New Papers in Fluid Mechanics

Countergradient diffusion of turbulent heat flux in turbulent Rayleigh flow

Physical Review Fluids - Fri, 01/10/2025 - 10:00

Author(s): Motonori Nakamura and Fujihiro Hamba

Motivated by countergradient diffusion phenomenon in the turbulent premixed flame, we discuss the effects of heat release on turbulence. We devised and implemented a direct numerical simulation of the turbulent Rayleigh flow generated by isotropic turbulence flowing into the Rayleigh flow. We show that countergradient diffusion of the turbulent heat flux occurs in the turbulent Rayleigh flow, and we discuss its mechanism based on the transport equations of the turbulent statistical quantities. It is shown that the released heat amplifies the density-internal energy correlation and contributes countergradient diffusion through the pressure gradient effect.


[Phys. Rev. Fluids 10, 014604] Published Fri Jan 10, 2025

Reducing foam friction with self-slippery liquid-infused porous surfaces

Physical Review Fluids - Fri, 01/10/2025 - 10:00

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

Mitigating energy consumption in fluid transportation is crucial for industrial processes. Our research addresses this by investigating the reduction of friction in liquid foams on Slippery Liquid-Infused Porous Surfaces (SLIPS). These surfaces have asperities filled with oil, significantly reducing friction compared to smooth surfaces. While oil is unsuitable due to its anti-foaming properties, we propose a unique approach using self-SLIPS, introducing the idea that the foam liquid can serve as a lubricant. Our findings demonstrate that these passive surfaces can achieve a 25% reduction in foam friction, promising significant advancements in energy-efficient fluid transport.


[Phys. Rev. Fluids 10, L011601] Published Fri Jan 10, 2025

Quasi-irrotational approximation for the Rayleigh-Taylor instability in a solid bounded by a rigid wall

Physical Review E - Fri, 01/10/2025 - 10:00

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

A quasi-irrotational approximation for the linear Rayleigh-Taylor instability in elastic solids with finite thickness has been developed for the case in which the slab is in contact with a rigid wall. The approximation yields simple but still reasonably accurate expressions for the instability growt…


[Phys. Rev. E 111, 015102] Published Fri Jan 10, 2025

Hydrodynamic instabilities of propagating interfaces under Darcy's law

Physical Review Fluids - Thu, 01/09/2025 - 10:00

Author(s): Joel Daou and Prabakaran Rajamanickam

The hydrodynamic instabilities of propagating interfaces in Hele-Shaw channels or porous media under the influence of an imposed flow and gravity are investigated within the framework of Darcy’s law. The stability analysis pertains to an interface between two fluids with different densities, viscosities, and permeabilities, which can be susceptible to Darrieus-Landau, Saffman-Taylor, and Rayleigh-Taylor instabilities. An explicit dispersion relation is derived.


[Phys. Rev. Fluids 10, 013201] Published Thu Jan 09, 2025

Laboratory model for barotropic vortices drifting towards a planetary pole

Physical Review Fluids - Thu, 01/09/2025 - 10:00

Author(s): Djihane Benzeggouta, Benjamin Favier, and Michael Le Bars

Inspired by recent observations of stable cyclone patterns on the North and the South poles of Jupiter, we present observations of monopolar barotropic cyclones in a model experiment for atmospheric polar flows. We show, both experimentally and with idealized quasi-geostrophic simulations, that starting from an initial vortex, there are two evolution regimes depending on vortex strength relative to the local beta-effect across its surface: a Rossby wave emission dominated regime and a strong vortex regime. In the latter regime, the generated cyclone drifts in a northwest direction. We show that the “beta gyre” induced “beta drift” mechanism locally applies in our experimental polar plane.


