| Volume 9, Issue 2 February 2024 | | Advertisement  | In this year, 2024, 156 Outstanding Referees were selected from the 91,600 currently active referees. The honorees come from over 42 different countries and will be recognized at the upcoming March Meeting. Read more. | | | | | |
Advertisement Don't miss these exciting Physical Review Journals events at the 2024 APS March Meeting | | | | Not an APS member? Join today to start connecting with a community of more than 50,000 physicists. | | | | Editors' Suggestion Thomasina V. Ball and Neil J. Balmforth Phys. Rev. Fluids 9, 023304 (2024) – Published 27 February 2024  | When a small fraction of viscoplastic fluid is placed into a rotating cylinder, steady states can be reached with pool at the bottom of the cylinder and a residual coating elsewhere. Lubrication theory used to model the film thickness builds on previous models by bridging between two asymptotic limits, incorporating both the gravitational force along the cylinder and the hydrostatic pressure gradients. The model predicts steady states are reached after a small number of rotations and allows exploration of drainage when the cylinder comes to a halt. Experiments using a Carbopol suspension provide a suitable comparison to test the thin film theory. | | | | | | Editors' Suggestion Ryuta X. Suzuki, Shoji Seya, Takahiko Ban, Manoranjan Mishra, and Yuichiro Nagatsu Phys. Rev. Fluids 9, 024003 (2024) – Published 14 February 2024  | We experimentally investigate displacement of a more viscous liquid by a less viscous one in a Hele-Shaw cell using an aqueous two phase system, where phase separation occurs in the growing liquid-liquid interfacial region, by varying the injection flow rate and the phase separation rate. We show that the degree of the interfacial phase separation scales as a unique function of the ratio of the flow and phase separation rates and it decreases with the ratio. These results demonstrate that the interfacial phase separation is arrested by the imposed flow and determined by competition between the flow and phase separation rates. The arresting effect and the mechanism are numerically verified. | | | | | | Editors' Suggestion Vira Dhaliwal, Christian Pedersen, Kheireddin Kadri, Guillaume Miquelard-Garnier, Cyrille Sollogoub, Jorge Peixinho, Thomas Salez, and Andreas Carlson Phys. Rev. Fluids 9, 024201 (2024) – Published 8 February 2024  | Liquid nanofilms are subject to rupture due to intermolecular forces triggered by surface perturbations arising from thermal fluctuations. When a shear stress is imposed at the free surface it becomes stable in the direction of shear, but perturbations can still grow in the perpendicular direction to the shear. | | | | | | Editors' Suggestion A. Bajpai, A. Bhateja, and R. Kumar Phys. Rev. Fluids 9, 024306 (2024) – Published 23 February 2024  | In this investigation, a novel two-way coupled gas-granular solver is developed, incorporating direct simulation Monte Carlo (DSMC) for gas particle collisions and discrete element method (DEM) for granular particle interactions. Gas-grain interaction model consists of momentum and energy exchange between the two phases. Using this framework, we have performed a comprehensive study of dust dispersion due to plume impingement on a lunar surface. We have predicted not only the velocity field of gas and grain phases, but also their temperature field, which can be meaningful information for spacecraft designers. | | | | | | Editors' Suggestion Brian F. Farrell and Petros J. Ioannou Phys. Rev. Fluids 9, 024605 (2024) – Published 21 February 2024  | In wall turbulence, the time-mean flow is returned to after almost any disturbance, which indicates that it is a stable statistical feature underlying the disorder of turbulence. However, the stability of this statistical feature can not be determined directly from the stability of the time-mean flow itself. What is required is a statistical stability analysis method. We determine the statistical stability of the time-mean state by averaging the dynamics of the return to the time-mean state over the turbulent attractor using the linear inverse model method. | | | | | | Editors' Suggestion Qinmin Zheng, Tianyi Li, Benteng Ma, Lin Fu, and Xiaomeng Li Phys. Rev. Fluids 9, 024608 (2024) – Published 29 February 2024  | In this work, we propose a novel framework for the high-fidelity reconstruction of large-area damaged turbulent fields with high resolution based on a physically constrained generative adversarial network. The network leverages