• Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-21
Stergios Katsanoulis, Mohammad Farazmand, Mattia Serra, and George Haller
更新日期：2020-02-21
• Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-21
Nazmi Burak Budanur, Elena Marensi, Ashley P. Willis, and Björn Hof
更新日期：2020-02-21
• Phys. Rev. Fluids (IF 2.442) Pub Date :
I. V. Kolokolov, L. L. Ogorodnikov, and S. S. Vergeles

It is known that the turbulence in a fast rotating volume becomes effectively two-dimensional one. The latter is characterized by inverse energy cascade leading to formation of coherent flow in finite systems. In rotating three-dimensional vessel this flow has the form of columnar vortices. Here we develop analytical theory describing interaction of the vortex with turbulent pulsations. This interaction results in energy transfer from small scale eddies to the large scale vortex. We derive the equation for radial velocity profile of the vortex and solve it for simplest boundary conditions. We indicate the domain of physical parameters where our theory works.

更新日期：2020-02-21
• Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-20
F. Tuerke, F. Lusseyran, D. Sciamarella, L. Pastur, and G. Artana
更新日期：2020-02-20
• Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-20
Antoine B. Blanchard and Arne J. Pearlstein
更新日期：2020-02-20
• Phys. Rev. Fluids (IF 2.442) Pub Date :
Aditya S. Khair and Jason K. Kabarowski

