas the peak farther away from the wall is attributed to the elongated streaks in the inactive region. In the low Re-ratio cases, the peaks of these two regions are close to each other during the entire transient, resulting in a single peak in the domain-averaged profile.},

}

RevDate: 2020-11-27

**Experimental visualization of sneezing and efficacy of face masks and shields.**

*Physics of fluids (Woodbury, N.Y. : 1994)*, **32(11):**115129.

In the present work, we propose and demonstrate a simple experimental visualization to simulate sneezing by maintaining dynamic similarity to actual sneezing. A pulsed jet with Reynolds number Re = 30 000 is created using compressed air and a solenoid valve. Tracer particles are introduced in the flow to capture the emulated turbulent jet formed due to a sneeze. The visualization is accomplished using a camera and laser illumination. It is observed that a typical sneeze can travel up to 25 ft in âˆ¼22 s in a quiescent environment. This highlights that the present widely accepted safe distance of 6 ft is highly underestimated, especially under the act of a sneeze. Our study demonstrates that a three-layer homemade mask is just adequate to impede the penetration of fine-sized particles, which may cause the spreading of the infectious pathogen responsible for COVID-19. However, a surgical mask cannot block the sneeze, and the sneeze particle can travel up to 2.5 ft. We strongly recommend using at least a three-layer homemade mask with a social distancing of 6 ft to combat the transmission of COVID-19 virus. In offices, we recommend the use of face masks and shields to prevent the spreading of droplets carrying the infectious pathogen. Interestingly, an N-95 mask blocks the sneeze in the forward direction; however, the leakage from the sides and top spreads the sneeze in the backward direction up to 2 ft. We strongly recommend using the elbow or hands to prevent droplet leakage even after wearing a mask during sneezing and coughing.

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@article {pmid33244217,

year = {2020},

author = {Arumuru, V and Pasa, J and Samantaray, SS},

title = {Experimental visualization of sneezing and efficacy of face masks and shields.},

journal = {Physics of fluids (Woodbury, N.Y. : 1994)},

volume = {32},

number = {11},

pages = {115129},

doi = {10.1063/5.0030101},

pmid = {33244217},

issn = {1070-6631},

abstract = {In the present work, we propose and demonstrate a simple experimental visualization to simulate sneezing by maintaining dynamic similarity to actual sneezing. A pulsed jet with Reynolds number Re = 30 000 is created using compressed air and a solenoid valve. Tracer particles are introduced in the flow to capture the emulated turbulent jet formed due to a sneeze. The visualization is accomplished using a camera and laser illumination. It is observed that a typical sneeze can travel up to 25 ft in âˆ¼22 s in a quiescent environment. This highlights that the present widely accepted safe distance of 6 ft is highly underestimated, especially under the act of a sneeze. Our study demonstrates that a three-layer homemade mask is just adequate to impede the penetration of fine-sized particles, which may cause the spreading of the infectious pathogen responsible for COVID-19. However, a surgical mask cannot block the sneeze, and the sneeze particle can travel up to 2.5 ft. We strongly recommend using at least a three-layer homemade mask with a social distancing of 6 ft to combat the transmission of COVID-19 virus. In offices, we recommend the use of face masks and shields to prevent the spreading of droplets carrying the infectious pathogen. Interestingly, an N-95 mask blocks the sneeze in the forward direction; however, the leakage from the sides and top spreads the sneeze in the backward direction up to 2 ft. We strongly recommend using the elbow or hands to prevent droplet leakage even after wearing a mask during sneezing and coughing.},

}

RevDate: 2020-11-27

**An experimental study of respiratory aerosol transport in phantom lung bronchioles.**

*Physics of fluids (Woodbury, N.Y. : 1994)*, **32(11):**111903.

The transport and deposition of micrometer-sized particles in the lung is the primary mechanism for the spread of aerosol borne diseases such as corona virus disease-19 (COVID-19). Considering the current situation, modeling the transport and deposition of drops in human lung bronchioles is of utmost importance to determine their consequences on human health. The current study reports experimental observations on deposition in micro-capillaries, representing distal lung bronchioles, over a wide range of Re that imitates the particle dynamics in the entire lung. The experiment investigated deposition in tubes of diameter ranging from 0.3 mm to 2 mm and over a wide range of Reynolds number (10-2 â©½ Re â©½ 103). The range of the tube diameter and Re used in this study is motivated by the dimensions of lung airways and typical breathing flow rates. The aerosol fluid was loaded with boron doped carbon quantum dots as fluorophores. An aerosol plume was generated from this mixture fluid using an ultrasonic nebulizer, producing droplets with 6.5 Âµm as a mean diameter and over a narrow distribution of sizes. The amount of aerosol deposited on the tube walls was measured using a spectrofluorometer. The experimental results show that dimensionless deposition (Î´) varies inversely with the bronchiole aspect ratio (L Â¯), with the effect of the Reynolds number (Re) being significant only at low L Â¯ . Î´ also increased with increasing dimensionless bronchiole diameter (D Â¯), but it is invariant with the particle size based Reynolds number. We show that Î´ L Â¯ âˆ¼ R e - 2 for 10-2 â©½ Re â©½ 1, which is typical of a diffusion dominated regime. For Re â©¾ 1, in the impaction dominated regime, Î´ L Â¯ is shown to be independent of Re. We also show a crossover regime where sedimentation becomes important. The experimental results conclude that lower breathing frequency and higher breath hold time could significantly increase the chances of getting infected with COVID-19 in crowded places.

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@article {pmid33244213,

year = {2020},

author = {Mallik, AK and Mukherjee, S and Panchagnula, MV},

title = {An experimental study of respiratory aerosol transport in phantom lung bronchioles.},

journal = {Physics of fluids (Woodbury, N.Y. : 1994)},

volume = {32},

number = {11},

pages = {111903},

doi = {10.1063/5.0029899},

pmid = {33244213},

issn = {1070-6631},

abstract = {The transport and deposition of micrometer-sized particles in the lung is the primary mechanism for the spread of aerosol borne diseases such as corona virus disease-19 (COVID-19). Considering the current situation, modeling the transport and deposition of drops in human lung bronchioles is of utmost importance to determine their consequences on human health. The current study reports experimental observations on deposition in micro-capillaries, representing distal lung bronchioles, over a wide range of Re that imitates the particle dynamics in the entire lung. The experiment investigated deposition in tubes of diameter ranging from 0.3 mm to 2 mm and over a wide range of Reynolds number (10-2 â©½ Re â©½ 103). The range of the tube diameter and Re used in this study is motivated by the dimensions of lung airways and typical breathing flow rates. The aerosol fluid was loaded with boron doped carbon quantum dots as fluorophores. An aerosol plume was generated from this mixture fluid using an ultrasonic nebulizer, producing droplets with 6.5 Âµm as a mean diameter and over a narrow distribution of sizes. The amount of aerosol deposited on the tube walls was measured using a spectrofluorometer. The experimental results show that dimensionless deposition (Î´) varies inversely with the bronchiole aspect ratio (L Â¯), with the effect of the Reynolds number (Re) being significant only at low L Â¯ . Î´ also increased with increasing dimensionless bronchiole diameter (D Â¯), but it is invariant with the particle size based Reynolds number. We show that Î´ L Â¯ âˆ¼ R e - 2 for 10-2 â©½ Re â©½ 1, which is typical of a diffusion dominated regime. For Re â©¾ 1, in the impaction dominated regime, Î´ L Â¯ is shown to be independent of Re. We also show a crossover regime where sedimentation becomes important. The experimental results conclude that lower breathing frequency and higher breath hold time could significantly increase the chances of getting infected with COVID-19 in crowded places.},

}

RevDate: 2020-11-21

**Diving into a Simple Anguilliform Swimmer's Sensitivity.**

*Integrative and comparative biology*, **60(5):**1236-1250.

Computational models of aquatic locomotion range from modest individual simple swimmers in 2D to sophisticated 3D multi-swimmer models that attempt to parse collective behavioral dynamics. Each of these models contain a multitude of model input parameters to which its outputs are inherently dependent, that is, various performance metrics. In this work, the swimming performance's sensitivity to parameters is investigated for an idealized, simple anguilliform swimming model in 2D. The swimmer considered here propagates forward by dynamically varying its body curvature, similar to motion of a Caenorhabditis elegans. The parameter sensitivities were explored with respect to the fluid scale (Reynolds number), stroke (undulation) frequency, as well as a kinematic parameter controlling the velocity and acceleration of each upstroke and downstroke. The input Reynolds number and stroke frequencies sampled were from [450, 2200] and [1, 3] Hz, respectively. In total, 5000 fluid-structure interaction simulations were performed, each with a unique parameter combination selected via a Sobol sequence, in order to conduct global sensitivity analysis. Results indicate that the swimmer's performance is most sensitive to variations in its stroke frequency. Trends in swimming performance were discovered by projecting the performance data onto particular 2D subspaces. Pareto-like optimal fronts were identified. This work is a natural extension of the parameter explorations of the same model from Battista in 2020.

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@article {pmid33220061,

year = {2020},

author = {Battista, NA},

title = {Diving into a Simple Anguilliform Swimmer's Sensitivity.},

journal = {Integrative and comparative biology},

volume = {60},

number = {5},

pages = {1236-1250},

doi = {10.1093/icb/icaa131},

pmid = {33220061},

issn = {1557-7023},

abstract = {Computational models of aquatic locomotion range from modest individual simple swimmers in 2D to sophisticated 3D multi-swimmer models that attempt to parse collective behavioral dynamics. Each of these models contain a multitude of model input parameters to which its outputs are inherently dependent, that is, various performance metrics. In this work, the swimming performance's sensitivity to parameters is investigated for an idealized, simple anguilliform swimming model in 2D. The swimmer considered here propagates forward by dynamically varying its body curvature, similar to motion of a Caenorhabditis elegans. The parameter sensitivities were explored with respect to the fluid scale (Reynolds number), stroke (undulation) frequency, as well as a kinematic parameter controlling the velocity and acceleration of each upstroke and downstroke. The input Reynolds number and stroke frequencies sampled were from [450, 2200] and [1, 3] Hz, respectively. In total, 5000 fluid-structure interaction simulations were performed, each with a unique parameter combination selected via a Sobol sequence, in order to conduct global sensitivity analysis. Results indicate that the swimmer's performance is most sensitive to variations in its stroke frequency. Trends in swimming performance were discovered by projecting the performance data onto particular 2D subspaces. Pareto-like optimal fronts were identified. This work is a natural extension of the parameter explorations of the same model from Battista in 2020.},

}

RevDate: 2020-11-20

**Wake symmetry impacts the performance of tandem hydrofoils during in-phase and out-of-phase oscillations differently.**

*Physical review. E*, **102(4-1):**043104.

