# Physical Review Fluids

### Oscillating grid generating turbulence near gas-liquid interfaces in shear-thinning dilute polymer solutions

Author(s): T. Lacassagne, S. Simoëns, M. EL Hajem, and J.-Y. Champagne

A first experimental characterization of low Reynolds number, oscillating grid generated, and near-surface turbulence in shear-thinning dilute polymer solutions is presented. Energy transfer and horizontal damping mechanisms are evidenced. The evolution of the viscous sublayer depth can be explained by both viscous and shear-thinning effects.

[Phys. Rev. Fluids 5, 033301] Published Tue Mar 31, 2020

### Differential diffusion effects on density-driven instability of reactive flows in porous media

Author(s): Timan Lei and Kai H. Luo

In diverse applications such as enhanced oil recovery and carbon sequestration, solute A in fluid 1 diffuses into fluid 2 where it reacts with solute B following A + B → C. Pore-scale simulations based on the lattice Boltzmann method reveal intricate density-driven instability and differential diffusion effects. New fingering regimes are observed under certain combinations of density ratios, diffusivity ratios, and Rayleigh number ratios among the participating fluids and species.

[Phys. Rev. Fluids 5, 033903] Published Tue Mar 31, 2020

### Generalized Shields criterion for weakly cohesive granular materials

Author(s): Florian Brunier-Coulin, Pablo Cuéllar, and Pierre Philippe

The erosion onset and kinetics of weakly cohesive granular materials based on experimental data from optically adapted jet erosion tests is investigated. The scouring kinetics are examined qualitatively for samples with an intergranular cohesion induced by either liquid or solid bonds. From a quantitative perspective, the erosion onset for the case of solid bond cohesion is here described by a generalized form of the usual Shields criterion including a cohesion number defined from yield tensile values considering both the micro- and macroscales.

[Phys. Rev. Fluids 5, 034308] Published Mon Mar 30, 2020

### Capillary bulldozing of sedimented granular material confined in a millifluidic tube

Author(s): Guillaume Dumazer, Bjørnar Sandnes, Knut Jørgen Måløy, and Eirik G. Flekkøy

When granular matter is involved in a confined two-phase flow, three ingredients that do not usually occur together must be considered in parallel: viscosity, capillarity, and solid friction. The displacement regimes of a gas-liquid interface along a horizontal millifluidic tube containing a sedimented granular phase are documented in detail. Experimental observations are made with a simple setup, and an associated theoretical interpretation that can describe a spectrum of capillary bulldozing regimes is developed.

[Phys. Rev. Fluids 5, 034309] Published Mon Mar 30, 2020

### Structure of coherent columnar vortices in three-dimensional rotating turbulent flow

Author(s): I. V. Kolokolov, L. L. Ogorodnikov, and S. S. Vergeles

The turbulence in a fast rotating fluid becomes effectively two-dimensional. The inverse energy cascade leads to formation of coherent columnar vortices. An analytical theory describing interaction of such a vortex with turbulent pulsations is developed. The radial velocity profile of the vortex is established.

[Phys. Rev. Fluids 5, 034604] Published Mon Mar 30, 2020

### From modulational instability to focusing dam breaks in water waves

Author(s): Félicien Bonnefoy, Alexey Tikan, François Copie, Pierre Suret, Guillaume Ducrozet, Gaurav Prabhudesai, Guillaume Michel, Annette Cazaubiel, Eric Falcon, Gennady El, and Stéphane Randoux

The Benjamin-Feir, or modulational, instability has been regarded as the main mechanism of spontaneous disintegration of broad weakly nonlinear wavepackets in deep water waves. We report water wave experiments with another result. For a range of initial parameters, a nearly rectangular initial wavepacket develops a strongly nonlinear modulation with two dispersive shock waves expanding from the edges of the wavepacket towards the center at finite speed. This is supported by theory based on the semiclassical limit of the one-dimensional focusing nonlinear Schrodinger equation.

