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Bibliography on: Reynolds Number

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ESP: PubMed Auto Bibliography 09 Apr 2020 at 01:33 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.

Created with PubMed® Query: "reynolds number" NOT pmcbook NOT ispreviousversion

Citations The Papers (from PubMed®)

RevDate: 2020-04-08

Asakawa J, Nishii K, Nakagawa Y, et al (2020)

Direct measurement of 1-mN-class thrust and 100-s-class specific impulse for a CubeSat propulsion system.

The Review of scientific instruments, 91(3):035116.

This paper presents the development of a thrust stand to enable direct measurement of thrust and specific impulse for a CubeSat propulsion system during firing. The thrust stand is an inverted pendulum and incorporates a mass balance for direct in situ mass measurement. The proposed calibration procedure allows precise performance characterization and achieves a resolution of 80 μN thrust and 0.01 g mass loss, by taking into account the drift of the thrust-stand zero caused by propellant consumption. The performance of a water micro-resistojet propulsion system for CubeSats was directly characterized as a proof of concept of the thrust stand. Continuous profiles of thrust, specific impulse, and mass consumption were acquired under various conditions in a single firing test. A thrust from 1 mN to 10 mN and a specific impulse from 45 s to 100 s with a maximum measurement uncertainty of ±15.3% were measured for the throat Reynolds number in the range 100-400.

RevDate: 2020-04-04

Joseph J, Rehman D, Delanaye M, et al (2020)

Numerical and Experimental Study of Microchannel Performance on Flow Maldistribution.

Micromachines, 11(3): pii:mi11030323.

Miniaturized heat exchangers are well known for their superior heat transfer capabilities in comparison to macro-scale devices. While in standard microchannel systems the improved performance is provided by miniaturized distances and very small hydraulic diameters, another approach can also be followed, namely, the generation of local turbulences. Localized turbulence enhances the heat exchanger performance in any channel or tube, but also includes an increased pressure loss. Shifting the critical Reynolds number to a lower value by introducing perturbators controls pressure losses and improves thermal efficiency to a considerable extent. The objective of this paper is to investigate in detail collector performance based on reduced-order modelling and validate the numerical model based on experimental observations of flow maldistribution and pressure losses. Two different types of perturbators, Wire-net and S-shape, were analyzed. For the former, a metallic wire mesh was inserted in the flow passages (hot and cold gas flow) to ensure stiffness and enhance microchannel efficiency. The wire-net perturbators were replaced using an S-shaped perturbator model for a comparative study in the second case mentioned above. An optimum mass flow rate could be found when the thermal efficiency reaches a maximum. Investigation of collectors with different microchannel configurations (s-shaped, wire-net and plane channels) showed that mass flow rate deviation decreases with an increase in microchannel resistance. The recirculation zones in the cylindrical collectors also changed the maldistribution pattern. From experiments, it could be observed that microchannels with S-shaped perturbators shifted the onset of turbulent transition to lower Reynolds number values. Experimental studies on pressure losses showed that the pressure losses obtained from numerical studies were in good agreement with the experiments (<4%).

RevDate: 2020-03-20

Sherlin GC (1960)

Behavior of Isolated Disturbances Superimposed on Laminar Flow in a Rectangular Pipe.

Journal of research of the National Bureau of Standards. Section A, Physics and chemistry, 64A(4):281-289.

An investigation was conducted in a horizontal transparent rectangular pipe to study the behavior, in laminar flow, of an isolated turbulent-like disturbance produced by injecting a quantity of dye into the pipe 39 feet from the entrance. As the resulting mass of colored water moved downstream, time-distance measurements were made for the front of the dye mass and for the rear of the disturbance. The experimental setup, which is described in some detail, permitted reasonable control over the mean flow rate from which Reynolds number was calculated. The utilization of the data unfolded a functional relationship among three quantities: The ratio of the velocity of the rear of the disturbance to the velocity of the front of the dye UR/UF ; the distance from the origin, XF ; and the Reynolds number R. The similarity of this work to that being done by Lindgren in Stockholm is mentioned.

RevDate: 2020-03-14

Thirani S, Gupta P, C Scalo (2020)

Knudsen number effects on the nonlinear acoustic spectral energy cascade.

Physical review. E, 101(2-1):023101.

We present a numerical investigation of the effects of gas rarefaction on the energy dynamics of resonating planar nonlinear acoustic waves. The problem setup is a gas-filled, adiabatic tube, excited from one end by a piston oscillating at the fundamental resonant frequency of the tube and closed at the other end; nonlinear wave steepening occurs until a limit cycle is reached, resulting in shock formation for sufficiently high densities. The Knudsen number, defined here as the ratio of the characteristic molecular collision timescale to the resonance period, is varied in the range Kn=10^{-1}-10^{-5}, from rarefied to dense regime, by changing the base density of the gas. The working fluid is Argon. A numerical solution of the Boltzmann equation, closed with the Bhatnagar-Gross-Krook model, is used to simulate cases for Kn≥0.01. The fully compressible one-dimensional Navier-Stokes equations are used for Kn<0.01 with adaptive mesh refinement to resolve the resonating weak shocks, reaching wave Mach numbers up to 1.01. Nonlinear wave steepening and shock formation are associated with spectral broadening of the acoustic energy in the wavenumber-frequency domain; the latter is defined based on the exact energy corollary for second-order nonlinear acoustics derived by Gupta and Scalo [Phys. Rev. E 98, 033117 (2018)2470-004510.1103/PhysRevE.98.033117], representing the Lyapunov function of the system. At the limit cycle, the acoustic energy spectra exhibit an equilibrium energy cascade with a -2 slope in the inertial range, also observed in freely decaying nonlinear acoustic waves by the same authors. In the present system, energy is introduced externally via a piston at low wavenumbers or frequencies and balanced by thermoviscous dissipation at high wavenumbers or frequencies, responsible for the base temperature increase in the system. The thermoviscous dissipation rate is shown to scale as Kn^{2} for fixed Reynolds number based on the maximum velocity amplitude, i.e., increasing with the degree of flow rarefaction; consistently, the smallest length scale of the steepened waves at the limit cycle, corresponding to the thickness of the shock (when present) also increases with Kn. For a given fixed piston velocity amplitude, the bandwidth of the inertial range of the spectral energy cascade decreases with increasing Knudsen numbers, resulting in a reduced resonant response of the system. By exploiting dimensionless scaling laws borrowed by Kolmogorov's theory of hydrodynamic turbulence, it is shown that an inertial range for spectral energy transfer can be expected for acoustic Reynolds numbers Re_{U_{max}}>100, based on the maximum acoustic velocity amplitude in the domain.

RevDate: 2020-03-10

Krieg M, K Mohseni (2020)

Transient Pressure Modeling in Jetting Animals.

Journal of theoretical biology pii:S0022-5193(20)30092-8 [Epub ahead of print].

There are many marine animals that employ a form of jet propulsion to move through the water, often creating the jets by expanding and collapsing internal fluid cavities. Due to the unsteady nature of this form of locomotion and complex body/nozzle geometries, standard modeling techniques prove insufficient at capturing internal pressure dynamics, and hence swimming forces. This issue has been resolved with a novel technique for predicting the pressure inside deformable jet producing cavities (M. Krieg and K. Mohseni, J. Fluid Mech., 769, 2015), which is derived from evolution of the surrounding fluid circulation. However, this model was only validated for an engineered jet thruster with simple geometry and relatively high Reynolds number (Re) jets. The purpose of this manuscript is twofold: (i) to demonstrate how the circulation based pressure model can be used to analyze different animal body motions as they relate to propulsive output, for multiple species of jetting animals, (ii) and to quantitatively validate the pressure modeling for biological jetting organisms (typically characterized by complicated cavity geometry and low/intermediate Re flows). Using jellyfish (Sarsia tubulosa) as an example, we show that the pressure model is insensitive to complex cavity geometry, and can be applied to lower Re swimming. By breaking down the swimming behavior of the jellyfish, as well as that of squid and dragonfly larvae, according to circulation generating mechanisms, we demonstrate that the body motions of Sarsia tubulosa are optimized for acceleration at the beginning of pulsation as a survival response. Whereas towards the end of jetting, the velar morphology is adjusted to decrease the energetic cost. Similarly, we show that mantle collapse rates in squid maximize propulsive efficiency. Finally, we observe that the hindgut geometry of dragonfly larvae minimizes the work required to refill the cavity. Date Received: 10-18-2019, Date Accepted: 99-99-9999 *kriegmw@hawaii.edu, UHM Ocean and Res Eng, 2540 Dole St, Honolulu, HI 96822.

RevDate: 2020-03-03

Fang WZ, Ham S, Qiao R, et al (2020)

Magnetic Actuation of Surface Walkers: The Effects of Confinement and Inertia.

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

Driven by a magnetic field, the rotation of a particle near a wall can be rectified into a net translation. The particles thus actuated, or surface walkers, are a kind of active colloids that find applications in biology and microfluidics. Here, we investigate the motion of spherical surface walkers confined between two walls using simulations based on the immersed-boundary lattice Boltzmann method. The degree of confinement and the nature of the confining walls (slip vs. no-slip) significantly affect a particle's translation speed and can even reverse its translation direction. When the rotational Reynolds number Reω is larger than 1, inertia effects reduce the critical frequency of the applied magnetic fields, beyond which the sphere can no longer follow the applied fields. The reduction of the critical frequency is especially pronounced when the sphere is confined near a no-slip wall. As Reω increases beyond 1, even when a sphere still rotates synchronously with the applied field, its translational Reynolds number no longer increases linearly with Reω and even decreases when Reω exceeds ~10.

RevDate: 2020-03-02

Mensch AE, TG Cleary (2019)

Measurements and predictions of thermophoretic soot deposition.

International journal of heat and mass transfer, 143:.

A thin laminar flow channel with a transverse temperature gradient was used to examine thermophoretic deposition of soot aerosol particles in experiments and modeled in Fire Dynamics Simulator (FDS) simulations. Conditions investigated included three flowrates, with nominal Reynolds number based on the hydraulic diameter of 55, 115 and 230, and two applied temperature gradients, nominally 10 °C/mm and 20 °C/mm, with repeats. Soot was generated from a propene diffusion flame. The burner exhaust was mixed with dilution air, and most large agglomerates greater than 1 μm aerodynamic diameter were removed prior to the channel inlet. The expected thermophoretic velocity of the aerosol was calculated from the applied temperature gradient. A calculated deposition velocity was determined from the mass of deposition, the channel inlet soot concentration, and the exposure time. Uniform soot deposition allowed targets to be used to measure the mass of deposition on the cold side of the channel. The mass of deposition was also determined by subtracting the mass of soot exiting the channel from the mass of soot entering the channel during the exposure time. The deposition velocities from these two methods generally agreed with the thermophoretic velocity and with each other. The deposition mass predicted by the FDS model also compared well with the experiments in most cases. The disagreements for the lowest flow rate cases are attributed to buoyant flow effects adding uncertainty to the actual temperature gradients present in the channel. (The opinions, findings, and conclusions expressed in this paper are the authors' and do not represent the views or policies of NIST or the United States Government.).

RevDate: 2020-02-26

Stixrude L, Scipioni R, MP Desjarlais (2020)

A silicate dynamo in the early Earth.

Nature communications, 11(1):935 pii:10.1038/s41467-020-14773-4.

The Earth's magnetic field has operated for at least 3.4 billion years, yet how the ancient field was produced is still unknown. The core in the early Earth was surrounded by a molten silicate layer, a basal magma ocean that may have survived for more than one billion years. Here we use density functional theory-based molecular dynamics simulations to predict the electrical conductivity of silicate liquid at the conditions of the basal magma ocean: 100-140 GPa, and 4000-6000 K. We find that the electrical conductivity exceeds 10,000 S/m, more than 100 times that measured in silicate liquids at low pressure and temperature. The magnetic Reynolds number computed from our results exceeds the threshold for dynamo activity and the magnetic field strength is similar to that observed in the Archean paleomagnetic record. We therefore conclude that the Archean field was produced by the basal magma ocean.

RevDate: 2020-02-25

Rehman D, Joseph J, Morini GL, et al (2020)

A Hybrid Numerical Methodology Based on CFD and Porous Medium for Thermal Performance Evaluation of Gas to Gas Micro Heat Exchanger.

Micromachines, 11(2): pii:mi11020218.

In micro heat exchangers, due to the presence of distributing and collecting manifolds as well as hundreds of parallel microchannels, a complete conjugate heat transfer analysis requires a large amount of computational power. Therefore in this study, a novel methodology is developed to model the microchannels as a porous medium where a compressible gas is used as a working fluid. With the help of such a reduced model, a detailed flow analysis through individual microchannels can be avoided by studying the device as a whole at a considerably less computational cost. A micro heat exchanger with 133 parallel microchannels (average hydraulic diameter of 200 μ m) in both cocurrent and counterflow configurations is investigated in the current study. Hot and cold streams are separated by a stainless-steel partition foil having a thickness of 100 μ m. Microchannels have a rectangular cross section of 200 μ m × 200 μ m with a wall thickness of 100 μ m in between. As a first step, a numerical study for conjugate heat transfer analysis of microchannels only, without distributing and collecting manifolds is performed. Mass flow inside hot and cold fluid domains is increased such that inlet Reynolds number for both domains remains within the laminar regime. Inertial and viscous coefficients extracted from this study are then utilized to model pressure and temperature trends within the porous medium model. To cater for the density dependence of inertial and viscous coefficients due to the compressible nature of gas flow in microchannels, a modified formulation of Darcy-Forschheimer law is adopted. A complete model of a double layer micro heat exchanger with collecting and distributing manifolds where microchannels are modeled as the porous medium is finally developed and used to estimate the overall heat exchanger effectiveness of the investigated micro heat exchanger. A comparison of computational results using proposed hybrid methodology with previously published experimental results of the same micro heat exchanger showed that adopted methodology can predict the heat exchanger effectiveness within the experimental uncertainty for both cocurrent and counterflow configurations.

RevDate: 2020-02-21

Luo J, Chen L, Li K, et al (2020)

Optimal kinematic dynamos in a sphere.

Proceedings. Mathematical, physical, and engineering sciences, 476(2233):20190675.

A variational optimization approach is used to optimize kinematic dynamos in a unit sphere and locate the enstrophy-based critical magnetic Reynolds number for dynamo action. The magnetic boundary condition is chosen to be either pseudo-vacuum or perfectly conducting. Spectra of the optimal flows corresponding to these two magnetic boundary conditions are identical since theory shows that they are relatable by reversing the flow field (Favier & Proctor 2013 Phys. Rev. E88, 031001 (doi:10.1103/physreve.88.031001)). A no-slip boundary for the flow field gives a critical magnetic Reynolds number of 62.06, while a free-slip boundary reduces this number to 57.07. Optimal solutions are found to possess certain rotation symmetries (or anti-symmetries) and optimal flows share certain common features. The flows localize in a small region near the sphere's centre and spiral upwards with very large velocity and vorticity, so that they are locally nearly Beltrami. We also derive a new lower bound on the magnetic Reynolds number for dynamo action, which, for the case of enstrophy normalization, is five times larger than the previous best bound.

RevDate: 2020-02-21

Jose S, R Govindarajan (2020)

Non-normal origin of modal instabilities in rotating plane shear flows.

Proceedings. Mathematical, physical, and engineering sciences, 476(2233):20190550.

Small variations introduced in shear flows are known to affect stability dramatically. Rotation of the flow system is one example, where the critical Reynolds number for exponential instabilities falls steeply with a small increase in rotation rate. We ask whether there is a fundamental reason for this sensitivity to rotation. We answer in the affirmative, showing that it is the non-normality of the stability operator in the absence of rotation which triggers this sensitivity. We treat the flow in the presence of rotation as a perturbation on the non-rotating case, and show that the rotating case is a special element of the pseudospectrum of the non-rotating case. Thus, while the non-rotating flow is always modally stable to streamwise-independent perturbations, rotating flows with the smallest rotation are unstable at zero streamwise wavenumber, with the spanwise wavenumbers close to that of disturbances with the highest transient growth in the non-rotating case. The instability critical rotation number scales inversely as the square of the Reynolds number, which we demonstrate is the same as the scaling obeyed by the minimum perturbation amplitude in non-rotating shear flow needed for the pseudospectrum to cross the neutral line. Plane Poiseuille flow and plane Couette flow are shown to behave similarly in this context.

