# Latest papers in fluid mechanics

### Logarithmic profiles of velocity in stably stratified atmospheric boundary layers

Author(s): Yu Cheng, Andrey Grachev, and Chiel van Heerwaarden

In global weather and climate models, the turbulent exchange of momentum, heat, moisture and carbon dioxide between the Earth’s surface and the atmosphere is described by Monin-Obukhov similarity theory (MOST) since 1954. A fundamental assumption of MOST is that velocity near the surface does not follow a logarithmic profile due to buoyancy effects driven by stratification. In contrast to MOST, we find that buoyancy does not change the logarithmic nature of velocity profiles but instead modifies the slope of the log law. The proposed logarithmic profile can serve as an alternative to MOST, possibly leading to more realistic predictions of weather and climate, especially in polar regions.

[Phys. Rev. Fluids 8, 114602] Published Thu Nov 16, 2023

### Two self-similar Reynolds-stress transport models with anisotropic eddy viscosity

Author(s): Brandon E. Morgan, Kevin Ferguson, and Britton J. Olson

Two Reynolds-averaged Navier-Stokes models with full Reynolds-stress transport (RST) and tensor eddy viscosity are presented. These new models represent RST extensions of the k−2L−a−C and k−ϕ−L−a−C models by Morgan [Phys. Rev. E **103**, 053108 (2021); Phys. Rev. E **105**, 045104 (2022)]. Self-similarity a…

[Phys. Rev. E 108, 055104] Published Wed Nov 15, 2023

### Chemomechanical model of sperm locomotion reveals two modes of swimming

Author(s): Chenji Li, Brato Chakrabarti, Pedro Castilla, Achal Mahajan, and David Saintillan

We present a chemomechanical model to analyze the propulsion of mammalian spermatozoa. The model accounts for motor kinetics, flagellar deformations, and the hydrodynamics of the suspending fluid. Simulations demonstrate spontaneous oscillations leading to realistic swimming patterns. Notably, the swimming velocity exhibits two distinct peaks as a function of the activity of the molecular motors. These peaks are characterized by distinct waveforms and trajectories. Our findings contribute to a deeper understanding of the biophysical mechanisms involved in sperm locomotion.

[Phys. Rev. Fluids 8, 113102] Published Wed Nov 15, 2023

### Vortex breakdown in the shear-driven flow in a rectangular cavity

Author(s): Haoyi Wang, Xinyi Yu, San To Chan, Guillaume Durey, Amy Q. Shen, and Jesse T. Ault

The vortex dynamics of laminar flow past a rectangular cavity is investigated using numerical simulations and microfluidic experiments. Classical bubble-type vortex breakdown is observed within the cavity, and the evolution and dynamical transitions of the breakdown regions are investigated. The stability and bifurcations of the stagnation points and their transitions to stable/unstable limit cycles are analyzed.

[Phys. Rev. Fluids 8, 114701] Published Wed Nov 15, 2023

### Geometry-induced rectification of looped oscillatory flows

Author(s): Ruy Ibanez, Aditya Raghunandan, and Douglas H. Kelley

We show a mechanism for rectifying an oscillating fluid flow in a looped channel with a T-junction using geometric features. We experimentally show that a net flow is produced in a looped channel with a forcing oscillating velocity. We identify that separation at the T-junction of the looped channel induces a net flow in the looped section. We present an analytic model that captures some of the basic flow features at the T-junction.

[Phys. Rev. Fluids 8, 113101] Published Thu Nov 09, 2023

### Spatial-temporal behaviors of low-Stokes-number particles forming coherent structures in high-aspect-ratio liquid bridges by thermocapillary effect

Author(s): Shin Noguchi and Ichiro Ueno

We experimentally investigate coherent structures of low-Stokes-number particles in the thermocapillary liquid bridge. Particle accumulation structures are found in the m=1 azimuthal wave number flow in the liquid bridge O(10−3 m), with spatial structure different from previous research for particles heavier than the test liquid. Coherent structures for particles suspended in the liquid bridge are illustrated in the laboratory and rotating frames. Particles with Stokes number St O(10−5) are tracked to examine formation of coherent structures. We find that two major coherent structures simultaneously emanate from inside the liquid bridge of aspect ratio Γ=1.6.