[Phys. Rev. Fluids 5, 034802] Published Fri Mar 27, 2020

### Interaction network analysis in shear thickening suspensions

Author(s): Marcio Gameiro, Abhinendra Singh, Lou Kondic, Konstantin Mischaikow, and Jeffrey F. Morris

Dense frictional particulate suspensions in a viscous liquid respond to applied shear in a complex manner that includes continuous or discontinuous shear thickening. This change in rheology as a function of particle packing fraction and/or applied stress is associated with a formation of complex interaction networks that develop on a mesoscopic scale. The properties of these networks, analyzed by the newly developed tools based on persistent homology in both two and three spatial dimensions, are found to be closely related to the macroscopic system response.

[Phys. Rev. Fluids 5, 034307] Published Mon Mar 23, 2020

### Local analysis of the clustering, velocities, and accelerations of particles settling in turbulence

Author(s): Mohammadreza Momenifar and Andrew D. Bragg

We use three-dimensional Voronoi tessellation to perform a local analysis of particle motion in isotropic turbulence while independently varying the flow Reynolds and Froude number, and particle Stokes number. Among other interesting findings, our local analysis shows that in flow regions where particles are clustered, fluid accelerations at the particle positions are significant. These results call into question the validity of the “sweep-stick” mechanism for particle clustering in turbulence, which states that inertial particles cluster in flow regions where the fluid acceleration is zero.

[Phys. Rev. Fluids 5, 034306] Published Fri Mar 20, 2020

### Shape oscillations of a viscoelastic droplet suspended in a viscoelastic host liquid

Author(s): Fang Li, Xie-Yuan Yin, and Xie-Zhen Yin

The small-amplitude oscillation of a liquid droplet suspended in an immiscible host liquid is studied, where both liquids are assumed viscoelastic. An analytical characteristic equation is derived, and the damping rate and angular frequency that describe the droplet oscillation are numerically solved. The effect of the properties of the host liquid, including its density, viscosity and elasticity, on the viscoelastic droplet oscillation is examined for the quadrupole mode. The host liquid is found to make the droplet oscillation more complex and the mechanisms behind that are discussed.

[Phys. Rev. Fluids 5, 033610] Published Thu Mar 19, 2020

### Triad resonant instability of horizontally periodic internal modes

Author(s): Bruce R. Sutherland and Riley Jefferson

Nonuniform stratification and vertical confinement significantly restricts the development of triad resonant instability for low mode internal gravity waves with and without background rotation.

[Phys. Rev. Fluids 5, 034801] Published Thu Mar 19, 2020

### Randomized resolvent analysis

Author(s): Jean Hélder Marques Ribeiro, Chi-An Yeh, and Kunihiko Taira

Randomized numerical algebra is incorporated into resolvent analysis to reduce large-scale resolvent operators to their low-rank approximations. The key to finding the resolvent modes accurately is to weigh the random test matrix using insights from the base flow. Turbulent flow over a NACA0012 airfoil at Re = 23,000 is used to demonstrate significant speedup and memory savings to accurately find principal resolvent modes.

[Phys. Rev. Fluids 5, 033902] Published Wed Mar 18, 2020

### Theory for undercompressive shocks in tears of wine

Author(s): Yonatan Dukler, Hangjie Ji, Claudia Falcon, and Andrea L. Bertozzi

Tears of wine, in which a thin layer of a water-ethanol mixture travel up an inclined surface against gravity and then fall down in the form of tears, have been observed in wine glasses for centuries. It has been modeled with a conservation law with a nonconvex flux and higher order diffusion due to the bulk surface tension. The resulting nonclassical “undercompressive” shock solutions are the main driver of the destabilizing front forming the “wine tears”. Prior modeling did not address the wine tears but rather the behavior of the film at earlier stages and the behavior of the meniscus.

[Phys. Rev. Fluids 5, 034002] Published Tue Mar 17, 2020

### Formation of vase-shaped drops

Author(s): Martin Coux, Pierre Chantelot, Lucie Domino, Christophe Clanet, Antonin Eddi, and David Quéré

Beautiful, elusive shapes are obtained when a water droplet deposited on a nonwetting substrate is subjected to a strong vertical impulse. Drops are highly reshaped to form truncated cones that eventually collapse. The evolution of the geometrical features of these so-called “vase-shaped droplets” is reported and discussed.