RevDate: 2020-02-18

Razavi Bazaz S, Mashhadian A, Ehsani A, et al (2020)

Computational inertial microfluidics: a review.

Lab on a chip [Epub ahead of print].

Since the discovery of inertial focusing in 1961, numerous theories have been put forward to explain the migration of particles in inertial flows, but a complete understanding is still lacking. Recently, computational approaches have been utilized to obtain better insights into the underlying physics. In particular, fundamental aspects of particle focusing inside straight and curved microchannels have been explored in detail to determine the dependence of focusing behavior on particle size, channel shape, and flow Reynolds number. In this review, we differentiate between the models developed for inertial particle motion on the basis of whether they are semi-analytical, Navier-Stokes-based, or built on the lattice Boltzmann method. This review provides a blueprint for the consideration of numerical solutions for modeling of inertial particle motion, whether deformable or rigid, spherical or non-spherical, and whether suspended in Newtonian or non-Newtonian fluids. In each section, we provide the general equations used to solve particle motion, followed by a tutorial appendix and specified sections to engage the reader with details of the numerical studies. Finally, we address the challenges ahead in the modeling of inertial particle microfluidics for future investigators.

RevDate: 2020-02-17

Tanveer A, Salahuddin T, Khan M, et al (2019)

Theoretical analysis of non-Newtonian blood flow in a microchannel.

Computer methods and programs in biomedicine, 191:105280 pii:S0169-2607(19)31978-9 [Epub ahead of print].

BACKGROUND: In this work the theoretical analysis is presented for a electroosmotic flow of Bingham nanofluid induced by applied electrostatic potential. The linearized Poisson-Boltzmann equation is considered in the presence of Electric double layer (EDL). A Bingham fluid model is employed to describe the rheological behavior of the non-Newtonian fluid. Mathematical formulation is presented under the assumption of long wavelength and small Reynolds number. Flow characteristics are investigated by employing Debye-Huckel linearization principle. Such preferences have not been reported previously for non-Newtonian Bingham nanofluid to the best of author's knowledge.

METHOD: The transformed equations for electroosmotic flow are solved to seek values for the nanofluid velocity, concentration and temperature along the channel length.

RESULTS: The effects of key parameters like Brinkmann number, Prandtl number, Debey Huckel parameter, thermophoresis parameter, Brownian motion parameter are plotted on velocity, temperature and concentration profiles. Graphical results for the flow phenomenon are discussed briefly.

CONCLUSIONS: Non-uniformity in channel as well as yield stress τ0 cause velocity declaration for both positive and negative values of U. Nanofluid temperature is found an increasing function of electro osmotic parameter κ if U is positive while it is a decreasing function if U is negative. A completely reverse response is seen in case of concentration profile. The thermophoresis parameter Nt, the Brow nian motion parameter Nb and Brinkman number Br cause an enhancement in temperature. The results are new in case of U.

RevDate: 2020-02-18

Yan SR, Sedeh S, Toghraie D, et al (2020)

Analysis and manegement of laminar blood flow inside a cerebral blood vessel using a finite volume software program for biomedical engineering.

Computer methods and programs in biomedicine, 190:105384 pii:S0169-2607(19)32471-X [Epub ahead of print].

BACKGROUND AND OBJECTIVE: Hemodynamic blood flow analysis in the cerebrovascular is has become one of the important research topics in the bio-mechanic in recent decades. The primary duty of the cerebral blood vessel is supplying Glucose and oxygen for the brain.

METHODS: In this investigation, the non-Newtonian blood flow in the cerebral blood vessels studied. For modeling the geometry of this problem, we used Magnetic Resonance Image (MRI) approach to take Digital Imaging and Communications in Medicine (DICOM) images and using an open-source software package to construct the geometry, which is a complicated one. The power-law indexes, heat flux, and Reynolds number range in the investigation are 0.6 ≤ n ≤ 0.8, 5 ≤ q ≤ 15Wm-2 and 160≤Re≤310. Effects of Reynolds number, power-law indexes and heat fluxes are investigated.

RESULTS: We found that the pressure drop increase with increasing the Reynolds number and power-law index. The maximum Nusselt number in the cerebral blood vessels accrued in the running position of the body in n = 0.8. Also, the highest average wall shear stress occurs in maximum power-law indexes and Reynolds number.

CONCLUSION: By increasing the power-law index and Reynolds number, the wall shear stress increases.

RevDate: 2020-02-13

Waheed S, Noreen S, Tripathi D, et al (2020)

Electrothermal transport of third-order fluids regulated by peristaltic pumping.

Journal of biological physics pii:10.1007/s10867-020-09540-x [Epub ahead of print].

The study of heat and electroosmotic characteristics in the flow of a third-order fluid regulated by peristaltic pumping is examined by using governing equations, i.e., the continuity equation, momentum equation, energy equation, and concentration equation. The wavelength is considered long compared to its height and a low Reynolds number is assumed. The velocity slip condition is employed. Analytical solutions are performed through the perturbation technique. The expressions for the dimensionless velocity components, temperature, concentration, and heat transfer rate are obtained. Pumping features were computed numerically for discussion of results. Trapping and heat transfer coefficient distributions were also studied graphically. The findings of the present study can be applied to design biomicrofluidic devices like tumor-on-a-chip and organ-on-a-chip.

RevDate: 2020-02-11

Cerbus RT, Liu CC, Gioia G, et al (2020)

Small-scale universality in the spectral structure of transitional pipe flows.

Science advances, 6(4):eaaw6256 pii:aaw6256.

Turbulent flows are not only everywhere, but every turbulent flow is the same at small scales. The extraordinary simplification engendered by this "small-scale universality" is a hallmark of turbulence theory. However, on the basis of the restrictive assumptions invoked by A. N. Kolmogorov to demonstrate this universality, it is widely thought that only idealized turbulent flows conform to this framework. Using experiments and simulations that span a wide range of Reynolds number, we show that small-scale universality governs the spectral structure of a class of flows with no apparent ties to the idealized flows: transitional pipe flows. Our results not only extend the universality of Kolmogorov's framework beyond expectation but also establish an unexpected link between transitional pipe flows and Kolmogorovian turbulence.

RevDate: 2020-02-11

Usherwood JR, Cheney JA, Song J, et al (2020)

High aerodynamic lift from the tail reduces drag in gliding raptors.

The Journal of experimental biology, 223(Pt 3): pii:223/3/jeb214809.

Many functions have been postulated for the aerodynamic role of the avian tail during steady-state flight. By analogy with conventional aircraft, the tail might provide passive pitch stability if it produced very low or negative lift. Alternatively, aeronautical principles might suggest strategies that allow the tail to reduce inviscid, induced drag: if the wings and tail act in different horizontal planes, they might benefit from biplane-like aerodynamics; if they act in the same plane, lift from the tail might compensate for lift lost over the fuselage (body), reducing induced drag with a more even downwash profile. However, textbook aeronautical principles should be applied with caution because birds have highly capable sensing and active control, presumably reducing the demand for passive aerodynamic stability, and, because of their small size and low flight speeds, operate at Reynolds numbers two orders of magnitude below those of light aircraft. Here, by tracking up to 20,000, 0.3 mm neutrally buoyant soap bubbles behind a gliding barn owl, tawny owl and goshawk, we found that downwash velocity due to the body/tail consistently exceeds that due to the wings. The downwash measured behind the centreline is quantitatively consistent with an alternative hypothesis: that of constant lift production per planform area, a requirement for minimizing viscous, profile drag. Gliding raptors use lift distributions that compromise both inviscid induced drag minimization and static pitch stability, instead adopting a strategy that reduces the viscous drag, which is of proportionately greater importance to lower Reynolds number fliers.

RevDate: 2020-02-06

Wu Z, Zaki TA, C Meneveau (2020)

High-Reynolds-number fractal signature of nascent turbulence during transition.

Proceedings of the National Academy of Sciences of the United States of America pii:1916636117 [Epub ahead of print].

Transition from laminar to turbulent flow occurring over a smooth surface is a particularly important route to chaos in fluid dynamics. It often occurs via sporadic inception of spatially localized patches (spots) of turbulence that grow and merge downstream to become the fully turbulent boundary layer. A long-standing question has been whether these incipient spots already contain properties of high-Reynolds-number, developed turbulence. In this study, the question is posed for geometric scaling properties of the interface separating turbulence within the spots from the outer flow. For high-Reynolds-number turbulence, such interfaces are known to display fractal scaling laws with a dimension [Formula: see text], where the 1/3 excess exponent above 2 (smooth surfaces) follows from Kolmogorov scaling of velocity fluctuations. The data used in this study are from a direct numerical simulation, and the spot boundaries (interfaces) are determined by using an unsupervised machine-learning method that can identify such interfaces without setting arbitrary thresholds. Wide separation between small and large scales during transition is provided by the large range of spot volumes, enabling accurate measurements of the volume-area fractal scaling exponent. Measurements show a dimension of [Formula: see text] over almost 5 decades of spot volume, i.e., trends fully consistent with high-Reynolds-number turbulence. Additional observations pertaining to the dependence on height above the surface are also presented. Results provide evidence that turbulent spots exhibit high-Reynolds-number fractal-scaling properties already during early transitional and nonisotropic stages of the flow evolution.

RevDate: 2020-02-06

Xu Z, Qin H, Li P, et al (2020)

Computational fluid dynamics approaches to drag and wake of a long-line mussel dropper under tidal current.

Science progress, 103(1):36850419901235.

Hydrodynamic effects of mussel farms have attracted increased research attentions in recent years. The understanding of the hydrodynamic impacts is essential for predicting the sustainability of mussel farms. A large mussel farm includes thousands of mussel droppers, and the combined drag on the mussel droppers is sufficient to possibly affect the longevity of the entire long-lines. This article intends to study the drag and wake of an individual long-line mussel dropper using computational fluid dynamics approaches. Two equivalent rough cylinders, namely, Curved-Model and Sharp-Model, have been utilized to simulate the mussel dropper, and each rough cylinder is assigned with surface roughness. The porosity is not considered in this article due to its complexity from inhalant and exhalant of mussels. Two-dimensional laminar simulations are conducted at Reynolds number from 10 to 200, and three-dimensional large eddy simulations are conducted at subcritical Reynolds number ranging from 3900 to 10 5 . The results show that larger drag coefficients and Strouhal numbers are attributed to surface roughness and sharp crowns on the rough cylinder. The obtained drag coefficient ranges from 1.1 to 1.2 with respect to the diameter of the mussel dropper and the peak value of the tidal velocities. Wakes behind rough cylinders fluctuate more actively compared to those of smooth cylinders. This research work provides new insight for further investigations on hydrodynamic interactions between fluid and mussel droppers.

RevDate: 2020-02-02

Ganta N, Mahato B, YG Bhumkar (2020)

Prediction of the aerodynamic sound generated due to flow over a cylinder performing combined steady rotation and rotary oscillations.

The Journal of the Acoustical Society of America, 147(1):325.

Analysis of sound generated due to a laminar flow past a circular cylinder subjected to the mean rotation along with the rotary oscillating motion has been performed for the Reynolds number Re = 150 and the Mach number M = 0.2. The direct numerical simulation approach has been used to study modifications in the generated sound field over a range of forcing parameters using disturbance pressure field information. Flow and sound fields are accurately resolved over a nondimensional radial distance r≤100 from the center of the cylinder. Frequencies, as well as wavelengths of generated sound waves, have been effectively altered by varying the forcing frequency-ratio, whereas the directivity nature of the radiated sound field has been modified by varying the forcing amplitude-ratio. Doak's decomposition technique has been used to understand the reasons behind changes in the radiated sound fields as the forcing parameters are varied.

RevDate: 2020-02-01

Alouges F, G Di Fratta (2020)

Parking 3-sphere swimmer: II. The long-arm asymptotic regime.

The European physical journal. E, Soft matter, 43(2):6 pii:10.1140/epje/i2020-11932-5.

The paper carries on our previous investigations on the complementary version of Purcell's rotator (sPr3): a low-Reynolds-number swimmer composed of three balls of equal radii. In the asymptotic regime of very long arms, the Stokes-induced governing dynamics is derived, and then experimented in the context of energy-minimizing self-propulsion characterized in the first part of the paper.

RevDate: 2020-01-30

Liu P, Liu H, Yang Y, et al (2020)

Comparison of design methods for negative pressure gradient rotary bodies: A CFD study.

PloS one, 15(1):e0228186 pii:PONE-D-19-20026.

Computational fluid dynamics (CFD) simulation is used to test two body design methods which use negative pressure gradient to suppress laminar flow separation and drag reduction. The steady-state model of the Transition SST model is used to calculate the pressure distribution, wall shear stress, and drag coefficient under zero angle of attack at different velocities. Four bodies designed by two different methods are considered. Our results show the first method is superior to the body of Hansen in drag reduction and the body designed by the first method is more likely to obtain the characteristics of suppressing or eliminating separation, which can effectively improve laminar flow coverage to achieve drag reduction under higher Reynolds number conditions. The results show that the negative pressure gradient method can suppress separation and drag reduction better than the second method. This successful design method is expected to open a promising prospect for its application in the design of small drag, small noise subsonic hydrodynamic hull and underwater weapons.

RevDate: 2020-01-28

Nguyen KH, Gemmell BJ, JR Rohr (2020)

Effects of temperature and viscosity on miracidial and cercarial movement of Schistosoma mansoni: ramifications for disease transmission.

International journal for parasitology pii:S0020-7519(20)30009-6 [Epub ahead of print].

Parasites with complex life cycles can be susceptible to temperature shifts associated with seasonal changes, especially as free-living larvae that depend on a fixed energy reserve to survive outside the host. The life cycle of Schistosoma, a trematode genus containing some species that cause human schistosomiasis, has free-living, aquatic miracidial and cercarial larval stages that swim using cilia or a forked tail, respectively. The small size of these swimmers (150-350 µm) dictates that their propulsion is dominated by viscous forces. Given that viscosity inhibits the swimming ability of small organisms and is inversely correlated with temperature, changes in temperature should affect the ability of free-living larval stages to swim and locate a host. By recording miracidial and cercarial movement of Schistosoma mansoni using a high-speed camera and manipulating temperature and viscosity independently, we assessed the role each factor plays in the swimming mechanics of the parasite. We found a positive effect of temperature and a negative effect of viscosity on miracidial and cercarial speed. Reynolds numbers, which describe the ratio of inertial to viscous forces exerted on an aquatic organism, were <1 across treatments. Q10 values were <2 when comparing viscosity treatments at 20°C and 30°C, further supporting the influence of viscosity on miracidial and cercarial speed. Given that both larval stages have limited energy reserves and infection takes considerable energy, successful transmission depends on both speed and lifespan. We coupled our speed data with mortality measurements across temperatures and discovered that the theoretical maximum distance travelled increased with temperature and decreased with viscosity for both larval stages. Thus, our results suggest that S. mansoni transmission is high during warm times of the year, partly due to improved swimming performance of the free-living larval stages, and that increases in temperature variation associated with climate change might further increase transmission.

RevDate: 2020-01-28

Fauzi FB, Ismail E, Syed Abu Bakar SN, et al (2020)

The role of gas-phase dynamics in interfacial phenomena during few-layer graphene growth through atmospheric pressure chemical vapour deposition.

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

The complicated chemical vapour deposition (CVD) is currently the most viable method of producing graphene. Most studies have extensively focused on chemical aspects either through experiments or computational studies. However, gas-phase dynamics in CVD reportedly plays an important role in improving graphene quality. Given that mass transport is the rate-limiting step for graphene deposition in atmospheric-pressure CVD (APCVD), the interfacial phenomena at the gas-solid interface (i.e., the boundary layer) are a crucial controlling factor. Accordingly, only by understanding and controlling the boundary-layer thickness can uniform full-coverage graphene deposition be achieved. In this study, a simplified computational fluid dynamics analysis of APCVD was performed to investigate gas-phase dynamics during deposition. Boundary-layer thickness was also estimated through the development of a customised homogeneous gas model. Interfacial phenomena, particularly the boundary layer and mass transport within it, were studied. The effects of Reynolds number on these factors were explored and compared with experimentally obtained results of the characterised graphene deposit. We then discussed and elucidated the important relation of fluid dynamics to graphene growth through APCVD.

RevDate: 2020-01-26

Asghar Z, Ali N, Javid K, et al (2020)

Bio-inspired propulsion of micro-swimmers within a passive cervix filled with couple stress mucus.

