# Physical Review Fluids

### Flow induced by the rotation of two circular cylinders in a viscous fluid

Author(s): E. Dormy and H. K. Moffatt

Stokes flow driven by rotation of two parallel cylinders inside a cylinder of large radius R0 is investigated, and the flow in this triply-connected domain is determined numerically, with particular focus on the narrow-gap situation, when the local behavior is well described by lubrication theory. The asymptotic situation for infinite R0 is inferred (i) when the cylinder axes are unconstrained, and (ii) when they are held fixed. Contributions to the far-field are identified: a torquelet and a radial quadrupole in the counter- and co-rotating cases, respectively. In the former case, when the cylinders make contact (zero gap) a contact force acting on the cylinder pair is identified.

[Phys. Rev. Fluids 9, 044102] Published Mon Apr 22, 2024

### Sedimentation of a single soluble particle at low Reynolds and high Péclet numbers

Author(s): Nan He, Yutong Cui, David Wai Quan Chin, Thierry Darnige, Philippe Claudin, and Benoît Semin

We report experiments on the dissolution of a single particle during its sedimentation in a quiescent aqueous solution in the regime of low Reynolds and high Péclet numbers. We use butyramide, a chemical which does not change the density of water when it dissolves. The particle shrinks at a rate independent of its initial radius, in agreement with the model that we derive assuming Stokes drag and a mass transfer rate given by Levich (1962). This model becomes quantitative when including two correction factors to account for the non-sphericity of the particle and for the inclusions of air bubbles inside the particle.

[Phys. Rev. Fluids 9, 044502] Published Mon Apr 22, 2024

### Straight to zigzag transition of foam pseudo-Plateau borders on textured surfaces

Author(s): Alexis Commereuc, Sandrine Mariot, Emmanuelle Rio, and François Boulogne

The structure of liquid foams follows simple geometric rules formulated by Plateau 150 years ago. On smooth surfaces, the foam liquid channels, also called pseudo Plateau borders, are straight between vertices. We demonstrate experimentally that on rough surfaces and under some conditions that we establish, the bubble footprint exhibits a morphological transition. The footprint can adopt a zigzag shape between vertices. We rationalize the number of zigzag segments by a geometric distribution describing the observations made with the footprint perimeter and the mesh size of the asperities.

[Phys. Rev. Fluids 9, L041601] Published Mon Apr 22, 2024

### Direct comparison of density-driven convective mixing in a three-dimensional porous medium using experiments and simulation

Author(s): Rebecca Liyanage, Xiaojing Fu, Ronny Pini, and Ruben Juanes

We examine how fluids mix in three-dimensional (3D) porous materials due to differences in density which represent one mechanism of underground carbon dioxide storage. The experiment closely matched the simulation in terms of the patterns and speed of mixing. Interestingly, the experiment reveals columnar plumes self-organizing into a reticular pattern, previously seen only in 3D simulations. Results demonstrate quantitative matching over time in concentration, variance, scalar dissipation rate, and dissolution flux. A new relation between dissipation rate and flux is established, highlighting a 30% higher flux in 3D versus 2D systems, affirming prior estimations.

[Phys. Rev. Fluids 9, 043802] Published Thu Apr 18, 2024

### Instability and trajectories of buoyancy-driven annular disks: A numerical study

Author(s): G. Corsi, P. G. Ledda, G. Vagnoli, F. Gallaire, and A. De Simone

Seed dispersal strategies exemplify the role of morphology in defining falling paths. Here, macroscopic geometry effects are systematically studied by considering trajectories of annular disks. Via linear stability analysis, we identify the stability boundary for vertical fall, with a non-monotonic behavior of the critical falling velocity with the hole size, before increasing for large holes. Nonlinear simulations confirm linear analyses and suggest strategies for annular seed release at different heights, with the emergence of paths possibly beneficial for controlled positioning or advantageous for covering large lateral distances, depending on the hole size and disk weight.

[Phys. Rev. Fluids 9, 043907] Published Wed Apr 17, 2024

### Extended spreading of saline droplets upon impact on a frosty surface

Author(s): Hao Zeng, Feng Wang, and Chao Sun

The impact and freezing process of a water droplet are studied experimentally, incorporating the presence of frost and salt. A distinct transition of the spreading dynamics is observed, altering from the well-known 1/2 inertial scaling law to a 1/10 capillary-viscous scaling law. By considering the effect of impact inertia, partial-wetting behavior, and salinity, the mechanism of this transition is elucidated, and a unified model for predicting the droplet arrested diameter is proposed.

[Phys. Rev. Fluids 9, 044001] Published Tue Apr 16, 2024

### Degrees of freedom and the dynamics of fully developed turbulence

Author(s): Diego Donzis and Shilpa Sajeev

Turbulent flows comprise a multitude of scales which make them extremely difficult to analyze and simulate. In this work, we study homogeneous isotropic turbulence using a novel approach which solves the Navier-Sokes equations on a reduced set of modes that are sampled stochastically. The complementary set are solved using trivial dynamics. This method, called Selected Eddy Simulations, is able to capture broad dynamics of turbulence with just 10% of resolved modes, suggesting the turbulence attractor may be smaller or more robust to modeling than previously thought. This result also holds promise for developing alternative low-cost numerical approaches to study turbulent flows.