complete/sparse fields of velocity components as physical constraints and adopts a PatchGAN discriminator network. The proposed reconstruction framework has been shown to achieve excellent reconstruction performance. The reconstructed flow fields are consistent with the raw flow fields in terms of magnitude, power spectrum, and two-point correlation function. | | | | | | Interfacial Phenomena and Flows | Letter Yu Zhao and Haitao Xu Phys. Rev. Fluids 9, L022001 (2024) – Published 8 February 2024  | We propose a focused laser heating method to study wave properties of flowing soap films. The laser-induced thermocapillary flows lead to symmetric disturbances in film thickness. The shock waves originating from the propagating symmetric elastic waves, which have remained elusive despite considerable experimental efforts, are thus stimulated on flowing soap films with low surfactant concentrations, or appreciable elasticities. Interestingly, on soap films with high surfactant concentrations, or vanishingly small elasticities, the laser-induced disturbances in film thickness remain unchanged and flow with the film without propagating, creating "laser-engraving" on free liquid films. | | | | | | Letter F. Novotný, Y. Huang, J. Kvorka, Š Midlik, D. Schmoranzer, Z. Xie, and L. Skrbek Phys. Rev. Fluids 9, L022601 (2024) – Published 28 February 2024  | Spherically symmetric thermal counterflow of superfluid 4He driven by a central heater is unaffected by shear thanks to the absence of walls. This quantum flow displays a two-fluid behavior and upon increasing drive undergoes a complex process of transition to quantum turbulence that involves formation of normal fluid turbulence above a certain critical threshold, drawing energy from a preexisting random tangle of quantized vortices. Spherically symmetric thermal counterflow can serve as a model flow for cosmological phenomena relating cosmic strings to quantized vortices, for processes occurring in neutron stars, or cosmological structure formation within superfluid models of dark matter. | | | | | | Biological and Biomedical Flows | Mohammad Mohaghar, Angelica A. Connor, Shuai Wu, Ruike Renee Zhao, and Donald R. Webster Phys. Rev. Fluids 9, 023101 (2024) – Published 13 February 2024  | This study quantifies the effects of breaking the symmetry of magnetic-responsive material appendage motion. Asymmetry is achieved through distinct shape changes and asymmetric cycle periods. The appendage with an asymmetric joint and asymmetric cycle demonstrates significantly faster downward motion (and enhances swimming efficiency) by reducing the vorticity strength and viscous energy dissipation in the surrounding fluid. The study provides insights into the induced flow and opens avenues for bio-inspired aquatic robots made from magnetic-responsive soft materials with the potential for fostering underwater propulsion and exploration in aquatic environments. | | | | | | Sankalp Nambiar and J. S. Wettlaufer Phys. Rev. Fluids 9, 023102 (2024) – Published 14 February 2024  | We study the fluid mediated hydrodynamics of an orientable microscopic swimmer that is near a deformable boundary, and that can intrinsically execute random orientation changes. Accounting for swimmer reorientations via orientation tumbles or active Brownian rotations on time scales comparable to the boundary deformations, we find that a pusher-type swimmer can rotate away from the interface, while its attraction towards the interface is enhanced. Depending on the viscosity of the fluids on either side of the interface, the swimmer can experience a stronger migration towards the interface at short times, and away from the interface in the long-time limit. | | | | | | Complex and Non-Newtonian Fluids | Sumit Kumar Mehta and Pranab Kumar Mondal Phys. Rev. Fluids 9, 023301 (2024) – Published 5 February 2024  | This work finds that the yield stress (YS) of electrically actuated viscoplastic fluids significantly influences mixing and aggregation phenomena. When shear force is applied below the YS limit, these fluids exhibit solid-like behavior due to their viscoplasticity. This is commonly observed in biological fluids like blood used in applications where rapid mixing is crucial. The local modulation of electrical forcing is achievable due to the ionic interaction between the solid and liquid phases. This leads to the observation of distinct flow structures, found to be influenced by the YS. Additionally, the likelihood of constituent particles aggregating is strongly correlated with