[Phys. Rev. Fluids 8, 114002] Published Thu Nov 09, 2023

### Incorporating intrinsic compressibility effects in velocity transformations for wall-bounded turbulent flows

Author(s): Asif Manzoor Hasan, Johan Larsson, Sergio Pirozzoli, and Rene Pecnik

The law of the wall states that, after appropriate scaling, the mean velocity of incompressible turbulent flows near flat solid walls is a universal function of the wall-normal distance. An analogous law does not exist for compressible flows due to complexities arising from mean property variations and intrinsic compressibility effects, which occur when pressure directly changes the density of fluid elements. By accounting for both these effects, we propose a new velocity scaling that transforms the mean velocity of compressible flows to the incompressible law of the wall. This transformation is more accurate than the state-of-the-art, and is applicable to a wider range of flows.

[Phys. Rev. Fluids 8, L112601] Published Thu Nov 09, 2023

### Instability of mixing layers: Momentum and thermal transport in the continuum breakdown regime

Author(s): Vishnu Mohan, A. Sameen, Balaji Srinivasan, and Sharath S. Girimaji

We examine the momentum and thermal transport in the continuum breakdown regime of a mixing layer flow, which exhibits Kelvin-Helmholtz instability under ideal continuum conditions. The Grad 13 moment model is used as it provides an adequate description of the flow physics (second-order accurate in …

[Phys. Rev. E 108, L053101] Published Wed Nov 08, 2023

### Direct numerical simulation of electroconvection under a uniform magnetic field

Author(s): Jinxiang Cai, Dongxiao Zhao, and Gaojin Li

Above a critical voltage, ion transport near ion-selective surfaces leads to electroconvection of the liquid electrolyte, causing significant fluctuations in ion flux. Our simulation shows that applying a magnetic field can greatly modify the electroconvection by inducing a Lorentz force. The combined electro-magneto-hydrodynamic effects produce a cross-flow that advects electroconvective vortices and effectively suppresses the local high-ion-flux regions from reaching the ion-selective surfaces.

[Phys. Rev. Fluids 8, 113701] Published Wed Nov 08, 2023

### Theoretical modeling of capillary surfer interactions on a vibrating fluid bath

Author(s): Anand U. Oza, Giuseppe Pucci, Ian Ho, and Daniel M. Harris

“Capillary surfers” are small objects that self-propel while floating at the interface of a vibrating fluid bath. In this paper, we construct and analyze a theoretical model for the waves generated by such surfers and thus the hydrodynamic forces exerted by one surfer on another. Our model recovers the dynamical modes of surfer pairs found in experiments, and predicts that surfer collectives may lock into a variety of quantized bound states. Generally, our work shows that capillary surfers are a promising platform for studying wave-coupled active matter.

[Phys. Rev. Fluids 8, 114001] Published Tue Nov 07, 2023

### Measuring scale-dependent shape anisotropy by coarse-graining: Application to inhomogeneous Rayleigh-Taylor turbulence

Author(s): Dongxiao Zhao and Hussein Aluie

A novel approach is introduced to measure flow anisotropy at all length scales. It is valid for general fields, including inhomogeneous flows with complex boundaries where traditional Fourier approaches face difficulties. Application to two- and three-dimensional Rayleigh-Taylor turbulence reveals the markedly distinct anisotropy at different scales due to differing energy cascade directions.

[Phys. Rev. Fluids 8, 114601] Published Tue Nov 07, 2023

### Capillary surfers: Wave-driven particles at a vibrating fluid interface

Author(s): Ian Ho, Giuseppe Pucci, Anand U. Oza, and Daniel M. Harris

A small solid particle resting atop a vibrating fluid interface generates a field of outwardly propagating capillary waves due to its relative vertical motion. In this paper, we show that if the particle’s symmetry is broken, the resultant unbalanced wave stresses enable steady self-propulsion along the interface. Such “capillary surfers” interact with each other hydrodynamically at long range via their mutual wavefield and form a number of dynamic bound states. This new active system bridges the gap between dissipation- and inertia-dominated regimes and promises a number of novel collective behaviors.

[Phys. Rev. Fluids 8, L112001] Published Tue Nov 07, 2023

### Two-dimensional simulations of Rayleigh-Taylor instability in elastic-plastic media

Author(s): A. R. Piriz, J. J. López Cela, S. A. Piriz, and N. A. Tahir

Two-dimensional numerical simulations for the Rayleigh-Taylor instability in an elastic-plastic medium are presented. Recent predictions of the theory regarding the asymmetric growth of peaks and valleys during the linear phase of the instability evolution are confirmed. Extension to the nonlinear r…

