# Physical Review Fluids

### Spontaneous dynamics of two-dimensional Leidenfrost wheels

Author(s): Rodolfo Brandão and Ory Schnitzer

In a two-dimensional model of a Leidenfrost drop levitating above a flat hot substrate, it is found that the lubrication layer of vapor can develop an asymmetry which has the effect of propelling the drop sideways.

[Phys. Rev. Fluids 5, 091601(R)] Published Fri Sep 18, 2020

### Deflection of phototactic microswimmers through obstacle arrays

Author(s): Marvin Brun-Cosme-Bruny, Andre Förtsch, Walter Zimmermann, Eric Bertin, Philippe Peyla, and Salima Rafaï

A study of the effect of inhomogeneous environments on the swimming direction of the microalgae *Chlamydomonas reinhardtii* in the presence of a light stimulus is presented. A mean deflection of microswimmers is measured that shows an interesting nonlinear dependence on the direction of the guiding light beam with respect to the symmetry axes of the pillar lattice. This is shown both in experiments and numerical simulations. On the basis of these results, an analytical model for microswimmers is suggested, where the pillar lattice is replaced by an anisotropic scattering medium.

[Phys. Rev. Fluids 5, 093302] Published Fri Sep 18, 2020

### Revisiting the Taylor-Culick approximation. II. Retraction of a viscous sheet

Author(s): Hiranya Deka and Jean-Lou Pierson

The retraction of a viscous liquid sheet is studied using direct numerical simulations and long-wave asymptotic models. In the viscous regime, there exists a self-similar solution for the interface and the velocity profiles of a retracting sheet. This similarity solution reveals that the tip speed decreases as a function of time for a finite liquid sheet in contrast to the steady speed reached in the inertia dominated regime. Direct numerical simulations corroborate these theoretical predictions.

[Phys. Rev. Fluids 5, 093603] Published Fri Sep 18, 2020

### Low Mach number fluctuating hydrodynamics model for ionic liquids

Author(s): Katherine Klymko, Andrew Nonaka, John B. Bell, Sean P. Carney, and Alejandro L. Garcia

Room temperature ionic liquids (RTILs) are mixtures of large ionic molecules of importance to energy technology applications, such as supercapacitors and high-performance batteries. A new computational model that uses fluctuating hydrodynamics to allow for efficient and accurate investigation of complex nanometer scale structures is presented. This hydrodynamic model is derived to be consistent with the thermodynamic and electrical properties ultimately responsible for the rich phenomena observed in RTILs. Simulation results demonstrate that the model reproduces important physical effects observed in RTIL experiments.

[Phys. Rev. Fluids 5, 093701] Published Fri Sep 18, 2020

### Gravity-driven thermoviscous liquid film down a heated or cooled vertical cylinder

Author(s): Sana Khanum and Naveen Tiwari

Gravity-driven flow of a liquid over an isothermal cylinder is unconditionally unstable. The flow of a thermoviscous fluid over a heated or cooled substrate shows interesting stability behavior. The relevant parameters in the model affect the spatiotemporal nature of the instability.

[Phys. Rev. Fluids 5, 094005] Published Fri Sep 18, 2020

### Symmetry-breaking waves and space-time modulation mechanisms in two-dimensional plane Poiseuille flow

Author(s): Roger Ayats, Alvaro Meseguer, and Fernando Mellibovsky

Two distinct scenarios of spatial modulation in two-dimensional plane Poiseuille flow have been studied. The first one is based on the identification of a new family of asymmetric Tollmien-Schlichting waves (TSW) breaking the reflectional symmetry about the channel midplane. The second follows the fate of a branch of time-periodic space-modulated waves that exclusively bridge upper-branch TSW-trains of different number of replicas by means of a codimension-2 bifurcation point. These modulated waves may therefore play a relevant role in the strange saddle governing domain-filling turbulent dynamics at high Reynolds numbers.

[Phys. Rev. Fluids 5, 094401] Published Fri Sep 18, 2020

### Local energy flux of turbulent flows

Author(s): Alexandros Alexakis and Sergio Chibbaro

The local energy flux rate toward small scales in isotropic turbulent flows is investigated. The joint probability density function is calculated, with the local filtered strain rate, for a scale in the inertial range. The flux shows good correlation with the strain, in support of the Smagorinsky eddy viscosity model. The implications of the results for subgrid scale models are discussed and new modeling directions are proposed.