[Phys. Rev. Fluids 9, 044605] Published Mon Apr 15, 2024

### Electrocapillary, thermocapillary, and buoyancy convection driven flows in the Melcher-Taylor experimental setup

Author(s): Alexander Yu. Gelfgat and Gerrit Maik Horstmann

Electrocapillary flows in a classical experiment of Melcher and Taylor are studied. The computed streamlines qualitatively represent the experimental image. With the increase of electrocapillary forcing, the main circulation localizes near a boundary with a larger electric potential. When a dielectric liquid is replaced by a poorly conducting one, the system becomes non-isothermal owing to the Joule heating, and the flow is driven also by buoyancy and thermocapillary convection. The results show that consideration of the two-phase model is mandatory. The Lippmann equation, connecting electrically induced surface tension with nonuniform surface electric potential, is numerically verified.

[Phys. Rev. Fluids 9, 044101] Published Fri Apr 12, 2024

### Turbulence modulation by suspended finite-sized particles: Toward physics-based multiphase subgrid modeling

Author(s): S. Balachandar, C. Peng, and L.-P. Wang

The presence of a dispersed phase substantially modifies small-scale turbulence. Here we present a comprehensive mechanistically based model to predict turbulence modulation, the predictions of which, compared with particle-resolved simulations and experiments, is shown.

[Phys. Rev. Fluids 9, 044304] Published Thu Apr 11, 2024

### Particle hydrodynamics in acoustic fields: Unifying acoustophoresis with streaming

Author(s): Xiaokang Zhang, Jake Minten, and Bhargav Rallabandi

Acoustic fields are used in numerous applications to induce movement of suspended particles. We develop a rigorous theory that systematically unifies inviscid acoustophoresis with viscous streaming. The theory connects particle motion to a generalized form of the secondary radiation force, which depends on the Stokes layer thickness around the particle, and the contrast of density and compressibility between the particle and the fluid. We identify a reversal of particle motion when inertial and viscous forces are comparable, validated with numerical solutions. This has significant implications for applications involving particle sorting or focusing based on size or material properties.

[Phys. Rev. Fluids 9, 044303] Published Wed Apr 10, 2024

### Transient energy growth in channel flow with compliant walls

Author(s): Frédéric Alizard, Benoît Pier, and Smail Lebbal

Shear flows in contact with compliant boundaries exhibit a rich dynamics involving traveling-wave-flutter, Tollmien-Schlichting, as well as divergence instabilities. In this context, maximum transient growth effects are the result of optimal energy exchanges during the fluid-structure interaction process. The present investigation studies the detailed contribution of the different interacting modes. In particular, it is found that the optimal gain may be associated with a large-amplitude oscillatory behavior and that wall-compliance enhances this phenomenon.

[Phys. Rev. Fluids 9, 043905] Published Tue Apr 09, 2024

### Lagrangian coherent structures control solute dispersion in heterogeneous poroelastic media

Author(s): Junhong Wu, Daniel Lester, Michael G. Trefry, and Guy Metcalfe

This study focuses on how Lagrangian coherent structures (LCSs) control solute dispersion in heterogeneous poroelastic media (HPM). We show how interactions between medium compressibility, conductivity heterogeneity, and periodic forcing give rise to complex flows and diverse LCS types (KAM islands, chaotic saddles, etc) that have profound impacts on diffusive solute transport (main image) that do not arise in the steady counterpart (inset). Strongly anomalous transport impacts both spatial moments and residence time distributions and persists at low Péclet numbers. This study reveals the complex transport phenomena that can arise in HPM and shows how LCSs govern solute dispersion.

[Phys. Rev. Fluids 9, 044501] Published Tue Apr 09, 2024

### Lagrangian modeling of a nonhomogeneous turbulent shear flow: Molding homogeneous and isotropic trajectories into a jet

Author(s): Bianca Viggiano, Thomas Basset, Mickaël Bourgoin, Raúl Bayoán Cal, Laurent Chevillard, Charles Meneveau, and Romain Volk

We propose a novel approach to accurately model complex flow, ultimately predicting the behavior of a turbulent jet by molding a set of velocity signals input from an idealized flow (readily available from numerical databases online). The model uses fundamental properties of the jet, such as velocity means and standard deviations, easily accessible from textbooks, experiments, or low-order simulations (RANS, LES). The modeled jet reproduces many subtle and intricate properties of the turbulent flow, including the intermittent extreme events known to exist in turbulent flows, which have been typically thought of as not capable of being captured with current modeling techniques.