The motion of a spherical particle undergoing electrophoresis in weakly inertial or viscoelastic shear flow is quantified via asymptotic analysis. We are motivated by several experimental studies reporting cross-streamline migration of electrophoretic colloids in Poiseuille microchannel flow. Specifically, particles migrate in a Newtonian liquid to the center(walls) of a channel when their electrophoretic velocity is in the opposite(same) direction to(as) the flow. Here, we calculate that weak fluid inertia causes a leading-order cross-streamline lift force of magnitude $5.50\veps|\zeta|a^3\rho\dot\gamma E^{\infty}/\mu$ for electrophoresis along the velocity axis of an unbounded simple shear flow, where ζ and a denote the particle zeta potential and radius, respectively; ρ, μ, and $\veps$ are the fluid density, viscosity, and permittivity, respectively; γ̇ is the shear rate of the ambient flow; and E∞ is the strength of the imposed electric field. This force acts to propel the sphere to shear streamlines that, in a frame translating with the particle, are directed reverse to the electrophoretic motion, which is consistent with the above-mentioned experiments. Other recent experiments have observed migration of electrophoretic particles in Poiseuille flow of a viscoelastic polymer solution: the migration direction is opposite to that in a Newtonian liquid. Here, we calculate a leading-order cross-streamline lift force of magnitude $7.07(1-3.33\Psi_2/\Psi_1)\veps|\zeta|a \Psi_1\dot\gamma E^{\infty}/\mu$ for electrophoresis in simple shear flow of a second-order fluid, where Ψ1 and Ψ2 are the first and second normal stress coefficients, respectively. The lift is toward streamlines moving in the direction of electrophoresis for _2/_1 更新日期：2020-02-20 • Phys. Rev. Fluids (IF 2.442) Pub Date : Félicien Bonnefoy, Alexey Tikan, Francois Copie, Pierre Suret, Guillaume Ducrozet, Gaurav Prabhudesai, Guillaume Michel, Annette Cazaubiel, Eric Falcon, Gennady El, and Stéphane Randoux We report water wave experiments performed in a long tank where we consider the evolution of nonlinear deep-water surface gravity waves with the envelope in the form of a large-scale rectangular barrier. Our experiments reveal that, for a range of initial parameters, the nonlinear wave packet is not disintegrated by the Benjamin-Feir instability but exhibits a specific, strongly nonlinear modulation, which propagates from the edges of the wavepacket towards the center with finite speed. Using numerical tools of nonlinear spectral analysis of experimental data we identify the observed envelope wave structures with focusing dispersive dam break flows, a peculiar type of dispersive shock waves recently described in the framework of the semi-classical limit of the 1D focusing nonlinear Schr"odinger equation (1D-NLSE). Our experimental results are shown to be in a good quantitative agreement with the predictions of the semi-classical 1D-NLSE theory. This is the first observation of the persisting dispersive shock wave dynamics in a modulationally unstable water wave system. 更新日期：2020-02-20 • Phys. Rev. Fluids (IF 2.442) Pub Date : Biao Shen, Jiewei Liu, Gustav Amberg, Minh Do-Quang, Junichiro Shiomi, Koji Takahashi, and Yasuyuki Takata Enhancement of boiling heat transfer on biphilic (mixed-wettability) surfaces faces a sudden reversal at low pressures, which is brought about by excessive contact-line spreading across the wetting heterogeneities. We employ the diffuse-interface approach to numerically study bubble expansion on a heating surface that consists of opposing wettabilities. The results show a dramatic shift in the dynamics of traversing contact line across the wettability divide under different gravities, which correspond to variable bubble growth rates. Specifically, it is found that the contact-line propagation tends to follow closely the rapidly expanding bubble at low gravity, with only a brief interruption at the border between the hydrophobic and hydrophilic sections of the surface. Only when the bubble growth becomes sufficiently weakened at high gravity does the contact line get slowed down drastically to the point of being nearly immobilized at the edge of the hydrophilic surface. The following bubble expansion, which faces strong limitations in the direction parallel to the surface, features a consistent apparent contact angle around 66.4o, regardless of the wettability combination. A simple theoretical model based on the force balance analysis is proposed to describe the physical mechanism behind such a dramatic transition in the contact-line behavior. 更新日期：2020-02-20 • Phys. Rev. Fluids (IF 2.442) Pub Date : Christopher R. Anthony, Michael T. Harris, and Osman A. Basaran When two drops are slowly brought together and first touch, a microscopic liquid neck or a bridge forms between them. The expansion of the neck is controlled by the capillary (Laplace) pressure which diverges when the curvature of the interface is infinite at the point where the drops first touch. The change in topology and the flows that ensue as time advances and the bridge grows from microscopic to macroscopic scales, and the two drops merge into one, are intimately coupled to this singularity in the dynamics. Despite the large volume of work dedicated to this problem, currently experiment, theory, and computation are not in complete agreement with respect to the earliest times following the initial contact of the two drops. Experiments, supported by simulations, report an initial regime where the radius of the connecting bridge grows linearly in time before a transition to either a Stokes regime or an inertial regime where either viscous or inertial force balances capillary (surface tension) force. In the initial linear regime, referred to as the inertially-limited viscous (ILV) regime, all three forces are thought to be important. This is in contrast to theory which predicts that all coalescence events begin in the Stokes regime where inertia is negligible. Here, we use high accuracy numerical simulations to show that the ILV regime is only realized when the two coalescing drops are initially separated by a finite distance. Moreover, for two drops that initially just touch at a point, coalescence always begins in the Stokes regime. It is demonstrated that the linear ILV regime is more akin to a Taylor-Culick type regime whose existence and duration are purely consequences of the use of an initial bridge of finite size that poorly approximates the point contact condition that is a cardinal feature of the coalescence singularity. 更新日期：2020-02-20 • Phys. Rev. Fluids (IF 2.442) Pub Date : Joseph G. Ballouz and Nicholas T. Ouellette The energy cascade is the most significant feature that separates turbulence from other unsteady flows, and results from the behavior of the nonlinear term in the Navier–Stokes equations. The mathematical form of of this term, however, places constraints on exactly how it can act. Here, we consider the action of the nonlinear term in physical space rather than in Fourier space, where the energy transfer between scales can be interpreted as a mechanical process where some scales do work on others. This formulation reveals the fundamental role played by geometry, as work can only be done when the eigenframes of the turbulent stress and strain rate are appropriately aligned. By comparing a direct numerical simulation of the Navier–Stokes equations, an ensemble of random solenoidal vector fields, and a random sampling of uniform eigenframe alignments, we show that this geometric alignment plays a much stronger role in determining the flux between scales than do the magnitudes of the stress and strain rate. We also show that when the alignment is effectively two-dimensional, even when embedded in a three-dimensional flow, that the energy flux is typically inverse, suggesting that the inverse cascade in two-dimensional turbulence may have a kinematic origin. Our results point to some potentially fruitful directions for turbulence modeling. 更新日期：2020-02-19 • Phys. Rev. Fluids (IF 2.442) Pub Date : Chengxi Zhao, Duncan A. Lockerby, and James E. Sprittles The instability and rupture of nanoscale liquid threads is shown to strongly depend on thermal fluctuations. These fluctuations are naturally occurring within molecular dynamics (MD) simulations and can be incorporated via fluctuating hydrodynamics into a stochastic lubrication equation (SLE). A simple and robust numerical scheme is developed for the SLE that is validated against MD for both the initial (linear) instability and the nonlinear rupture process. Particular attention is paid to the rupture process and its statistics, where the `double-cone’ profile reported by Moseler & Landmann is observed, as well as other distinct profile forms depending on the flow conditions. Comparison to the similarity solution in Eggers , a power-law of the minimum thread radius against time to rupture, shows agreement only at low surface tension; indicating that surface tension cannot generally be neglected when considering rupture dynamics. 更新日期：2020-02-19 • Phys. Rev. Fluids (IF 2.442) Pub Date : Marcio Gameiro, Abhinendra Singh, Lou Kondic, Konstantin Mischaikow, and Jeffrey F. Morris Dense frictional particulate suspensions in a viscous liquid undergo increasingly strong continuous shear thickening (CST) as the solid packing fraction, ϕ, increases above a critical volume fraction, and discontinuous shear thickening (DST) is observed for even higher packing fractions. Recent studies have related shear thickening to a transition from mostly lubricated to predominantly frictional contacts with the increase in stress, with the transition determined by overcoming a repulsive force. The rheology and networks of frictional forces from two and three dimensional simulations of shear-thickening suspensions are studied. These are analyzed using measures of the topology of the network, including tools of persistent homology. We observe that at low stress the frictional interaction networks are predominantly quasi-linear along the compression axis. With an increase in stress, the force networks become more isotropic, forming loops in addition to chain-like structures. The topological measures of Betti numbers and total persistence provide a compact means of describing the mean properties of the frictional force networks, and provide a link between macroscopic rheology and the microscopic interactions. A total persistence measure describing the significance of loops in the force network structure, as a function of stress and packing fraction, shows behavior similar to that of relative viscosity, and displays a scaling law near the jamming fraction for both two and three dimensional systems considered. The total persistence measures for both dimensions are found to be very similar. 