[Phys. Rev. Fluids 10, 014701] Published Thu Jan 09, 2025

Instability characteristics induced by roughness elements in the rotating-disk boundary layer of a rotor-stator cavity

Physical Review Fluids - Wed, 01/08/2025 - 10:00

Author(s): Qiang Du, Yaguang Xie, Lei Xie, Ruonan Wang, Qingzong Xu, and Junqiang Zhu

Due to the inviscid cross-flow instability caused by the inflection point in radial velocity, both convective and absolute instabilities coexist in the rotating-disk boundary layer within rotor-stator cavities. To investigate the laminar-turbulent transition process through convective instability under specific roughness element excitation, this study employs numerical simulation with body force to simulate roughness elements. The findings reveal that under convective instability, the rotating-disk boundary layer initially exhibits stationary spiral waves mode, followed by the development of traveling spiral waves mode in the radial direction, ultimately transitioning to turbulence.


[Phys. Rev. Fluids 10, 013901] Published Wed Jan 08, 2025

Electric field-dependent scaling law for overdamped (di)electrowetting and dewetting on dielectric

Physical Review Fluids - Wed, 01/08/2025 - 10:00

Author(s): Shreyank Goel, Rakshith Gowda BT, and Dipin S. Pillai

Droplet dynamics in (di)electrowetting and dewetting on dielectric configuration is elucidated using a lubrication model. Electric field-modified Frumkin-Derjaguin theory through Lippmann’s principle of electrocapillarity is employed. The research uncovers a field strength dependent power-law relationship for the contact line motion that conforms to Tanner’s law for the case of critical field leading to complete wetting. Electrodewetting shows a faster retraction compared to wetting due to reduced viscous dissipation. These findings provide a new theoretical framework for electric field-driven wetting and dewetting, enabling advancements in digital microfluidics.


[Phys. Rev. Fluids 10, 014201] Published Wed Jan 08, 2025

Characteristics of the meandering effect in a stratified wake

Physical Review Fluids - Wed, 01/08/2025 - 10:00

Author(s): Xinyi Huang and Jiaqi J. L. Li

This work focuses on the impact of meandering on wake statistics in the stratified environment. The range of wake meandering increases in the vertical direction while wake height does not. Meandering does not directly change the velocity scaling, or the wake width and height, but it leads to deviation of the velocity profile from self-similarity, and the development of layered flow structures. We can accurately measure how meandering distorts the self-similar velocity profile and impacts scaling of the width and height, and thus how meandering changes the scaling of the velocity deficit.


[Phys. Rev. Fluids 10, 014602] Published Wed Jan 08, 2025

Exploring external rarefied gas flows through the method of fundamental solutions

Physical Review E - Tue, 01/07/2025 - 10:00

Author(s): Himanshi, Anirudh Singh Rana, and Vinay Kumar Gupta

The well-known Navier-Stokes-Fourier equations of fluid dynamics are, in general, not adequate for describing rarefied gas flows. Moreover, while the Stokes equations—a simplified version of the Navier-Stokes-Fourier equations—are effective in modeling slow and steady liquid flow past a sphere, they…


[Phys. Rev. E 111, 015101] Published Tue Jan 07, 2025

Nonresonant effects in pilot-wave hydrodynamics

Physical Review Fluids - Mon, 01/06/2025 - 10:00

Author(s): Bauyrzhan K. Primkulov, Davis J. Evans, Joel B. Been, and John W. M. Bush

Pilot-wave hydrodynamics concerns the dynamics of droplets walking on a vibrating liquid bath, and forms the basis for the field of hydrodynamic quantum analogs. We here investigate a theoretical model that captures both vertical and horizontal drop dynamics. The model provides new rationale for a number of phenomena, including colinear swaying, intermittent walking, and chaotic speed oscillations, all of which are linked to variability in the droplet’s impact phase. Our study also highlights the degeneracy in the droplet’s vertical dynamics, consideration of which is essential for understanding the dynamics of droplets in confined geometries and interacting with standing Faraday waves.