[Phys. Rev. Fluids 9, 044604] Published Tue Apr 09, 2024

### Experimental study of the turbulence ingestion noise of rotor blades

Author(s): Han Wu, Yuhong Li, Xin Zhang, Siyang Zhong, and Xun Huang

In this work, we experimentally investigate turbulence ingesting rotor noise under various thrusting states. The broadband noise caused by turbulence ingestion is found to dominate at the normalized frequency range of fR/U∞ = 20 to 80 when the rotor is under low-thrusting conditions. Results also suggest that the turbulence ingestion broadband noise can be scaled by Mach number scaling of M∞2Mc4, where M∞ is the freestream Mach number, and Mc is the corresponding blade tip Mach number.

[Phys. Rev. Fluids 9, 044801] Published Tue Apr 09, 2024

### Internally heated and fully compressible convection: Flow morphology and scaling laws

Author(s): Whitney T. Powers, Evan H. Anders, and Benjamin P. Brown

In stars and planets natural processes heat convective flows in the bulk of a convective region rather than at hard boundaries. Internally heated convection has been studied extensively in incompressible fluids, but the effects of stratification and compressibility have not been examined in detail. In this work, we study fully compressible convection driven by a spatially uniform heating source in a suite of two- and three-dimensional Cartesian, hydrodynamic simulations. We characterize how Mach, Reynolds, and Nusselt numbers scale with the characteristic strength of the internal heat source. We also measure kinetic energy power spectra and discuss the flow morphologies.

[Phys. Rev. Fluids 9, 043501] Published Tue Apr 09, 2024

### Floquet stability analysis of pulsatile flow in toroidal pipes

Author(s): J. Simon Kern, Valerio Lupi, and Dan S. Henningson

Unsteady flows in curved pipes are ubiquitous in science and engineering but their stability characteristics are not well understood in most cases of practical interest. We study the linear stability of pulsatile flow in the archetypal configuration of a toroidal pipe, which appears e.g. in aortic blood flow. The Floquet stability analysis of the harmonically forced system reveals that the curvature leads to nonlinear interactions in the baseflow andconsiderable stabilization that can be orders of magnitude larger than in the corresponding planar case. The figure shows a typical snapshot of the streamwise velocity field in a torus subject to a pulsating pressure gradient.

[Phys. Rev. Fluids 9, 043906] Published Tue Apr 09, 2024

### Dense fluid transport through nanoporous graphene membranes in the limit of steric exclusion

Author(s): Runfeng Zhou, Mathew M. Swisher, Akshay Deshmukh, Chengzhen Sun, John H. Lienhard, and Nicolas G. Hadjiconstantinou

We develop a model that describes the permeance of simple fluids as well as small hydrocarbon molecules through nanoporous, atomically thin membranes. The model is in agreement with molecular dynamics simulations for a wide range of pore sizes, including pores approaching the steric exclusion limit, as needed for understanding separation processes using such membranes.

[Phys. Rev. Fluids 9, 044202] Published Tue Apr 09, 2024

### Effects of wall conductivities on magnetoconvection in a cube

Author(s): Hai-Tao Zhu, Long Chen, and Ming-Jiu Ni

This numerical simulation investigates the vertical convection of liquid metal with varying magnetic fields and wall conductivities. The applied horizontal magnetic field alters plume dynamics and topology, leading to a more coherent large-scale flow structure but weakening convection through Joule dissipation. This competition between rectification and magnetic damping determines the magnetic field’s impact on heat transfer, with the quasi-two-dimensional state being the threshold. Our analysis demonstrates that while the plume area remains constant, condensation of coherent structures enhances horizontal heat transport per unit area, significantly improving overall heat transfer.

[Phys. Rev. Fluids 9, 043701] Published Mon Apr 08, 2024

### Dynamics of rigid fibers interacting with triangular obstacles in microchannel flows

Author(s): Zhibo Li, Clément Bielinski, Anke Lindner, Olivia du Roure, and Blaise Delmotte

We combine experiments and numerical simulations to investigate the interaction between a rigid fiber and a triangular obstacle in a microfluidic channel. We find different dynamics depending on the initial position and orientation of the fiber. We show that these dynamics are dictated by the fiber configuration in the vicinity of the obstacle. Some dynamics induce a cross-stream migration which grows with the fiber length. Our findings could in the future be used to design and optimize microfluidic sorting devices to sort rigid fibers by length.

[Phys. Rev. Fluids 9, 044302] Published Mon Apr 08, 2024

### Transitional pressure drop in a cavitied microchannel

Author(s): Yan Yan Liu, Sjouke W. Schekman, Mo Xiao Li, Tian Jian Lu, and Tongbeum Kim

A flow progression from laminar flow slippage to rotational vortices in cavitied microchannels is shown to cause a reversal of flow resistance, i.e. a reduced flow resistance at low Reynolds numbers compared to an unmodified microchannel but a comparatively higher flow resistance at high Reynolds numbers. Furthermore, an earlier transition of initial laminar flow to turbulent flow is suggested to be triggered by instabilities generated along shear layers, formed between the mainstream flow and rotational vortices in each cavity that modifies the sidewalls of microchannels.

[Phys. Rev. Fluids 9, 044201] Published Fri Apr 05, 2024