YS. | | | | | | Rajnandan Borthakur and Uddipta Ghosh Phys. Rev. Fluids 9, 023302 (2024) – Published 13 February 2024  | Electrophoresis often coexists with imposed background flows in many applications. When particles carry nonuniform surface charge, and the fluid itself is complex, the particles may follow fascinating trajectories, as shown in this work. Indeed, the combined action of a background flow and complex fluidic medium may cause particles to undergo cross-stream migration. However, the very nature of its trajectory and the extent of its migration depends on whether the imposed flow or the electrophoretic propulsion dominates the motion. The insights provided here may be exploited for improving electrophoretic separation and sorting of particles based on their size and surface charge. | | | | | | Ilya Barmak, Davide Picchi, Alexander Gelfgat, and Neima Brauner Phys. Rev. Fluids 9, 023303 (2024) – Published 13 February 2024  | A rigorous numerical solution for steady laminar flows of shear-thinning fluids in rectangular ducts is presented for the first time. We derive universal scaling laws for the effective viscosity that depends on the dimensionless rheological parameters and the effective channel size that is a function of the aspect ratio of the duct. These allow us to generalize the classical formula for the friction factor of Newtonian flows (12/Re) to Carreau fluids flowing in rectangular ducts of any aspect ratio, where the Reynolds number is based on the effective channel size and the effective viscosity. | | | | | | Editors' Suggestion Thomasina V. Ball and Neil J. Balmforth Phys. Rev. Fluids 9, 023304 (2024) – Published 27 February 2024 | When a small fraction of viscoplastic fluid is placed into a rotating cylinder, steady states can be reached with pool at the bottom of the cylinder and a residual coating elsewhere. Lubrication theory used to model the film thickness builds on previous models by bridging between two asymptotic limits, incorporating both the gravitational force along the cylinder and the hydrostatic pressure gradients. The model predicts steady states are reached after a small number of rotations and allows exploration of drainage when the cylinder comes to a halt. Experiments using a Carbopol suspension provide a suitable comparison to test the thin film theory. | | | | | | Camille Steinik, Davide Picchi, Gianluca Lavalle, and Pietro Poesio Phys. Rev. Fluids 9, 023305 (2024) – Published 28 February 2024  | We studied the filling dynamics of a shear-thinning fluid in a capillary tube. In regimes where inertial effects can be neglected, we generalize the Lucas-Washburn scaling relation to shear-thinning fluids, showing that the classical 1/2 scaling law holds only if an ad hoc time-dependent effective viscosity that applies to both Newtonian and shear-thinning fluids is introduced. In regimes where inertia competes with viscous and gravity effects, the system shows an oscillating behavior. The shear-thinning effect acts on the system, favoring such oscillating behavior. | | | | | | Compressible and Rarefied Flows, Kinetic Theory | Sergiu Busuioc and Victor Sofonea Phys. Rev. Fluids 9, 023401 (2024) – Published 22 February 2024  | Numerical solutions of the Enskog equation obtained employing a Finite-Difference Lattice Boltzmann (FDLB) with half-range Gauss-Hermite quadratures and a Direct Simulation Monte Carlo (DSMC)-like particle method (PM), are systematically compared to determine the range of applicability of the simplified Enskog collision operator implemented in the Lattice Boltzmann framework. For low to moderate reduced density, the proposed FDLB model exhibits commendable accuracy for all bounded flows tested in this study, with substantially lower computational cost than the PM method. | | | | | | Ronald du Puits Phys. Rev. Fluids 9, 023501 (2024) – Published 13 February 2024  | Roughness at a surface hotter or colder than its environment may significantly enhance the convective heat transfer coefficient. This enhancement results from modification of the near-wall flow field. Our work demonstrates how roughness elements deform the temperature field compared to a smooth surface, and, under which conditions the heat transfer coefficient is enhanced. | | | | | | Roshan Samuel and Mahendra K. Verma Phys. Rev. Fluids 9, 023502 (2024) – Published 16 February 2024  | Through high-resolution direct numerical simulations, we demonstrate that energy transfers in two-dimensional (2D) thermal convection exhibit Bolgiano-Obukhov scaling. This is