The hydrodynamics of two oscillating foils in side-by-side configuration is numerically investigated for in-phase and out-of-phase pitching at Reynolds number of 4000 and Strouhal numbers of St=0.25-0.5. The effects of phase difference (in-phase and out-of-phase) and Strouhal number on symmetric attributes of the wake and unsteady propulsive performance of the foils are studied in detail. At lower Strouhal numbers, there is a quasisteady performance in both thrust generation and power consumption, which coincides with persistence of the wake symmetry. As Strouhal number increases, however, in-phase and out-of-phase pitching display unsteady cycle-averaged behavior with very different wake characteristics. The asymmetric wake of in-phase pitching foils at high Strouhal numbers transitions to a quasisymmetric wake, when an extensive interaction between the two vortex streets is observed in the wake. This coincides with an improvement on the propulsive performance of the foils. In contrast, the symmetric wake of the out-of-phase pitching foils at a high Strouhal number transitions to an asymmetric wake. The adverse effect of this transition is only observed on the propulsive performance of one foil while the other exploits the wake towards a better performance. The collective performance of the the out-of-phase pitching system, however, remains unchanged. There is also a strong correlation between the wake symmetric characteristics and total nonzero side-force production.

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@article {pmid33212661,

year = {2020},

author = {Gungor, A and Hemmati, A},

title = {Wake symmetry impacts the performance of tandem hydrofoils during in-phase and out-of-phase oscillations differently.},

journal = {Physical review. E},

volume = {102},

number = {4-1},

pages = {043104},

doi = {10.1103/PhysRevE.102.043104},

pmid = {33212661},

issn = {2470-0053},

abstract = {The hydrodynamics of two oscillating foils in side-by-side configuration is numerically investigated for in-phase and out-of-phase pitching at Reynolds number of 4000 and Strouhal numbers of St=0.25-0.5. The effects of phase difference (in-phase and out-of-phase) and Strouhal number on symmetric attributes of the wake and unsteady propulsive performance of the foils are studied in detail. At lower Strouhal numbers, there is a quasisteady performance in both thrust generation and power consumption, which coincides with persistence of the wake symmetry. As Strouhal number increases, however, in-phase and out-of-phase pitching display unsteady cycle-averaged behavior with very different wake characteristics. The asymmetric wake of in-phase pitching foils at high Strouhal numbers transitions to a quasisymmetric wake, when an extensive interaction between the two vortex streets is observed in the wake. This coincides with an improvement on the propulsive performance of the foils. In contrast, the symmetric wake of the out-of-phase pitching foils at a high Strouhal number transitions to an asymmetric wake. The adverse effect of this transition is only observed on the propulsive performance of one foil while the other exploits the wake towards a better performance. The collective performance of the the out-of-phase pitching system, however, remains unchanged. There is also a strong correlation between the wake symmetric characteristics and total nonzero side-force production.},

}

RevDate: 2020-11-20

**Transfer learning for nonlinear dynamics and its application to fluid turbulence.**

*Physical review. E*, **102(4-1):**043301.

We introduce transfer learning for nonlinear dynamics, which enables efficient predictions of chaotic dynamics by utilizing a small amount of data. For the Lorenz chaos, by optimizing the transfer rate, we accomplish more accurate inference than the conventional method by an order of magnitude. Moreover, a surprisingly small amount of learning is enough to infer the energy dissipation rate of the Navier-Stokes turbulence because we can, thanks to the small-scale universality of turbulence, transfer a large amount of the knowledge learned from turbulence data at lower Reynolds number.

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@article {pmid33212599,

year = {2020},

author = {Inubushi, M and Goto, S},

title = {Transfer learning for nonlinear dynamics and its application to fluid turbulence.},

journal = {Physical review. E},

volume = {102},

number = {4-1},

pages = {043301},

doi = {10.1103/PhysRevE.102.043301},

pmid = {33212599},

issn = {2470-0053},

abstract = {We introduce transfer learning for nonlinear dynamics, which enables efficient predictions of chaotic dynamics by utilizing a small amount of data. For the Lorenz chaos, by optimizing the transfer rate, we accomplish more accurate inference than the conventional method by an order of magnitude. Moreover, a surprisingly small amount of learning is enough to infer the energy dissipation rate of the Navier-Stokes turbulence because we can, thanks to the small-scale universality of turbulence, transfer a large amount of the knowledge learned from turbulence data at lower Reynolds number.},

}

RevDate: 2020-11-19

**Early changes of pulmonary arterial hemodynamics in patients with systemic sclerosis: flow pattern, WSS, and OSI analysis with 4D flow MRI.**

*European radiology* pii:10.1007/s00330-020-07301-x [Epub ahead of print].

OBJECTIVES: To study the pulmonary artery (PA) hemodynamics in patients with systemic sclerosis (SSc) using 4D flow MRI (4D-flow).

METHODS: Twenty-three patients with SSc (M/F: 2/21, 57 Â± 15 years, 3 manifest PA hypertension (PAH) by right heart catheterization) and 10 control subjects (M/F: 1/9, 55 Â± 17 years) underwent 4D-flow for the in vivo measurement of 3D blood flow velocities in the PA. Data analysis included area-averaged flow quantification at the main PA, 3D wall shear stress (WSS), oscillatory shear index (OSI) calculation along the PA surface, and Reynolds number. The composite outcome of all-cause death and major adverse cardiac events was also investigated.

RESULTS: The maximum PA flow at the systole did not differ, but the minimum flow at the diastole was significantly greater in patients with SSc compared with that in control subjects (7.7 Â± 16.0 ml/s vs. â€‘ 13.0 Â± 17.3 ml/s, p < 0.01). The maximum WSS at the peak systole was significantly lower and OSI was significantly greater in patients with SSc compared with those in control subjects (maximum WSS: 1.04 Â± 0.20 Pa vs. 1.33 Â± 0.34 Pa, p < 0.01, OSI: 0.139 Â± 0.031 vs. 0.101 Â± 0.037, p < 0.01). The cumulative event-free rate for the composite event was significantly lower in patients with minimum flow in main PA â‰¤ 9.22 ml/s (p = 0.012) and in patients with Reynolds number â‰¤ 2560 (p < 0.001).

CONCLUSIONS: 4D-flow has the potential to detect changes of PA hemodynamics noninvasively and predict the outcome in patients with SSc at the stage before manifest PAH.

KEY POINTS: â€¢ The WSS at the peak systolic phase was significantly lower (p < 0.05), whereas OSI was greater (p < 0.01) in patients with SSc without manifest PAH than in controls. â€¢ The hemodynamic change detected by 4D-flow may help patient management even at the stage before manifest PAH in SSc. â€¢ The minimum PA flow and Reynolds number by 4D-flow will serve as a predictive marker for SSc.

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@article {pmid33211148,

year = {2020},

author = {Ikoma, T and Suwa, K and Sano, M and Ushio, T and Saotome, M and Ogawa, N and Satoh, H and Maekawa, Y},

title = {Early changes of pulmonary arterial hemodynamics in patients with systemic sclerosis: flow pattern, WSS, and OSI analysis with 4D flow MRI.},

journal = {European radiology},

volume = {},

number = {},

pages = {},

doi = {10.1007/s00330-020-07301-x},

pmid = {33211148},

issn = {1432-1084},

support = {26461065//Ministry of Education, Culture, Sports, Science and Technology/ ; },

abstract = {OBJECTIVES: To study the pulmonary artery (PA) hemodynamics in patients with systemic sclerosis (SSc) using 4D flow MRI (4D-flow).

METHODS: Twenty-three patients with SSc (M/F: 2/21, 57 Â± 15 years, 3 manifest PA hypertension (PAH) by right heart catheterization) and 10 control subjects (M/F: 1/9, 55 Â± 17 years) underwent 4D-flow for the in vivo measurement of 3D blood flow velocities in the PA. Data analysis included area-averaged flow quantification at the main PA, 3D wall shear stress (WSS), oscillatory shear index (OSI) calculation along the PA surface, and Reynolds number. The composite outcome of all-cause death and major adverse cardiac events was also investigated.

RESULTS: The maximum PA flow at the systole did not differ, but the minimum flow at the diastole was significantly greater in patients with SSc compared with that in control subjects (7.7 Â± 16.0 ml/s vs. â€‘ 13.0 Â± 17.3 ml/s, p < 0.01). The maximum WSS at the peak systole was significantly lower and OSI was significantly greater in patients with SSc compared with those in control subjects (maximum WSS: 1.04 Â± 0.20 Pa vs. 1.33 Â± 0.34 Pa, p < 0.01, OSI: 0.139 Â± 0.031 vs. 0.101 Â± 0.037, p < 0.01). The cumulative event-free rate for the composite event was significantly lower in patients with minimum flow in main PA â‰¤ 9.22 ml/s (p = 0.012) and in patients with Reynolds number â‰¤ 2560 (p < 0.001).

CONCLUSIONS: 4D-flow has the potential to detect changes of PA hemodynamics noninvasively and predict the outcome in patients with SSc at the stage before manifest PAH.

KEY POINTS: â€¢ The WSS at the peak systolic phase was significantly lower (p < 0.05), whereas OSI was greater (p < 0.01) in patients with SSc without manifest PAH than in controls. â€¢ The hemodynamic change detected by 4D-flow may help patient management even at the stage before manifest PAH in SSc. â€¢ The minimum PA flow and Reynolds number by 4D-flow will serve as a predictive marker for SSc.},

}

RevDate: 2020-11-17

**Combined electrokinetic and shear flows control colloidal particle distribution across microchannel cross-sections.**

*Soft matter* [Epub ahead of print].