Computer methods and programs in biomedicine, 189:105313 pii:S0169-2607(19)31516-0 [Epub ahead of print].

BACKGROUND AND OBJECTIVE: The swimming mechanism of self-propelling organisms has been imitated by biomedical engineers to design the mechanical micro bots. The interaction of these swimmers with surrounding environment is another important aspect. The present swimming problem integrates Taylor sheet model with couple stress fluid model. The thin passage containing micro-swimmers and mucus is approximated as a rigid (passive) two-dimensional channel. The spermatozoa forms a pack quite similar as a complex wavy sheet.

METHODS: Swimming problem with couple stress cervical liquid (at low Reynolds number) leads to a linear sixth order differential equation. The boundary value problem (BVP) is solved analytically with two unknowns i.e. speed of complex wavy sheet and flow rate of couple stress mucus. After utilizing this solution into equilibrium conditions these unknowns can be computed via Newton-Raphson algorithm. Furthermore, the pairs of numerically calculated organism speed and flow rate are utilized in the expression of power dissipation.

RESULTS: This work describes that the speed of micro-swimmers can be enhanced by suitable rheology of the surrounding liquid. The usage of couple stress fluid as compared to Newtonian fluid enhances the energy dissipation and reduces the flow rate. On the other hand complex wavy surface also aids the organisms to swim faster.

RevDate: 2020-01-24

Zhu Y, Yang G, Zhuang C, et al (2020)

Oral cavity flow distribution and pressure drop in balaenid whales feeding: A theoretical analysis.

Bioinspiration & biomimetics [Epub ahead of print].

Balaenid whales, as continuous ram filter feeders, can efficiently separate prey from water by baleen. The feeding process of balaenid whales is extremely complex, in which the flow distribution and pressure drop in the oral cavity play a significant role. In this paper, a theoretical model coupled with oral cavity velocity and pressure in balaenid whales is established based on mass conservation, momentum conservation and pressure drop equations, considering both the inertial and the friction terms. A discrete method with section-by-section calculation is adopted to solve the theoretical model. The effects of four crucial parameters, i.e., the ratio of filtration area to inlet area (S), the Reynolds number of entrance (Rein), the ratio of thickness to permeability of the porous media formed by the fringe layer (φ) and the width ratio of the anteroposterior canal within the mouth along the tongue (APT channel) to that along the lip (APL channel) (H) are discussed. The results show that, for a give case, the flow distribution and the pressure drop both show increasing trends with the flow direction. For different cases, whenSis small,Reinis small andφis large, a good flow pattern emerges with a smoother flow speed near the oropharynx, better drainage, better shunting and filtration, and higher energy efficiency. However, for smaller values ofH, some energy efficiency is sacrificed to achieve additional average transverse flow in order to produce better shunting and filtration. The research in this paper provides a reference for the design of high-efficiency bionic filters.

RevDate: 2020-01-24

Afrouzi HH, Ahmadian M, Hosseini M, et al (2020)

Simulation of blood flow in arteries with aneurysm: Lattice Boltzmann Approach (LBM).

Computer methods and programs in biomedicine, 187:105312 pii:S0169-2607(19)31835-8 [Epub ahead of print].

BACKGROUND AND OBJECTIVE: In most countries, the higher death rates are due to cardiovascular disease and stroke. These problems often derive from irregular blood flow and the circulatory system disorder.

METHODS: In this paper, the blood flow is simulated in a created aneurysm in the artery upon using Lattice Boltzmann Method (LBM). Blood is selected as a non-Newtonian fluid which was simulated with power-law model. The lattice Boltzmann results for non-Newtonian fluid flow with power-law model and the curved boundary are compared and validated with previous studies which show a good agreement. In this study, simulations are carried out for two types of aneurysms. For the first aneurysm, three power-law exponents of 0.6, 0.8 and 1.0 at Reynolds number of 100 for three different cases are investigated.

RESULTS: The results show that the wall shear stress increases with increasing the power-law exponent. In addition, in the main duct of artery where the velocity is larger, shear stress is lower due to the smaller velocity gradient. For the second Aneurysm, the simulations are done for three Reynolds numbers of 100, 150 and 200, and three Womersley numbers of 4, 12 and 20. The blood flow is pulsating at the inlet such as the real pulsating wave in the blood. Results show that with increasing the Womersley number, the velocity profiles in the middle of the aneurysm are closer at a constant Reynolds number.

CONCLUSIONS: With increasing the Reynolds number, the range of vortices and values of velocity and tension grow in the aneurysm.

RevDate: 2020-01-24

Porté-Agel F, Bastankhah M, S Shamsoddin (2020)

Wind-Turbine and Wind-Farm Flows: A Review.

Boundary-layer meteorology, 174(1):1-59.

Wind energy, together with other renewable energy sources, are expected to grow substantially in the coming decades and play a key role in mitigating climate change and achieving energy sustainability. One of the main challenges in optimizing the design, operation, control, and grid integration of wind farms is the prediction of their performance, owing to the complex multiscale two-way interactions between wind farms and the turbulent atmospheric boundary layer (ABL). From a fluid mechanical perspective, these interactions are complicated by the high Reynolds number of the ABL flow, its inherent unsteadiness due to the diurnal cycle and synoptic-forcing variability, the ubiquitous nature of thermal effects, and the heterogeneity of the terrain. Particularly important is the effect of ABL turbulence on wind-turbine wake flows and their superposition, as they are responsible for considerable turbine power losses and fatigue loads in wind farms. These flow interactions affect, in turn, the structure of the ABL and the turbulent fluxes of momentum and scalars. This review summarizes recent experimental, computational, and theoretical research efforts that have contributed to improving our understanding and ability to predict the interactions of ABL flow with wind turbines and wind farms.

RevDate: 2020-01-22

Kaminsky J, Klewicki J, B Birnir (2019)

Application of the stochastic closure theory to the Townsend-Perry constants.

Physical review. E, 100(6-1):061101.

We compare the stochastic closure theory (SCT) to the Townsend-Perry constants as estimated from measurements in the Flow Physic Facility (FPF) at the University of New Hampshire. First, we explain the derivation of the Townsend-Perry constants, which were originally formulated by Meneveau and Marusic, in analogy with a Gaussian distribution. However, this was not supported by the data. Instead, the data show a sub-Gaussian relation that was explained by Birnir and Chen. We show herein how the SCT can be used to compute the constants, which explains their sub-Gaussian relations. We then compare the SCT theory predictions, including Reynolds-number-dependent corrections, with the data, showing good agreement.

RevDate: 2020-01-22

Jin Y, Cheng S, LP Chamorro (2019)

Active pitching of short splitters past a cylinder: Drag increase and wake.

Physical review. E, 100(6-1):063106.

The flow and drag induced by active pitching of plates in the wake of a cylinder of diameter d were experimentally studied for various plate lengths L as well as pitching frequencies f_{p} and amplitudes A_{0} at Reynolds number Re=1.6×10^{4}. Planar particle image velocimetry and a load cell were used to characterize the flow statistics and mean drag of a variety of cylinder-splitter assemblies. Results show the distinctive effect of active pitching on these quantities. In particular, flow recovery was significantly modulated by L, f_{p}, or A_{0}. Specific pitching settings resulted in a wake with dominant meandering patterns and faster flow recovery. We defined a modified version of the amplitude-based Strouhal number of the system St_{A} to account for the effect of the cylinder in active pitching. It characterizes the drag coefficient C_{d} across all the cases studied, and reveals two regions intersecting at a critical value of St_{A}≈0.035. Below this value, the C_{d} remained nearly constant; however, it exhibited a linear increase with increasing St_{A} past this critical point. Inspection of the integral momentum equation showed the dominant role of velocity fluctuations in modulating C_{d} past the critical St_{A}.

RevDate: 2020-01-19

Rajwa-Kuligiewicz A, Radecki-Pawlik A, Skalski T, et al (2020)

Hydromorphologically-driven variability of thermal and oxygen conditions at the block ramp hydraulic structure: The Porębianka River, Polish Carpathians.

The Science of the total environment, 713:136661 pii:S0048-9697(20)30171-6 [Epub ahead of print].

Growing anthropopressure in mountain streams aimed at limiting erosion and flood protection often caused adverse effects on the natural environment. In recent years, great attention has been paid to the restoration and conservation of natural habitats in mountain streams using environmentally friendly solutions such as the Block Ramp (BR) Hydraulic Structures. In this study we investigated the factors responsible for spatial variability in thermal and oxygen conditions at the single BR structure in the growing season, and the relation between water temperature and dissolved oxygen (DO) concentration. This has been done by measurements of hydraulic characteristics along with physicochemical properties of water, such as water temperature and DO concentration, at two different discharges. The redundancy analysis has been applied in order to describe the relationships among hydraulic parameters and physicochemical variables, and extract potential sources of water temperature and DO variability within the BR hydraulic structure. Results have shown that DO and water temperature distributions within the BR hydraulic structure depend on discharge conditions and are associated with the submergence of the block ramp. The highest heterogeneity in hydraulic, DO and water temperature conditions occurs at low flow and is associated with the presence of crevices between protruding cobbles at the block ramp. The lowest variability, in turn, occurs at high discharge, when the block ramp is completely submerged. The results indicated that thermal and oxygen conditions within the BR hydraulic structure are independent of hydraulic parameters at low flow. Moreover, the relation between DO concentration and water temperature is positive at low flow indicating potential impact of biological processes. On the contrary, at high discharge both, the DO concentrations and water temperature within the BR structure, depend on bed shear velocity and maximum Reynolds number.

RevDate: 2020-01-17

Haghighinia A, S Movahedirad (2020)

Mass transfer in a novel passive micro-mixer: Flow tortuosity effects.

Analytica chimica acta, 1098:75-85.

Hydroynamic fluid tortuosity is a parameter to describe the fluid streamlines average elongation. The motivation of the present study is introducing a new concept for theoretical predictions of dynamic tortuosity effects on mass transfer in a novel three-dimensional passive T-shape micro-mixer both experimentally and by numerical simulation. In the numerical analysis, continuity, motion, and diffusion-convection equations were solved, and the amount of mass transfer and the fluid tortuosity was calculated for different rectangular winglet angles. The Reynolds number is considered in the range of 0.1-93. The results show that when the angle of winglet tends to 22.5°, the fluid tortuosity, lateral velocity, and fluid mass transfer tend to maximum values. Furthermore, the effect of fluid tortuosity on the fluid stretching as a theory of chaotic mixing is investigated.

RevDate: 2020-01-17

Rhoades T, Kothapalli CR, PS Fodor (2020)

Mixing Optimization in Grooved Serpentine Microchannels.

Micromachines, 11(1): pii:mi11010061.

Computational fluid dynamics modeling at Reynolds numbers ranging from 10 to 100 was used to characterize the performance of a new type of micromixer employing a serpentine channel with a grooved surface. The new topology exploits the overlap between the typical Dean flows present in curved channels due to the centrifugal forces experienced by the fluids, and the helical flows induced by slanted groove-ridge patterns with respect to the direction of the flow. The resulting flows are complex, with multiple vortices and saddle points, leading to enhanced mixing across the section of the channel. The optimization of the mixers with respect to the inner radius of curvature (Rin) of the serpentine channel identifies the designs in which the mixing index quality is both high (M > 0.95) and independent of the Reynolds number across all the values investigated.

RevDate: 2020-01-17

Oktamuliani S, Hasegawa K, Y Saijo (2019)

Left Ventricular Vortices in Myocardial Infarction Observed with Echodynamography.

Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, 2019:5816-5819.

Echodynamography (EDG) is a computational method to deduce two-dimensional (2D) blood flow vector from conventional color Doppler ultrasound image by considering that the blood flow is divided into vortex and base flow components. Left ventricular (LV) vortices indicate cardiac flow status influenced by LV wall motion. Thus, quantitative assessment of LV vortices may become new and sensitive parameters for cardiac function. In the present study, quantitative parameters of LV vortices such as vortex index, vortex size, and Reynolds number were calculated and relation between each parameter was assessed. Six healthy volunteers and three patients with myocardial infarction (MI) who underwent color Doppler echocardiography (CDE) were involved in the study. Serial CDE images in apical three-chamber view were recorded and 2D blood flow vector was superimposed on the CDE image. Vortex index, vortex size, and Reynolds number were compared between the normal volunteers and the MI patients. The results showed that vortex index (3.09±2.06 vs. 3.34±2.33, p<; 0.05), vortex size (1.76 0.69 vs. 2.01 ±0.68, p<; 0.05), Reynolds number (1020±603 vs.±1312 1046, p<; ±0.05) were significantly greater in the MI patients than in the healthy volunteers. Vortex equivalent diameter in LV showed significant positive correlation with Reynolds number (R2 = 0.799, y = 0.001x + 0.7098, p <; 0.05) in healthy volunteers and (R2 = 0.6404, y = 0.0005x+1.3185, p<; 0.05) in MI patients. Vortex index showed positive correlation with Reynolds number (R2 = 0.9351, y = 0.002x+0.1397, p<; 0.05) in healthy volunteers and (R2 = 0.758, y = 0.0019x+0.7957, p<; 0.05) in MI patients. In conclusion, EDG provides information on LV hemodynamics by quantitative LV vortices parameters both in healthy volunteers and MI patients.

RevDate: 2020-01-17

Hamad EM, Sawalmeh B, Mhawsh AA, et al (2019)

Investigation of Bifurcation Effect on Various Microfluidic Designs for Blood Separation.

Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, 2019:1097-1100.

In this project, a microfluidic device for blood separation will be designed and tested in order to separate plasma from whole blood for diagnostic purposes. The design will be based on previously implemented designs that will be further discussed in the next sections. When designing microfluidic devices, it is essential to consider the different physical phenomena that arise from switching from the macro scale to the micro scale. Parameters such as the Reynolds number and the forces affecting the fluid must be studied in order to produce a suitable and effective design. Finite element methods have been implemented prior to the production of the microfluidic devices. Various geometries/designs have been tested using Fluent ANSYS software. Later on, the successful design was fabricated using micromachining on an acrylic substrate and was tested using simulated blood through of a syringe pump.

RevDate: 2020-01-16

Martínez-Merino P, Midgley S, Martín EI, et al (2020)

Novel WS2-based nanofluids for concentrating solar power: performance characterization and molecular-level insights.

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

Nano-colloidal suspensions of nanomaterials in a fluid, nanofluids, are appealing because of their interesting properties related to heat transfer processes. Whilst nanomaterials based on transition metal chalcogenides (TMCs) have been widely studied in catalysis, sensing, and energy storage applications, there are few studies of nanofluids based on TMCs for heat transfer applications. In this study, the preparation and analysis of nanofluids based on 2D-WS2 in a typical heat transfer fluid (HTF) used in concentrating solar power (CSP) plants is reported. Nanofluids prepared using a exfoliation process exhibited well-defined nanosheets and were highly stable. The nanofluids were characterized in terms of properties related to their application in CSP. The presence of WS2 nanosheets did not modify significantly the surface tension, the viscosity, or the isobaric specific heat, but the thermal conductivity was improved by up to 30%. The Ur factor, which characterizes the thermal efficiency of the fluid in the solar collector, shows an enhancement of up to 22% in the nanofluid, demonstrating great promise for CSP applications. The Reynolds number and friction factor of the fluid were not significantly modified by the addition of the nanomaterial to the HTF, which is also positive for practical applications in CSP plants. Ab initio molecular dynamics simulations of the nanoparticle/fluid interface showed an irreversible dissociative adsorption of diphenyl oxide molecules on the WS2 edge, with very low kinetic barrier. The resulting 'decoration' of the WS2 edge dramatically affects the nature of the interface interactions and is therefore expected to affect significantly the rheological and transport properties of the nanofluids.

RevDate: 2020-01-15

Zhang T, Moreau D, Geyer T, et al (2020)

Dataset on tip vortex formation noise produced by wall-mounted finite airfoils with flat and rounded tip geometries.

Data in brief, 28:105058 pii:105058.

The vortex generated at the tip of an airfoil such as an aircraft wing, wind turbine blade, submarine fin or propeller blade can dominate its wake and be a significant source of unwanted noise. The data collection presented in this paper consists of measurements of tip vortex formation noise produced by finite length airfoils with flat and rounded tips. These data were obtained using the specialist aeroacoustic test facilities at the Brandenburg University of Technology (BTU) in Cottbus, Germany and a 47-channel planar microphone array. Over 1200 unique test cases with variations in airfoil profile shape, tip geometry, angle of attack and Reynolds number were measured during the experimental campaign. The dataset contains one-third-octave band tip noise spectra that have been processed using Acoular, a Python module for acoustic beamforming.

