# Physical Review Fluids

### Turbulence statistics and transport in compressible mixing driven by spherical implosions with narrowband and broadband initial perturbations

Author(s): Moutassem El Rafei and Ben Thornber

We investigate compressible turbulent mixing evolving in spherical implosions with differing initial conditions using high-resolution implicit large eddy simulations. We examine in detail temporal and spatial turbulent transport budgets including density self-correlation, turbulent mass flux, and turbulent kinetic energy. This analysis provides improved understanding of the mixing process initiated by Richtmyer-Meshkov and Rayleigh-Taylor instabilities including quantification of contributions to asymmetries in the mixing layer and numerical dissipation.

[Phys. Rev. Fluids 9, 034501] Published Mon Mar 04, 2024

### Drop encapsulation and bubble bursting in surfactant-laden flows in capillary channels

Author(s): P. Pico, L. Kahouadji, S. Shin, J. Chergui, D. Juric, and O. K. Matar

In this investigation, we dive into the phenomenon of drop encapsulation in elongated bubbles travelling through liquid-filled capillary channels in the presence of surface-active material. Our numerical results reveal that complex interactions between surfactant parameters, Marangoni stresses, viscosity, and inertia are responsible for dramatically altering the pinch-off times, along with the number, size, and velocity of the encapsulated drops. We summarize these interactions in three distinct encapsulation morphological regimes, providing a structured overview of the underlying dynamics.

[Phys. Rev. Fluids 9, 034001] Published Fri Mar 01, 2024

### Effects of the Saffman lift force on particle statistics and turbulence modulation in two-phase flow

Author(s): Jinchi Li, Ping Wang, and Xiaojing Zheng

The Saffman force is one of the key factors for particle transport and hence the interaction among phases in two-phase wall-bounded turbulence. This numerical work finds that the accumulation of particles near the wall and preferential concentration in low-speed streaks are suppressed by the lift force, leading to destruction of the conditional hairpin vortices and decreasing velocity fluctuations near the wall. However, in the outer layer, the particle-turbulence interaction is increased by the lift force because of higher particle concentration.

[Phys. Rev. Fluids 9, 034301] Published Fri Mar 01, 2024

### Experimental study of a helical acoustic streaming flow

Author(s): Bjarne Vincent, Sophie Miralles, Daniel Henry, Valéry Botton, and Alban Pothérat

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.

[Phys. Rev. Fluids 9, 024101] Published Thu Feb 29, 2024

### High-fidelity reconstruction of large-area damaged turbulent fields with a physically constrained generative adversarial network

Author(s): Qinmin Zheng, Tianyi Li, Benteng Ma, Lin Fu, and Xiaomeng Li

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.

[Phys. Rev. Fluids 9, 024608] Published Thu Feb 29, 2024

### Stable, entropy-consistent, and localized artificial-diffusivity method for capturing discontinuities

Author(s): Suhas S. Jain, Rahul Agrawal, and Parviz Moin

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.

[Phys. Rev. Fluids 9, 024609] Published Thu Feb 29, 2024

### Capillary imbibition of shear-thinning fluids: From Lucas-Washburn to oscillatory regimes

Author(s): Camille Steinik, Davide Picchi, Gianluca Lavalle, and Pietro Poesio

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.

[Phys. Rev. Fluids 9, 023305] Published Wed Feb 28, 2024

### Hysteresis and ribbons in Taylor-Couette flow of a semidilute noncolloidal suspension

Author(s): Changwoo Kang, Michael F. Schatz, and Parisa Mirbod

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.

[Phys. Rev. Fluids 9, 023901] Published Wed Feb 28, 2024

### Spherical thermal counterflow of superfluid $^{4}\mathrm{He}$

Author(s): F. Novotný, Y. Huang, J. Kvorka, Š Midlik, D. Schmoranzer, Z. Xie, and L. Skrbek

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.

[Phys. Rev. Fluids 9, L022601] Published Wed Feb 28, 2024

### Viscoplastic rimming flow inside a rotating cylinder

Author(s): Thomasina V. Ball and Neil J. Balmforth

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.

[Phys. Rev. Fluids 9, 023304] Published Tue Feb 27, 2024

### Neighbor-induced unsteady force in the interaction of a cylindrical shock wave with an annular particle cloud

Author(s): Sam Briney, Andreas N. Osnes, Magnus Vartdal, Thomas L. Jackson, and S. Balachandar

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.

[Phys. Rev. Fluids 9, 024308] Published Tue Feb 27, 2024

### Grid-generated velocity fields at very small Reynolds numbers

Author(s): Dana Duong and Stavros Tavoularis

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.

[Phys. Rev. Fluids 9, 024607] Published Tue Feb 27, 2024

### Artificially thickened boundary layer turbulence due to trip wires of varying diameter

Author(s): Zhanqi Tang, Nan Jiang, Zhiming Lu, and Quan Zhou

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.

[Phys. Rev. Fluids 9, 024606] Published Mon Feb 26, 2024

### Superflow passing over a rough surface: Vortex nucleation

Author(s): Thomas Frisch, Sergey Nazarenko, and Sergio Rica

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.

[Phys. Rev. Fluids 9, 024701] Published Mon Feb 26, 2024

### Measurement of an eddy diffusivity for chaotic electroconvection using combined computational and experimental techniques

Author(s): Arunraj Balaji-Wright, Felix Stockmeier, Richard Dunkel, Matthias Wessling, and Ali Mani

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.

[Phys. Rev. Fluids 9, 023701] Published Fri Feb 23, 2024

### Plume-surface interaction during lunar landing using a two-way coupled DSMC-DEM approach

Author(s): A. Bajpai, A. Bhateja, and R. Kumar

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.

[Phys. Rev. Fluids 9, 024306] Published Fri Feb 23, 2024

### Trapping of inertial particles in a two-dimensional unequal-strength counterrotating vortex pair flow

Author(s): Zilong Zhao, Zhiwei Guo, Zhigang Zuo, and Zhongdong Qian

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.

[Phys. Rev. Fluids 9, 024307] Published Fri Feb 23, 2024

### Bounded flows of dense gases

Author(s): Sergiu Busuioc and Victor Sofonea

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.

[Phys. Rev. Fluids 9, 023401] Published Thu Feb 22, 2024