Recent experimental observations on combined electrokinetic and shear flows of colloidal suspensions in rectangular cross-section microfluidic channels have shown unusual cross-stream colloidal particle migration and dynamic assembly. Although a new electrophoresis-induced lift force has been postulated to cause the lateral migration of colloidal particles, little is known about how fluid properties and flow conditions impact this force and therefore subsequent colloidal particle migration. Furthermore, no experimental quantification of this electrophoresis-induced lift force is available. We report several key advances by demonstrating that the kinematic viscosity of the fluid can be used to modulate the spatial distribution of particles over the entire microchannel cross-section, with suppression of the colloidal particle migration observed with increase in fluid kinematic viscosity. Colloidal particle migration of âˆ¼10 Î¼m from not only the top and bottom microchannel walls but also from the side walls is shown with the corresponding electrophoresis-induced lift force of up to âˆ¼30 fN. The breadth of flow conditions tested capture the channel Reynolds number in the 0.1-1.1 range, with inertial migration of colloidal particles shown in flow regimes where the migration was previously thought to be ineffective, if not for the electrophoresis-induced lift force. The ability of the electrophoresis-induced lift force to migrate colloidal particles across the entire microchannel cross-section establishes a new paradigm for three-dimensional control of colloidal particles within confined microchannels.

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@article {pmid33201951,

year = {2020},

author = {Lochab, V and Prakash, S},

title = {Combined electrokinetic and shear flows control colloidal particle distribution across microchannel cross-sections.},

journal = {Soft matter},

volume = {},

number = {},

pages = {},

doi = {10.1039/d0sm01646b},

pmid = {33201951},

issn = {1744-6848},

abstract = {Recent experimental observations on combined electrokinetic and shear flows of colloidal suspensions in rectangular cross-section microfluidic channels have shown unusual cross-stream colloidal particle migration and dynamic assembly. Although a new electrophoresis-induced lift force has been postulated to cause the lateral migration of colloidal particles, little is known about how fluid properties and flow conditions impact this force and therefore subsequent colloidal particle migration. Furthermore, no experimental quantification of this electrophoresis-induced lift force is available. We report several key advances by demonstrating that the kinematic viscosity of the fluid can be used to modulate the spatial distribution of particles over the entire microchannel cross-section, with suppression of the colloidal particle migration observed with increase in fluid kinematic viscosity. Colloidal particle migration of âˆ¼10 Î¼m from not only the top and bottom microchannel walls but also from the side walls is shown with the corresponding electrophoresis-induced lift force of up to âˆ¼30 fN. The breadth of flow conditions tested capture the channel Reynolds number in the 0.1-1.1 range, with inertial migration of colloidal particles shown in flow regimes where the migration was previously thought to be ineffective, if not for the electrophoresis-induced lift force. The ability of the electrophoresis-induced lift force to migrate colloidal particles across the entire microchannel cross-section establishes a new paradigm for three-dimensional control of colloidal particles within confined microchannels.},

}

RevDate: 2020-11-17

**Swimming microorganisms acquire optimal efficiency with multiple cilia.**

*Proceedings of the National Academy of Sciences of the United States of America* pii:2011146117 [Epub ahead of print].

Planktonic microorganisms are ubiquitous in water, and their population dynamics are essential for forecasting the behavior of global aquatic ecosystems. Their population dynamics are strongly affected by these organisms' motility, which is generated by their hair-like organelles, called cilia or flagella. However, because of the complexity of ciliary dynamics, the precise role of ciliary flow in microbial life remains unclear. Here, we have used ciliary hydrodynamics to show that ciliates acquire the optimal propulsion efficiency. We found that ciliary flow highly resists an organism's propulsion and that the swimming velocity rapidly decreases with body size, proportional to the power of minus two. Accordingly, the propulsion efficiency decreases as the cube of body length. By increasing the number of cilia, however, efficiency can be significantly improved, up to 100-fold. We found that there exists an optimal number density of cilia, which provides the maximum propulsion efficiency for all ciliates. The propulsion efficiency in this case decreases inversely proportionally to body length. Our estimated optimal density of cilia corresponds to those of actual microorganisms, including species of ciliates and microalgae, which suggests that now-existing motile ciliates and microalgae have survived by acquiring the optimal propulsion efficiency. These conclusions are helpful for better understanding the ecology of microorganisms, such as the energetic costs and benefits of multicellularity in Volvocaceae, as well as for the optimal design of artificial microswimmers.

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@article {pmid33199601,

year = {2020},

author = {Omori, T and Ito, H and Ishikawa, T},

title = {Swimming microorganisms acquire optimal efficiency with multiple cilia.},

journal = {Proceedings of the National Academy of Sciences of the United States of America},

volume = {},

number = {},

pages = {},

doi = {10.1073/pnas.2011146117},

pmid = {33199601},

issn = {1091-6490},

abstract = {Planktonic microorganisms are ubiquitous in water, and their population dynamics are essential for forecasting the behavior of global aquatic ecosystems. Their population dynamics are strongly affected by these organisms' motility, which is generated by their hair-like organelles, called cilia or flagella. However, because of the complexity of ciliary dynamics, the precise role of ciliary flow in microbial life remains unclear. Here, we have used ciliary hydrodynamics to show that ciliates acquire the optimal propulsion efficiency. We found that ciliary flow highly resists an organism's propulsion and that the swimming velocity rapidly decreases with body size, proportional to the power of minus two. Accordingly, the propulsion efficiency decreases as the cube of body length. By increasing the number of cilia, however, efficiency can be significantly improved, up to 100-fold. We found that there exists an optimal number density of cilia, which provides the maximum propulsion efficiency for all ciliates. The propulsion efficiency in this case decreases inversely proportionally to body length. Our estimated optimal density of cilia corresponds to those of actual microorganisms, including species of ciliates and microalgae, which suggests that now-existing motile ciliates and microalgae have survived by acquiring the optimal propulsion efficiency. These conclusions are helpful for better understanding the ecology of microorganisms, such as the energetic costs and benefits of multicellularity in Volvocaceae, as well as for the optimal design of artificial microswimmers.},

}

RevDate: 2020-11-17

**The effect of tethering on the clearance rate of suspension-feeding plankton.**

*Proceedings of the National Academy of Sciences of the United States of America* pii:2017441117 [Epub ahead of print].

Many planktonic suspension feeders are attached to particles or tethered by gravity when feeding. It is commonly accepted that the feeding flows of tethered suspension feeders are stronger than those of their freely swimming counterparts. However, recent flow simulations indicate the opposite, and the cause of the opposing conclusions is not clear. To explore the effect of tethering on suspension feeding, we use a low-Reynolds-number flow model. We find that it is favorable to be freely swimming instead of tethered since the resulting feeding flow past the cell body is stronger, leading to a higher clearance rate. Our result underscores the significance of the near-field flow in shaping planktonic feeding modes, and it suggests that organisms tether for reasons that are not directly fluid dynamical (e.g., to stay near surfaces where the concentration of bacterial prey is high).

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@article {pmid33199599,

year = {2020},

author = {Andersen, A and KiÃ¸rboe, T},

title = {The effect of tethering on the clearance rate of suspension-feeding plankton.},

journal = {Proceedings of the National Academy of Sciences of the United States of America},

volume = {},

number = {},

pages = {},

doi = {10.1073/pnas.2017441117},

pmid = {33199599},

issn = {1091-6490},

abstract = {Many planktonic suspension feeders are attached to particles or tethered by gravity when feeding. It is commonly accepted that the feeding flows of tethered suspension feeders are stronger than those of their freely swimming counterparts. However, recent flow simulations indicate the opposite, and the cause of the opposing conclusions is not clear. To explore the effect of tethering on suspension feeding, we use a low-Reynolds-number flow model. We find that it is favorable to be freely swimming instead of tethered since the resulting feeding flow past the cell body is stronger, leading to a higher clearance rate. Our result underscores the significance of the near-field flow in shaping planktonic feeding modes, and it suggests that organisms tether for reasons that are not directly fluid dynamical (e.g., to stay near surfaces where the concentration of bacterial prey is high).},

}

RevDate: 2020-11-16

**Fractal Model for Drag Reduction on Multiscale Nonwetting Rough Surfaces.**

*Langmuir : the ACS journal of surfaces and colloids* [Epub ahead of print].

Rough surfaces in contact with a flow of fluid exhibit alternating no-slip and free shear boundary conditions at the solid-liquid and air-liquid interfaces, respectively, thereby potentially offering drag reduction benefits. The balance between the dynamic pressure in the flow and the restoring capillary pressure in the interasperity spaces determines the stability of the Cassie state of wettability and is a function of the relative extent of no-slip and free shear regions per unit surface area. In the present study, using a fractal representation of rough surface topography, an analytical model is developed to quantify the stability of the Cassie state of wettability as well as drag reduction and the friction factor for laminar flow in a rectangular channel between nonwetting multiscale rough surfaces. A systematic study is conducted to quantify the effects of fractal parameters of the surfaces and the flow Reynolds number on drag reduction and the friction factor. The studies are used to develop friction factor curves extending the classical Moody diagram to hydrophobic and superhydrophobic surfaces. On the basis of the studies, regime maps are derived for estimating the extent of drag reduction offered by hydrophobic and superhydrophobic surfaces, revealing that superhydrophobic surfaces do not always offer the best drag reduction performance. The application of the fractal model to practical topographies of nonwetting surfaces of copper, aluminum, and zinc oxide fabricated via electrodeposition and etching is also discussed.