RevDate: 2020-01-14

Salazar-Magallón JA, A Huerta de la Peña (2020)

Production of antifungal saponins in an airlift bioreactor with a cell line transformed from Solanum chrysotrichum and its activity against strawberry phytopathogens.

Preparative biochemistry & biotechnology [Epub ahead of print].

Biotechnology through plant cell cultures in bioreactors is a tool that allows increasing the production of secondary metabolites of commercial interest. The hydrodynamic characterization, in addition to the transfer (OTR) and uptake (OUR) of oxygen through the dynamic method with different aeration rate, were used to see their influence on the production of biomass and saponins. The culture poisoning technique was used to determine the antifungal activity of the SC-2 and SC-3 saponins in vitro. Likewise, the shear or hydrodynamic stress of 273.6 mN/m2 were calculated based on the Reynolds Number. The oxygen supply (OTR) was always greater than the demand (OUR) for all the aeration rate evaluated. Dry weight values of 8.6 gDW/L and a concentration of 2.7 mg/L and 187.3 mg/L of the saponins SC-2 and SC-3 respectively were obtained with an air flow of 0.1 vvm. In addition, it was possible to inhibit the growth of phytopathogenic fungi in vitro by up to 93%, while in vivo it was possible to reduce the infections of strawberry seeds inoculated with phytopathogens, obtaining up to 94% of germinated seeds. This information will facilitate the rational operation of the bioreactor culture system that produces secondary metabolites.

RevDate: 2020-01-17

Hu X, Lin J, Chen D, et al (2020)

Influence of non-Newtonian power law rheology on inertial migration of particles in channel flow.

Biomicrofluidics, 14(1):014105.

In this paper, the inertial migration of particles in the channel flow of power-law fluid is numerically investigated. The effects of the power-law index (n), Reynolds number (Re), blockage ratio (k), and channel aspect ratio (AR) on the inertial migration of particles and equilibrium position are explored. The results show that there exist two stages of particle migration and four stable equilibrium positions for particles in the cross section of a square channel. The particle equilibrium positions in a rectangular channel are much different from those in a square channel. In shear-thinning fluids, the long channel face equilibrium position and two kinds of particle trajectories are found at low Re. With increasing Re, the short channel face equilibrium position turns to be stable, multiequilibrium positions, and three kinds of particle trajectories along the long wall start to form. Only two stable equilibrium positions exist in shear-thickening fluids. The equilibrium positions are getting closer to the channel centerline with increasing n and k and with decreasing Re. The inertial focusing length L2 in the second stage of particle migration is much longer than inertial focusing length L1 in the first stage. In the square channel, L2 is decreased with increasing Re and k and with decreasing n. In the rectangular channel, L2 is the shortest in the shear-thinning fluid.

RevDate: 2020-01-17

Feng Y, Wang Q, Duan JL, et al (2020)

Attachment and adhesion force between biogas bubbles and anaerobic granular sludge in the up-flow anaerobic sludge blanket.

Water research, 171:115458 pii:S0043-1354(19)31235-7 [Epub ahead of print].

The performance of the up-flow anaerobic sludge blanket (UASB) is significantly governed by the hydrodynamics of the reactor. Though the influence of hydrodynamics on mass transfer, granular size distribution, and biogas production was well studied, the interaction between biogas bubbles and anaerobic granular sludge (AGS) is poorly understood. This study used the impinging-jet technique and bubble probe atomic force microscope (AFM) to investigate the attachment and adhesion force between biogas bubbles (CH4 and CO2) and AGS. The fluxes of normalized CH4 or CO2 bubble-attachment on two kinds of AGS were directly affected by gas velocity and decreased with an increase in the Reynolds number ranged from 40 to 140. The bubble-attachment had a positive linear relationship with the contact angles, ratio of exopolymeric protein and polysaccharide, and hydrophilic functional groups of AGS. A bubble probe AFM was used to explore the adhesion force between a single bubble and AGS. The results indicated that the adhesion force between the bubbles and the two kinds of AGS also decreased with increasing approach velocity. Overall, these results contribute to a new insight into the understanding of interaction between biogas bubbles and AGS in UASB reactors.

RevDate: 2020-01-17

Muhammad R, Khan MI, Jameel M, et al (2019)

Fully developed Darcy-Forchheimer mixed convective flow over a curved surface with activation energy and entropy generation.

Computer methods and programs in biomedicine, 188:105298 pii:S0169-2607(19)31617-7 [Epub ahead of print].

BACKGROUND: Mixed convection (forced+natural convection) is frequently observed in exceptionally high output devices where the forced convection isn't sufficient to dissipate all of the heat essential. At this point, consolidating natural convection with forced convection will frequently convey the ideal outcomes. Nuclear reactor technology and a few features of electronic cooling are the examples of these processes. Mixed convection problems are categorized by Richardson number (Ri), which is the ratio of Grashof number (for natural convection) and Reynolds number (for forced convection). For buoyancy or mixed convection the relative effect can be addressed by Richardson number. Typically, the natural convection is negligible when Richardson number is less than 0.1 (Ri < 0.1), forced convection is negligible when Richardson number is greater than 10 (Ri > 10) and neither is negligible when (0.1 < Ri < 10). It might be noticed that generally the forced convection is large comparative with natural convection except in case of remarkably low forced flow velocities. The current work gives significant insights regarding dissipative mixed convective Darcy-Forchheimer flow with entropy generation over a stretched curved surface. The energy equation is developed with respect to nonlinear radiation, dissipation and Ohmic heating (Joule heating). Binary reaction via activation energy is accounted.

METHOD: Curvilinear transformations are utilized to change the nonlinear PDE's into ordinary ones. Computational outcomes are obtained via NDSolve MATHEMATICA. The results are computed and discussed graphically.

RESULTS: Velocity decays for Forchheimer number. Entropy generation enhances for diffusion parameter and chemical reaction parameter. Concentration profile reduces chemical reaction parameter and enhances for activation parameter.

RevDate: 2020-01-17

Rane YS, JO Marston (2020)

Computational study of fluid flow in tapered orifices for needle-free injectors.

Journal of controlled release : official journal of the Controlled Release Society, 319:382-396 pii:S0168-3659(20)30019-5 [Epub ahead of print].

Transdermal drug delivery using spring-powered jet injection has been studied for several decades and continues to be highly sought after due to the advent of targeted needle-free techniques, especially for viscous and complex fluids. As such, this paper reports results from numerical simulations to study the role of fluid rheology and cartridge geometry on characteristics such as jet exit velocity, total pressure drop and boundary layer thickness, since these all factor in to jet stability and collimation. The numerical approach involves incompressible steady flow with turbulence modelling based on the system Reynolds number at the orifice (Re = ρdovj/μ). The results are experimentally validated for a given geometry over a wide range of Reynolds numbers (101 < Re < 104), and our results indicate a sharp decrease in dimensionless pressure drop (Eu = 2∆P/ρvj2) for Re < 102) and gradually approaching the inviscid limit at Re ≥ 104. By extending the study to non-Newtonian fluids, whose rheological profile is approximated by the Carreau model, we also elucidated the effect of different rheological parameters. Lastly by studying a range of nozzle geometries such as conical, sigmoid taper and multi-tier tapers, we observe that fluid acceleration suppresses the boundary layer growth, which indicates there may be optimal geometries for creating jets to target specific tissue depths.

RevDate: 2020-01-07

Pan X, Tang L, Feng J, et al (2020)

Experimental Research on the Degradation Coefficient of Ammonia Nitrogen Under Different Hydrodynamic Conditions.

Bulletin of environmental contamination and toxicology pii:10.1007/s00128-019-02781-0 [Epub ahead of print].

Degradation coefficients for pollutants in water are important parameters that are significantly influenced by environmental conditions. In controlled experiments, the processes and trends of ammonia nitrogen (NH3-N) degradation in raw waters were studied under different flow conditions using a laboratory annular flume. Analysis of the observed change in NH3-N concentration with time under various flow conditions allowed calculation of a degradation efficiency (concentration change amount/initial concentration) which for NH3-N increased as the flow velocity increased. According to a first-order kinetic equation to fit the experimental data, the range of variation of the degradation coefficient of NH3-N at different flowrates was between 0.047 per day (0.01 m/s) and 0.203 per day (0.30 m/s). Dimensional analysis was used to analyze the relationship between the degradation coefficient and flow velocity (v), water depth (H), Froude number (Fr), and Reynolds number (Re), which was verified through field data collected in the Chishui River.

RevDate: 2020-01-08

Sobecki C, Zhang J, C Wang (2019)

Numerical Study of Paramagnetic Elliptical Microparticles in Curved Channels and Uniform Magnetic Fields.

Micromachines, 11(1): pii:mi11010037.

We numerically investigated the dynamics of a paramagnetic elliptical particle immersed in a low Reynolds number Poiseuille flow in a curved channel and under a uniform magnetic field by direct numerical simulation. A finite element method, based on an arbitrary Lagrangian-Eulerian approach, analyzed how the channel geometry, the strength and direction of the magnetic field, and the particle shape affected the rotation and radial migration of the particle. The net radial migration of the particle was analyzed after executing a π rotation and at the exit of the curved channel with and without a magnetic field. In the absence of a magnetic field, the rotation is symmetric, but the particle-wall distance remains the same. When a magnetic field is applied, the rotation of symmetry is broken, and the particle-wall distance increases as the magnetic field strength increases. The causation of the radial migration is due to the magnetic angular velocity caused by the magnetic torque that constantly changes directions during particle transportation. This research provides a method of magnetically manipulating non-spherical particles on lab-on-a-chip devices for industrial and biological applications.

RevDate: 2020-01-08
CmpDate: 2020-01-06

Bahrami A, Hoseinzadeh S, Heyns PS, et al (2019)

Experimental investigation of co-flow jet's airfoil flow control by hot wire anemometer.

The Review of scientific instruments, 90(12):125107.

An experimental flow control technique is given in this paper to study the jet effect on the coflow jet's airfoil with injection and suction and compared with the jet-off condition. The airfoil is CFJ0025-065-196, and the Reynolds number based on the airfoil's chord length is 105. To measure the turbulence components of flow, a hot wire anemometry apparatus in a wind tunnel has been used. In this paper, the effect of the average velocity and boundary layer thickness on the coflow jet's airfoil is analyzed. The test is done for two different coflow velocities and for different angles of attack. It is also shown that, by increasing the velocity difference between the jet and the main flow, separation is delayed, and this delay can be preserved by raising coflow velocity at higher angles of attack. So, this flow control method has a good efficiency, and it is possible to reach higher numbers of lift and lower numbers of drag coefficients.

RevDate: 2020-01-02

Pendse V, Mazumdar B, H Kumar (2020)

Formulation of experimental data based model for solid-liquid mass transfer enhancement in three phase fluidized bed using nanofluid.

Data in brief, 28:104990.

This experimental data based model in three phase fluidized bed was designed to enhance the solid-liquid mass transfer. This data focuses on mass transfer enhancement using nanomaterial. In present investigation benzoic acid-water-air system was used as three phases ie solid, liquid and gas respectively with Arachitol nano as nanomaterial in different volume percent in three phase fluidized bed. Data from experiment were collected by varying gas velocity, bed height, nanomaterial percentage and time. After a convenient selection various correlation have been derived. The data presented here is the full set of experimental value and coefficients and exponents in correlation were estimated from nonlinear optimization technique in MATLAB.

RevDate: 2020-01-08

Krishnan SR, Bal J, SA Putnam (2019)

A simple analytic model for predicting the wicking velocity in micropillar arrays.

Scientific reports, 9(1):20074.

Hemiwicking is the phenomena where a liquid wets a textured surface beyond its intrinsic wetting length due to capillary action and imbibition. In this work, we derive a simple analytical model for hemiwicking in micropillar arrays. The model is based on the combined effects of capillary action dictated by interfacial and intermolecular pressures gradients within the curved liquid meniscus and fluid drag from the pillars at ultra-low Reynolds numbers [Formula: see text]. Fluid drag is conceptualized via a critical Reynolds number: [Formula: see text], where v0 corresponds to the maximum wetting speed on a flat, dry surface and x0 is the extension length of the liquid meniscus that drives the bulk fluid toward the adsorbed thin-film region. The model is validated with wicking experiments on different hemiwicking surfaces in conjunction with v0 and x0 measurements using Water [Formula: see text], viscous FC-70 [Formula: see text] and lower viscosity Ethanol [Formula: see text].

RevDate: 2020-01-08

Li H, Li Y, Huang B, et al (2019)

Flow Characteristics of the Entrance Region with Roughness Effect within Rectangular Microchannels.

Micromachines, 11(1): pii:mi11010030.

We conducted systematic numerical investigations of the flow characteristics within the entrance region of rectangular microchannels. The effects of the geometrical aspect ratio and roughness on entrance lengths were analyzed. The incompressible laminar Navier-Stokes equations were solved using finite volume method (FVM). In the simulation, hydraulic diameters (Dh) ranging from 50 to 200 µm were studied, and aspect ratios of 1, 1.25, 1.5, 1.75, and 2 were considered as well. The working fluid was set as water, and the Reynolds number ranged from 0.5 to 100. The results showed a good agreement with the conducted experiment. Correlations are proposed to predict the entrance lengths of microchannels with respect to different aspect ratios. Compared with other correlations, these new correlations are more reliable because a more practical inlet condition was considered in our investigations. Instead of considering the influence of the width and height of the microchannels, in our investigation we proved that the critical role is played by the aspect ratio, representing the combination of the aforementioned parameters. Furthermore, the existence of rough elements obviously shortens the entrance region, and this effect became more pronounced with increasing relative roughness and Reynolds number. A similar effect could be seen by shortening the roughness spacing. An asymmetric distribution of rough elements decreased the entrance length compared with a symmetric distribution, which can be extrapolated to other irregularly distributed forms.

RevDate: 2020-01-08

Kang DJ (2019)

Effects of Channel Wall Twisting on the Mixing in a T-Shaped Micro-Channel.

Micromachines, 11(1): pii:mi11010026.

A new design scheme is proposed for twisting the walls of a microchannel, and its performance is demonstrated numerically. The numerical study was carried out for a T-shaped microchannel with twist angles in the range of 0 to 34π. The Reynolds number range was 0.15 to 6. The T-shaped microchannel consists of two inlet branches and an outlet branch. The mixing performance was analyzed in terms of the degree of mixing and relative mixing cost. All numerical results show that the twisting scheme is an effective way to enhance the mixing in a T-shaped microchannel. The mixing enhancement is realized by the swirling of two fluids in the cross section and is more prominent as the Reynolds number decreases. The twist angle was optimized to maximize the degree of mixing (DOM), which increases with the length of the outlet branch. The twist angle was also optimized in terms of the relative mixing cost (MC). The two optimum twisting angles are generally not coincident. The optimum twist angle shows a dependence on the length of the outlet branch but it is not affected much by the Reynolds number.

RevDate: 2019-12-26

Zhang L, Zhou J, Zhang B, et al (2019)

Numerical investigation on the solid particle erosion in elbow with water-hydrate-solid flow.

Science progress [Epub ahead of print].

Erosion in pipeline caused by solid particles, which may lead to premature failure of the pipe system, is regarded as one of the most important concerns in the field of oil and gas. Therefore, the Euler-Lagrange, erosion model, and discrete phase model are applied for the purpose of simulating the erosion of water-hydrate-solid flow in submarine hydrate transportation pipeline. In this article, the flow and erosion characteristics are well verified on the basis of experiments. Moreover, analysis is conducted to have a good understanding of the effects of hydrate volume, mean curvature radius/pipe diameter (R/D) rate, flow velocity, and particle diameter on elbow erosion. It is finally obtained that the hydrate volume directly affects the Reynolds number through viscosity and the trend of the Reynolds number is consistent with the trend of erosion rate. Taking into account different R/D rates, the same Stokes number reflects different dynamic transforms of the maximum erosion zone. However, the outmost wall (zone D) will be the final erosion zone when the value of the Stokes number increases to a certain degree. In addition, the erosion rate increases sharply along with the increase of flow velocity and particle diameter. The effect of flow velocity on the erosion zone can be ignored in comparison with the particle diameter. Moreover, it is observed that flow velocity is deemed as the most sensitive factor on erosion rate among these factors employed in the orthogonal experiment.