更新日期：2020-02-18 • Phys. Rev. Fluids (IF 2.442) Pub Date : Bruce R. Sutherland and Riley Jefferson Theory is developed to predict the growth and structure of sibling'' waves developing through triadic resonant instability of a vertically confined mode-1 internalparent’’ wave in uniform stratification including the influence of background rotation. For a sufficiently hydrostatic parent wave, two branches for growth of sibling waves are dominant. The branch with largest growth rate corresponds to sibling waves having frequencies much larger than that of the parent; the other branch corresponds to sibling waves having frequencies close to half the frequency of the parent. Numerical simulations show that sibling waves corresponding to the subharmonic branch appear in practice. In the absence of rotation, the sibling waves corresponding to this branch are predicted to have near-constant growth rate as their horizontal wavenumber increases. With rotation, however, the growth rate peaks at moderate wavenumber. In all cases, as confirmed by numerical simulations, the e-folding time for the growth of the sibling waves can be thousands of buoyancy periods for parent waves having amplitudes typical of realistic oceanic internal modes. In non-uniform stratification, the parent wave self-interacts immediately to force superharmonics. Nonetheless, numerical simulations with symmetric top-hat stratification show that triadic resonant instability eventually emerges. Such emergence is not evident in simulations with stratification more representative of the ocean. The results suggest a reconsideration of the efficacy of parametric subharmonic instability in leading to the breakdown of low-mode internal tides in the ocean. 更新日期：2020-02-17 • Phys. Rev. Fluids (IF 2.442) Pub Date : A. Poyé, O. Agullo, N. Plihon, W. J. T. Bos, V. Desangles, and G. Bousselin We report on a numerical study of axisymmetric flow of liquid metal in a circular duct with rectangular cross-section. The flow is forced through the combination of an axial magnetic field and a radial current. Sweeping a wide range of forcing parameters, we identify the different regimes which characterize the flows and explicit the associate scaling laws. Results from different studies in the literature are interpreted in the light of our numerical simulations. 更新日期：2020-02-17 • Phys. Rev. Fluids (IF 2.442) Pub Date : Kartik P. Iyer, Jörg Schumacher, Katepalli R. Sreenivasan, and P. K. Yeung We study the fractal scaling of iso-level sets of a passive scalar mixed by three-dimensional homogeneous and isotropic turbulence at high Reynolds numbers. The scalar field is maintained by a linear mean scalar gradient and the Schmidt number is unity. A fractal box-counting dimension DF can be obtained for iso-levels below about 3 standard deviations of the scalar fluctuation on either side of its mean value. The dimension varies systematically with the iso-level, with a maximum of about 8/3 for the iso-level at the mean scalar value; this maximum dimension also follows as an upper bound from the geometric measure theory. We interpret this result to mean that mixing in turbulence is always incomplete. A unique box-counting dimension for all iso-levels results when we consider the spatial support of the steep cliffs of the scalar conditioned on local strain-rate; that unique dimension, independent of the iso-level set, is about 4/3. 更新日期：2020-02-17 • Phys. Rev. Fluids (IF 2.442) Pub Date : Sumit Malik, Olga M. Lavrenteva, and Avinoam Nir The deformation, stationarity and stability of a drop rotating axisymmetrically in an immiscible viscous fluid, under the influence of an externally imposed flow are studied. The ambient fluid, slightly heavier or lighter than the drop is under simultaneous rotating and compressional or extensional forcing flow at infinity. The problem is formulated and solved via an integral equation having unknown surface velocity considering low Reynolds number with equal viscosity inside the drop and of the ambient fluid. Numerical simulations are carried out by using the boundary integral method. The drop stationarity is discussed for a variety of Bond numbers, Bo, and capillary numbers, Ca, for the simultaneous action of both flow fields. The critical bounds of Ca for the stability of stationary flat and extended shapes of the drop were established for the considered range of Bo. 更新日期：2020-02-14 • Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-13 Sanjay C. P. and Ashwin Joy 更新日期：2020-02-13 • Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-13 Jiwen Gong, Jason P. Monty, and Simon J. Illingworth 更新日期：2020-02-13 • Phys. Rev. Fluids (IF 2.442) Pub Date : Alexis Berny, Luc Deike, Thomas Séon, and Stéphane Popinet When a bubble bursts at the surface of a liquid, it creates a jet that may break up and produce jet droplets. This phenomenon has motivated numerous studies due to its multiple applications, from bubbles in a glass of champagne to ocean/atmosphere interactions. We simulate the bursting of a single bubble by direct numerical simulations of the axisymmetric two-phase liquid-gas Navier-Stokes equations. We describe the number, size and velocity of all the ejected droplets, for a wide range of control parameters, defined as non-dimensional numbers, the Laplace number which compares capillary and viscous forces and the Bond number which compares gravity and capillarity. The total vertical momentum of the ejected droplets is shown to follow a simple scaling relationship with a primary dependency on the Laplace number. Through a simple evaporation model, coupled with the dynamics obtained numerically, it is shown that all the jet droplets (up to fourteen) produced by the bursting event must be taken into account as they all contribute to the total amount of evaporated water. A simple scaling relationship is obtained for the total amount of evaporated water as a function of the bubble size and fluid properties. This relationship is a first important step toward building a physics-based model of the ocean-atmosphere water vapour fluxes controlled by bubbles bursting at the surface. 更新日期：2020-02-13 • Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-12 Namshad Thekkethil, Atul Sharma, and Amit Agrawal 更新日期：2020-02-12 • Phys. Rev. Fluids (IF 2.442) Pub Date : Felix Milan, Luca Biferale, Mauro Sbragaglia, and Federico Toschi We study droplet dynamics and breakup in generic time-dependent flows via a multicomponent Lattice Boltzmann algorithm, with emphasis on flow start up conditions. We first study droplet breakup in a confined oscillatory shear flow via two different protocols. In one set up, we start from an initially spherical droplet and turn on the flow abruptly (shock method''); in the other protocol, we start from an initially spherical droplet as well, but we progressively increase the amplitude of the flow, by allowing the droplet to relax to the steady state for each increase in amplitude, before increasing the flow amplitude again (relaxation method’’). The two protocols are shown to produce substantially different breakup scenarios. The mismatch between these two protocols is also studied for variations in the flow topology, the degree of confinement and the inertia of the fluid. All results point to the fact that under extreme conditions of confinement the relaxation protocols can drive the droplets into metastable states, which break only for very intense flow amplitudes, but their stability is prone to external perturbations, such as an oscillatory driving force . 更新日期：2020-02-12 • Phys. Rev. Fluids (IF 2.442) Pub Date : Yonatan Dukler, Hangjie Ji, Claudia Falcon, and Andrea L. Bertozzi We revisit the tears of wine problem for thin films in water-ethanol mixtures and present a new model for the climbing dynamics. The new formulation includes a Marangoni stress balanced by both the normal and tangential components of gravity as well as surface tension which lead to distinctly different behavior. The prior literature did not address the wine tears but rather the behavior of the film at earlier stages and the behavior of the meniscus. In the lubrication limit we obtain an equation that is already well-known for rising films in the presence of thermal gradients. Such models can exhibit non-classical shocks that are undercompressive. We present basic theory that allows one to identify the signature of an undercompressive (UC) wave. We observe both compressive and undercompressive waves in new experiments and we argue that, in the case of a pre-coated glass, the famous "wine tears" emerge from a reverse undercompressive shock originating at the meniscus. 更新日期：2020-02-12 • Phys. Rev. Fluids (IF 2.442) Pub Date : Gianluca Lavalle, Nicolas Grenier, Sophie Mergui, and Georg F. Dietze Solitary traveling waves are prominent features on the surface of a falling liquid film and are known to promote heat/mass transfer. We focus on the little studied case where they are subject to an extremely confined counter-current gas flow, and we identify two novel secondary instabilities. At high gas velocities, a catastrophic instability develops, leading to flooding through wave reversal. At lower gas velocities, an oscillatory instability occurs, producing a high-frequency periodic modulation of the wave height. Conjunction of this self-sustained oscillatory state and vortices forming in the liquid is shown to enhance mixing. Also, we uncover a new hydrodynamic route toward film rupture. 更新日期：2020-02-12 • Phys. Rev. Fluids (IF 2.442) Pub Date : Felix Eich, Charitha M. de Silva, Ivan Marusic, and Christian J. Kähler This study compares the predicted synthetic flow fields generated based on the representative structures of the attached eddy model to experimental data captured using Particle Image Velocimetry of a turbulent boundary layer. To this end, wall-parallel and cross-stream planar fields are analysed qualitatively and quantitatively by examining instantaneous flow features and by statistical two-point correlation functions, respectively. Our results reveal that although single-point flow statistics are in good agreement with the experimental data, a comparison of instantaneous flow fields and multipoint statistics between the attached eddy model and experiments shows differences in the spatial coherence. Based on these observations, a modification to the placement of the representative eddies in the attached eddy model is proposed that incorporates the meandering of these flow structures, which has been extensively reported in turbulent boundary layers. Our results reveal that this subtle modification provides a superior spatial representation of a turbulent boundary layer from the attached eddy model by reducing periodic effects and the overestimated spatial coherence. Similar improvements are also reported for the spatial representation of the spanwise velocity component. Finally, further refinements to the model are identified and discussed towards future improvements to the model. 更新日期：2020-02-12 • Phys. Rev. Fluids (IF 2.442) Pub Date : Sanjay C. P. and Ashwin Joy Understanding the role of frictional drag in diffusive transport is an important problem in the field of active turbulence. Using a generic continuum model that applies well to living systems, we investigate the role of Ekman friction on the passive transport of Lagrangian tracers that go with the local flow. We find that the crossover from ballistic to diffusive regime happens at a time scale τc that attains a minimum at zero friction, meaning that both injection and dissipation of energy delay the relaxation of tracers. We explain this by proposing that τc∼ℓ*/urms, where ℓ* is a dominant length scale extracted from energy spectrum peak, and urms is a velocity scale that sets the kinetic energy at steady state, both scales monotonically decrease with friction. Finally, we predict friction scaling laws for ℓ*, urms and the diffusion coefficient 𝒟∼ℓ*urms/2, that are valid over a wide range of fluid friction. The findings of our report should apply to transport phenomena in generic active systems such as dense bacterial suspensions, micro-tubule networks or even artificial swimmers, to name a few. 