in contrast with convection in three dimensions which follows the Kolmogorov-Obukhov phenomenology. This difference arises from the presence of inverse cascade in 2D which leads to a negative kinetic energy flux at small wavenumbers. The magnitude of this flux decreases with wavenumber due to the effect of buoyancy. We also demonstrate that entropy dissipation in the thermal boundary layers displays a scaling law that is observable from the entropy flux curves. | | | | | | Xin-Yuan Chen (陈新元), Juan-Cheng Yang (阳倦成), and Ming-Jiu Ni (倪明玖) Phys. Rev. Fluids 9, 023503 (2024) – Published 20 February 2024  | In this experimental investigation we examine the dynamics of liquid metal thermal convection within an elongated cuboid cell with aspect ratio 1/3. We highlight the evolution of flow modes, transitioning from single- to double-roll mode, and transitional modes which are reconstructed by visualizing temperature data on the sidewall. As the Rayleigh number surpasses the critical value, the flow state transforms from a multiple-mode coexistence state to a single-roll mode-dominated configuration. Flow modes and global transport properties offer profound insights into the fundamental characteristics of thermal convection in liquid metals under strong lateral geometric confinement. | | | | | | Drops, Bubbles, Capsules, and Vesicles | Pengyu Shi Phys. Rev. Fluids 9, 023601 (2024) – Published 6 February 2024  | Bubbles rising near a vertical wall are known to bounce repeatedly when the Reynolds number (Re) exceeds about 65. This behavior contradicts potential flow theory, which suggests a consistently attractive transverse force. In this study, we demonstrate through numerical simulations that below a critical Re-dependent separation, the transverse force decreases with decreasing separation, potentially reversing from attractive to repulsive. We believe this reversal is responsible for the bouncing motion observed in freely near-wall rising bubbles. | | | | | | Alireza Mashayekhi, Coralie Vazquez, Hongying Zhao, Michael Gattrell, James F. Gilchrist, and John M. Frostad Phys. Rev. Fluids 9, 023602 (2024) – Published 8 February 2024  | In prior work, it was observed that some bitumen droplets coalesced faster when colliding in shear than colliding head-on. Inspired by this observation, we aimed to reproduce the same behavior in a simpler system composed of pure oil, water, and surfactants/particles. Using a cantilevered-capillary force apparatus we observed this phenomenon for droplets stabilized by cellulose nanocrystals and coined the term "shear-triggered coalescence" to describe it. | | | | | | Electrokinetic Phenomena, Electrohydrodynamics, and Magnetohydrodynamics | Arunraj Balaji-Wright, Felix Stockmeier, Richard Dunkel, Matthias Wessling, and Ali Mani Phys. Rev. Fluids 9, 023701 (2024) – Published 23 February 2024  | The Poisson-Nernst-Planck-Stokes equations capture the chaotic dynamics of electroconvection accurately, but direct numerical simulation of electroconvection is prohibitively expensive. Furthermore, prediction of the mean fields via application of Reynolds averaging leads to a closure problem. In this work, we combine the macroscopic forcing method, a numerical technique for measurement of closure operators in Reynolds-averaged equations, with high-fidelity experimental data in order to determine a leading order closure for chaotic electroconvection. Simulations of the Reynolds-averaged equations using the leading order closure accurately predict experimental polarization curves. | | | | | | Instability, Transition, and Control | Changwoo Kang, Michael F. Schatz, and Parisa Mirbod Phys. Rev. Fluids 9, 023901 (2024) – Published 28 February 2024  | Flow states in dispersed particle flow determine the performance in industrial applications such as chemical mixers and bioreactors. Hysteresis in flow transitions can modify the flow condition and thus can affect the efficiency. We numerically show hysteretic behaviors in the Taylor-Couette flow of a noncolloidal suspension with a rotating inner cylinder and a stationary outer one. We also examine a standing wave of weak counterrotating vortices, known as ribbons, that occurs as the primary instability. | | | | | | Interfacial Phenomena and Flows | Juan S. Marin Quintero, Butunath Majhy, and Prashant R. Waghmare Phys. Rev. Fluids 9, 024001 (2024) – Published 7 February 2024  | This work analyzes the transient wetting dynamics of droplets in viscous media with appropriate governing parameters and examines the drop's transient