RevDate: 2020-01-08

Liu X, Fan D, Yu X, et al (2019)

Effects of Simulated Gravel on Hydraulic Characteristics of Overland Flow Under Varying Flow Discharges, Slope Gradients and Gravel Coverage Degrees.

Scientific reports, 9(1):19781 pii:10.1038/s41598-019-56223-2.

To quantify the hydraulic characteristics of overland flow on gravel-covered slopes, eight flow discharges (Q) (8.44-122 L/min), five slope gradients (J) (2°-10°) and four gravel coverage degrees (Cr) (0-30%) were examined via a laboratory flume. The results showed that (1) gravel changed flow regime. Gravel increased the Reynolds number (Re) by 2.94-33.03%. Re were less affected by J and positively correlated with Cr and Q. Gravel decreased the Froude number (Fr) by 6.83-77.31%. Fr was positively correlated with Q and J and negatively correlated with Cr. (2) Gravel delayed the flow velocity (u) and increased the flow depth (h) and flow resistance (f). Gravel reduced u by 1.20-58.95%. u was positively correlated with Q and J and negatively correlated with Cr. Gravel increased h by 0.12-2.41 times. h was positively correlated with Q and Cr and negatively correlated with J. Gravel increased f by 0.15-18.42 times. f were less affected by J, positively correlated with Cr and negatively correlated with Q. (3) The relationships between hydraulic parameters and Q, J and Cr identified good power functions. Hydraulic parameters were mainly affected by Cr. These results can guide the ecological construction of soil and water conservation.

RevDate: 2020-01-08
CmpDate: 2019-12-30

Panickacheril John J, Donzis DA, KR Sreenivasan (2019)

Solenoidal Scaling Laws for Compressible Mixing.

Physical review letters, 123(22):224501.

Mixing of passive scalars in compressible turbulence does not obey the same classical Reynolds number scaling as its incompressible counterpart. We first show from a large database of direct numerical simulations that even the solenoidal part of the velocity field fails to follow the classical incompressible scaling when the forcing includes a substantial dilatational component. Though the dilatational effects on the flow remain significant, our main results are that both the solenoidal energy spectrum and the passive scalar spectrum assume incompressible forms, and that the scalar gradient essentially aligns with the most compressive eigenvalue of the solenoidal part, provided that only the solenoidal components are consistently used for scaling. A slight refinement of this statement is also pointed out.

RevDate: 2020-01-08

Nath R, M Krishnan (2019)

Optimization of double diffusive mixed convection in a BFS channel filled with Alumina nanoparticle using Taguchi method and utility concept.

Scientific reports, 9(1):19536.

This research work focuses on the implementation of Taguchi method and utility concept for optimization of flow, geometrical and thermo-physical parameters for mixed convective heat and mass transfer in a backward facing step (BFS) channel filled with Alumina nanoparticle doped in water-ethylene glycol mixture. Mass, momentum, energy and solutal conservation equations for the flow field are cast in velocity-vorticity form of Navier-Stokes equations, which are solved using Galerkin's weighted residual finite element method through isoparametric formulation. The following six parameters, expansion ratio of the BFS channel (H/h), Reynolds number (Re), buoyancy ratio (N), nanoparticle volume fraction (χ), shape of nanoparticles and thermal Grashof number (GrT) at three levels are considered as controlling parameters for optimization using Taguchi method. An L27 orthogonal array has been chosen to get the levels of the six parameters for the 27 trial runs. Simulation results were obtained for 27 trial runs from which three different sets of optimum levels of the control parameters were obtained for maximum Nu and Sh and minimum wall shear stress during double diffusive mixed convection in the channel. Then, in order to obtain a single set of optimum levels of the control parameters to achieve maximum heat and mass transfer and minimum wall shear stress concurrently, utility concept has been implemented. Taguchi results indicate that expansion ratio and volume fraction of nanoparticles are the significant contributing parameters to achieve maximum heat and mass transfer and minimum wall shear stress. Utility concept predicts the average Nusselt number less by 2% and Sherwood number less by 3% compared to the Taguchi method with equal weightage of 40% assumed for Nusselt and Sherwood numbers and 20% for wall shear stress.

RevDate: 2020-01-08

Maklad O, Eliasy A, Chen KJ, et al (2019)

Simulation of Air Puff Tonometry Test Using Arbitrary Lagrangian-Eulerian (ALE) Deforming Mesh for Corneal Material Characterisation.

International journal of environmental research and public health, 17(1): pii:ijerph17010054.

: Purpose: To improve numerical simulation of the non-contact tonometry test by using arbitrary Lagrangian-Eulerian deforming mesh in the coupling between computational fluid dynamics model of an air jet and finite element model of the human eye.

METHODS: Computational fluid dynamics model simulated impingement of the air puff and employed Spallart-Allmaras model to capture turbulence of the air jet. The time span of the jet was 30 ms and maximum Reynolds number was Re=2.3×104, with jet orifice diameter 2.4 mm and impinging distance 11 mm. The model of the human eye was analysed using finite element method with regional hyperelastic material variation and corneal patient-specific topography starting from stress-free configuration. The cornea was free to deform as a response to the air puff using an adaptive deforming mesh at every time step of the solution. Aqueous and vitreous humours were simulated as a fluid cavity filled with incompressible fluid with a density of 1000 kg/m3.

RESULTS: Using the adaptive deforming mesh in numerical simulation of the air puff test improved the traditional understanding of how pressure distribution on cornea changes with time of the test. There was a mean decrease in maximum pressure (at corneal apex) of 6.29 ± 2.2% and a development of negative pressure on a peripheral corneal region 2-4 mm away from cornea centre.

CONCLUSIONS: The study presented an improvement of numerical simulation of the air puff test, which will lead to more accurate intraocular pressure (IOP) and corneal material behaviour estimation. The parametric study showed that pressure of the air puff is different from one model to another, value-wise and distribution-wise, based on cornea biomechanical parameters.

RevDate: 2019-12-31

Sum Wu K, Nowak J, KS Breuer (2019)

Scaling of the performance of insect-inspired passive-pitching flapping wings.

Journal of the Royal Society, Interface, 16(161):20190609.

Flapping flight using passive pitch regulation is a commonly used mode of thrust and lift generation in insects and has been widely emulated in flying vehicles because it allows for simple implementation of the complex kinematics associated with flapping wing systems. Although robotic flight employing passive pitching to regulate angle of attack has been previously demonstrated, there does not exist a comprehensive understanding of the effectiveness of this mode of aerodynamic force generation, nor a method to accurately predict its performance over a range of relevant scales. Here, we present such scaling laws, incorporating aerodynamic, inertial and structural elements of the flapping-wing system, validating the theoretical considerations using a mechanical model which is tested for a linear elastic hinge and near-sinusoidal stroke kinematics over a range of scales, hinge stiffnesses and flapping frequencies. We find that suitably defined dimensionless parameters, including the Reynolds number, Re, the Cauchy number, Ch, and a newly defined 'inertial-elastic' number, IE, can reliably predict the kinematic and aerodynamic performance of the system. Our results also reveal a consistent dependency of pitching kinematics on these dimensionless parameters, providing a connection between lift coefficient and kinematic features such as angle of attack and wing rotation.

RevDate: 2020-01-08

Tan L, Ali J, Cheang UK, et al (2019)

µ-PIV Measurements of Flows Generated by Photolithography-Fabricated Achiral Microswimmers.

Micromachines, 10(12): pii:mi10120865.

Robotic micro/nanoswimmers can potentially be used as tools for medical applications, such as drug delivery and noninvasive surgery. Recently, achiral microswimmers have gained significant attention because of their simple structures, which enables high-throughput fabrication and size scalability. Here, microparticle image velocimetry (µ-PIV) was used to study the hydrodynamics of achiral microswimmers near a boundary. The structures of these microswimmers resemble the letter L and were fabricated using photolithography and thin-film deposition. Through µ-PIV measurements, the velocity flow fields of the microswimmers rotating at different frequencies were observed. The results herein yield an understanding of the hydrodynamics of the L-shaped microswimmers, which will be useful in applications such as fluidic manipulation.

RevDate: 2019-12-18

Ford MP, Lai HK, Samaee M, et al (2019)

Hydrodynamics of metachronal paddling: effects of varying Reynolds number and phase lag.

Royal Society open science, 6(10):191387.

Negatively buoyant freely swimming crustaceans such as krill must generate downward momentum in order to maintain their position in the water column. These animals use a drag-based propulsion strategy, where pairs of closely spaced swimming limbs are oscillated rhythmically from the tail to head. Each pair is oscillated with a phase delay relative to the neighbouring pair, resulting in a metachronal wave travelling in the direction of animal motion. It remains unclear how oscillations of limbs in the horizontal plane can generate vertical momentum. Using particle image velocimetry measurements on a robotic model, we observed that metachronal paddling with non-zero phase lag created geometries of adjacent paddles that promote the formation of counter-rotating vortices. The interaction of these vortices resulted in generating large-scale angled downward jets. Increasing phase lag resulted in more vertical orientation of the jet, and phase lags in the range used by Antarctic krill produced the most total momentum. Synchronous paddling produced lower total momentum when compared with metachronal paddling. Lowering Reynolds number by an order of magnitude below the range of adult krill (250-1000) showed diminished downward propagation of the jet and lower vertical momentum. Our findings show that metachronal paddling is capable of producing flows that can generate both lift (vertical) and thrust (horizontal) forces needed for fast forward swimming and hovering.

RevDate: 2019-12-18

Wang X, IC Christov (2019)

Theory of the flow-induced deformation of shallow compliant microchannels with thick walls.

Proceedings. Mathematical, physical, and engineering sciences, 475(2231):20190513.

Long, shallow microchannels embedded in thick, soft materials are widely used in microfluidic devices for lab-on-a-chip applications. However, the bulging effect caused by fluid-structure interactions between the internal viscous flow and the soft walls has not been completely understood. Previous models either contain a fitting parameter or are specialized to channels with plate-like walls. This work is a theoretical study of the steady-state response of a compliant microchannel with a thick wall. Using lubrication theory for low-Reynolds-number flows and the theory for linearly elastic isotropic solids, we obtain perturbative solutions for the flow and deformation. Specifically, only the channel's top wall deformation is considered, and the ratio between its thickness t and width w is assumed to be (t/w)2≫1. We show that the deformation at each stream-wise cross section can be considered independently, and that the top wall can be regarded as a simply supported rectangle subject to uniform pressure at its bottom. The stress and displacement fields are found using Fourier series, based on which the channel shape and the hydrodynamic resistance are calculated, yielding a new flow rate-pressure drop relation without fitting parameters. Our results agree favourably with, and thus rationalize, previous experiments.

RevDate: 2020-01-08

Raza W, KY Kim (2019)

Asymmetrical Split-and-Recombine Micromixer with Baffles.

Micromachines, 10(12): pii:mi10120844.

The present work proposes a planar micromixer design comprising hybrid mixing modules of split-and-recombine units and curved channels with radial baffles. The mixing performance was evaluated numerically by solving the continuity and momentum equations along with the advection-diffusion equation in a Reynolds number range of 0.1-80. The variance of the concentration of the mixed species was considered to quantify the mixing index. The micromixer showed far better mixing performance over whole Reynolds number range than an earlier split-and-recombine micromixer. The mixer achieved mixing indices greater than 90% at Re ≥ 20 and a mixing index of 99.8% at Re = 80. The response of the mixing quality to the change of three geometrical parameters was also studied. A mixing index over 80% was achieved within 63% of the full length at Re = 20.

RevDate: 2020-01-08

Abay A, Recktenwald SM, John T, et al (2020)

Cross-sectional focusing of red blood cells in a constricted microfluidic channel.

Soft matter, 16(2):534-543.

Constrictions in blood vessels and microfluidic devices can dramatically change the spatial distribution of passing cells or particles and are commonly used in biomedical cell sorting applications. However, the three-dimensional nature of cell focusing in the channel cross-section remains poorly investigated. Here, we explore the cross-sectional distribution of living and rigid red blood cells passing a constricted microfluidic channel by tracking individual cells in multiple layers across the channel depth and across the channel width. While cells are homogeneously distributed in the channel cross-section pre-contraction, we observe a strong geometry-induced focusing towards the four channel faces post-contraction. The magnitude of this cross-sectional focusing effect increases with increasing Reynolds number for both living and rigid red blood cells. We discuss how this non-uniform cell distribution downstream of the contraction results in an apparent double-peaked velocity profile in particle image velocimetry analysis and show that trapping of red blood cells in the recirculation zones of the abrupt construction depends on cell deformability.

RevDate: 2019-12-17

Shah Z, Khan A, Khan W, et al (2019)

Micropolar gold blood nanofluid flow and radiative heat transfer between permeable channels.

Computer methods and programs in biomedicine, 186:105197 pii:S0169-2607(19)31672-4 [Epub ahead of print].

This article characterizes flow and heat transmission of blood that carries the micropolar nanofluid of gold in a permeable channel. The thermal radiations are also present in the channel while its walls are either moving or stationary. The base-fluid is considered as blood while micro polar nanofluid is taken as gold. By using similarity transformations along with dimensionless quantities the modeled equations of the problem are transmuted into a system of non-linear ODEs with a set of appropriate boundary conditions. The semi-analytical method, HAM is then applied to determine the solution of a set of resultant equations. The results obtained by HAM have also compared with numerical solutions. The influence of non-dimensional parameters like fractional parameter suction/injection β, Reynolds Number Re, Darcys Number Da, micropolar parameter K, Prandtl number Pr and Radiation parameter Rd etc., which provides physical interpretations of temperature, microrotation n and velocity fields are discussed in detail with the help of graphical representations. Nusselt number is calculated and presented through table. This study determined that the temperature of micropolar nanofluid augmented along with augmentation in the volume fraction. Radiation Rd augmented the heat transfer rate at the upper wall and reduce it at the lower wall. The suction/injection parameter 'β' reduces the heat transfer rate in case of β < 0 at the upper wall, where it is augmented at lower wall.

RevDate: 2020-01-16

Pandey R, Kumar M, Majdoubi J, et al (2019)

A review study on blood in human coronary artery: Numerical approach.

Computer methods and programs in biomedicine, 187:105243 pii:S0169-2607(19)31946-7 [Epub ahead of print].

Computational fluid dynamics (CFD) study of blood flow in human coronary artery is one of the emerging fields of Biomed- ical engineering. In present review paper, Finite Volume Method with governing equations and boundary conditions are briefly discussed for different coronary models. Many researchers have come up with astonishing results related to the various factors (blood viscosity, rate of blood flow, shear stress on the arterial wall, Reynolds number, etc.) affecting the hemodynamic of blood in the right/left coronary artery. The aim of this paper is to present an overview of all those work done by the researchers to justify their work related to factors which hampers proper functioning of heart and lead to Coronary Artery Disease (CAD). Governing equations like Navier-stokes equations, continuity equations etc. are widely used and are solved using CFD solver to get a clearer view of coronary artery blockage. Different boundary conditions and blood properties published in the last ten years are summarized in the tabulated form. This table will help new researchers to work on this area.

RevDate: 2019-12-04

Poehnl R, Popescu MN, W Uspal (2019)

Axisymmetric spheroidal squirmers and self-diffusiophoretic particles.

Journal of physics. Condensed matter : an Institute of Physics journal [Epub ahead of print].

We study, by means of an exact analytical solution, the motion of a spheroidal, axisymmetric squirmer in an unbounded fluid, as well as the low Reynolds number hydrodynamic flow associated to it. In contrast to the case of a spherical squirmer --- for which, e.g., the velocity of the squirmer and the magnitude of the stresslet associated with the flow induced by the squirmer are respectively determined by the amplitudes of the first two slip (``squirming'') modes --- for the spheroidal squirmer each squirming mode either contributes to the velocity, or contributes to the stresslet. The results are straightforwardly extended to the self-phoresis of axisymmetric, spheroidal, chemically active particles in the case when the phoretic slip approximation holds.

RevDate: 2020-01-08

Rajappan A, GH McKinley (2019)

Epidermal biopolysaccharides from plant seeds enable biodegradable turbulent drag reduction.

Scientific reports, 9(1):18263.