[Phys. Rev. Fluids 5, 033609] Published Mon Mar 16, 2020

### Bifurcations in the dynamics of a dipolar spheroid in a shear flow subjected to an external field

Author(s): V. Kumaran

The dynamical phase behavior of a spheroid with a magnetic dipole rotating in a shear flow and external field is investigated theoretically. Depending on the ratio of the torques due to the shear flow, a sequence of bifurcations leads to a rich complexity in the phase portraits in orientation space. We also find that the dynamics of an ideal thin rod could be very different from that of a high aspect ratio spheroid.

[Phys. Rev. Fluids 5, 033701] Published Mon Mar 16, 2020

### Migration of an electrophoretic particle in a weakly inertial or viscoelastic shear flow

Author(s): Aditya S. Khair and Jason K. Kabarowski

The cross-streamline migration of a spherical particle undergoing electrophoresis in weakly inertial or viscoelastic simple shear flow is quantified via asymptotic analysis. The findings for the migration direction and speed are in reasonable agreement with previous experimental studies on migration of electrophoretic colloids in Poiseuille microchannel flow.

[Phys. Rev. Fluids 5, 033702] Published Mon Mar 16, 2020

### Improvement of the parabolized stability equation to predict the linear evolution of disturbances in three-dimensional boundary layers based on ray tracing theory

Author(s): Runjie Song, Lei Zhao, and Zhangfeng Huang

A new method to predict the linear evolution of disturbances in three-dimensional inhomogeneous boundary layers is proposed, named RTPSE, in which the line-marching parabolized stability equation (PSE) is improved by applying ray tracing (RT) theory. Results show that RTPSE can accurately predict the spanwise wave number and amplitude ratio for both stationary and traveling crossflow waves, while the traditional PSE could not.

[Phys. Rev. Fluids 5, 033901] Published Mon Mar 16, 2020

### Collapse of a bubble injected side-by-side with another bubble into an incipiently fluidized bed: A CFD-DEM study

Author(s): A. Padash and C. M. Boyce

Computational fluid dynamics–discrete element method simulation results demonstrate the bubble collapse phenomenon observed in a prior experimental study when two bubbles are injected side-by-side into an incipiently fluidized bed. Results confirm that one of the bubbles collapses when there is a slight size difference between the two bubbles and its extent is beyond a critical value. The collapse occurs because of a preferential gas channeling toward the larger bubble, which leaves the smaller bubble without sufficient gas flow to support its shape.

[Phys. Rev. Fluids 5, 034304] Published Mon Mar 16, 2020

### Can preferential concentration of finite-size particles in plane Couette turbulence be reproduced with the aid of equilibrium solutions?

Author(s): Tiago Pestana, Markus Uhlmann, and Genta Kawahara

Fluid-particle interaction is studied in plane Couette flow by considering a nontrivial equilibrium solution that features exact coherent structures representative of wall-bounded shear flows. Despite the advantage of a much reduced complexity in comparison with a true turbulent flow, this strategy is shown to reproduce the phenomena of particle preferential concentration. The proposed approach is expected to be fruitful in future studies on various aspects of particulate flow, and perspectives for future works are also discussed.

[Phys. Rev. Fluids 5, 034305] Published Mon Mar 16, 2020

### Initial regime of drop coalescence

Author(s): Christopher R. Anthony, Michael T. Harris, and Osman A. Basaran

What is the initial regime of coalescence when two drops just touch at a point? This long-standing problem has proven formidable because of the smallness of the bridge that is required to connect the drops at the initial instant in any continuum simulation. Here, initial bridge radii, heights, and radii of curvature as small as 10−6, 10−12, and 10−18 are achieved, showing that coalescence always begins in a Stokes regime. The inertially limited viscous regime is shown to be a Taylor-Culick-like regime seen only when drops are initially separated by a finite distance.

[Phys. Rev. Fluids 5, 033608] Published Fri Mar 13, 2020

### Length of standing jumps along granular flows down smooth inclines

Author(s): Ségolène Méjean, François Guillard, Thierry Faug, and Itai Einav

Predicting the flows of dense granular media is still a challenging issue. Here, numerical discrete element method simulations are used to investigate the diversity of patterns within granular jumps that accompany height, velocity, and density discontinuities in gravity-driven free-surface flows. It is demonstrated that standing granular jumps can serve as a useful gauge for evaluating the dissipation mechanisms that govern the flowability of granular media.

[Phys. Rev. Fluids 5, 034303] Published Fri Mar 13, 2020