response immediately after the actuation, the drop's retraction and resultant dynamics, and the effect of multiple wave actuation on the droplet transition. | | | | | | Garima Singh and Naveen Tiwari Phys. Rev. Fluids 9, 024002 (2024) – Published 13 February 2024  | The stability of a traveling wave on the outside of a heated solid vertical cylinder is considered. The axisymmetric traveling wave becomes unstable and leads to the formation of asymmetric droplets over the cylinder. Eigenvalue analysis indicates that at smaller wavenumbers, the gravity mode exists, and a thermocapillary mode also exists but at larger wavenumbers. Patterns generated using nonlinear analysis show that the selected mode is governed by the geometric confinement in the azimuthal direction. Interesting nonlinear patterns are obtained due to an interesting interplay between the gravity mode and thermocapillary mode. | | | | | | Editors' Suggestion Ryuta X. Suzuki, Shoji Seya, Takahiko Ban, Manoranjan Mishra, and Yuichiro Nagatsu Phys. Rev. Fluids 9, 024003 (2024) – Published 14 February 2024 | We experimentally investigate displacement of a more viscous liquid by a less viscous one in a Hele-Shaw cell using an aqueous two phase system, where phase separation occurs in the growing liquid-liquid interfacial region, by varying the injection flow rate and the phase separation rate. We show that the degree of the interfacial phase separation scales as a unique function of the ratio of the flow and phase separation rates and it decreases with the ratio. These results demonstrate that the interfacial phase separation is arrested by the imposed flow and determined by competition between the flow and phase separation rates. The arresting effect and the mechanism are numerically verified. | | | | | | Laminar and Viscous Flows | Bjarne Vincent, Sophie Miralles, Daniel Henry, Valéry Botton, and Alban Pothérat Phys. Rev. Fluids 9, 024101 (2024) – Published 29 February 2024  | The acoustofluidic helix stirs fluid within a closed container efficiently with only one ultrasound source. The helical shape is obtained by reflecting the acoustic beam on the cavity walls. This acoustic forcing drives multiple descending jets, each impinging a vertical wall and wrapping around the central axis. Time-averaged and low-frequency unsteady flow structures have been obtained by three-dimensional particle tracking velocimetry and Eulerian field reconstructions. Both the velocity amplitudes of the overall time-averaged flow, and the vortex dynamics, depend on the dimensionless acoustic force magnitude called the acoustic Grashof number. | | | | | | Editors' Suggestion Vira Dhaliwal, Christian Pedersen, Kheireddin Kadri, Guillaume Miquelard-Garnier, Cyrille Sollogoub, Jorge Peixinho, Thomas Salez, and Andreas Carlson Phys. Rev. Fluids 9, 024201 (2024) – Published 8 February 2024 | Liquid nanofilms are subject to rupture due to intermolecular forces triggered by surface perturbations arising from thermal fluctuations. When a shear stress is imposed at the free surface it becomes stable in the direction of shear, but perturbations can still grow in the perpendicular direction to the shear. | | | | | | S. Tomasi Masoni, A. Mariotti, M. Antognoli, C. Galletti, R. Mauri, M. V. Salvetti, and E. Brunazzi Phys. Rev. Fluids 9, 024202 (2024) – Published 15 February 2024  | Understanding flow regimes and mixing in microreactors is crucial for achieving high reaction yields. This study combines simulations and experiments in an X-microreactor up to Reynolds number (Re) 600. For Re > 375, an unsteady periodic regime is observed with a collapsing central vortical structure and symmetric vorticity shedding. Counterrotating vortices form, merge, and recreate the central vortex. Despite increased mixing in this regime, reaction yield remains similar due to reduced reactant residence time. A model predicting reaction yield based on mixing degree and nominal kinetic constant is developed, successfully encompassing all flow regimes and Damköhler numbers (0.1 Da 103). | | | | | | Calum Mallorie, Rohan Vernekar, Benjamin Owen, David W. Inglis, and Timm Krüger Phys. Rev. Fluids 9, 024203 (2024) – Published 21 February 2024  | Deterministic lateral displacement (DLD) is a common method of separating suspensions of particles by their physical properties. DLD devices are typically limited to operation in the Stokes flow regime, which leads to high processing times, because their behavior becomes unpredictable at flow rates where fluid inertia is important. In