[Phys. Rev. Fluids 5, 094604] Published Fri Sep 18, 2020

### Instability of natural convection of air in a laterally heated cube with perfectly insulated horizontal boundaries and perfectly conducting spanwise boundaries

Author(s): Alexander Yu. Gelfgat

Primary and secondary instabilities of buoyancy convection in a laterally heated cube with perfectly thermally insulated horizontal boundaries and perfectly thermally conducting spanwise boundaries are studied. It is revealed that en route to unsteadiness, the flow undergoes a steady symmetry-breaking pitchfork bifurcation. With a further increase of the Grashof number, the nonsymmetric flow bifurcates into an oscillatory state via a Hopf bifurcation.

[Phys. Rev. Fluids 5, 093901] Published Thu Sep 17, 2020

### Yield stress fluid behavior of foam in porous media

Author(s): Alexis Mauray, Max Chabert, and Hugues Bodiguel

Flow of foams is studied in a model porous medium, in a large range of capillary numbers, Ca, and relative gas flow rates, fg. Pressure measurements show that the effective viscosity is a decreasing power-law function of Ca, with the exponent ranging from −1 to −0.75. Direct observation reveals that the flow is heterogeneous and the fraction of preferential paths increases with both fg and Ca. In a straight channel of varying cross section, a bubble train behaves as a shear-thinning yield stress fluid, accounting quantitatively for the effective viscosity in the micromodel.

[Phys. Rev. Fluids 5, 094004] Published Thu Sep 17, 2020

### Drag enhancement in a dusty Kolmogorov flow

Author(s): A. Sozza, M. Cencini, S. Musacchio, and G. Boffetta

Particles suspended in a fluid exert feedback forces that can significantly impact the flow by altering turbulent drag. Flow modulation induced by small spherical heavy particles is studied by means of numerical simulations of an Eulerian two-way coupling model. The amplitude of the mean flow and the turbulence intensity are found to be reduced by increasing particle mass loading and fluid friction is enhanced. Surprisingly, these effects are stronger for particles of smaller inertia.

[Phys. Rev. Fluids 5, 094302] Published Thu Sep 17, 2020

### Time-averaged transport in oscillatory squeeze flow of a viscoelastic fluid

Author(s): Rui Yang, Ivan C. Christov, Ian M. Griffiths, and Guy Z. Ramon

An investigation of the Taylor–Aris dispersion in an oscillatory axisymmetric squeeze flow, driven periodically by the motion of one of the confining, parallel planes is presented. Using the method of multiple timescale homogenization, the mass-heat balance equation in this flow is reduced to a one-dimensional equation, indicating three effective mechanisms: diffusion, advection, and reaction. The results show that the transport in the oscillatory squeeze flow can be either enhanced or diminished, depending on the interplay of these three effective (homogenized) mechanisms.

[Phys. Rev. Fluids 5, 094501] Published Tue Sep 15, 2020

### Mean-flow data assimilation based on minimal correction of turbulence models: Application to turbulent high Reynolds number backward-facing step

Author(s): Lucas Franceschini, Denis Sipp, and Olivier Marquet

We perform mean-flow reconstruction through variational data assimilation using Reynolds-Averaged Navier-Stokes equations closed by the Spalart-Allmaras model in a high Reynolds number Backward-Facing Step configuration. Two correction terms are used: a force term in the momentum equations and a scalar term in the turbulence model. We show that the force term perfectly reconstructs the target data but only for dense measurements, while the scalar solution is slightly less accurate, but independent of the number of measurements. An observability Gramian analysis shows that the scalar term is much less flexible than the force term.

[Phys. Rev. Fluids 5, 094603] Published Mon Sep 14, 2020

### Chemically reacting transverse plume

Author(s): P. Ranjan, K. Perez, T. Alvarado, B. Potter, and R. E. Breidenthal

A simple model of the transverse plume predicts that at a certain freestream speed, the entrainment rate is a maximum and the flame length is a minimum. This presumably corresponds to the most intense combustion in a compact wildfire. The model is in accord with laboratory experiments of a chemically reacting plume.

[Phys. Rev. Fluids 5, 094701] Published Mon Sep 14, 2020

### Understanding pulsed jet impingement cooling by instantaneous heat flux matching at solid-liquid interfaces

Author(s): Khan Md. Rabbi, Jake Carter, and Shawn A. Putnam

Introducing flow pulsation into a water-jet cooling system allows for flow-field control for more efficient heat removal. Here, transient thermal mapping is used to quantify how a pulsed water jet in a falling-film geometry can enhance the overall cooling performance. It is found that the influence of the jet-pulsation frequency on the maximum cooling performance can be predicted by heat flux matching at the solid-liquid interface, while the optimum pulsation frequency is dictated by the thermofluid instabilities that occur in the falling film.