The high cost of synthetic polymers has been a key impediment limiting the widespread adoption of polymer drag reduction techniques in large-scale engineering applications, such as marine drag reduction. To address consumable cost constraints, we investigate the use of high molar mass biopolysaccharides, present in the mucilaginous epidermis of plant seeds, as inexpensive drag reducers in large Reynolds number turbulent flows. Specifically, we study the aqueous mucilage extracted from flax seeds (Linum usitatissimum) and compare its drag reduction efficacy to that of poly(ethylene oxide) or PEO, a common synthetic polymer widely used as a drag reducing agent in aqueous flows. Macromolecular and rheological characterisation confirm the presence of high molar mass (≥2 MDa) polysaccharides in the extracted mucilage, with an acidic fraction comprising negatively charged chains. Frictional drag measurements, performed inside a bespoke Taylor-Couette apparatus, show that the as-extracted mucilage has comparable drag reduction performance under turbulent flow conditions as aqueous PEO solutions, while concurrently offering advantages in terms of raw material cost, availability, and bio-compatibility. Our results indicate that plant-sourced mucilage can potentially serve as a cost-effective and eco-friendly substitute for synthetic drag reducing polymers in large scale turbulent flow applications.

RevDate: 2020-01-16

Ibrahim M, M Ijaz Khan (2019)

Mathematical modeling and analysis of SWCNT-Water and MWCNT-Water flow over a stretchable sheet.

Computer methods and programs in biomedicine, 187:105222 pii:S0169-2607(19)31683-9 [Epub ahead of print].

In this article we focused on the mixed convection flow of SWCNT-Water and MWCNT-Water over a stretchable permeable sheet. The nanofluid occupied porous medium. Darcy's law is used to characterize porous medium. The impact of viscous dissipation is considered. Transformation procedure is adopted to transform the governing PDE's system into dimensionless form. In order to solve the dimensionless PDE's system we used numerical method known as Finite difference method. Effects of flow variables i.e porosity parameter, suction parameter, Grashof number and Reynolds number on velocity, skin friction, temperature and Nusselt number are described graphically. The obtained results shows that velocity is dominant in SWCNT-Water over MWCNT-Water. Temperature is dominant in MWCNT-Water over SWCNT-Water.

RevDate: 2020-01-16

Turkyilmazoglu M (2019)

Single phase nanofluids in fluid mechanics and their hydrodynamic linear stability analysis.

Computer methods and programs in biomedicine, 187:105171 pii:S0169-2607(19)31698-0 [Epub ahead of print].

BACKGROUND AND OBJECTIVE: The hydrodynamic stability of nanofluids of one phase is investigated in this paper based on linear stability theory. The overall thrust here is that the linear stability features of nanofluids can be estimated from their corresponding working fluid, at least in special circumstances.

METHODS: The approach uses the adjusting parameter to make assertions about stability. This is possible by certain correlations between the resulting eigenvalues.

RESULTS: It is shown that as the nanoparticles are added, the mean flow of nanofluids is slightly modified and the resulting eigen space of nano disturbances is built on the corresponding pure flow eigen space of perturbations. Several fluid dynamics problems are revisited to verify the usefulness of the obtained correlations.

CONCLUSION: The presented approach in this work serves us to understand the stabilizing/destabilizing effects of nanofluids as compared to the standard base fluids in terms of stability of viscous/inviscid and temporal/spatial senses. To illustrate, the critical Reynolds number in a traditional boundary layer flow is shown to be pushed to higher values with the dispersed nanoparticles in a working fluid, clearly implying the delay in transition from laminar to turbulent state.

RevDate: 2020-01-13
CmpDate: 2020-01-13

Chen J, He Y, Wang J, et al (2019)

Dynamics of nitrogen transformation and bacterial community with different aeration depths in malodorous river.

World journal of microbiology & biotechnology, 35(12):196.

In this research, the dynamics of nitrogen transformation and bacterial community in malodorous river were investigated with different aeration depths. Computational flow dynamics (CFD) and Reynolds number (Re) were specially used to characterize the hydrodynamics condition under different aeration depths. The results indicated that aeration depth had vital impact on nitrogen transformation and bacterial community structure. It was found that a range of aeration depth (0.20-0.45 m above sediment-water interface) facilitated the removal of NH4+-N and TN with Re ranging between 6211 and 8930. Proteobacteria took over Firmicutes to become the predominant phylum (36-78%) under aeration, and the main subdivisions of γ-, β- and δ-Proteobacteria also varied greatly with different aeration depths. Interestingly, there was a marked shift of the inferentially identified dominant functional role within Proteobacteria from organic-matter degradation to nitrogen metabolism and then to sulfur metabolism as well as the coupling of nitrogen and sulfur with the increase of disturbance. The redundancy analysis (RDA) further confirmed the importance of aeration disturbance in shaping bacterial community. These findings help to gain improved understanding of endogenous N-behavior and aquatic microbial ecology, and underline the need for integrating the hydrodynamics factors with microbial community.

RevDate: 2020-01-08
CmpDate: 2019-12-02

Singh H, Bonnesoeur A, Besnard H, et al (2019)

A large thermal turbulent Taylor-Couette (THETACO) facility for investigation of turbulence induced by simultaneous action of rotation and radial temperature gradient.

The Review of scientific instruments, 90(11):115112.

A thermal turbulent Taylor-Couette facility has been designed to investigate turbulent flows generated by differential rotation and radial temperature gradient. It consists of a cylindrical annulus with a rotating inner cylinder and a fixed outer cylinder. The electric heating system is installed inside the inner cylinder, and the annulus is immersed in a large cylindrical container filled with cooling fluid. Temperature regulators independently control the temperature of the inner surface of the inner cylinder and that of the cooling fluid. The facility allows us to reach values of the Reynolds number (Re ∼ 5 × 105) and of the Rayleigh number (Ra ∼ 3 × 106) for water as the working fluid. The facility provides torque measurements, a full optical access at the side and from the bottom for velocity measurements using particle image velocimetry (2D, stereoscopic, and tomographic). Temperature measurements in the flow can be performed by thermochromic liquid crystals or laser induced fluorescence.

RevDate: 2019-12-01

Zhang X, Lam WA, MD Graham (2019)

Dynamics of deformable straight and curved prolate capsules in simple shear flow.

Physical review fluids, 4(4):.

This work investigates the motion of neutrally-buoyant, slightly deformable straight and curved prolate fluid-filled capsules in unbounded simple shear flow at zero Reynolds number using direct simulations. The curved capsules serve as a model for the typical crescent-shaped sickle red blood cells in sickle cell disease (SCD). The effects of deformability and curvature on the dynamics are revealed. We show that with low deformability, straight prolate spheroidal capsules exhibit tumbling in the shear plane as their unique asymptotically stable orbit. This result contrasts with that for rigid spheroids, where infinitely many neutrally stable Jeffery orbits exist. The dynamics of curved prolate capsules are more complicated due to a combined effect of deformability and curvature. At short times, depending on the initial orientation, slightly deformable curved prolate capsules exhibit either a Jeffery-like motion such as tumbling or kayaking, or a non-Jeffery-like behavior in which the director (end-to-end vector) of the capsule crosses the shear-gradient plane back and forth. At long times, however, a Jeffery-like quasiperiodic orbit is taken regardless of the initial orientation. We further show that the average of the long-time trajectory can be well approximated using the analytical solution for Jeffery orbits with an effective orbit constant Ceff and aspect ratio ℓeff. These parameters are useful for characterizing the dynamics of curved capsules as a function of given deformability and curvature. As the capsule becomes more deformable or curved, Ceff decreases, indicating a shift of the orbit towards log-rolling motion, while ℓeff increases weakly as the degree of curvature increases but shows negligible dependency on deformability. These features are not changed substantially as the viscosity ratio between the inner and outer fluids is changed from 1 to 5. As cell deformability, cell shape, and cell-cell interactions are all pathologically altered in blood disorders such as SCD, these results will have clear implications on improving our understanding of the pathophysiology of hematologic disease.

RevDate: 2020-01-08
CmpDate: 2019-11-29

Shan X (2019)

Central-moment-based Galilean-invariant multiple-relaxation-time collision model.

Physical review. E, 100(4-1):043308.

Aiming at systematically correcting the non-Galilean-invariant thermal diffusivity in the previous multiple-relaxation-time Boltzmann equation collision model [Shan and Chen, Int. J. Mod. Phys. C 18, 635 (2007)IJMPEO0129-183110.1142/S0129183107010887], we show that by separately relaxing the central moments of the distribution function, Chapman-Enskog calculation leads to the correct hydrodynamic equations with mutually independent and Galilean invariant viscosity and thermal diffusivity, provided the velocity-space discretization preserves moments up to the fourth order. By transforming the central moments back to the absolute reference frame and evaluating using fixed discrete velocities, the efficient and accurate streaming-collision time-stepping algorithm is preserved. The lattice Boltzmann model is found to have excellent numerical stability in high-Reynolds-number simulations.

RevDate: 2020-01-08
CmpDate: 2019-11-29

Berera A, D Clark (2019)

Information production in homogeneous isotropic turbulence.

Physical review. E, 100(4-1):041101.

We study the Reynolds number scaling of the Kolmogorov-Sinai entropy and attractor dimension for three-dimensional homogeneous isotropic turbulence through the use of direct numerical simulation. To do so, we obtain Lyapunov spectra for a range of different Reynolds numbers by following the divergence of a large number of orthogonal fluid trajectories. We find that the attractor dimension grows with the Reynolds number as Re^{2.35} with this exponent being larger than predicted by either dimensional arguments or intermittency models. The distribution of Lyapunov exponents is found to be finite around λ≈0 contrary to a possible divergence suggested by Ruelle. The relevance of the Kolmogorov-Sinai entropy and Lyapunov spectra in comparing complex physical systems is discussed.

RevDate: 2020-01-08
CmpDate: 2019-11-29

Chitsaz M, M Fathali (2019)

Impact of an initial random magnetic field on the evolution of two-dimensional shearless mixing layers.

Physical review. E, 100(4-1):043106.

The impact of an initial random magnetic field on the temporal evolution of a two-dimensional incompressible turbulent shearless mixing layer is investigated using direct numerical simulation. Different intensities of the initial random magnetic field are imposed with uniform probability distribution on an identical flow field. The initial flow field condition is the turbulent shearless mixing layer with different kinetic energy ratio (E_{H}/E_{L}=6.7) and identical integral length scale. Simulations are carried out in a moderate magnetic Reynolds number, which causes a two-way interaction between the velocity and magnetic fields. In order to analyze the effect of the initial random magnetic field on the mixing characteristics, the intermittency inside the mixing layer and the mixing evolution parameters are investigated. It is found that with small initial magnetic field intensity, the intermittency in both large and small scales are larger than those values in hydrodynamic flow. However, increasing the intensity of the initial magnetic field reduces the intermittency in the mixing region to lower values compared to the hydrodynamic flow. The mixing layer growth rate and the mixing efficiency both show reduction by increasing the initial magnetic field intensity, which is attributed to the reduction of the averaged Reynolds number of both homogenous isotropic turbulent regions due to the suppressing effect of the Lorentz force on the velocity fields of these regions.

RevDate: 2020-01-11

Ćmiel AM, Strużyński A, Wyrębek M, et al (2020)

Response of freshwater mussel recruitment to hydrological changes in a eutrophic floodplain lake.

The Science of the total environment, 703:135467.

Although eutrophication of freshwaters is a natural process, the human impact often leads to inland waters becoming overloaded with nutrients, impoverishing many valuable and vanishing habitats, such as floodplain lakes. These changes need to be reversed if the occurrence of endangered aquatic species is to be restored. In this paper we analyse the impact of a change in the water regime of a naturally eutrophic floodplain lake, which harbours a large diversity of Unionidae (large freshwater mussels), a globally threatened taxonomic group that provides important ecosystem functions and services. We found that a slight increase in the discharge from this waterbody, following the construction of an additional outflow pipe, positively influenced recruitment in three of the five mussel species inhabiting the lake. We also found that, after the construction of this additional outflow, the niches of juveniles of Anodonta cygnea and Unio spp. changed, revealing differences in their hydrological requirements. Our results suggest that, as in lotic habitats, complex hydraulic parameters are highly significant to unionid mussels in lentic conditions.

RevDate: 2020-01-08

Taheri RA, Goodarzi V, A Allahverdi (2019)

Mixing Performance of a Cost-effective Split-and-Recombine 3D Micromixer Fabricated by Xurographic Method.

Micromachines, 10(11):.

This paper presents experimental and numerical investigations of a novel passive micromixer based on the lamination of fluid layers. Lamination-based mixers benefit from increasing the contact surface between two fluid phases by enhancing molecular diffusion to achieve a faster mixing. Novel three-dimensional split and recombine (SAR) structures are proposed to generate fluid laminations. Numerical simulations were conducted to model the mixer performance. Furthermore, experiments were conducted using dyes to observe fluid laminations and evaluate the proposed mixer's characteristics. Mixing quality was experimentally obtained by means of image-based mixing index (MI) measurement. The multi-layer device was fabricated utilizing the Xurography method, which is a simple and low-cost method to fabricate 3D microfluidic devices. Mixing indexes of 96% and 90% were obtained at Reynolds numbers of 0.1 and 1, respectively. Moreover, the device had an MI value of 67% at a Reynolds number of 10 (flow rate of 116 µL/min for each inlet). The proposed micromixer, with its novel design and fabrication method, is expected to benefit a wide range of lab-on-a-chip applications, due to its high efficiency, low cost, high throughput and ease of fabrication.

RevDate: 2019-11-18

Shaikh MM, Massan SU, AI Wagan (2019)

A sixteen decimal places' accurate Darcy friction factor database using non-linear Colebrook's equation with a million nodes: A way forward to the soft computing techniques.

Data in brief, 27:104733 pii:104733.

The Colebrook's equation is considered as an empirical model to accurately compute the Darcy friction factor in pipes under fully-developed turbulent flow. Due to non-linearity and implicitness of the Colebrook's equation, one needs to use numerical methods to acquire reasonably good approximation to the true friction factor values. However, such idea is not preferred by practitioners as it demands use of computers - also more computational time and effort. To overcome this, explicit equations that can describe Darcy friction factor directly in terms of the Reynolds number and relative roughness are essential. Using Fixed point iteration method in the MATLAB software, we have developed a 16 decimal places' accurate friction factor database for the Darcy friction factor for a 1000 by 1000 mesh of Reynolds number and relative roughness values. The accurate dataset described in this work will serve to be basis for the construction of new and more reliable explicit equations using regression modeling, artificial intelligence techniques and other soft computing methods.

RevDate: 2019-11-18

Papageorgiou DT, S Tanveer (2019)

Analysis and computations of a non-local thin-film model for two-fluid shear driven flows.

Proceedings. Mathematical, physical, and engineering sciences, 475(2230):20190367.

This paper is concerned with analysis and computations of a non-local thin-film model developed in Kalogirou & Papageorgiou (J. Fluid Mech.802, 5-36, 2016) for a perturbed two-layer Couette flow when the thickness of the more viscous fluid layer next to the stationary wall is small compared to the thickness of the less viscous fluid. Travelling wave solutions and their stability are determined numerically, and secondary bifurcation points are identified in the process. We also determine regions in parameter space where bistability is observed with two branches being linearly stable at the same time. The travelling wave solutions are mathematically justified through a quasi-solution analysis in a neighbourhood of an empirically constructed approximate solution. This relies in part on precise asymptotics of integrals of Airy functions for large wave numbers. The primary bifurcation about the trivial state is shown rigorously to be supercritical, and the dependence of bifurcation points, as a function of Reynolds number R and the primary wavelength 2πν-1/2 of the disturbance, is determined analytically.

RevDate: 2019-12-17

Farooq S, Hayat T, Khan MI, et al (2019)

Entropy generation minimization (EGM) in magneto peristalsis with variable properties.

Computer methods and programs in biomedicine, 186:105045 pii:S0169-2607(19)30711-4 [Epub ahead of print].

OBJECTIVE AND BACKGROUND: This article featuring the peristaltic transport of viscous material with variable properties (i.e. temperature dependent viscosity and thermal conductivity) through curved configuration. Fluid saturating through porous channel walls of uniform space. Entropy generation consideration here is to analyze irreversibility aspects. Channel boundaries retain the velocity and thermal slip conditions.