this study, we show that the average flow direction in a typical DLD device can diverge from the direction of the applied pressure drop due to inertial effects, and present an explanation for why this happens. This new understanding may contribute to improved DLD designs for operation at high flow rates. | | | | | | Multiphase, Granular, and Particle-Laden Flows | Hamad El Kahza and Pejman Sanaei Phys. Rev. Fluids 9, 024301 (2024) – Published 16 February 2024  | Using the Stokes equation for fluid flow and the advection-diffusion equation for the transport of solids, alongside threshold laws governing erosion and deposition, we present a model aimed at conducting a comprehensive analysis of both erosion and deposition processes within a porous medium composed of axisymmetric channels. | | | | | | J. Meibohm, L. Sundberg, B. Mehlig, and K. Gustavsson Phys. Rev. Fluids 9, 024302 (2024) – Published 16 February 2024  | Caustic singularities of the spatial distribution of particles in turbulent aerosols enhance collision rates and accelerate coagulation. The rate of caustic formation depends sensitively on the particle inertia. We study caustic formation in a non-Gaussian statistical model to understand why there is a significant difference in formation rates between direct numerical simulations and Gaussian models. In the limit of small inertia, caustics form due to an optimal fluctuation of the Lagrangian fluid-velocity gradients, and we show that the formation rate depends sensitively on the tails of the gradient distribution, explaining the observed mismatch | | | | | | Rasmus Korslund Schlander, Stelios Rigopoulos, and George Papadakis Phys. Rev. Fluids 9, 024303 (2024) – Published 20 February 2024  | We characterize the role of coherent structures on the transport and wall deposition of low-inertia particles in a turbulent pipe flow using extended proper orthogonal decomposition (EPOD) and spectral analysis. The equilibrium Eulerian approach is employed to model particle velocity and concentration. A new Fukagata-Iwamoto-Kasagi (FIK) identity is derived for the wall deposition rate coefficient (Sherwood number) and employed to quantify the contributions of the mean and fluctuating velocity and particle concentration fields for different Stokes, Froude and Reynolds numbers. New terms appear due to the inertia of the particles that encapsulate the turbophoresis effect. | | | | | | Benjamin Fry, Laurent Lacaze, Thomas Bonometti, Pierre Elyakime, and François Charru Phys. Rev. Fluids 9, 024304 (2024) – Published 20 February 2024  | Granular bedload plays a crucial role in shaping streams and influencing their development over time. This process involves the movement of grains along the stream bed surface driven by the shear stress from the flowing water. For an accurate model on a practical scale, it is essential to grasp the key properties of the granular layer interacting with the water. Our focus lies in understanding the rheological characteristics of the water grain mixture when grain movement is localized within a thin layer near the bed surface and under a weakly inertial regime. This aims to expand our understanding of viscous-laminar properties mostly described for a thick shear layer in the literature. | | | | | | O. M. Lavrenteva, I. Smagin, and A. Nir Phys. Rev. Fluids 9, 024305 (2024) – Published 20 February 2024  | A novel approach to study of the shear-induced diffusion of particles in suspensions with continuous particle size distribution is suggested. It addresses the migration of local moments of the size distribution. Particle size distribution at each point is consequently obtained from the resulting moments, by solving inverse problems locally. For a particular example of stationary flow in a circular tube, we present results that include concentration inhomogeneity, moments' distributions and the consequent local continuous particle size distributions. The similarity and difference from cases of monodispersed suspensions are discussed. | | | | | | Editors' Suggestion A. Bajpai, A. Bhateja, and R. Kumar Phys. Rev. Fluids 9, 024306 (2024) – Published 23 February 2024 | In this investigation, a novel two-way coupled gas-granular solver is developed, incorporating direct simulation Monte Carlo (DSMC) for gas particle collisions and discrete element method (DEM) for granular particle interactions. Gas-grain interaction model consists of momentum and energy exchange between the two phases. Using this framework, we have performed a comprehensive study of dust dispersion