[Phys. Rev. Fluids 5, 094003] Published Fri Sep 11, 2020

### Particle-induced miscible fingering: Continuum limit

Author(s): Rui Luo, Yun Chen, and Sungyon Lee

Oil is injected into a mixture of the same oil and noncolloidal particles inside a Hele-Shaw cell to investigate the connection between miscible fingering and the interfacial structure that develops inside the thin gap. By tuning the channel confinement relative to the particle size, it is demonstrated that shear-induced diffusion of particles can be enhanced and the interfacial shape caused to become more rounded, which results in changes in fingering morphologies. The results of the study suggest a potential use of the wall confinement to control hydrodynamic instabilities uniquely in suspensions.

[Phys. Rev. Fluids 5, 094301] Published Fri Sep 11, 2020

### Quasi-two-dimensional foam flow through and around a permeable obstacle

Author(s): Natalia Shmakova, Thibaud Chevalier, Antti Puisto, Mikko Alava, Christophe Raufaste, and Stéphane Santucci

In a study of the evolving structure of two-dimensional liquid foams flowing through an inhomogeneous confining cell, the motion and deformation of their elementary components, the bubbles, are quantified for various confinement ratios. The flow experiments highlight the elastoplastic properties of foams, controlled by their liquid fraction, which are notably responsible for the symmetry breaking of the flow, with multipolar deformation and velocity fields around the localized inhomogeneity.

[Phys. Rev. Fluids 5, 093301] Published Thu Sep 10, 2020

### Spiral defect chaos in Rayleigh-Bénard convection: Asymptotic and numerical studies of azimuthal flows induced by rotating spirals

Author(s): Eduardo Vitral, Saikat Mukherjee, Perry H. Leo, Jorge Viñals, Mark R. Paul, and Zhi-Feng Huang

Spiral defect chaos appears near the onset of Rayleigh-Bénard convection for a low Prandtl number fluid. In this state, rotating spirals are continuously nucleated and eliminated, yielding a persistent dynamics. Here we derive an equation for the azimuthal flow induced by an effective body force originating from rotating spirals. This result is verified numerically using a two-dimensional generalized Swift-Hohenberg model and the three-dimensional Boussinesq equations. We identify a correlation between the appearance of spiral defect chaos and the balance between mean-flow advection and diffusive dynamics related to roll unwinding.

[Phys. Rev. Fluids 5, 093501] Published Thu Sep 10, 2020

### Impact and lifecycle of superfluid helium drops on a solid surface

Author(s): Matthew L. Wallace, David Mallin, Michael Milgie, Andres A. Aguirre-Pablo, Kenneth R. Langley, Sigurdur T. Thoroddsen, and Peter Taborek

Superfluid helium droplets impacting on a solid surface behave much differently than any other fluid. After a short period of initial spreading that is similar to classical fluids, superfluid helium drops quickly shrink and disappear as the superfluid drains out through a thin adsorbed layer of helium on the surface. The lifetime and contact angle of these drops is strongly temperature-dependent, and colder drops (with high superfluid fractions) maintain a constant contact angle throughout the contraction. Above Tlambda, helium spreads slowly like other classical fluids.

[Phys. Rev. Fluids 5, 093602] Published Thu Sep 10, 2020

### Electro-osmotic instability of concentration enrichment in curved geometries for an aqueous electrolyte

Author(s): Bingrui Xu, Zhibo Gu, Wei Liu, Peng Huo, Yueting Zhou, S. M. Rubinstein, M. Z. Bazant, B. Zaltzman, I. Rubinstein, and Daosheng Deng

The critical Peclet number for an electro-osmotic instability is reduced in inverse proportion to the geometrically increased electric fields due to curvature of the electrode. This electro-osmotic instability, in contrast to the well-known one, appears exclusively at the enriched interface (anode), rather than at the depleted one (cathode).

[Phys. Rev. Fluids 5, 091701(R)] Published Wed Sep 09, 2020

### Regularized representation of bacterial hydrodynamics

Author(s): Kenta Ishimoto, Eamonn A. Gaffney, and Benjamin J. Walker

Fluid flows induced by a flagellated bacterial swimmer are often modeled as a simple force dipole, valid in the far field. A refined swimmer representation is presented that makes use of regularized singularities, retaining simplicity while capturing details of the complex flow field near the swimmer. A simple model system is then considered, demonstrating that these nuanced hydrodynamics are significant for bacterial interactions.

[Phys. Rev. Fluids 5, 093101] Published Tue Sep 08, 2020