更新日期：2020-02-11 • Phys. Rev. Fluids (IF 2.442) Pub Date : Na Zhao, Bo Wang, Qianqian Kang, and Jingtao Wang In this paper, the numerical investigation of the bubble formation and rising in gas-liquid-solid flow systems has been done through a coupled numerical method of Volume of fluid (VOF) method and discrete element model (DEM). VOF is used to capture the interface of the bubble and DEM is employed to track the movement of particles. This coupled numerical method is validated through the good consistency of our calculated results to those experimental data (from other literatures) of the bubble rising in a gas-liquid-solid system. The calculation results in this paper disclose that the detachment time of the first formed bubble is longer for case I (when particles are laying at the bottom of the container) than for case II (when the particles are settling freely). The main reason is that a stronger circulation (induced by the sedimentation of particles) near the bottom of the container plays a stronger cutting role and speeds up the detachment of the bubble. Then, the effects of some factors including particle volume fractions, gas velocities, orifice sizes, surface tensions and liquid densities on the bubble formation and rising have also been investigated systematically. This work is useful for further researches on the gas-liquid-solid fluidized bed system. 1 Introduction The multiphase flow is a flow system of two or more coexisting phases with clear interfaces Currently, multiphase flows are generally divided into two categories: two phase flows and three phase flows1. Gas-liquid-solid three-phase flows have been widely used in chemical, petroleum, energy and environmental industries because of their large interphase heattransfer areas and good masstransfer efficiencies2−6 In a multiphase flow system, bubble rheological behaviors play important roles to the flow as the formation and rising of the bubbles could stir the liquid and enhance the disturbance of the flow. Hence, the study of the bubble formation and rising in multiphase flows has a remarkable increase in recent years. In order to understand the complexity of the multiphase flows, more and more attention has been paid to the numerical simulation of multiphase flows through different numerical methods of computational fluid dynamics (CFD)7 There are two major numerical methods for the study of multiphase flows: Eulerian–Eulerian (E-E)^{5,\thinspace 8-12}$and Eulerian-Lagrangian (E-L)13−17,19−20 method In the E-E method, different phases are treated as interpenetrating continuous media. In the E-L method, Navier-Stokes equations are solved in Eulerian frame for the continuous phase, and the particle orbital equation is solved in Lagrangian frame for the particles By using the E-E method, Chen et al.$^{\thinspace 9}$calculated the axisymmetric gas-liquid transient flow, and Lu et al.$^{\mathrm{\thinspace 18\thinspace }}$calculated the diameter and rising speed of bubbles in a free bubbling fluidized bed. As for the E-L method, Wen et al.19 combined the two-fluid model with the discrete element method and established a closed E-E-L model to calculate gas-liquid-solid three-phase flows Volume of fluid (VOF) method21 could capture the motion of the gas-liquid interface and has been applied for the study of bubble rheological behaviors in gas-liquid-solid three-phase flows Li et … 更新日期：2020-02-11 • Phys. Rev. Fluids (IF 2.442) Pub Date : Tiago Pestana, Markus Uhlmann, and Genta Kawahara This work employs for the first time invariant solutions of the Navier-Stokes equations to study the interaction between finite-size particles and near-wall coherent structures. We consider horizontal plane Couette flow and focus on Nagata’s upper-branch equilibrium solution at low Reynolds numbers where this solution is linearly stable. When adding a single heavy particle with a diameter equivalent to 2.5 wall units (one twelfth of the gap width), we observe that the solution is essentially maintained while the particle motion attains a periodic state after migrating towards the region occupied by a low-speed streak - irrespective of its initial position. As a result of the ensuing preferential particle location, the time-average streamwise particle velocity differs from the plane- average fluid-phase velocity at the same wall-distance as the particle center, as previously observed in experiments and in numerical data for fully turbulent wall-bounded flows. The same mechanism has been elucidated through an analysis of spanwise hydrodynamic forces acting on laterally constrained particles in additional simulations. Finally, a parametric study with different particle to fluid density ratios is conducted which shows that the amplitude of oscillations of the spanwise particle velocity decreases linearly with density. 更新日期：2020-02-11 • Phys. Rev. Fluids (IF 2.442) Pub Date : Ségolène Méjean, François Guillard, Thierry Faug, and Itai Einav Granular jumps—the change in height and depth-averaged velocity during granular flows—occur during transitions from thin and fast flows (supercritical) to thick and slow flows (subcritical). The present paper describes discrete element method simulations inspired by recent laboratory experiments, which produce standing jumps in two-dimensional free-surface dry granular flows down a slope. Special attention is paid to characterizing and measuring the finite length of those standing granular jumps, as well as to deciphering their internal structure. By varying macroscopic quantities, such as slope angle and mass discharge, and microscopic properties, such as interparticle friction and grain diameter, a rich variety of granular jump patterns is observed. Hydraulic-like granular jumps with an internal water-like roller are identified, in addition to diffuse laminar granular jumps and steep colliding granular jumps. Moreover, a recently established depth-averaged relation for the prediction of jump heights is fed with the measured jump length and fitted against the numerical simulations to examine the effective friction in the granular medium for each type of jump. The dominant component of the general friction law is found to be different when transitioning from one jump pattern to the others. This study demonstrates that the granular jump pattern, and particularly its geometry and internal structure, can offer a stringent test for addressing the dissipation mechanisms that govern the flowability of granular media. ay be entered using the command. 更新日期：2020-02-11 • Phys. Rev. Fluids (IF 2.442) Pub Date : Y. Ben-Ami and A. Manela We study the propagation of acoustic waves in a viscous heat-conducting gas at non-adiabatic conditions. Considering a planar slab configuration, constant wall heating is applied at the confining walls to maintain non-uniform temperature and density reference distributions. Acoustic excitation is then imposed via small-amplitude harmonic wall oscillations and normal heat-flux perturbations. Focusing on continuum-limit conditions of small Knudsen numbers and high actuation frequencies (yet small compared with the mean collision frequency), the gas domain affected by wall excitation is confined to a thin layer (termed “acoustic layer”) in the vicinity of the excited boundary, and an approximate solution is derived based on asymptotic expansion of the acoustic fields. The application of thermoacoustic wall excitation necessitates the formation of an ever thinner “thermal layer” that governs the transmission of wall unsteady heat flux into sound waves. The results of the approximate analysis, supported by continuum-model finite-differences and direct simulation Monte Carlo calculations, clarify the impacts of system non-adiabaticity and gas kinetic model of interaction on sound propagation. Primarily, reference wall heating results in an extension of the acoustic layer and consequent sound wave radiation over larger distances from the wall source. Considering the entire range of inverse power law (repulsion point center) interactions, it is also found that wave attenuation is affected by the kinetic model of gas collisions, yielding stronger decay rates in gases with softer molecular interactions. The results are used to generalize the counterpart adiabatic-system findings for the amount of boundary heat-flux required for the silencing of vibroacoustic sound at non-adiabatic reference conditions. 更新日期：2020-02-11 • Phys. Rev. Fluids (IF 2.442) Pub Date : Kevin Lippera, Michael Benzaquen, and Sébastien Michelin Chemically-active droplets exhibit complex avoiding trajectories. While heterogeneity is inevitable in active matter experiments, it is mostly overlooked in their modelling. Exploiting its geometric simplicity, we fully-resolve the head-on collision of two swimming droplets of different radii and demonstrate that even a small contrast in size critically conditions their collision and subsequent dynamics. We identify three fundamentally-different regimes. The resulting high sensitivity of pairwise collisions is expected to profoundly affect their collective dynamics. 更新日期：2020-02-11 • Phys. Rev. Fluids (IF 2.442) Pub Date : Si Suo, Mingchao Liu, and Yixiang Gan Porous media with hierarchical structures are commonly encountered in both natural and synthetic materials, e.g., fractured rock formations, porous electrodes and fibrous materials, which generally consist of two or more distinguishable levels of pore structure with different characteristic lengths. The multiphase flow behaviours in hierarchical porous media have remained elusive. In this study, we investigate the influences of hierarchical structures in porous media on the dynamics of immiscible fingering during fluid-fluid displacement. Divided by the breakthrough, such displacement process includes pre- and post-breakthrough stages during which the fingering evolution is dominated by viscous and capillary effects, respectively. Through conducting a series of numerical simulations, we found that the immiscible fingering can be suppressed due to the existence of secondary porous structures. To characterise the fingering dynamics in hierarchical porous media, a phase diagram, which describes the switch among the three fingering modes (the suppressing, crossover and dendrite mode), is constructed by introducing a scaling parameter, i.e., the ratio of time scales considering the combined effect of characteristic pore sizes and wettability. The findings presented in this work provide a basis for further research on the application of hierarchical porous media for controlling immiscible fingerings. . 更新日期：2020-02-11 • Phys. Rev. Fluids (IF 2.442) Pub Date : Pascal S. Raux, Anthony Troger, Pierre Jop, and Alban Sauret The fragmentation of liquid sheets produces a collection of droplets. The size distribution of the droplets has a considerable impact on the coating efficiency of sprays and the transport of contaminants. Although many processes commonly used particulate suspensions, the influence of the particles on the spreading dynamics of the sheet and its subsequent fragmentation has so far been considered negligible. In this paper, we consider experimentally a transient suspension sheet that expands radially. We characterize the influence of the particles on the dynamics of the liquid sheet and the fragmentation process. We highlight that the presence of particles modifies the thickness and reduces the stability of the liquid sheet. Our study suggests that particles can significantly modify the dynamics of liquid films through capillary effects, even for volume fractions much smaller than the maximum packing. 