[Phys. Rev. Fluids 10, 013601] Published Mon Jan 06, 2025

Experimental study of the Richtmyer-Meshkov instability in spherical geometry

Physical Review Fluids - Mon, 01/06/2025 - 10:00

Author(s): Mathieu Brasseur, Georges Jourdan, Christian Mariani, Diogo C. Barros, Marc Vandenboomgaerde, and Denis Souffland

An experimental investigation of the Richtmyer-Meshkov instability is conducted in spherical geometry where the displacement and the growth of the perturbations at the interface are given and compared to numerical simulations and new theoretical predictions. The results show that the instability amplitude initially grows, stabilizes, and then reduces before the arrival of the reflected shock wave. The theoretical model developed here agrees well with the experiments, although a time shift is observed in the stabilization regime. Furthermore, we show that convergent Rayleigh-Taylor effects are the main stabilizing mechanisms, and that compressibility has a negligible effect.


[Phys. Rev. Fluids 10, 014001] Published Mon Jan 06, 2025

Two neural network Unet architecture for subfilter stress modeling

Physical Review Fluids - Mon, 01/06/2025 - 10:00

Author(s): Andy Wu and Sanjiva K. Lele

Neural networks applied to turbulence modeling often do not learn locality or generalize to very high Reynolds number reasonably. Here, a two neural network architecture is introduced that learns the relevant neighborhood needed for sub-filter stress modeling through convolutions and the U-net architecture while generalizing reasonably to Reynolds numbers far larger than the training set on forced homogeneous isotropic turbulence and channel flow.


[Phys. Rev. Fluids 10, 014601] Published Mon Jan 06, 2025

Optimization framework for analyzing nonlinear stability due to sparse finite-amplitude perturbations

Physical Review Fluids - Thu, 01/02/2025 - 10:00

Author(s): A. Leonid Heide and Maziar S. Hemati

This paper introduces an optimization framework for identifying sparse finite-amplitude perturbations that maximize transient growth in nonlinear systems. An iterative direct-adjoint looping algorithm is formulated based on the first-order necessary conditions for optimality. The method is applied to a reduced-order model of sinusoidal shear flow. Our results show that optimal sparse perturbations can achieve comparable energy amplification as the optimal non-sparse solution by triggering many of the same nonlinear modal interactions responsible for driving transient growth. We anticipate the approach will be a useful tool in future investigations into flow stability and control.


[Phys. Rev. Fluids 10, 014401] Published Thu Jan 02, 2025

Micro liquid bridge in periodic electric pulses: The impact of frequency

Physical Review Fluids - Mon, 12/30/2024 - 10:00

Author(s): Miao Sun and Yanbo Xie

Previous work showed that a floating liquid bridge can be sustained under DC or high-frequency AC voltage, though the effects of frequency remain unclear. We investigated the stability of a micro-floating liquid bridge under periodic voltage pulses. The recorded current reveals the formation and breakup of the bridge as six distinct states of stability beyond high-speed imaging. Our results show that both pulse frequency and the electrocapillary number are crucial for liquid bridge stability. Considering the charging/discharging process of the system, we corrected the formation and breakup time, which well explained the observed delay in these processes.


[Phys. Rev. Fluids 9, 123701] Published Mon Dec 30, 2024

Unified view of elastic and elasto-inertial turbulence in channel flows at low and moderate Reynolds numbers

Physical Review Fluids - Mon, 12/30/2024 - 10:00

Author(s): Giulio Foggi Rota, Christian Amor, Soledad Le Clainche, and Marco Edoardo Rosti

Viscoelastic fluids like DNA solutions and polymer melts yield chaotic flows even with small inertial effects (quantified by the Reynolds number). Such turbulent motion is conventionally classified as elasto-inertial turbulence (EIT) or elastic turbulence (ET) when inertial effects are finite or vanishing. Our numerical study investigates the turbulent flow of viscoelastic fluids in planar channel flows over a wide range of Reynolds numbers. We discover that EIT and ET exhibit the same dynamical features and are thus the same. Our finding sheds light on low Reynolds number turbulence, with broader implications for materials science, industrial processes, and biology.