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@article {pmid33197195,

year = {2020},

author = {Hatte, S and Pitchumani, R},

title = {Fractal Model for Drag Reduction on Multiscale Nonwetting Rough Surfaces.},

journal = {Langmuir : the ACS journal of surfaces and colloids},

volume = {},

number = {},

pages = {},

doi = {10.1021/acs.langmuir.0c02790},

pmid = {33197195},

issn = {1520-5827},

abstract = {Rough surfaces in contact with a flow of fluid exhibit alternating no-slip and free shear boundary conditions at the solid-liquid and air-liquid interfaces, respectively, thereby potentially offering drag reduction benefits. The balance between the dynamic pressure in the flow and the restoring capillary pressure in the interasperity spaces determines the stability of the Cassie state of wettability and is a function of the relative extent of no-slip and free shear regions per unit surface area. In the present study, using a fractal representation of rough surface topography, an analytical model is developed to quantify the stability of the Cassie state of wettability as well as drag reduction and the friction factor for laminar flow in a rectangular channel between nonwetting multiscale rough surfaces. A systematic study is conducted to quantify the effects of fractal parameters of the surfaces and the flow Reynolds number on drag reduction and the friction factor. The studies are used to develop friction factor curves extending the classical Moody diagram to hydrophobic and superhydrophobic surfaces. On the basis of the studies, regime maps are derived for estimating the extent of drag reduction offered by hydrophobic and superhydrophobic surfaces, revealing that superhydrophobic surfaces do not always offer the best drag reduction performance. The application of the fractal model to practical topographies of nonwetting surfaces of copper, aluminum, and zinc oxide fabricated via electrodeposition and etching is also discussed.},

}

RevDate: 2020-11-13

**Functional Morphology of Gliding Flight II. Morphology Follows Predictions of Gliding Performance.**

*Integrative and comparative biology* pii:5903741 [Epub ahead of print].

The evolution of wing morphology among birds, and its functional consequences, remains an open question, despite much attention. This is in part because the connection between form and function is difficult to test directly. To address this deficit, in prior work, we used computational modeling and sensitivity analysis to interrogate the impact of altering wing aspect ratio (AR), camber, and Reynolds number on aerodynamic performance, revealing the performance landscapes that avian evolution has explored. In the present work, we used a dataset of three-dimensionally scanned bird wings coupled with the performance landscapes to test two hypotheses regarding the evolutionary diversification of wing morphology associated with gliding flight behavior: (1) gliding birds would exhibit higher wing AR and greater chordwise camber than their non-gliding counterparts; and (2) that two strategies for gliding flight exist, with divergent morphological conformations. In support of our first hypothesis, we found evidence of morphological divergence in both wing AR and camber between gliders and non-gliders, suggesting that wing morphology of birds that utilize gliding flight is under different selective pressures than the wings of non-gliding taxa. Furthermore, we found that these morphological differences also yielded differences in coefficient of lift measured both at the maximum lift to drag ratio and at minimum sinking speed, with gliding taxa exhibiting higher coefficient of lift in both cases. Minimum sinking speed was also lower in gliders than non-gliders. However, contrary to our hypothesis, we found that the maximum ratio of the coefficient of lift to the coefficient of drag differed between gliders and non-gliders. This may point to the need for gliders to maintain high lift capability for takeoff and landing independent of gliding performance or could be due to the divergence in flight styles among gliders, as not all gliders are predicted to optimize either quantity. However, direct evidence for the existence of two morphologically defined gliding flight strategies was equivocal, with only slightly stronger support for an evolutionary model positing separate morphological optima for these strategies than an alternative model positing a single peak. The absence of a clear result may be an artifact of low statistical power owing to a relatively small sample size of gliding flyers expected to follow the "aerial search" strategy.

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@article {pmid33184652,

year = {2020},

author = {Rader, JA and Hedrick, TL and He, Y and Waldrop, LD},

title = {Functional Morphology of Gliding Flight II. Morphology Follows Predictions of Gliding Performance.},

journal = {Integrative and comparative biology},

volume = {},

number = {},

pages = {},

doi = {10.1093/icb/icaa126},

pmid = {33184652},

issn = {1557-7023},

abstract = {The evolution of wing morphology among birds, and its functional consequences, remains an open question, despite much attention. This is in part because the connection between form and function is difficult to test directly. To address this deficit, in prior work, we used computational modeling and sensitivity analysis to interrogate the impact of altering wing aspect ratio (AR), camber, and Reynolds number on aerodynamic performance, revealing the performance landscapes that avian evolution has explored. In the present work, we used a dataset of three-dimensionally scanned bird wings coupled with the performance landscapes to test two hypotheses regarding the evolutionary diversification of wing morphology associated with gliding flight behavior: (1) gliding birds would exhibit higher wing AR and greater chordwise camber than their non-gliding counterparts; and (2) that two strategies for gliding flight exist, with divergent morphological conformations. In support of our first hypothesis, we found evidence of morphological divergence in both wing AR and camber between gliders and non-gliders, suggesting that wing morphology of birds that utilize gliding flight is under different selective pressures than the wings of non-gliding taxa. Furthermore, we found that these morphological differences also yielded differences in coefficient of lift measured both at the maximum lift to drag ratio and at minimum sinking speed, with gliding taxa exhibiting higher coefficient of lift in both cases. Minimum sinking speed was also lower in gliders than non-gliders. However, contrary to our hypothesis, we found that the maximum ratio of the coefficient of lift to the coefficient of drag differed between gliders and non-gliders. This may point to the need for gliders to maintain high lift capability for takeoff and landing independent of gliding performance or could be due to the divergence in flight styles among gliders, as not all gliders are predicted to optimize either quantity. However, direct evidence for the existence of two morphologically defined gliding flight strategies was equivocal, with only slightly stronger support for an evolutionary model positing separate morphological optima for these strategies than an alternative model positing a single peak. The absence of a clear result may be an artifact of low statistical power owing to a relatively small sample size of gliding flyers expected to follow the "aerial search" strategy.},

}

RevDate: 2020-11-15

**Hydrodynamics of Vortical Gas Jets Coupled to Point-Like Suction.**

*Physics of fluids (Woodbury, N.Y. : 1994)*, **32(10):**.

Vortical jet flows in the Reynolds number (Re) range from 1000 to 3425 and swirl number (S) below 0.5, alone and in combination with suction through a small aperture, are experimentally investigated using optical visualization. Schlieren photography is employed to assess the vortical flow structure and establish the fundamental understanding of the source-to-sink gas-dynamic coupling, including the role played by flow rate, jet diameter, and the separation distance between the gas jet source and the suction sink. Compared to vortex-free jets, vortical jets for Re>2700 with swirl number S>0.27 experience earlier laminar-to-turbulent transition, with resulting rapid growth of the jet boundary. The ability to control growth of the jet expansion and mass and momentum dissipation into the surrounding is demonstrated via use of a coaxially aligned flow suction placed in the path of a jet. When a swirling jet is completely coupled with a flow suction, jet expansion is significantly suppressed. The suction/sink flow rate imposes a limit on the maximum input/source flow rate of gas jet to achieve complete coupling. Furthermore, there is a maximum distance over which effective coupling can occur, and for all Reynolds numbers considered this distance is shorter than the distance at which the jet structure breaks up into turbulent eddies in the absence of a sink.

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@article {pmid33184554,

year = {2020},

author = {Lee, JY and Kottke, PA and Fedorov, AG},

title = {Hydrodynamics of Vortical Gas Jets Coupled to Point-Like Suction.},

journal = {Physics of fluids (Woodbury, N.Y. : 1994)},

volume = {32},

number = {10},

pages = {},

pmid = {33184554},

issn = {1070-6631},

support = {R01 GM112662/GM/NIGMS NIH HHS/United States ; },

abstract = {Vortical jet flows in the Reynolds number (Re) range from 1000 to 3425 and swirl number (S) below 0.5, alone and in combination with suction through a small aperture, are experimentally investigated using optical visualization. Schlieren photography is employed to assess the vortical flow structure and establish the fundamental understanding of the source-to-sink gas-dynamic coupling, including the role played by flow rate, jet diameter, and the separation distance between the gas jet source and the suction sink. Compared to vortex-free jets, vortical jets for Re>2700 with swirl number S>0.27 experience earlier laminar-to-turbulent transition, with resulting rapid growth of the jet boundary. The ability to control growth of the jet expansion and mass and momentum dissipation into the surrounding is demonstrated via use of a coaxially aligned flow suction placed in the path of a jet. When a swirling jet is completely coupled with a flow suction, jet expansion is significantly suppressed. The suction/sink flow rate imposes a limit on the maximum input/source flow rate of gas jet to achieve complete coupling. Furthermore, there is a maximum distance over which effective coupling can occur, and for all Reynolds numbers considered this distance is shorter than the distance at which the jet structure breaks up into turbulent eddies in the absence of a sink.},

}

RevDate: 2020-11-11

**Prediction of the Limiting Flux and Its Correlation with the Reynolds Number during the Microfiltration of Skim Milk Using an Improved Model.**

*Foods (Basel, Switzerland)*, **9(11):** pii:foods9111621.

Limiting flux (JL) determination is a critical issue for membrane processing. This work presents a modified exponential model for JL calculation, based on a previously published version. Our research focused on skim milk microfiltrations. The processing variables studied were the crossflow velocity (CFV), membrane hydraulic diameter (dh), temperature, and concentration factor, totaling 62 experimental runs. Results showed that, by adding a new parameter called minimum transmembrane pressure, the modified model not only improved the fit of the experimental data compared to the former version (R2 > 97.00%), but also revealed the existence of a minimum transmembrane pressure required to obtain flux (J). This result is observed as a small shift to the right on J versus transmembrane pressure curves, and this shift increases with the flow velocity. This fact was reported in other investigations, but so far has gone uninvestigated. The JL predicted values were correlated with the Reynolds number (Re) for each dh tested. Results showed that for a same Re; JL increased as dh decreased; in a wide range of Re within the turbulent regime. Finally, from dimensionless correlations; a unique expression JL = f (Re, dh) was obtained; predicting satisfactorily JL (R2 = 84.11%) for the whole set of experiments.