更新日期：2020-02-11 • Phys. Rev. Fluids (IF 2.442) Pub Date : A. Padash and C. M. Boyce A recent magnetic resonance imaging (MRI) study (Boyce et al., Phys. Rev. Fluids, 2019, 034303) found that when two bubbles were injected side-by-side into an incipiently fluidized bed, below a critical injection volume, one bubble collapsed, while the other bubble survived, reaching the bed surface. Limitations in the experimental measurements left open questions about the apparent collapse: (1) did the bubble actually collapse or rather move out of the imaging plane and (2) if the bubble collapsed, was it due to gas transfer between bubbles or a preference for gas to channel toward the surviving bubble. Here, we demonstrate that computational fluid dynamics - discrete element method (CFD-DEM) simulations can recreate this phenomenon. Simulation results reveal that the bubble does in fact collapse and that this collapse occurs because the slight size difference between the two bubbles causes gas flow to channel preferentially to the larger bubble, leaving the smaller bubble without enough gas flow to support its roof. 更新日期：2020-02-11 • Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-10 Richard D. J. G. Ho, Andres Armua, and Arjun Berera 更新日期：2020-02-10 • Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-10 Jean-Baptiste Salmon and Frédéric Doumenc 更新日期：2020-02-10 • Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-10 Kishan Bellur, Ezequiel F. Médici, Chang Kyoung Choi, James C. Hermanson, and Jeffrey S. Allen 更新日期：2020-02-10 • Phys. Rev. Fluids (IF 2.442) Pub Date : Nicholas Z. Liu, Daniel R. Ladiges, Jason Nassios, and John E. Sader The Boltzmann equation provides a rigorous description of gas flows at all degrees of gas rarefaction. Asymptotic analyses of this equation yields valuable insight into the physical mechanisms underlying gas flows. In this article, we report an asymptotic analysis of the Boltzmann-BGK equation for a slightly rarefied gas when the acoustic wavelength is comparable to the macroscopic characteristic length scale of the flow. This is performed using a three-way matched asymptotic expansion, which accounts for the Knudsen layer, the viscous layer and the outer Hilbert region—these are separated by asymptotically disparate length scales. Transport equations and boundary conditions for these regions are derived. The utility of this theory is demonstrated by application to three problems: (1) flow generated by uniformly heating two plates, (2) oscillatory thermal creep induced between two plates, and (3) the flow generated by an oscillating sphere. Comparisons to numerical simulations of the Boltzmann-BGK equation and previous asymptotic theories (for long wavelength) are performed. The present theory is distinct from previous asymptotic analyses that implicitly assume long or short acoustic wavelength. This theory is expected to find application in the design and characterisation of nanoelectromechanical devices, which often generate acoustic oscillatory flows of a rarefied nature. 更新日期：2020-02-10 • Phys. Rev. Fluids (IF 2.442) Pub Date : Aaron Madden, Juan Fernandez de la Mora, Hadi Sabouri, Nguyen Pham, and Brian Hawkett The tip radius Rtip of ferrofluids deformed by magnetization is typically ~ 100 $$m, much larger than in conducting electrified liquids, even though in both cases the capillary stress {2$$/R}$_{tip}$balances the electrostatic or magnetostatic stress. The reason for the relative dullness of magnetic tips appears to be the saturation of magnetization, Mmax=Msat. This hypothesis lacks quantitative experimental confirmation but is supported theoretically by the recently proposed high field asymptote Rtip{$$}4γμoMsat2 obtained for isolated drops in strong and uniform magnetic fields. Here we confirm experimentally the controlling role of the magnetic saturation effect under more general conditions, by showing that the dimensionless group μoMsat2Rtip/(4γ) is always a quantity of order unity, even for ferrofluid menisci subject to the highly non-uniform magnetic fields created by supporting them at the tip of a ferrous needle facing a strong magnet. Four different ferrofluids are used, with high, medium and low surface tensions, including highly polar and nonpolar solvents. Substantial changes in Msat are achieved by letting the suspending solvent evaporate, with record tip radii down to 9.5$$m demonstrated at high volume fractions of magnetic particles. 更新日期：2020-02-10 • Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-07 Xander M. de Wit, Andrés J. Aguirre Guzmán, Matteo Madonia, Jonathan S. Cheng, Herman J. H. Clercx, and Rudie P. J. Kunnen 更新日期：2020-02-07 • Phys. Rev. Fluids (IF 2.442) Pub Date : Nazmi Burak Budanur, Elena Marensi, Ashley P. Willis, and Björn Hof In the past two decades, our understanding of the transition to turbulence in shear flows with linearly stable laminar solutions has greatly improved. Regarding the susceptibility of the laminar flow, two concepts have been particularly useful: the edge states and the minimal seeds. In this nonlinear picture of the transition, the basin boundary of turbulence is set by the edge state’s stable manifold and this manifold comes closest in energy to the laminar equilibrium at the minimal seed. We begin this paper by presenting numerical experiments in which three-dimensional perturbations are too energetic to trigger turbulence in pipe flow but they do lead to turbulence when their amplitude is reduced. We show that this seemingly counter-intuitive observation is in fact consistent with the fully nonlinear description of the transition mediated by the edge state. In order to understand the physical mechanisms behind this process, we measure the turbulent kinetic energy production and dissipation rates as a function of the radial coordinate. Our main observation is that the transition to turbulence relies on the energy amplification away from the wall, as opposed to the turbulence itself, whose energy is predominantly produced near the wall. This observation is further supported by the similar analyses on the minimal seeds and the edge states. Furthermore, we show that the time-evolution of production-over-dissipation curves provide a clear distinction between the different initial amplification stages of the transition to turbulence from the minimal seed. 更新日期：2020-02-07 • Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-05 P. Léard, B. Favier, P. Le Gal, and M. Le Bars 更新日期：2020-02-06 • Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-05 Rahul Agrawal, Alexandros Alexakis, Marc E. Brachet, and Laurette S. Tuckerman 更新日期：2020-02-06 • Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-05 Arash Alizad Banaei, Mona Rahmani, D. Mark Martinez, and Luca Brandt 更新日期：2020-02-06 • Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-05 J. Purseed, B. Favier, L. Duchemin, and E. W. Hester 更新日期：2020-02-06 • Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-05 John O. Dabiri 更新日期：2020-02-06 • Phys. Rev. Fluids (IF 2.442) Pub Date : W. Till Kranz, Fabian Frahsa, Annette Zippelius, Matthias Fuchs, and Matthias Sperl We compute the rheological properties of inelastic hard spheres in steady shear flow for general shear rates and densities. Starting from the microscopic dynamics we generalise the Integration Through Transients (itt) formalism to a fluid of dissipative, randomly driven granular particles. The stress relaxation function is computed approximately within a mode-coupling theory—based on the physical picture, that relaxation of shear is dominated by slow structural relaxation, as the glass transition is approached. The transient build-up of stress in steady shear is thus traced back to transient density correlations which are computed self-consistently within mode-coupling theory. The glass transition is signalled by the appearance of a yield stress and a divergence of the Newtonian viscosity, characterizing linear response. For shear rates comparable to the structural relaxation time, the stress becomes independent of shear rate and we observe shear thinning, while for the largest shear rates Bagnold scaling, i.e., a quadratic increase of shear stress with shear rate, is recovered. The rheological properties are qualitatively similar for all values of ε, the coefficient of restitution; however, the magnitude of the stress as well as the range of shear thinning and thickening show significant dependence on the inelasticity. 更新日期：2020-02-06 • Phys. Rev. Fluids (IF 2.442) Pub Date : Chance Parrish and Satish Kumar The coating and drying of non-flat discrete objects is a key manufacturing step for a wide variety of products. Flow of a thin, non-volatile liquid film on the outside of a rotating cylinder is commonly used as a model problem to study the coating of discrete objects. However, the behavior of a volatile, particle-laden coating has yet to be studied and remains an important open problem. In this work, we use lubrication theory to study the evolution of a liquid film laden with colloidal particles in the presence of solvent evaporation. Two coupled evolution equations describing variations in coating thickness and composition as a function of time and the angular coordinate are solved numerically. In the limit of a rapidly rotating cylinder, gravitational effects are negligible, and linear stability analysis and nonlinear simulations demonstrate that non-uniform drying at larger drying rates may cause thickness and composition disturbances to re-grow after initially decaying. When gravitational effects are significant, poor liquid redistribution at lower rotation rates and larger drying rates leads to less uniform coatings. Colloidal particles hinder liquid redistribution at high concentrations by increasing the viscosity, but help prevent rupture of the coating at more moderate concentrations. A parametric study reveals that both thickness and composition variations are minimized at large rotation rate, low drying rate, and moderate initial particle concentration. 更新日期：2020-02-06 • Phys. Rev. Fluids (IF 2.442) Pub Date : Xiao Zhang and Michael D. Graham This work investigates the orbital dynamics of a fluid-filled deformable prolate capsule in unbounded simple shear flow at zero Reynolds number using direct simulations. The motion of the capsule is simulated using a model that incorporates shear elasticity, area dilatation, and bending resistance. Here the deformability of the capsule is characterized by the nondimensional capillary number$\Ca$, which represents the ratio of viscous stresses to elastic restoring stresses on the capsule. For a capsule with small bending stiffness, at a given$\Ca$, the orientation converges over time towards a unique stable orbit independent of the initial orientation. With increasing$\Ca$, four dynamical modes are found for the stable orbit, namely, rolling, wobbling, oscillating-swinging, and swinging. On the other hand, for a capsule with large bending stiffness, multiplicity in the orbit dynamics is observed. When the viscosity ratio λ≲1, the long-axis of the capsule always tends towards a stable orbit in the flow-gradient plane, either tumbling or swinging, depending on$\Ca$. When λ≳1, the stable orbit of the capsule is a tumbling motion at low$\Ca$, irrespective of the initial orientation. Upon increasing$\Ca$, there is a symmetry-breaking bifurcation away from the tumbling orbit, and the capsule is observed to adopt multiple stable orbital modes including nonsymmetric precessing and rolling, depending on the initial orientation. As$\Ca$further increases, the nonsymmetric stable orbit loses existence at a saddle-node bifurcation, and rolling becomes the only attractor at high$\Ca$, whereas the rolling state coexists with the nonsymmetric state at intermediate values of$\Ca$. A symmetry-breaking bifurcation away from the rolling orbit is also found upon decreasing$\Ca\$. The regime with multiple attractors becomes broader as the aspect ratio of the capsule increases, while narrowing as viscosity ratio increases. We also report the particle contribution to the stress, which also displays multiplicity.