METHOD: Wave frame of reference is attained with the utilization of long wavelength and small Reynolds number approach. Solution of the simplified coupled system of dimensionless constraints is obtained numerically. Detailed analysis of important quantities of interest has been presented in discussion portion.

RESULTS: Entropy generation variation near center is very small whereas in the vicinity of the channel wall is larger. Bejan number has reverse variation as observed for entropy generation.

CONCLUSION: Variable characteristics of viscosity has opposite impact on velocity and temperature is observed. It is also noticed small irreversibility effects are obtained for higher varying viscosity and thermal conductivity near the vicinity of the channel walls.

RevDate: 2020-01-08

Ying Y, Y Lin (2019)

Inertial Focusing and Separation of Particles in Similar Curved Channels.

Scientific reports, 9(1):16575.

Inertial particle focusing in curved channels has enormous potential for lab-on-a-chip applications. This paper compares a zigzag channel, which has not been used previously for inertial focusing studies, with a serpentine channel and a square wave channel to explore their differences in terms of focusing performance and separation possibilities. The particle trajectories and fluid fields in the curved channels are studied by a numerical simulation. The effects of different conditions (structure, Reynolds number, and particle size) on the competition between forces and the focusing performance are studied. The results indicate that the zigzag channel has the best focusing effect at a high Reynolds number and that the serpentine channel is second in terms of performance. Regarding the particle separation potential, the zigzag channel has a good performance in separating 5 μm and 10 μm particles at ReC = 62.5. In addition, the pressure drop of the channel is also considered to evaluate the channel performance, which has not been taken into account in the literature on inertial microfluidics. This result is expected to be instructive for the selection and optimization of inertial microchannel structures.

RevDate: 2019-12-10

Zhang Z, P Zhang (2019)

Numerical Interpretation to the Roles of Liquid Viscosity in Droplet Spreading at Small Weber Numbers.

Langmuir : the ACS journal of surfaces and colloids, 35(49):16164-16171.

Droplet impacting a free-slip plane at small Weber numbers (We < 30) was numerically investigated by a front tracking method, with particular emphasis on clarifying the roles of the liquid viscosity and the "left-over" internal kinetic energy in droplet spreading. The most interesting discovery is that there exists a certain range of We in which the maximum diameter rate, D̃m, shows a nonmonotonic variation with the Reynolds number, Re. This non-monotonic variation is owing to the dual role of liquid viscosity in influencing droplet spreading. Specifically, when the initial surface energy is comparable to the initial kinetic energy (the corresponding We is around 10-30), the high strain rates of the droplet internal flow dominate its viscous dissipation at a relatively large Re, while the liquid viscosity dominates the viscous dissipation at a relatively small Re. Furthermore, to unravel the influence of droplet attachment and detachment on droplet spreading, we considered two limiting situations such as full attachment (with no gas film throughout droplet spreading) and full detachment (with a gas film throughout droplet spreading). The results show that the droplet with a gas film tends to generate a stronger vortical motion in its rim, results in a larger left-over kinetic energy, and hence causes a smaller spreading.

RevDate: 2020-01-08

Kottmeier J, Wullenweber M, Blahout S, et al (2019)

Accelerated Particle Separation in a DLD Device at Re > 1 Investigated by Means of µPIV.

Micromachines, 10(11):.

A pressure resistant and optically accessible deterministic lateral displacement (DLD) device was designed and microfabricated from silicon and glass for high-throughput fractionation of particles between 3.0 and 7.0 µm comprising array segments of varying tilt angles with a post size of 5 µm. The design was supported by computational fluid dynamic (CFD) simulations using OpenFOAM software. Simulations indicated a change in the critical particle diameter for fractionation at higher Reynolds numbers. This was experimentally confirmed by microparticle image velocimetry (µPIV) in the DLD device with tracer particles of 0.86 µm. At Reynolds numbers above 8 an asymmetric flow field pattern between posts could be observed. Furthermore, the new DLD device allowed successful fractionation of 2 µm and 5 µm fluorescent polystyrene particles at Re = 0.5-25.

RevDate: 2019-12-07

Di D, Qu X, Liu C, et al (2019)

Continuous production of celecoxib nanoparticles using a three-dimensional-coaxial-flow microfluidic platform.

International journal of pharmaceutics, 572:118831.

Increasing the dissolution rate of water insoluble drugs by decreasing the particle size of the drugs into nano-size is a well-known strategy. However, continuous production of drug nanoparticles with uniform particle size is critical for clinical application of the strategy. Here we report a simple microfluidic mixing method that can achieve continuous production of celecoxib nanoparticles with uniform particle size and high dissolution rate. A three-dimensional (3D) coaxial-flow microfluidic device was fabricated by assembling two coaxial aligned borosilicate glass capillaries on a glass slide, and a tapered glass capillary was inserted into another bigger cylindrical one with coaxial alignment. Celecoxib nanoparticles were prepared by the microfluidic device under the turbulent jet regime. The 3D-coaxial-flow pattern and high Reynolds number ensured the extremely short mixing time, consequently, resulted in the high throughput production of drug nanoparticles with uniform particle size. The obtained nanoparticles were spherical in shape, and showed superior dissolution rate compared with the coarse powder both in sink and non-sink conditions. The bioavailability of the water insoluble drug was also significantly improved by the reduction of particle size into nano-size.

RevDate: 2019-11-10

Herrmann N, Neubauer P, M Birkholz (2019)

Spiral microfluidic devices for cell separation and sorting in bioprocesses.

Biomicrofluidics, 13(6):061501.

Inertial microfluidic systems have been arousing interest in medical applications due to their simple and cost-efficient use. However, comparably small sample volumes in the microliter and milliliter ranges have so far prevented efficient applications in continuous bioprocesses. Nevertheless, recent studies suggest that these systems are well suited for cell separation in bioprocesses because of their facile adaptability to various reactor sizes and cell types. This review will discuss potential applications of inertial microfluidic cell separation systems in downstream bioprocesses and depict recent advances in inertial microfluidics for bioprocess intensification. This review thereby focusses on spiral microchannels that separate particles at a moderate Reynolds number in a laminar flow (Re < 2300) according to their size by applying lateral hydrodynamic forces. Spiral microchannels have already been shown to be capable of replacing microfilters, extracting dead cells and debris in perfusion processes, and removing contaminant microalgae species. Recent advances in parallelization made it possible to process media on a liter-scale, which might pave the way toward industrial applications.

RevDate: 2019-11-25

Sharzehee M, Chang Y, Song JP, et al (2019)

Hemodynamic effects of myocardial bridging in patients with hypertrophic cardiomyopathy.

American journal of physiology. Heart and circulatory physiology, 317(6):H1282-H1291.

Myocardial bridging (MB) is linked to angina and myocardial ischemia and may lead to sudden cardiac death in patients with hypertrophic cardiomyopathy (HCM). However, it remains unclear how MB affect the coronary blood flow in HCM patients. The aim of this study was to assess the effects of MB on coronary hemodynamics in HCM patients. Fifteen patients with MB (7 HCM and 8 non-HCM controls) in their left anterior descending (LAD) coronary artery were chosen. Transient computational fluid dynamics (CFD) simulations were conducted in anatomically realistic models of diseased (with MB) and virtually healthy (without MB) LAD from these patients, reconstructed from biplane angiograms. Our CFD simulation results demonstrated that dynamic compression of MB led to diastolic flow disturbances and could significantly reduce the coronary flow in HCM patients as compared with non-HCM group (P < 0.01). The pressure drop coefficient was remarkably higher (P < 0.05) in HCM patients. The flow rate change is strongly correlated with both upstream Reynolds number and MB compression ratio, while the MB length has less impact on coronary flow. The hemodynamic results and clinical outcomes revealed that HCM patients with an MB compression ratio higher than 65% required a surgical intervention. In conclusion, the transient MB compression can significantly alter the diastolic flow pattern and wall shear stress distribution in HCM patients. HCM patients with severe MB may need a surgical intervention.NEW & NOTEWORTHY In this study, the hemodynamic significance of myocardial bridging (MB) in patients with hypertrophic cardiomyopathy (HCM) was investigated to provide valuable information for surgical decision-making. Our results illustrated that the transient MB compression led to complex flow patterns, which can significantly alter the diastolic flow and wall shear stress distribution. The hemodynamic results and clinical outcomes demonstrated that patients with HCM and an MB compression ratio higher than 65% required a surgical intervention.

RevDate: 2020-01-08

Shanko ES, van de Burgt Y, Anderson PD, et al (2019)

Microfluidic Magnetic Mixing at Low Reynolds Numbers and in Stagnant Fluids.

Micromachines, 10(11):.

Microfluidic mixing becomes a necessity when thorough sample homogenization is required in small volumes of fluid, such as in lab-on-a-chip devices. For example, efficient mixing is extraordinarily challenging in capillary-filling microfluidic devices and in microchambers with stagnant fluids. To address this issue, specifically designed geometrical features can enhance the effect of diffusion and provide efficient mixing by inducing chaotic fluid flow. This scheme is known as "passive" mixing. In addition, when rapid and global mixing is essential, "active" mixing can be applied by exploiting an external source. In particular, magnetic mixing (where a magnetic field acts to stimulate mixing) shows great potential for high mixing efficiency. This method generally involves magnetic beads and external (or integrated) magnets for the creation of chaotic motion in the device. However, there is still plenty of room for exploiting the potential of magnetic beads for mixing applications. Therefore, this review article focuses on the advantages of magnetic bead mixing along with recommendations on improving mixing in low Reynolds number flows (Re ≤ 1) and in stagnant fluids.

RevDate: 2019-12-02

Ijaz Khan M, Ali A, Hayat T, et al (2019)

Entropy optimized dissipative CNTs based flow with probable error and statistical declaration.

Computer methods and programs in biomedicine, 185:105137 pii:S0169-2607(19)31706-7 [Epub ahead of print].

BACKGROUND: CNTs are categorized subject to their structures i.e., SWCNTs (single wall nanotubes), DWCNTs (double wall nanotubes) and MWCNTs (multi-wall nanotubes). The various structures have distinct characteristics that make the nanotubes suitable for various physical applications. It is due their unique electrical, mechanical and thermal attributes CNTs present thrilling opportunities for mechanical engineering, industrial, scientific research and commercial applications. There is fruitful potential for carbon nanotubes in the composites business and industry. Today, CNTs find utilization in frequent various products, and analyst continue to explore new applications. Currently applications comprise wind turbines, bicycle components, scanning probe microscopes, flat panel displays, marine paints, sensing devices, electronics, batteries with longer lifetime and electrical circuitry etc. Such applications in mind, entropy optimized dissipative CNTs based flow of nanomaterial by a stretched surface. Flow is caused due to stretching phenomenon and studied in 3D coordinates. Both types of CNTs are studied i.e., SWCNTs and MWCNTs. CNTs are considered for nanoparticles and water for continuous phase fluid. Special consideration is given to the analysis of statistical declaration and probable error for skin friction and Nusselt number. Furthermore, entropy rate is calculated. Entropy rate is discussed in the presence of four main irreversibilities i.e., heat transfer, Joule heating, porosity and dissipation.

METHOD: Homotopy technique is utilized to develop the convergence series solutions.

RESULTS: Impacts of sundry variables subject to both SWCNTs (single) and MWCNTs (multi) are graphically discussed. Statistical analysis and probable error for surface drag force and Nusselt number are numerically calculated subject to various flow variables. Numerical results for such engineering quantities are displayed through tables. In addition, comparative analysis for SWCNTs and MWCNTs are presented for the velocity, concentration and thermal fields.

CONCLUSIONS: Results for entropy rate is calculated in the presence of various sundry variable through implementation of second law of thermodynamics. It is examined from the results that velocity decreases for both CNTs via higher magnetic, inertia coefficient and porosity parameters. Secondary velocity i.e., velocity in g-direction boosts up versus rotation parameter while it declines for larger slip parameter for both CNTs. thermal field intensifies for both CNTs via larger heat generation/absorption parameter. Concentration which shows the mass transfer of species increases subject to higher homogeneous parameter and Schmidt number in case of both CNTs. Entropy rate in more for larger magnetic, Reynolds number and slip parameter. Bejan number boosts up for higher Reynold number and slip parameter while it declines for magnetic parameter.

RevDate: 2020-01-08

Dincau B, Dressaire E, A Sauret (2019)

Pulsatile Flow in Microfluidic Systems.

Small (Weinheim an der Bergstrasse, Germany) [Epub ahead of print].

This review describes the current knowledge and applications of pulsatile flow in microfluidic systems. Elements of fluid dynamics at low Reynolds number are first described in the context of pulsatile flow. Then the practical applications in microfluidic processes are presented: the methods to generate a pulsatile flow, the generation of emulsion droplets through harmonic flow rate perturbation, the applications in mixing and particle separation, and the benefits of pulsatile flow for clog mitigation. The second part of the review is devoted to pulsatile flow in biological applications. Pulsatile flows can be used for mimicking physiological systems, to alter or enhance cell cultures, and for bioassay automation. Pulsatile flows offer unique advantages over a steady flow, especially in microfluidic systems, but also require some new physical insights and more rigorous investigation to fully benefit future applications.

RevDate: 2019-11-01

Folkersma M, Schmehl R, A Viré (2019)

Boundary layer transition modeling on leading edge inflatable kite airfoils.

Wind energy (Chichester, England), 22(7):908-921.

We present a computational fluid dynamic analysis of boundary layer transition on leading edge inflatable kite airfoils used for airborne wind energy generation. Because of the operation in pumping cycles, the airfoil is generally subject to a wide range of Reynolds numbers. The analysis is based on the combination of the shear stress transport turbulence model with the γ - R ˜ e θ t transition model, which can handle the laminar boundary layer and its transition to turbulence. The implementation of both models in OpenFOAM is described. We show a validation of the method for a sailwing (ie, a wing with a membrane) airfoil and an application to a leading edge inflatable kite airfoil. For the sailwing airfoil, the results computed with transition model agree well with the existing low Reynolds number experiment over the whole range of angles of attack. For the leading edge inflatable kite airfoil, the transition modeling has both favorable and unfavorable effects on the aerodynamics. On the one hand, the aerodynamics suffer from the laminar separation. But, on the other hand, the laminar boundary layer thickens slower than the turbulent counterpart, which, in combination with transition, delays the separation. The results also indicate that the aerodynamics of the kite airfoil could be improved by delaying the boundary layer transition during the traction phase and tripping the transition in the retraction phase.

RevDate: 2019-12-20

Ozden K, Sert C, Y Yazicioglu (2020)

Numerical investigation of wall pressure fluctuations downstream of concentric and eccentric blunt stenosis models.

Proceedings of the Institution of Mechanical Engineers. Part H, Journal of engineering in medicine, 234(1):48-60.

Pressure fluctuations that cause acoustic radiation from vessel models with concentric and eccentric blunt stenoses are investigated. Large eddy simulations of non-pulsatile flow condition are performed using OpenFOAM. Calculated amplitude and spatial-spectral distribution of acoustic pressures at the post-stenotic region are compared with previous experimental and theoretical results. It is found that increasing the Reynolds number does not change the location of the maximum root mean square wall pressure, but causes a general increase in the spectrum level, although the change in the shape of the spectrum is not significant. On the contrary, compared to the concentric model at the same Reynolds number, eccentricity leads to an increase both at the distance of the location of the maximum root mean square wall pressure from the stenosis exit and the spectrum level. This effect becomes more distinct when radial eccentricity of the stenosis increases. Both the flow rate and the eccentricity of the stenosis shape are evaluated to be clinically important parameters in diagnosing stenosis.

RevDate: 2020-01-08

Su L, Duan Z, He B, et al (2019)

Thermally Developing Flow and Heat Transfer in Elliptical Minichannels with Constant Wall Temperature.

Micromachines, 10(10):.

Laminar convective heat transfer of elliptical minichannels is investigated for hydrodynamically fully developed but thermal developing flow with no-slip condition. A three-dimensional numerical model is developed in different elliptical geometries with the aspect ratio varying from 0.2 to 1. The effect of Reynolds number (25 ≤ Re ≤ 2000) on the local Nusselt number is examined in detail. The results indicate that the local Nusselt number is a decreasing function of Reynolds number and it is sensitive to Reynolds number especially for Re less than 250. The effect of aspect ratio on local Nusselt number is small when compared with the effect of Reynolds number on local Nusselt number. The local Nusselt number is independent of cross-section geometry at the inlet. The maximum effect of aspect ratio on local Nusselt number arises at the transition section rather than the fully developed region. However, the non-dimensional thermal entrance length is a monotonic decreasing concave function of aspect ratio but a weak function of Reynolds number. Correlations for the local Nusselt number and the thermal developing length for elliptical channels are developed with good accuracy, which may provide guidance for design and optimization of elliptical minichannel heat sinks.