Additional Links: PMID-33172214

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@article {pmid33172214,

year = {2020},

author = {Astudillo-Castro, C and Cordova, A and Oyanedel-Craver, V and Soto-Maldonado, C and Valencia, P and Henriquez, P and Jimenez-Flores, R},

title = {Prediction of the Limiting Flux and Its Correlation with the Reynolds Number during the Microfiltration of Skim Milk Using an Improved Model.},

journal = {Foods (Basel, Switzerland)},

volume = {9},

number = {11},

pages = {},

doi = {10.3390/foods9111621},

pmid = {33172214},

issn = {2304-8158},

support = {11110402//Fondo Nacional de Desarrollo CientÃfico y TecnolÃ³gico/ ; },

abstract = {Limiting flux (JL) determination is a critical issue for membrane processing. This work presents a modified exponential model for JL calculation, based on a previously published version. Our research focused on skim milk microfiltrations. The processing variables studied were the crossflow velocity (CFV), membrane hydraulic diameter (dh), temperature, and concentration factor, totaling 62 experimental runs. Results showed that, by adding a new parameter called minimum transmembrane pressure, the modified model not only improved the fit of the experimental data compared to the former version (R2 > 97.00%), but also revealed the existence of a minimum transmembrane pressure required to obtain flux (J). This result is observed as a small shift to the right on J versus transmembrane pressure curves, and this shift increases with the flow velocity. This fact was reported in other investigations, but so far has gone uninvestigated. The JL predicted values were correlated with the Reynolds number (Re) for each dh tested. Results showed that for a same Re; JL increased as dh decreased; in a wide range of Re within the turbulent regime. Finally, from dimensionless correlations; a unique expression JL = f (Re, dh) was obtained; predicting satisfactorily JL (R2 = 84.11%) for the whole set of experiments.},

}

RevDate: 2020-11-10

**On the role of phase lag in multi-appendage metachronal swimming of euphausiids.**

*Bioinspiration & biomimetics* [Epub ahead of print].

Metachronal paddling is a common method of drag-based aquatic propulsion, in which a series of swimming appendages are oscillated, with the motion of each appendage phase-shifted relative to the neighboring appendages. Ecologically and economically important Euphausiid species such as Antarctic krill (E. superba) swim constantly in the pelagic zone by stroking their paddling appendages (pleopods), with locomotion accounting for the bulk of their metabolic expenditure. They tailor their metachronal swimming gaits for behavioral and energetic needs by changing pleopod kinematics. The functional importance of inter-pleopod phase lag (Ï•) to metachronal swimming performance and wake structure is unknown. To examine this relation, we developed a geometrically and dynamically scaled robot ('krillbot') capable of self-propulsion. Krillbot pleopods were prescribed to mimic published kinematics of fast-forward swimming (FFW) and hovering (HOV) gaits of E. superba, and the Reynolds number and Strouhal number of the krillbot matched well with those calculated for freely-swimming E. superba. In addition to examining published kinematics with uneven Ï• between pleopod pairs, we modified E. superba kinematics to uniformly vary Ï• from 0% to 50% of the cycle. Swimming speed and thrust were largest for FFW with Ï• between 15%-25%, coincident with Ï• range observed in FFW gait of E. superba. In contrast to synchronous rowing (Ï•=0%) where distances between hinged joints of adjacent pleopods were nearly constant throughout the cycle, metachronal rowing (Ï•>0%) brought adjacent pleopods closer together and moved them farther apart. This factor minimized body position fluctuation and augmented metachronal swimming speed. Though swimming speed was lowest for HOV, a ventrally angled downward jet was generated that can assist with weight support during feeding. In summary, our findings show that inter-appendage phase lag can drastically alter both metachronal swimming speed and the large-scale wake structure.

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@article {pmid33171451,

year = {2020},

author = {Ford, M and Santhanakrishnan, A},

title = {On the role of phase lag in multi-appendage metachronal swimming of euphausiids.},

journal = {Bioinspiration & biomimetics},

volume = {},

number = {},

pages = {},

doi = {10.1088/1748-3190/abc930},

pmid = {33171451},

issn = {1748-3190},

abstract = {Metachronal paddling is a common method of drag-based aquatic propulsion, in which a series of swimming appendages are oscillated, with the motion of each appendage phase-shifted relative to the neighboring appendages. Ecologically and economically important Euphausiid species such as Antarctic krill (E. superba) swim constantly in the pelagic zone by stroking their paddling appendages (pleopods), with locomotion accounting for the bulk of their metabolic expenditure. They tailor their metachronal swimming gaits for behavioral and energetic needs by changing pleopod kinematics. The functional importance of inter-pleopod phase lag (Ï•) to metachronal swimming performance and wake structure is unknown. To examine this relation, we developed a geometrically and dynamically scaled robot ('krillbot') capable of self-propulsion. Krillbot pleopods were prescribed to mimic published kinematics of fast-forward swimming (FFW) and hovering (HOV) gaits of E. superba, and the Reynolds number and Strouhal number of the krillbot matched well with those calculated for freely-swimming E. superba. In addition to examining published kinematics with uneven Ï• between pleopod pairs, we modified E. superba kinematics to uniformly vary Ï• from 0% to 50% of the cycle. Swimming speed and thrust were largest for FFW with Ï• between 15%-25%, coincident with Ï• range observed in FFW gait of E. superba. In contrast to synchronous rowing (Ï•=0%) where distances between hinged joints of adjacent pleopods were nearly constant throughout the cycle, metachronal rowing (Ï•>0%) brought adjacent pleopods closer together and moved them farther apart. This factor minimized body position fluctuation and augmented metachronal swimming speed. Though swimming speed was lowest for HOV, a ventrally angled downward jet was generated that can assist with weight support during feeding. In summary, our findings show that inter-appendage phase lag can drastically alter both metachronal swimming speed and the large-scale wake structure.},

}

RevDate: 2020-11-07

**Phenomenological analysis on whipping behavior of rice flour batter.**

*Journal of food science* [Epub ahead of print].

In this study, the bubbles in rice flour batter were investigated under a constant temperature, because the bubble size distribution is important for the control of food texture. We obtained experimental data using a hand mixer and compared the properties of doughs prepared using six rice flours; each flour was prepared through a different milling process. We also added the size effect of the rice flour particles as the Bond number. Furthermore, we performed a dynamic wettability test to estimate the wettability of the rice flour surface. The results of this test were described well by the Washburn equation, and dc cosÎ¸/dp was calculated as a wettability parameter (where, dc = effective diameter of a capillary in a powder bed, cosÎ¸ = the contact angle, dp = mean particle diameter of rice flour). If bubble sizes depend mainly on the inertial force, viscous force, surface tension, and gravity, then the normalized mean bubble diameter should be a function of the Reynolds number, Weber number, and Froude number. The mean bubble diameter (dbm) generated by whipping was expected to be affected by the thickness (d) of the rod of the mixer, its movement speed, and physical properties of the material. Therefore, dimensionless mean diameter (dbm /d) was expressed based on a dimensionless equation. In the three-phase dispersion, different empirical equations were obtained depending on the amount of rice flour added, and the bubble diameter could be predicted using dimensionless parameters. In addition, the equations were generally applicable to the various materials selected for this study. PRACTICAL APPLICATION: The powder properties of rice flour were investigated, and dimensionless parameters were analyzed to construct an appropriate process control system for rice flour-based food products. Although the process method optimized for flour products is also used for rice flour products in practical situations, the comprehensive evaluation based on dimensionless parameters leads to optimization of the process for rice-flour based products. Moreover, this optimization might strongly support the creation of a new texture, and thus, the potential for market expansion of rice-flour based products is considerable.

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@article {pmid33159347,

year = {2020},

author = {Ichikawa, C and Ishikawa, D and Yang, JM and Fujii, T},

title = {Phenomenological analysis on whipping behavior of rice flour batter.},

journal = {Journal of food science},

volume = {},

number = {},

pages = {},

doi = {10.1111/1750-3841.15452},

pmid = {33159347},

issn = {1750-3841},

abstract = {In this study, the bubbles in rice flour batter were investigated under a constant temperature, because the bubble size distribution is important for the control of food texture. We obtained experimental data using a hand mixer and compared the properties of doughs prepared using six rice flours; each flour was prepared through a different milling process. We also added the size effect of the rice flour particles as the Bond number. Furthermore, we performed a dynamic wettability test to estimate the wettability of the rice flour surface. The results of this test were described well by the Washburn equation, and dc cosÎ¸/dp was calculated as a wettability parameter (where, dc = effective diameter of a capillary in a powder bed, cosÎ¸ = the contact angle, dp = mean particle diameter of rice flour). If bubble sizes depend mainly on the inertial force, viscous force, surface tension, and gravity, then the normalized mean bubble diameter should be a function of the Reynolds number, Weber number, and Froude number. The mean bubble diameter (dbm) generated by whipping was expected to be affected by the thickness (d) of the rod of the mixer, its movement speed, and physical properties of the material. Therefore, dimensionless mean diameter (dbm /d) was expressed based on a dimensionless equation. In the three-phase dispersion, different empirical equations were obtained depending on the amount of rice flour added, and the bubble diameter could be predicted using dimensionless parameters. In addition, the equations were generally applicable to the various materials selected for this study. PRACTICAL APPLICATION: The powder properties of rice flour were investigated, and dimensionless parameters were analyzed to construct an appropriate process control system for rice flour-based food products. Although the process method optimized for flour products is also used for rice flour products in practical situations, the comprehensive evaluation based on dimensionless parameters leads to optimization of the process for rice-flour based products. Moreover, this optimization might strongly support the creation of a new texture, and thus, the potential for market expansion of rice-flour based products is considerable.},

}

RevDate: 2020-11-10

**Empirical Modelling of Hydrodynamic Effects on Starch Nanoparticles Precipitation in a Spinning Disc Reactor.**

*Nanomaterials (Basel, Switzerland)*, **10(11):** pii:nano10112202.

Empirical correlations have been developed to relate experimentally determined starch nanoparticle size obtained in a solvent-antisolvent precipitation process with key hydrodynamic parameters of a spinning disc reactor (SDR). Three different combinations of dimensionless groups including a conventional Reynolds number (Re), rotational Reynolds number (ReÏ‰) and Rossby number (Ro) have been applied in individual models for two disc surfaces (smooth and grooved) to represent operating variables affecting film flow such as liquid flowrate and disc rotational speed, whilst initial supersaturation (S) has been included to represent varying antisolvent concentrations. Model 1 featuring a combination of Re, ReÏ‰ and S shows good agreement with the experimental data for both the grooved and smooth discs. For the grooved disc, Re has a greater impact on particle size, whereas ReÏ‰ is more influential on the smooth disc surface, the difference likely being due to the passive mixing induced by the grooves irrespective of the magnitude of the disc speed. Supersaturation has little impact on particle size within the limited initial supersaturation range studied. Model 2 which characterises both flow rate and disc rotational speed through Ro alone and combined with Re was less accurate in predicting particle size due to several inherent limitations.