更新日期：2020-02-06
• Phys. Rev. Fluids (IF 2.442) Pub Date :
Wen Yang, Ivan Delbende, Yann Fraigneau, and Laurent Martin Witkowski

In this paper, we aim at characterizing the spin-up process and the permanent regime of a rotating flow with free surface. The motion is created by the quasi-impulsive rotation of a disk located at the bottom of a cylindrical tank partially filled with a Newtonian water-glycerol mixture at a fixed initial aspect ratio (height to radius) of 0.25. Two experimental setups with different radius sizes and two numerical codes with distinct formulations are used, allowing for numerous comparisons and cross validations. Time-dependent surface height profiles and azimuthal velocity profiles are accurately measured with a laser line sensor and Laser Doppler Velocimetry respectively. These measurements are compared to numerical results showing an overall excellent agreement and whenever a discrepancy exists, explanations are given. The spin-up and permanent regime for two typical flow cases are detailed: (i) one for which the angular speed moderately deforms the free surface and (ii) one at higher angular speed for which the disk becomes partially dewetted. The shape of the free surface in the permanent regime for intermediate rotation speeds is also reported. The flow parameters are chosen in such a way that large deformations of the free surface are achieved, yet remaining under the threshold of appearance of rotating polygonal patterns. This enables us to characterize realistic base flows on which this instability may develop.