RevDate: 2020-01-08

Raza W, Ma SB, KY Kim (2019)

Single and Multi-Objective Optimization of a Three-Dimensional Unbalanced Split-and-Recombine Micromixer.

Micromachines, 10(10):.

The three-dimensional geometry of a micromixer with an asymmetrical split-and-recombine mechanism was optimized to enhance the fluid-mixing capability at a Reynolds number of 20. Single and multi-objective optimizations were carried out by using particle swarm optimization and a genetic algorithm on a modeled surrogate surface. Surrogate modeling was performed using the computational results for the mixing. Mixing and flow analyses were carried out by solving the convection-diffusion equation in combination with the three-dimensional continuity and momentum equations. The optimization was carried out with two design variables related to dimensionless geometric parameters. The mixing effectiveness was chosen as the objective function for the single-objective optimization, and the pressure drop and mixing index at the outlet were chosen for the multi-objective optimization. The sampling points in the design space were determined using a design of experiment technique called Latin hypercube sampling. The surrogates for the objective functions were developed using a Kriging model. The single-objective optimization resulted in 58.9% enhancement of the mixing effectiveness compared to the reference design. The multi-objective optimization provided Pareto-optimal solutions that showed a maximum increase of 48.5% in the mixing index and a maximum decrease of 55.0% in the pressure drop in comparison to the reference design.

RevDate: 2020-01-08
CmpDate: 2019-10-28

Basu A, JK Bhattacharjee (2019)

Kolmogorov or Bolgiano-Obukhov scaling: Universal energy spectra in stably stratified turbulent fluids.

Physical review. E, 100(3-1):033117.

We set up the scaling theory for stably stratified turbulent fluids. For a system having infinite extent in the horizontal directions, but with a finite width in the vertical direction, this theory predicts that the inertial range can display three possible scaling behavior, which are essentially parametrized by the buoyancy frequency N, or dimensionless horizontal Froude number F_{h}, and the vertical length scale l_{v} that sets the scale of variation of the velocity field in the vertical direction for a fixed Reynolds number. For very low N or very high Re_{b} or F_{h}, and with l_{v}≫l_{h}, the typical horizontal length scale, buoyancy forces are irrelevant and hence, unsurprisingly, the kinetic energy spectra show the well-known K41 scaling in the inertial range. In this regime, the local temperature behaves as a passively advected scalar, without any effect on the flow fields. For intermediate ranges of values of N,F_{h}∼O(1), corresponding to moderate stratification, buoyancy forces are important enough to affect the scaling. This leads to the Bolgiano-Obukhov scaling which is isotropic, when l_{v}∼l_{h}. Finally, for very large N, corresponding to strong stratification, together with a very small l_{v}, the inertial-range flow fields effectively two-dimensionalize. The kinetic energy spectra are predicted to be anisotropic with only the horizontal part of the kinetic energy spectra following the K41 scaling. This suggests an intriguing re-entrant K41 scaling, as a function of stratification, for the horizontal components of the velocity field in this regime. The scaling theory further predicts the scaling of the thermal energy in each of these three scaling regimes. Our theory can be tested in large-scale simulations and appropriate laboratory-based experiments.

RevDate: 2020-01-08

Bae HJ, Lozano-Durán A, Bose ST, et al (2019)

Dynamic slip wall model for large-eddy simulation.

Journal of fluid mechanics, 859:400-432.

Wall modelling in large-eddy simulation (LES) is necessary to overcome the prohibitive near-wall resolution requirements in high-Reynolds-number turbulent flows. Most existing wall models rely on assumptions about the state of the boundary layer and require a priori prescription of tunable coefficients. They also impose the predicted wall stress by replacing the no-slip boundary condition at the wall with a Neumann boundary condition in the wall-parallel directions while maintaining the no-transpiration condition in the wall-normal direction. In the present study, we first motivate and analyse the Robin (slip) boundary condition with transpiration (non-zero wall-normal velocity) in the context of wall-modelled LES. The effect of the slip boundary condition on the one-point statistics of the flow is investigated in LES of turbulent channel flow and a flat-plate turbulent boundary layer. It is shown that the slip condition provides a framework to compensate for the deficit or excess of mean momentum at the wall. Moreover, the resulting non-zero stress at the wall alleviates the well-known problem of the wall-stress under-estimation by current subgrid-scale (SGS) models (Jiménez & Moser, AIAA J., vol. 38 (4), 2000, pp. 605-612). Second, we discuss the requirements for the slip condition to be used in conjunction with wall models and derive the equation that connects the slip boundary condition with the stress at the wall. Finally, a dynamic procedure for the slip coefficients is formulated, providing a dynamic slip wall model free of a priori specified coefficients. The performance of the proposed dynamic wall model is tested in a series of LES of turbulent channel flow at varying Reynolds numbers, non-equilibrium three-dimensional transient channel flow and a zero-pressure-gradient flat-plate turbulent boundary layer. The results show that the dynamic wall model is able to accurately predict one-point turbulence statistics for various flow configurations, Reynolds numbers and grid resolutions.

RevDate: 2020-01-08

Lozano-Durán A, HJ Bae (2019)

Error scaling of large-eddy simulation in the outer region of wall-bounded turbulence.

Journal of computational physics, 392:532-555.

We study the error scaling properties of large-eddy simulation (LES) in the outer region of wall-bounded turbulence at moderately high Reynolds numbers. In order to avoid the additional complexity of wall-modeling, we perform LES of turbulent channel flows in which the no-slip condition at the wall is replaced by a Neumann condition supplying the exact mean wall-stress. The statistics investigated are the mean velocity profile, turbulence intensities, and kinetic energy spectra. The errors follow (Δ / L) α R e τ γ , where Δ is the characteristic grid resolution, Reτ is the friction Reynolds number, and L is the meaningful length-scale to normalize Δ in order to collapse the errors across the wall-normal distance. We show that Δ can be expressed as the L2-norm of the grid vector and that L is well represented by the ratio of the friction velocity and mean shear. The exponent α is estimated from theoretical arguments for each statistical quantity of interest and shown to roughly match the values computed by numerical simulations. For the mean profile and kinetic energy spectra, α ≈ 1, whereas the turbulence intensities converge at a slower rate α < 1. The exponent γ is approximately 0, i.e. the LES solution is independent of the Reynolds number. The expected behavior of the turbulence intensities at high Reynolds numbers is also derived and shown to agree with the classic log-layer profiles for grid resolutions lying within the inertial range. Further examination of the LES turbulence intensities and spectra reveals that both quantities resemble their filtered counterparts from direct numerical simulation (DNS) data, but that the mechanism responsible for this similarity is related to the balance between the input power and dissipation rather than to filtering.

RevDate: 2020-01-07

Hayat T, Waqar Ahmad M, Ijaz Khan M, et al (2019)

Entropy optimization in CNTs based nanomaterial flow induced by rotating disks: A study on the accuracy of statistical declaration and probable error.

Computer methods and programs in biomedicine, 184:105105 pii:S0169-2607(19)31590-1 [Epub ahead of print].

BACKGROUND: CNTs (Carbon nanotubes) being allotropes of carbon, made of graphene and diameters of single and multi-walls carbon nanotubes are typically 0.8 to 2 nm and 5 to 20 mn, although diameter of MWCNTs can exceed 100 nm. Carbon nanotubes lengths range from less than 100 nm to 0.5 m. Their impressive structural, electronic and mechanical attributes subject to their small size and mass, their high electrical and thermal conductivities, and their strong mechanical potency. CNTs based materials are successfully applied in medicine and pharmacy subject to their huge surface area that is proficient of conjugating or adsorbing with a wide variety of genes, drugs, antibodies, vaccines and biosensors etc. Therefore, we have presented a theoretical study about mathematical modeling of CNTs based viscous material flow between two rotating disks. Both types of nanotubes i.e., SWCNTs and MWCNTs are considered. Xue model is used for the mathematical modeling. Fluid flow is due to rotating disks. Main focus here is given to probable error and statistical declaration. Entropy is calculated for both single and multi-walls nanotubes.

METHOD: Nonlinear PDEs are first converted into ODEs and then computed for homotopy convergent solutions.

RESULTS AND CONCLUSION: Statistical declaration and probable error for skin friction and Nusselt number are numerically computed and discussed through Tables. From obtained outcomes it is concluded that magnitude of skin friction increases at both disks surface for higher values of Reynolds number, lower stretching parameter and porosity parameter while it decays for both of disks versus larger rotation parameter. Nusselt number or heat transfer rate also enhances at both disks in the presence of radiation and Reynolds number while it decays against Eckert number.

RevDate: 2020-01-08
CmpDate: 2019-12-02

Bukowicki M, ML Ekiel-Jeżewska (2019)

Sedimenting pairs of elastic microfilaments.

Soft matter, 15(46):9405-9417.

The dynamics of two identical elastic filaments settling under gravity in a viscous fluid in the low Reynolds number regime is investigated numerically. A large family of initial configurations symmetric with respect to a vertical plane is considered, as well as their non-symmetric perturbations. The behaviour of the filaments is primarily governed by the elasto-gravitational number, which depends on the filament's length and flexibility, and the strength of the external force. Flexible filaments usually converge toward horizontal and parallel orientation. We explain this phenomenon and show that it occurs also for curved rigid particles of similar shapes. Once aligned, the two fibres either converge toward a stationary, flexibility-dependent distance, or tend to collide or continuously repel each other. Rigid and straight rods perform periodic motions while settling down. Apart from very stiff particles, the dynamics is robust to non-symmetric perturbations.

RevDate: 2019-10-23

Holdenried-Chernoff D, Chen L, A Jackson (2019)

A trio of simple optimized axisymmetric kinematic dynamos in a sphere.

Proceedings. Mathematical, physical, and engineering sciences, 475(2229):20190308.

Planetary magnetic fields are generated by the motion of conductive fluid in the planet's interior. Complex flows are not required for dynamo action; simple flows have been shown to act as efficient kinematic dynamos, whose physical characteristics are more straightforward to study. Recently, Chen et al. (2018, J. Fluid Mech.839, 1-32. (doi:10.1017/jfm.2017.924)) found the optimal, unconstrained kinematic dynamo in a sphere, which, despite being of theoretical importance, is of limited practical use. We extend their work by restricting the optimization to three simple two-mode axisymmetric flows based on the kinematic dynamos of Dudley & James (1989, Proc. R. Soc. Lond. A425, 407-429. (doi:10.1098/rspa.1989.0112)). Using a Lagrangian optimization, we find the smallest critical magnetic Reynolds number for each flow type, measured using an enstrophy-based norm. A Galerkin method is used, in which the spectral coefficients of the fluid flow and magnetic field are updated in order to maximize the final magnetic energy. We consider the t01s01, t01s02 and t02s02 flows and find enstrophy-based critical magnetic Reynolds numbers of 107.7, 142.4 and 125.5 (13.7, 19.6 and 16.4, respectively, with the energy-based definition). These are up to four times smaller than the original flows. These simple and efficient flows may be used as benchmarks in future studies.

RevDate: 2020-01-08

Browne CA, Shih A, SS Datta (2019)

Pore-Scale Flow Characterization of Polymer Solutions in Microfluidic Porous Media.

Small (Weinheim an der Bergstrasse, Germany) [Epub ahead of print].

Polymer solutions are frequently used in enhanced oil recovery and groundwater remediation to improve the recovery of trapped nonaqueous fluids. However, applications are limited by an incomplete understanding of the flow in porous media. The tortuous pore structure imposes both shear and extension, which elongates polymers; moreover, the flow is often at large Weissenberg numbers, Wi, at which polymer elasticity in turn strongly alters the flow. This dynamic elongation can even produce flow instabilities with strong spatial and temporal fluctuations despite the low Reynolds number, Re. Unfortunately, macroscopic approaches are limited in their ability to characterize the pore-scale flow. Thus, understanding how polymer conformations, flow dynamics, and pore geometry together determine these nontrivial flow patterns and impact macroscopic transport remains an outstanding challenge. This review describes how microfluidic tools can shed light on the physics underlying the flow of polymer solutions in porous media at high Wi and low Re. Specifically, microfluidic studies elucidate how steady and unsteady flow behavior depends on pore geometry and solution properties, and how polymer-induced effects impact nonaqueous fluid recovery. This work thus provides new insights for polymer dynamics, non-Newtonian fluid mechanics, and applications such as enhanced oil recovery and groundwater remediation.

RevDate: 2019-10-23

Houba T, Dasgupta A, Gopalakrishnan S, et al (2019)

Supersonic turbulent flow simulation using a scalable parallel modal discontinuous Galerkin numerical method.

Scientific reports, 9(1):14442 pii:10.1038/s41598-019-50546-w.

The scalability and efficiency of numerical methods on parallel computer architectures is of prime importance as we march towards exascale computing. Classical methods like finite difference schemes and finite volume methods have inherent roadblocks in their mathematical construction to achieve good scalability. These methods are popularly used to solve the Navier-Stokes equations for fluid flow simulations. The discontinuous Galerkin family of methods for solving continuum partial differential equations has shown promise in realizing parallel efficiency and scalability when approaching petascale computations. In this paper an explicit modal discontinuous Galerkin (DG) method utilizing Implicit Large Eddy Simulation (ILES) is proposed for unsteady turbulent flow simulations involving the three-dimensional Navier-Stokes equations. A study of the method was performed for the Taylor-Green vortex case at a Reynolds number ranging from 100 to 1600. The polynomial order P = 2 (third order accurate) was found to closely match the Direct Navier-Stokes (DNS) results for all Reynolds numbers tested outside of Re = 1600, which had a normalized RMS error of 3.43 × 10-4 in the dissipation rate for a 603 element mesh. The scalability and performance study of the method was then conducted for a Reynolds number of 1600 for polynomials orders from P = 2 to P = 6. The highest order polynomial that was tested (P = 6) was found to have the most efficient scalability using both the MPI and OpenMP implementations.

RevDate: 2019-10-08

Porteous R, Moreau DJ, CJ Doolan (2019)

The effect of the incoming boundary layer thickness on the aeroacoustics of finite wall-mounted square cylinders.

The Journal of the Acoustical Society of America, 146(3):1808.

This paper is concerned with the influence of the incoming wall boundary layer thickness on the noise produced by a square finite wall-mounted cylinder in cross-flow. Acoustic and near wake velocity measurements have been taken in an anechoic wind tunnel for a cylinder in two different near-zero-pressure gradient turbulent boundary layers with thicknesses of 130% and 370% of the cylinder width, W. The cylinders have an aspect ratio of 0.29≤L/W≤22.9 (where L is the cylinder span) and were examined at a Reynolds number, based on width, of ReW = 1.4 × 104. The results presented in this paper demonstrate that increasing the height of the boundary layer delays the production of acoustic tones to higher aspect ratios. The height of the boundary layer changes the balance between upwash and downwash across the cylinder span, resulting in a delayed onset of the shedding regimes and correspondingly, the production of acoustic tones.

RevDate: 2019-11-26

Kawaguchi M, Fukui T, Funamoto K, et al (2019)

Viscosity Estimation of a Suspension with Rigid Spheres in Circular Microchannels Using Particle Tracking Velocimetry.

Micromachines, 10(10):.

Suspension flows are ubiquitous in industry and nature. Therefore, it is important to understand the rheological properties of a suspension. The key to understanding the mechanism of suspension rheology is considering changes in its microstructure. It is difficult to evaluate the influence of change in the microstructure on the rheological properties affected by the macroscopic flow field for non-colloidal particles. In this study, we propose a new method to evaluate the changes in both the microstructure and rheological properties of a suspension using particle tracking velocimetry (PTV) and a power-law fluid model. Dilute suspension (0.38%) flows with fluorescent particles in a microchannel with a circular cross section were measured under low Reynolds number conditions (Re ≈ 10-4). Furthermore, the distribution of suspended particles in the radial direction was obtained from the measured images. Based on the power-law index and dependence of relative viscosity on the shear rate, we observed that the non-Newtonian properties of the suspension showed shear-thinning. This method will be useful in revealing the relationship between microstructural changes in a suspension and its rheology.

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