[Phys. Rev. E 108, 055102] Published Mon Nov 06, 2023

### Boundary-bound reactions: Pattern formation with and without hydrodynamics

Author(s): Aiden Huffman and Henry Shum

We study chemical pattern formation in a fluid between two flat plates and the effect of such patterns on the formation of convective cells. This patterning is made possible by assuming the plates are chemically reactive or release reagents into the fluid, both of which we model as chemical fluxes. …

[Phys. Rev. E 108, 055103] Published Mon Nov 06, 2023

### Dynamics of spheroids in pressure-driven flows of shear thinning fluids

Author(s): Vishal Anand and Vivek Narsimhan

Tumbling behavior of spheroidal particles in inertialess flows of shear thinning fluids is analyzed. (Top Row) A spheroid in a linear shear flow of a shear thinning fluid and the variation of its tumbling time period with Carreau number. (Bottom Row) A spheroid in a pressure driven flow of a shear thinning flow and the variation of its tumbling time period with Carreau number. The red curves illustrate trends for shear thinning fluids. The dotted black lines show the corresponding Newtonian plateaus. For linear flows, the time period shows a nonmonotonic trend with the Carreau number, while for pressure driven flows the time period decreases monotonically between the two Newtonian plateaus.

[Phys. Rev. Fluids 8, 113302] Published Mon Nov 06, 2023

### Three-dimensional structure of cold-water gravity currents

Author(s): N. Castro-Folker, A. P. Grace, and M. Stastna

A typical fluid gets denser as it gets colder. Cold, fresh water is an exception to this rule, as it reaches a temperature of maximum density at 4 degrees Celsius. We can do experiments where we release a volume of fluid at one temperature into an ambient volume at a different temperature. When one uses a “typical” fluid, some experiments with different initial set-ups create intruding volumes that evolve into identical shapes. For our investigation we performed those experiments with cold, fresh water instead and found that the intruding volumes evolved into very different shapes. In the manuscript we discuss the geophysical implications of these results.

[Phys. Rev. Fluids 8, 113901] Published Mon Nov 06, 2023

### Wall-pressure fluctuations in attached compressible turbulent cavitating flows

Author(s): Changchang Wang, Guoyu Wang, Mindi Zhang, Biao Huang, and Yi-Qing Ni

Wall-pressure fluctuations are of greatest interest in cavitating flows - a threshold phenomenon determined by pressure fields, which are an important source of the corresponding significant vibrations and loud noise in many practical applications. In this article, we measure the wall pressure fluctuations in attached compressible turbulent cavitating flows, and firstly report their statistical behaviors which are independent of cavity regimes.

[Phys. Rev. Fluids 8, 114301] Published Mon Nov 06, 2023

### Thin film extensional flow of a transversely isotropic viscous fluid

Author(s): M. J. Hopwood, B. Harding, J. E. F. Green, and R. J. Dyson

Biological materials such as cervical mucus and collagen gel can possess a fibrous microstructure that affects their functional behavior. Using numerical and asymptotic techniques we solve a model for stretching a thin sheet of transversely isotropic viscous fluid. We show that the emergent bulk properties are governed by an effective viscosity dependent on the evolving angle of the fibers. We also demonstrate that the center-line of the sheet need not be flat, in contrast to the Newtonian case, and capture the full center-line behavior.

[Phys. Rev. Fluids 8, 113301] Published Fri Nov 03, 2023

### Fast-slow wave transitions induced by a random mean flow

Author(s): S. Boury, O. Bühler, and J. Shatah

Motivated by recent asymptotic results in atmosphere-ocean fluid dynamics, we present an idealized numerical and theoretical study of two-dimensional dispersive waves propagating through a small-amplitude random mean flow. The objective is to delineate clearly the conditions under which the cumulati…

[Phys. Rev. E 108, 055101] Published Thu Nov 02, 2023

### Characteristic rupture height of the mediating air film beneath an impacting drop on atomically smooth mica

Author(s): Ramin Kaviani and John M. Kolinski

Before a droplet can contact a solid surface, it must drain the air beneath it. In our daily experience, the air doesn’t alter the global outcome of contact formation in the blink of an eye. In this work, we find that the lubricating air film formed beneath an impacting droplet can resist rupture for droplets impacting at a velocity exceeding 1/2 a meter per second; however, beyond a critical impact velocity, contact always occurs from the liquid’s closest approach to the solid surface through the air, at a distance of 20 - 30 nanometers. Below the critical velocity, contact is suppressed for impact upon atomically smooth, cleaved mica, shedding light on the critical phase of liquid-solid contact.

[Phys. Rev. Fluids 8, 103602] Published Tue Oct 31, 2023