due to plume impingement on a lunar surface. We have predicted not only the velocity field of gas and grain phases, but also their temperature field, which can be meaningful information for spacecraft designers. | | | | | | Zilong Zhao, Zhiwei Guo, Zhigang Zuo, and Zhongdong Qian Phys. Rev. Fluids 9, 024307 (2024) – Published 23 February 2024  | This study indicates that small inertia particles can be trapped in a two-dimensional unequal-strength counter-rotating vortex pair (CVP) flow. Through analytical derivations of the particle motion in the potential CVP flow, this study first identifies a particle-attracting ring S0. | | | | | | Sam Briney, Andreas N. Osnes, Magnus Vartdal, Thomas L. Jackson, and S. Balachandar Phys. Rev. Fluids 9, 024308 (2024) – Published 27 February 2024  | A shock propagating through a cloud of particles results in highly unsteady forces on each particle in the cloud. In such a finite particle volume fraction scenario, reflected shocks from neighboring particles perturb the force on each particle from its value in the dilute limit. These forces have a delayed onset since the reflected shocks travel at finite speeds. This phenomenon is explored in detail using three-dimensional particle resolved simulations and a model is proposed to account for the unsteady nature of these forces. | | | | | | Nonlinear Dynamical Systems | T. Lichtenegger Phys. Rev. Fluids 9, 024401 (2024) – Published 20 February 2024  | Computational fluid dynamics simulations of dynamic flows usually entail large numerical costs. Over the last few years, several data-driven methods, partly of significant complexity, have been devised to mitigate CPU times while still capturing the relevant physics. In this work, a very simple approach with a clear physical interpretation is presented that splits the problem into a linear and a nonlinear part. Based on a database of precomputed reference states, predictions are made using the method of analogues (nonlinear dynamics) together with physics-informed propagators that capture and correct for any deviation from the nearest reference state in a linear fashion. | | | | | | Daniel Morón and Marc Avila Phys. Rev. Fluids 9, 024601 (2024) – Published 8 February 2024  | Pulsatile pipe flow, or the flow in a pipe with a mean and one or more harmonic bulk velocity components, is a benchmark to study unsteady driven flows in industrial and biological applications. We study the transitional regime of pulsatile pipe flow at moderate-to-high amplitudes and intermediate pulsation frequencies. We show that, as in steady driven pipe flow, the first long-lived turbulent structures are localized. We combine direct numerical simulations, causal analysis and turbulence modeling to describe the behavior of these turbulent patches in pulsatile pipe flow, and to determine the physical mechanisms by which they survive the pulsation. | | | | | | Vitaliy Kaminker Phys. Rev. Fluids 9, 024602 (2024) – Published 13 February 2024  | Turbulent power is introduced in to Jupiter's magnetosphere near the cen- trifugal equator of the plasma disc. Turbulent fluctuations are generated within the plasma fluid. Turbulent energy then travels out of the plasma disc via shear Alfvén waves, dissipating out of the system along the way. | | | | | | Linqi Yu, Mustafa Z. Yousif, Young-Woo Lee, Xiaojue Zhu, Meng Zhang, Paraskovia Kolesova, and Hee-Chang Lim Phys. Rev. Fluids 9, 024603 (2024) – Published 20 February 2024  | This study developed a mapping generative adversarial network (M-GAN) to predict unavailable parameters: streamwise velocity, temperature, and pressure from available velocity components. Two-dimensional Rayleigh–Bénard flow and turbulent channel flow are used to evaluate M-GAN performance. The results indicate that the proposed model has good capability to map the available parameters to unavailable parameters. Furthermore, M-GAN also has good generalization to predict the parameters from channel flows at different Reynolds numbers. | | | | | | Vincent Labarre, Pierre Augier, Giorgio Krstulovic, and Sergey Nazarenko Phys. Rev. Fluids 9, 024604 (2024) – Published 20 February 2024  | We analyze direct numerical simulations of stratified turbulence without shear modes, and with or without vortical modes at various Froude and buoyancy Reynolds numbers. It allows us to investigate the effects of vortical modes on the dynamics of stratified flows. A spatiotemporal analysis reveals slow internal gravity waves interacting by triadic resonance instabilities in our strongly