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@article {pmid33158219,

year = {2020},

author = {Sana, S and Zivkovic, V and Boodhoo, K},

title = {Empirical Modelling of Hydrodynamic Effects on Starch Nanoparticles Precipitation in a Spinning Disc Reactor.},

journal = {Nanomaterials (Basel, Switzerland)},

volume = {10},

number = {11},

pages = {},

doi = {10.3390/nano10112202},

pmid = {33158219},

issn = {2079-4991},

abstract = {Empirical correlations have been developed to relate experimentally determined starch nanoparticle size obtained in a solvent-antisolvent precipitation process with key hydrodynamic parameters of a spinning disc reactor (SDR). Three different combinations of dimensionless groups including a conventional Reynolds number (Re), rotational Reynolds number (ReÏ‰) and Rossby number (Ro) have been applied in individual models for two disc surfaces (smooth and grooved) to represent operating variables affecting film flow such as liquid flowrate and disc rotational speed, whilst initial supersaturation (S) has been included to represent varying antisolvent concentrations. Model 1 featuring a combination of Re, ReÏ‰ and S shows good agreement with the experimental data for both the grooved and smooth discs. For the grooved disc, Re has a greater impact on particle size, whereas ReÏ‰ is more influential on the smooth disc surface, the difference likely being due to the passive mixing induced by the grooves irrespective of the magnitude of the disc speed. Supersaturation has little impact on particle size within the limited initial supersaturation range studied. Model 2 which characterises both flow rate and disc rotational speed through Ro alone and combined with Re was less accurate in predicting particle size due to several inherent limitations.},

}

RevDate: 2020-11-06

**Aerodynamic efficiency of gliding birds vs. comparable UAVs: a review.**

*Bioinspiration & biomimetics* [Epub ahead of print].

Here, we reviewed published aerodynamic efficiencies of gliding birds and similar sized unmanned aerial vehicles (UAVs) motivated by a fundamental question: are gliding birds more efficient than comparable UAVs? Despite a multitude of studies that have quantified the aerodynamic efficiency of gliding birds, there is no comprehensive summary of these results. This lack of consolidated information inhibits a true comparison between birds and UAVs. Such a comparison is complicated by variable uncertainty levels between the different techniques used to predict avian efficiency. To support our comparative approach, we began by surveying theoretical and experimental estimates of avian aerodynamic efficiency and investigating the uncertainty associated with each estimation method. We found that the methodology used by a study affects the estimated efficiency and can lead to incongruent conclusions on gliding bird aerodynamic efficiency. Our survey showed that studies on live birds gliding in wind tunnels provide a reliable minimum estimate of a birds' aerodynamic efficiency while simultaneously quantifying the wing configurations used in flight. Next, we surveyed the aeronautical literature to collect the published aerodynamic efficiencies of similar-sized, non-copter UAVs. The compiled information allowed a direct comparison of UAVs and gliding birds. Contrary to our expectation, we found that there is no definitive evidence that any gliding bird species is either more or less efficient than a comparable UAV. This non-result highlights a critical need for new technology and analytical advances that can reduce the uncertainty associated with estimating a gliding bird's aerodynamic efficiency. Nevertheless, our survey indicated that species flying within subcritical Reynolds number regimes may inspire UAV designs that can extend their operational range to efficiently operate in subcritical regimes. The survey results provided here point the way forward for research into avian gliding flight and enable informed UAV designs.

Additional Links: PMID-33157545

Publisher:

PubMed:

Citation:

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hide bibtex listing

@article {pmid33157545,

year = {2020},

author = {Harvey, C and Inman, DJ},

title = {Aerodynamic efficiency of gliding birds vs. comparable UAVs: a review.},

journal = {Bioinspiration & biomimetics},

volume = {},

number = {},

pages = {},

doi = {10.1088/1748-3190/abc86a},

pmid = {33157545},

issn = {1748-3190},

abstract = {Here, we reviewed published aerodynamic efficiencies of gliding birds and similar sized unmanned aerial vehicles (UAVs) motivated by a fundamental question: are gliding birds more efficient than comparable UAVs? Despite a multitude of studies that have quantified the aerodynamic efficiency of gliding birds, there is no comprehensive summary of these results. This lack of consolidated information inhibits a true comparison between birds and UAVs. Such a comparison is complicated by variable uncertainty levels between the different techniques used to predict avian efficiency. To support our comparative approach, we began by surveying theoretical and experimental estimates of avian aerodynamic efficiency and investigating the uncertainty associated with each estimation method. We found that the methodology used by a study affects the estimated efficiency and can lead to incongruent conclusions on gliding bird aerodynamic efficiency. Our survey showed that studies on live birds gliding in wind tunnels provide a reliable minimum estimate of a birds' aerodynamic efficiency while simultaneously quantifying the wing configurations used in flight. Next, we surveyed the aeronautical literature to collect the published aerodynamic efficiencies of similar-sized, non-copter UAVs. The compiled information allowed a direct comparison of UAVs and gliding birds. Contrary to our expectation, we found that there is no definitive evidence that any gliding bird species is either more or less efficient than a comparable UAV. This non-result highlights a critical need for new technology and analytical advances that can reduce the uncertainty associated with estimating a gliding bird's aerodynamic efficiency. Nevertheless, our survey indicated that species flying within subcritical Reynolds number regimes may inspire UAV designs that can extend their operational range to efficiently operate in subcritical regimes. The survey results provided here point the way forward for research into avian gliding flight and enable informed UAV designs.},

}

RevDate: 2020-11-10

**Stochastic Interpolation of Sparsely Sampled Time Series via Multipoint Fractional Brownian Bridges.**

*Physical review letters*, **125(17):**170602.

We propose and test a method to interpolate sparsely sampled signals by a stochastic process with a broad range of spatial and/or temporal scales. To this end, we extend the notion of a fractional Brownian bridge, defined as fractional Brownian motion with a given scaling (Hurst) exponent H and with prescribed start and end points, to a bridge process with an arbitrary number of intermediate and nonequidistant points. Determining the optimal value of the Hurst exponent H_{opt}, appropriate to interpolate the sparse signal, is a very important step of our method. We demonstrate the validity of our method on a signal from fluid turbulence in a high Reynolds number flow and discuss the implications of the non-self-similar character of the signal. The method introduced here could be instrumental in several physical problems, including astrophysics, particle tracking, and specific tailoring of surrogate data, as well as in domains of natural and social sciences.

Additional Links: PMID-33156686

Publisher:

PubMed:

Citation:

show bibtex listing

hide bibtex listing

@article {pmid33156686,

year = {2020},

author = {Friedrich, J and Gallon, S and Pumir, A and Grauer, R},

title = {Stochastic Interpolation of Sparsely Sampled Time Series via Multipoint Fractional Brownian Bridges.},

journal = {Physical review letters},

volume = {125},

number = {17},

pages = {170602},

doi = {10.1103/PhysRevLett.125.170602},

pmid = {33156686},

issn = {1079-7114},

abstract = {We propose and test a method to interpolate sparsely sampled signals by a stochastic process with a broad range of spatial and/or temporal scales. To this end, we extend the notion of a fractional Brownian bridge, defined as fractional Brownian motion with a given scaling (Hurst) exponent H and with prescribed start and end points, to a bridge process with an arbitrary number of intermediate and nonequidistant points. Determining the optimal value of the Hurst exponent H_{opt},

appropriate to interpolate the sparse signal, is a very important step of our method. We demonstrate the validity of our method on a signal from fluid turbulence in a high Reynolds number flow and discuss the implications of the non-self-similar character of the signal. The method introduced here could be instrumental in several physical problems, including astrophysics, particle tracking, and specific tailoring of surrogate data, as well as in domains of natural and social sciences.},

}

RevDate: 2020-11-06

**Capsules Rheology in Carreau-Yasuda Fluids.**

*Nanomaterials (Basel, Switzerland)*, **10(11):** pii:nano10112190.

In this paper, a Multi Relaxation Time Lattice Boltzmann scheme is used to describe the evolution of a non-Newtonian fluid. Such method is coupled with an Immersed-Boundary technique for the transport of arbitrarily shaped objects navigating the flow. The no-slip boundary conditions on immersed bodies are imposed through a convenient forcing term accounting for the hydrodynamic force generated by the presence of immersed geometries added to momentum equation. Moreover, such forcing term accounts also for the force induced by the shear-dependent viscosity model characterizing the non-Newtonian behavior of the considered fluid. Firstly, the present model is validated against well-known benchmarks, namely the parabolic velocity profile obtained for the flow within two infinite laminae for five values of the viscosity model exponent, n = 0.25, 0.50, 0.75, 1.0, and 1.5. Then, the flow within a squared lid-driven cavity for Re = 1000 and 5000 (being Re the Reynolds number) is computed as a function of n for a shear-thinning (n < 1) fluid. Indeed, the local decrements in the viscosity field achieved in high-shear zones implies the increment in the local Reynolds number, thus moving the position of near-walls minima towards lateral walls. Moreover, the revolution under shear of neutrally buoyant plain elliptical capsules with different Aspect Ratio (AR = 2 and 3) is analyzed for shear-thinning (n < 1), Newtonian (n = 1), and shear-thickening (n > 1) surrounding fluids. Interestingly, the power law by Huang et al. describing the revolution period of such capsules as a function of the Reynolds number and the existence of a critical value, Rec, after which the tumbling is inhibited in confirmed also for non-Newtonian fluids. Analogously, the equilibrium lateral position yeq of such neutrally buoyant capsules when transported in a plane-Couette flow is studied detailing the variation of yeq as a function of the Reynolds number as well as of the exponent n.