更新日期：2020-02-06
• Phys. Rev. Fluids (IF 2.442) Pub Date :
Feng Gao, John W. Chew, and Olaf Marxen

Rotating fluids are well-known to be susceptible to waves. This has received much attention from the geophysics, oceanographic and atmospheric research communities. Inertial waves, which are driven by restoring forces, for example the Coriolis force, have been detected in the research fields mentioned above. This paper investigates inertial waves in turbine rim seal flows in turbomachinery. These are associated with the large-scale unsteady flow structures having distinct frequencies, unrelated to the main annulus blading, identified in many experimental and numerical studies. These unsteady flow structures have been shown in some cases to reduce sealing effectiveness and are difficult to predict with conventional steady Reynolds-averaged Navier-Stokes (RANS) approaches. Improved understanding of the underlying flow mechanisms and how these could be controlled is needed to improve the efficiency and stability of gas turbines. This study presents large-eddy simulations for three rim seal configurations – chute, axial and radial rim seals – representative of those used in gas turbines. Evidence of inertial waves is shown in the axial and chute seals, with characteristic wave frequencies limited within the threshold for inertial waves given by classic linear theory (i.e. |f*/frel|≤2), and instantaneous flow fields showing helical characteristics. The radial seal, which limits the radial fluid motion with the seal geometry, restricts the Coriolis force and suppresses the inertial wave.