stratified flow simulations such as the one in the figure. We observe that removing vortical modes helps to concentrate the energy around the wave frequency, but it is not enough to observe a weak internal gravity wave's turbulence regime. | | | | | | Editors' Suggestion Brian F. Farrell and Petros J. Ioannou Phys. Rev. Fluids 9, 024605 (2024) – Published 21 February 2024 | In wall turbulence, the time-mean flow is returned to after almost any disturbance, which indicates that it is a stable statistical feature underlying the disorder of turbulence. However, the stability of this statistical feature can not be determined directly from the stability of the time-mean flow itself. What is required is a statistical stability analysis method. We determine the statistical stability of the time-mean state by averaging the dynamics of the return to the time-mean state over the turbulent attractor using the linear inverse model method. | | | | | | Zhanqi Tang, Nan Jiang, Zhiming Lu, and Quan Zhou Phys. Rev. Fluids 9, 024606 (2024) – Published 26 February 2024  | Tripping effects are studied in artificially thickened turbulent boundary layers (AT-TBLs) within a finite-length test section. The emergence of the generated large-scale structures highlights the potential of the AT-TBLs to simulate high-Reτ boundary layer turbulence. We examine the noncanonical behaviors and external similarity under over-tripped conditions. The results emphasize the need for caution when pursuing excessive thickening of the boundary layer through leading-edge trips for generating high-Reτ canonical TBLs in a finite-length test section. | | | | | | Dana Duong and Stavros Tavoularis Phys. Rev. Fluids 9, 024607 (2024) – Published 27 February 2024  | This article is the first to investigate velocity fields behind grids at very small Reynolds numbers that include flows with negligible fluctuations. Measurements were taken behind four square-mesh grids with varying designs, mesh sizes and solidities. We have documented and quantified the weakening of turbulent behavior as the Reynolds number diminishes and identified trends and patterns of the large- and small-scale anisotropies, the skewness and flatness factors of the velocity derivative and the dissipation parameter that have not been reported previously. | | | | | | Editors' Suggestion Qinmin Zheng, Tianyi Li, Benteng Ma, Lin Fu, and Xiaomeng Li Phys. Rev. Fluids 9, 024608 (2024) – Published 29 February 2024 | In this work, we propose a novel framework for the high-fidelity reconstruction of large-area damaged turbulent fields with high resolution based on a physically constrained generative adversarial network. The network leverages complete/sparse fields of velocity components as physical constraints and adopts a PatchGAN discriminator network. The proposed reconstruction framework has been shown to achieve excellent reconstruction performance. The reconstructed flow fields are consistent with the raw flow fields in terms of magnitude, power spectrum, and two-point correlation function. | | | | | | Suhas S. Jain, Rahul Agrawal, and Parviz Moin Phys. Rev. Fluids 9, 024609 (2024) – Published 29 February 2024  | A localized artificial-diffusivity method is developed for capturing discontinuities, such as shocks and contacts, in compressible flows. A new sensor for contact discontinuity makes the method more localized, and a discretely consistent formulation eliminates the need for filtering the solution or filtering the sensors to obtain robust solutions. Improved predictions are observed in canonical shock-tube problems and large-eddy simulations of homogeneous isotropic turbulence. | | | | | | Thomas Frisch, Sergey Nazarenko, and Sergio Rica Phys. Rev. Fluids 9, 024701 (2024) – Published 26 February 2024  | The dynamics of a superfluid over a surface exhibits significant differences from compressible flow in ordinary fluids. In ordinary fluids, when the local speed exceeds the sound speed, intrinsic dissipation due to viscosity can enable a shock wave. However, in superfluids, lack of dissipation prevents shock waves. Instead, spatial modulation of a wave train of solitons, as in the figure, allows for a smooth transonic transition. This wave train has been observed to eventually become unstable, leading to quantized vortices and an effective drag on a rough surface. The wave train occurs beneath a lambda-shaped structure with a fore and back-front, reminiscent of ordinary compressible fluids. | | | | | | | |
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