Additional Links: PMID-33153075

Publisher:

PubMed:

Citation:

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@article {pmid33153075,

year = {2020},

author = {Coclite, A and Coclite, GM and De Tommasi, D},

title = {Capsules Rheology in Carreau-Yasuda Fluids.},

journal = {Nanomaterials (Basel, Switzerland)},

volume = {10},

number = {11},

pages = {},

doi = {10.3390/nano10112190},

pmid = {33153075},

issn = {2079-4991},

support = {Prin 2017, 267 project code 2017J4EAYB//Ministero dell'Istruzione, dell'UniversitÃ e della Ricerca/ ; CUP - D94I18000260001//Ministero dell'Istruzione, dell'UniversitÃ e della Ricerca/ ; },

abstract = {In this paper, a Multi Relaxation Time Lattice Boltzmann scheme is used to describe the evolution of a non-Newtonian fluid. Such method is coupled with an Immersed-Boundary technique for the transport of arbitrarily shaped objects navigating the flow. The no-slip boundary conditions on immersed bodies are imposed through a convenient forcing term accounting for the hydrodynamic force generated by the presence of immersed geometries added to momentum equation. Moreover, such forcing term accounts also for the force induced by the shear-dependent viscosity model characterizing the non-Newtonian behavior of the considered fluid. Firstly, the present model is validated against well-known benchmarks, namely the parabolic velocity profile obtained for the flow within two infinite laminae for five values of the viscosity model exponent, n = 0.25, 0.50, 0.75, 1.0, and 1.5. Then, the flow within a squared lid-driven cavity for Re = 1000 and 5000 (being Re the Reynolds number) is computed as a function of n for a shear-thinning (n < 1) fluid. Indeed, the local decrements in the viscosity field achieved in high-shear zones implies the increment in the local Reynolds number, thus moving the position of near-walls minima towards lateral walls. Moreover, the revolution under shear of neutrally buoyant plain elliptical capsules with different Aspect Ratio (AR = 2 and 3) is analyzed for shear-thinning (n < 1), Newtonian (n = 1), and shear-thickening (n > 1) surrounding fluids. Interestingly, the power law by Huang et al. describing the revolution period of such capsules as a function of the Reynolds number and the existence of a critical value, Rec, after which the tumbling is inhibited in confirmed also for non-Newtonian fluids. Analogously, the equilibrium lateral position yeq of such neutrally buoyant capsules when transported in a plane-Couette flow is studied detailing the variation of yeq as a function of the Reynolds number as well as of the exponent n.},

}

RevDate: 2020-11-11

**A zero-dimensional predictive model for the pressure drop in the stenotic coronary artery based on its geometric characteristics.**

*Journal of biomechanics*, **113:**110076 pii:S0021-9290(20)30500-5 [Epub ahead of print].

The diameter- or area-reduction ratio measured from coronary angiography, commonly used in clinical practice, is not accurate enough to represent the functional significance of the stenosis, i.e., the pressure drop across the stenosis. We propose a new zero-dimensional model for the pressure drop across the stenosis considering its geometric characteristics and flow rate. To identify the geometric parameters affecting the pressure drop, we perform three-dimensional numerical simulations for thirty-three patient-specific coronary stenoses. From these numerical simulations, we show that the pressure drop is mostly determined by the curvature as well as the area-reduction ratio of the stenosis before the minimal luminal area (MLA), but heavily depends on the area-expansion ratio after the MLA due to flow separation. Based on this result, we divide the stenosis into the converging and diverging parts in the present zero-dimensional model. The converging part is segmented into a series of straight and curved pipes with curvatures, and the loss of each pipe is estimated by an empirical relation between the total pressure drop, flow rate, and pipe geometric parameters (length, diameter, and curvature). The loss in the diverging part is predicted by a relation among the total pressure drop, Reynolds number, and area expansion ratio with the coefficients determined by a machine learning method. The pressure drops across the stenoses predicted by the present zero-dimensional model agree very well with those obtained from three-dimensional numerical simulations.

Additional Links: PMID-33152635

Publisher:

PubMed:

Citation:

show bibtex listing

hide bibtex listing

@article {pmid33152635,

year = {2020},

author = {Kim, J and Jin, D and Choi, H and Kweon, J and Yang, DH and Kim, YH},

title = {A zero-dimensional predictive model for the pressure drop in the stenotic coronary artery based on its geometric characteristics.},

journal = {Journal of biomechanics},

volume = {113},

number = {},

pages = {110076},

doi = {10.1016/j.jbiomech.2020.110076},

pmid = {33152635},

issn = {1873-2380},

abstract = {The diameter- or area-reduction ratio measured from coronary angiography, commonly used in clinical practice, is not accurate enough to represent the functional significance of the stenosis, i.e., the pressure drop across the stenosis. We propose a new zero-dimensional model for the pressure drop across the stenosis considering its geometric characteristics and flow rate. To identify the geometric parameters affecting the pressure drop, we perform three-dimensional numerical simulations for thirty-three patient-specific coronary stenoses. From these numerical simulations, we show that the pressure drop is mostly determined by the curvature as well as the area-reduction ratio of the stenosis before the minimal luminal area (MLA), but heavily depends on the area-expansion ratio after the MLA due to flow separation. Based on this result, we divide the stenosis into the converging and diverging parts in the present zero-dimensional model. The converging part is segmented into a series of straight and curved pipes with curvatures, and the loss of each pipe is estimated by an empirical relation between the total pressure drop, flow rate, and pipe geometric parameters (length, diameter, and curvature). The loss in the diverging part is predicted by a relation among the total pressure drop, Reynolds number, and area expansion ratio with the coefficients determined by a machine learning method. The pressure drops across the stenoses predicted by the present zero-dimensional model agree very well with those obtained from three-dimensional numerical simulations.},

}

RevDate: 2020-11-05

**Construction of Natural Loofah/Poly(vinylidene fluoride) Core-Shell Electrospun Nanofibers via a Controllable Janus Nozzle for Switchable Oil-Water Separation.**

*ACS applied materials & interfaces* [Epub ahead of print].

Developing microstructure and multifunctional membranes toward switchable oil-water separation has been highly desired in oily wastewater treatment. Herein, a controllable Janus nozzle was employed to innovatively electrospin natural loofah/poly(vinylidene fluoride) (PVDF) nanofibers with a core-shell structure for gravity-driven water purification. By adjusting flow rates of the PVDF component, a core-shell structure of the composite fibers was obtained caused by the lower viscosity and surface tension of PVDF. In addition, a steady laminar motion of fluids was constructed based on the Reynolds number of flow fields being less than 2300. In order to investigate the formation mechanism of the microstructure, a series of Janus nozzles with different lengths were controlled to study the blending of the two immiscible components. The gravity difference between the two components might cause disturbance of the jet motion, and the PVDF component unidirectionally encapsulated the loofah to form the shell layer. Most importantly, the dry loofah/PVDF membranes could separate oil from an oil-water mixture, while the water-wetted membrane exhibited switchable separation that could separate water from the mixtures because of the hydroxyl groups of the hydrophilic loofah hydrogen-bonding with water molecules and forming a hydration layer. The composite fibers can be applied in water remediation in practice, and the method to produce core-shell structures seems attractive for technological applications involving macroscopic core-shell nano- or microfibers.

Additional Links: PMID-33147949

Publisher:

PubMed:

Citation:

show bibtex listing

hide bibtex listing

@article {pmid33147949,

year = {2020},

author = {Wang, Y and Zhou, G and Yan, Y and Shao, B and Hou, J},

title = {Construction of Natural Loofah/Poly(vinylidene fluoride) Core-Shell Electrospun Nanofibers via a Controllable Janus Nozzle for Switchable Oil-Water Separation.},

journal = {ACS applied materials & interfaces},

volume = {},

number = {},

pages = {},

doi = {10.1021/acsami.0c12912},

pmid = {33147949},

issn = {1944-8252},

abstract = {Developing microstructure and multifunctional membranes toward switchable oil-water separation has been highly desired in oily wastewater treatment. Herein, a controllable Janus nozzle was employed to innovatively electrospin natural loofah/poly(vinylidene fluoride) (PVDF) nanofibers with a core-shell structure for gravity-driven water purification. By adjusting flow rates of the PVDF component, a core-shell structure of the composite fibers was obtained caused by the lower viscosity and surface tension of PVDF. In addition, a steady laminar motion of fluids was constructed based on the Reynolds number of flow fields being less than 2300. In order to investigate the formation mechanism of the microstructure, a series of Janus nozzles with different lengths were controlled to study the blending of the two immiscible components. The gravity difference between the two components might cause disturbance of the jet motion, and the PVDF component unidirectionally encapsulated the loofah to form the shell layer. Most importantly, the dry loofah/PVDF membranes could separate oil from an oil-water mixture, while the water-wetted membrane exhibited switchable separation that could separate water from the mixtures because of the hydroxyl groups of the hydrophilic loofah hydrogen-bonding with water molecules and forming a hydration layer. The composite fibers can be applied in water remediation in practice, and the method to produce core-shell structures seems attractive for technological applications involving macroscopic core-shell nano- or microfibers.},

}

RevDate: 2020-11-11

**Intertube Aggregation-Dependent Convective Heat Transfer in Vertically Aligned Carbon Nanotube Channels.**

*ACS applied materials & interfaces*, **12(45):**50355-50364.

The heat transfer of carbon nanotube fin geometry has received considerable attention. However, the flow typically occurred over or around the pillars of nanotubes due to the greater flow resistance between the tubes. Here, we investigated the forced convective heat transfer of water through the interstitial space of vertically aligned multiwalled carbon nanotubes (VAMWNTs, intertube distance = 69 nm). The water flow provided significantly a greater Reynolds number (Re) and Nusselt number (Nu) than air flow due to the greater density, heat capacity, and thermal conductivity. However, it resulted in surface tension-induced nanotube aggregation after the flow and drying process, generating random voids in the nanotube channel. This increased permeability (1.27 Ã— 10-11 m2) and Re (2.83 Ã— 10-1) but decreased the heat transfer coefficient (h, 9900 W m-2 K-1) and Nu (53.77), demonstrating a trade-off relationship. The h (25,927 W m-2 K-1) and Nu (153.49) could be further increased, at an equivalent permeability or Re, by increasing nanotube areal density from 2.08 Ã— 1010 to 1.04 Ã— 1011 cm-2. The area-normalized thermal resistance of the densified and aggregated VAMWNTs was smaller than those of the Ni foam, Si microchannel, and carbon nanotube fin array, demonstrating excellent heat transfer characteristics.