更新日期：2020-02-06
• Phys. Rev. Fluids (IF 2.442) Pub Date :
Martin Coux, Pierre Chantelot, Lucie Domino, Christophe Clanet, Antonin Eddi, and David Quéré

Vibrated substrates are useful tools to move and reshape drops. Experiments usually involve periodical excitation at small amplitude but new effects can be generated by using larger and/or quicker movements. Recently, impulsive motion of a plate has been shown to enhance drop takeoff. In this article we discuss the beautiful, elusive shapes obtained when a water droplet deposited on a non-wetting substrate is subjected to a strong vertical impulse. Drops are highly reshaped to form truncated cones that eventually collapse. We report and discuss the evolution of the geometrical features of these so-called “vase-shaped droplets”. Our understanding of the physical phenomena at stake in the experiment allows us to model the evolution of the shape of the droplet. In particular, we show that the top and the bottom of the vase follow dynamics with different timescales, leading to the truncated cone shape. We show that it is possible to play with one of these dynamics by changing the wetting of the liquid on the rising substrate, and are thus able to change the shape of the vase from cylindrical to completely flat.

更新日期：2020-02-06
• Phys. Rev. Fluids (IF 2.442) Pub Date :
M. H. Lakshminarayana Reddy and Meheboob Alam

The regularized versions of extended-hydrodynamic equations for a dilute granular gas, in terms of 10-, 13- and 14-moments, are derived from the inelastic Boltzmann equation. The regularization/parabolization is achieved by adding gradient terms that are derived following a Chapman-Enskog like gradient-expansion (H. Struchtrup, 2004, Phys. Fluids). For both granular and molecular gases, the resulting moment equations are found to be free from the well-known finite Mach-number singularity (that occurs in the Riemann problem of planar shock-waves) since the regularized gradient terms yield parabolic equations in contrast to the hyperbolic nature of original moment equations. In order to clarify the advantage of these regularized equations, the plane shock-wave problem is solved numerically for both molecular and granular gases; the calculated hydrodynamic profiles compare favourably with previous simulation results for molecular gases. For a granular gas, both regularized and non-regularized moment models predict asymmetric density and temperature profiles, with the maxima of both density and temperature occurring within the shock-layer, and the hydrodynamic fields are found to be smooth for the regularized models for all Mach-numbers studied. It is demonstrated that, unlike in the case of molecular gases, a "second" regularization of the regularized moment equations must be carried out in order to arrest the unbounded growth of density within the shock-layer in a granular gas.

更新日期：2020-02-06
• Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-01-31
Pedro O. S. Livera and José A. Miranda
更新日期：2020-02-03
• Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-03
Maxime Costalonga and Philippe Brunet
更新日期：2020-02-03
• Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-02-03
Shima Parsa, Enric Santanach-Carreras, Lizhi Xiao, and David A. Weitz
更新日期：2020-02-03
• Phys. Rev. Fluids (IF 2.442) Pub Date :
Bruno Van Ruymbeke, Noureddine Latrache, Céline Gabillet, and Catherine Colin

We investigate experimentally the defect-mediated turbulence (DMT) which is induced by bubbles injection in a Taylor-Couette flow when the inner cylinder is rotated while the outer cylinder is fixed. Bubbles of 1.2 mm in diameter are injected at the bottom of a Taylor Couette device of radii ratio equal to 0.91. The tangential Reynolds number range is [2200,19300] and the air injection rate varies up to 800 ml/min. For these conditions of the experiments, bubbles are trapped in the gap by the Taylor vortices and arranged as patterns (toroidal, wavy toroidal, spirals and wavy spirals). Visualizations of the bubble patterns were carried out. When decreasing the Reynolds number or increasing the air injection rate, spiral and toroidal patterns can coexist in a composite flow. Defects occur in the bubble’s patterns (merging or splitting of the Taylor vortices pairs). By analyzing the space time diagram of bubbles patterns and their complex demodulation, we highlight different regimes and transitions in the DMT of the bubbly Taylor Couette flow. The control parameter of the transitions is the air volumetric fraction, which evolves as the ratio between the axial injection Reynolds number and the tangential Reynolds number. By increasing the air volumetric fraction, the defects in the DMT flows are classified as three flow regimes: i) structured composite flow where the defects are periodic in space and time, ii) intermittency defects chaos where the defects zones alternate randomly with the patterns in the time and space ii) developed defects chaos with a large defects density. The statistical properties of these three regimes of the DMT are analyzed in the framework of the complex Ginzburg-Landau equation.

更新日期：2020-02-03
• Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-01-30
Rui Wu, Tao Zhang, Chao Ye, C. Y. Zhao, Evangelos Tsotsas, and Abdolreza Kharaghani
更新日期：2020-01-31
• Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-01-30
Qing Li and Baylor Fox-Kemper
更新日期：2020-01-31
• Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-01-31
Vilas Shinde
更新日期：2020-01-31
• Phys. Rev. Fluids (IF 2.442) Pub Date : 2020-01-28
Jason Laurie and Andrew W. Baggaley
更新日期：2020-01-29
Contents have been reproduced by permission of the publishers.

down
wechat
bug