[Phys. Rev. Fluids 9, L122602] Published Mon Dec 30, 2024

Polymer stretching and alignment under the hierarchy of coherent vortices in turbulence

Physical Review Fluids - Fri, 12/27/2024 - 10:00

Author(s): Yusuke Koide and Susumu Goto

At which scales do vortices in turbulence effectively stretch polymers? To answer this question, we conduct direct numerical simulations of turbulence and the Brownian dynamics simulations of the finitely extensible nonlinear elastic (FENE) dumbbell model. A scale-decomposition analysis based on a bandpass filter allows us to identify the dominant scale for polymer stretching. Furthermore, we explain the scale-dependent contribution to polymer stretching by focusing on the persistence of the stretching process of polymers induced by each-scale vortices.


[Phys. Rev. Fluids 9, 123303] Published Fri Dec 27, 2024

Bouncing oil-in-water compound droplets on superamphiphobic surfaces

Physical Review Fluids - Fri, 12/27/2024 - 10:00

Author(s): Shiji Lin, Lijie Sun, Zhiming Zhang, Yile Wang, Yakang Jin, Qin Xu, Zhigang Li, and Longquan Chen

We demonstrate that adding an immiscible oil core into impinging water droplets can strongly dampen the surface capillary wave propagation, which suppresses the air bubble entrapment in droplet impact on superamphiphobic surfaces at low Weber numbers; but instead, it facilitates the entrapment of a water drop, resulting in complex water-in-oil-in-water droplet configuration after rebound.


[Phys. Rev. Fluids 9, 123606] Published Fri Dec 27, 2024

Enhancing heat transfer in a channel with unsteady flow perturbations

Physical Review Fluids - Fri, 12/27/2024 - 10:00

Author(s): Silas Alben, Shivani Prabala, and Mitchell Godek

Recent studies have used optimization to determine fluid flows that can efficiently cool heated objects. This paper examines recently discovered steady optimal flows through a heated channel, and uses a perturbation method to find nearby unsteady flows that convect more heat - up to 80% - for a given amount of power needed to move the flow. The unsteady perturbations consist of vortices, small or large, that move along the channel walls and disrupt the thermal boundary layer.


[Phys. Rev. Fluids 9, 124503] Published Fri Dec 27, 2024

Force-dependence of the rigid-body motion for an arbitrarily shaped particle in a forced, incompressible Stokes flow

Physical Review Fluids - Fri, 12/27/2024 - 10:00

Author(s): Alvaro Domínguez and Mihail N. Popescu

A rigid body immersed in a fluid will generically move when the latter experiences a local force field. In the creeping flow regime, the body velocities (translational and angular) will be linear functionals of this field (“force representation”). Due to the incompressibility constraint, however, it should be possible to express them equivalently as linear functionals of the curl of the force (“curl representation”). Explicit expressions for this alternative formulation are derived, and illustrated with the example of self-chemophoresis.


[Phys. Rev. Fluids 9, L122101] Published Fri Dec 27, 2024

Second-order nonlinear analysis of instability in three-layer nanoscale composite planar liquid sheets

Physical Review Fluids - Thu, 12/26/2024 - 10:00

Author(s): Xiaocong Yang, Wentong Qiao, Hui Deng, Qingchang Meng, Bingrui Xu, and Qingfei Fu

The thermal drawing method has been widely used in fiber fabrication with the thickness down to the microscopic scale, where the flow instability plays an important role in obtaining nanowires or many intriguing patterns. Inspired by this, we performed an exploratory second-order nonlinear analysis to investigate the nonlinear instabilities of a planar liquid sheet under dual-mode, which can provide guidance to achieve sophisticated nanostructures for functional devices in a single fiber or integrated fabrics.


[Phys. Rev. Fluids 9, 123902] Published Thu Dec 26, 2024

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