Additional Links: PMID-33136360

Publisher:

PubMed:

Citation:

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@article {pmid33136360,

year = {2020},

author = {Jeon, W and Ahn, J and Kim, T and Kim, SM and Baik, S},

title = {Intertube Aggregation-Dependent Convective Heat Transfer in Vertically Aligned Carbon Nanotube Channels.},

journal = {ACS applied materials & interfaces},

volume = {12},

number = {45},

pages = {50355-50364},

doi = {10.1021/acsami.0c13361},

pmid = {33136360},

issn = {1944-8252},

abstract = {The heat transfer of carbon nanotube fin geometry has received considerable attention. However, the flow typically occurred over or around the pillars of nanotubes due to the greater flow resistance between the tubes. Here, we investigated the forced convective heat transfer of water through the interstitial space of vertically aligned multiwalled carbon nanotubes (VAMWNTs, intertube distance = 69 nm). The water flow provided significantly a greater Reynolds number (Re) and Nusselt number (Nu) than air flow due to the greater density, heat capacity, and thermal conductivity. However, it resulted in surface tension-induced nanotube aggregation after the flow and drying process, generating random voids in the nanotube channel. This increased permeability (1.27 Ã— 10-11 m2) and Re (2.83 Ã— 10-1) but decreased the heat transfer coefficient (h, 9900 W m-2 K-1) and Nu (53.77), demonstrating a trade-off relationship. The h (25,927 W m-2 K-1) and Nu (153.49) could be further increased, at an equivalent permeability or Re, by increasing nanotube areal density from 2.08 Ã— 1010 to 1.04 Ã— 1011 cm-2. The area-normalized thermal resistance of the densified and aggregated VAMWNTs was smaller than those of the Ni foam, Si microchannel, and carbon nanotube fin array, demonstrating excellent heat transfer characteristics.},

}

RevDate: 2020-11-03

**Flow over seal whiskers: Importance of geometric features for force and frequency response.**

*PloS one*, **15(10):**e0241142.

The complex undulated geometry of seal whiskers has been shown to substantially modify the turbulent structures directly downstream, resulting in a reduction of hydrodynamic forces as well as modified vortex-induced-vibration response when compared with smooth whiskers. Although the unique hydrodynamic response has been well documented, an understanding of the fluid flow effects from each geometric feature remains incomplete. In this computational investigation, nondimensional geometric parameters of the seal whisker morphology are defined in terms of their hydrodynamic relevance, such that wavelength, aspect ratio, undulation amplitudes, symmetry and undulation off-set can be varied independently of one another. A two-factor fractional factorial design of experiments procedure is used to create 16 unique geometries, each of which dramatically amplifies or attenuates the geometric parameters compared with the baseline model. The flow over each unique topography is computed with a large-eddy simulation at a Reynolds number of 500 with respect to the mean whisker thickness and the effects on force and frequency are recorded. The results determine the specific fluid flow impact of each geometric feature which will inform both biologists and engineers who seek to understand the impact of whisker morphology or lay out a framework for biomimetic design of undulated structures.

Additional Links: PMID-33119653

PubMed:

Citation:

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@article {pmid33119653,

year = {2020},

author = {Lyons, K and Murphy, CT and Franck, JA},

title = {Flow over seal whiskers: Importance of geometric features for force and frequency response.},

journal = {PloS one},

volume = {15},

number = {10},

pages = {e0241142},

pmid = {33119653},

issn = {1932-6203},

abstract = {The complex undulated geometry of seal whiskers has been shown to substantially modify the turbulent structures directly downstream, resulting in a reduction of hydrodynamic forces as well as modified vortex-induced-vibration response when compared with smooth whiskers. Although the unique hydrodynamic response has been well documented, an understanding of the fluid flow effects from each geometric feature remains incomplete. In this computational investigation, nondimensional geometric parameters of the seal whisker morphology are defined in terms of their hydrodynamic relevance, such that wavelength, aspect ratio, undulation amplitudes, symmetry and undulation off-set can be varied independently of one another. A two-factor fractional factorial design of experiments procedure is used to create 16 unique geometries, each of which dramatically amplifies or attenuates the geometric parameters compared with the baseline model. The flow over each unique topography is computed with a large-eddy simulation at a Reynolds number of 500 with respect to the mean whisker thickness and the effects on force and frequency are recorded. The results determine the specific fluid flow impact of each geometric feature which will inform both biologists and engineers who seek to understand the impact of whisker morphology or lay out a framework for biomimetic design of undulated structures.},

}

RevDate: 2020-10-26

**Generation of Atmospheric Turbulence with Unprecedentedly Large Reynolds Number in a Wind Tunnel.**

*Physical review letters*, **125(15):**154503.

Generating laboratory flows resembling atmospheric turbulence is of prime importance to study the effect of wind fluctuations on objects such as buildings, vehicles, or wind turbines. A novel driving of an active grid following a stochastic process is used to generate velocity fluctuations with correlation lengths, and, thus, integral scales, much larger than the transverse dimension of the wind tunnel. The combined action of the active grid and a modulation of the fan speed allows one to generate a flow characterized by a four-decade inertial range and an integral scale Reynolds number of 2Ã—10^{7}.

Additional Links: PMID-33095615

Publisher:

PubMed:

Citation:

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@article {pmid33095615,

year = {2020},

author = {Neuhaus, L and HÃ¶lling, M and Bos, WJT and Peinke, J},

title = {Generation of Atmospheric Turbulence with Unprecedentedly Large Reynolds Number in a Wind Tunnel.},

journal = {Physical review letters},

volume = {125},

number = {15},

pages = {154503},

doi = {10.1103/PhysRevLett.125.154503},

pmid = {33095615},

issn = {1079-7114},

abstract = {Generating laboratory flows resembling atmospheric turbulence is of prime importance to study the effect of wind fluctuations on objects such as buildings, vehicles, or wind turbines. A novel driving of an active grid following a stochastic process is used to generate velocity fluctuations with correlation lengths, and, thus, integral scales, much larger than the transverse dimension of the wind tunnel. The combined action of the active grid and a modulation of the fan speed allows one to generate a flow characterized by a four-decade inertial range and an integral scale Reynolds number of 2Ã—10^{7}.

},

}

RevDate: 2020-10-30

**Analyzing the Performance of a Miniature 3D Wind Sensor for Mars.**

*Sensors (Basel, Switzerland)*, **20(20):**.

This paper analyzes the behavior of a miniature 3D wind sensor designed for Mars atmosphere. The sensor is a spherical structure of 10 mm diameter divided in four sectors. By setting all the sectors to constant temperature, above that of the air, the 3D wind velocity vector can be measured. Two sets of experiments have been performed. First, an experimental campaign made under typical Mars conditions at the Aarhus Wind Tunnel Simulator is presented. The results demonstrate that both wind speed and angle can be efficiently measured, using a simple inverse algorithm. The effect of sudden wind changes is also analyzed and fast response times in the range of 0.7 s are obtained. The second set of experiments is focused on analyzing the performance of the sensor under extreme Martian wind conditions, reaching and going beyond the Dust Devil scale. To this purpose, both high-fidelity numerical simulations of fluid dynamics and heat transfer and experiments with the sensor have been performed. The results of the experiments, made for winds in the Reynolds number 1000-2000 range, which represent 65-130 m/s of wind speed under typical Mars conditions, further confirm the simulation predictions and show that it will be possible to successfully measure wind speed and direction even under these extreme regimes.

Additional Links: PMID-33092016

PubMed:

Citation:

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@article {pmid33092016,

year = {2020},

author = {DomÃnguez-Pumar, M and Kowalski, L and JimÃ©nez, V and RodrÃguez, I and Soria, M and Bermejo, S and Pons-Nin, J},

title = {Analyzing the Performance of a Miniature 3D Wind Sensor for Mars.},

journal = {Sensors (Basel, Switzerland)},

volume = {20},

number = {20},

pages = {},

pmid = {33092016},

issn = {1424-8220},

support = {RTI2018-098728-B-C33//Ministerio de EconomÃa y Competitividad/ ; },

abstract = {This paper analyzes the behavior of a miniature 3D wind sensor designed for Mars atmosphere. The sensor is a spherical structure of 10 mm diameter divided in four sectors. By setting all the sectors to constant temperature, above that of the air, the 3D wind velocity vector can be measured. Two sets of experiments have been performed. First, an experimental campaign made under typical Mars conditions at the Aarhus Wind Tunnel Simulator is presented. The results demonstrate that both wind speed and angle can be efficiently measured, using a simple inverse algorithm. The effect of sudden wind changes is also analyzed and fast response times in the range of 0.7 s are obtained. The second set of experiments is focused on analyzing the performance of the sensor under extreme Martian wind conditions, reaching and going beyond the Dust Devil scale. To this purpose, both high-fidelity numerical simulations of fluid dynamics and heat transfer and experiments with the sensor have been performed. The results of the experiments, made for winds in the Reynolds number 1000-2000 range, which represent 65-130 m/s of wind speed under typical Mars conditions, further confirm the simulation predictions and show that it will be possible to successfully measure wind speed and direction even under these extreme regimes.},

}

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Early support from the DOE component of the Human Genome Project was critically important for getting the ESP project on a firm foundation. Since that funding ended (nearly 20 years ago), the project has been operated as a purely volunteer effort. Anyone wishing to assist in these efforts should send an email to Robbins.

ESP Plans

With the development of methods for adding typeset side notes to PDF files, the ESP project now plans to add annotated versions of some classical papers to its holdings. We also plan to add new reference and pedagogical material. We have already started providing regularly updated, comprehensive bibliographies to the ESP.ORG site.

ESP Picks from Around the Web (updated 07 JUL 2018 )

Old Science

Weird Science

Treating Disease with Fecal Transplantation

Fossils of miniature humans (hobbits) discovered in Indonesia

Paleontology

Dinosaur tail, complete with feathers, found preserved in amber.

Astronomy

Mysterious fast radio burst (FRB) detected in the distant universe.

Big Data & Informatics

Big Data: Buzzword or Big Deal?

Hacking the genome: Identifying anonymized human subjects using publicly available data.