Latest papers in fluid mechanics

Activity-induced asymmetric dispersion in confined channels with constriction

Physical Review Fluids - Thu, 01/18/2024 - 10:00

Author(s): Armin Maleki, Malihe Ghodrat, and Ignacio Pagonabarraga

Our Brownian dynamics model uncovers response mechanisms of microorganisms such as Escherichia coli to shear flows and constrictions, shedding light on anomalous accumulation of active particles in confined channels. Our findings highlight the butterfly-like attractors that trap particles past the constriction and characterize relevant dynamical regimes of active particle accumulation. We also explore the impact of particle size, channel geometry, and fluid velocity on trapping probability and accumulation strength. This intriguing phenomenon could be utilized as geometrical sift for mixture of active particles with different sizes/velocities and lead to innovations in particle manipulation.


[Phys. Rev. Fluids 9, 013103] Published Thu Jan 18, 2024

Onset of Lagrangian chaos: From fractal power spectrum to the absolutely continuous one

Physical Review Fluids - Thu, 01/18/2024 - 10:00

Author(s): Rafil V. Sagitov, Igor I. Wertgeim, and Michael A. Zaks

We study fluid motions excited by a spatially periodic force in a plane region with periodic boundary conditions; mean drift in both directions across the domain is nonzero. Under weak force, the flow is stationary; Fourier spectra of velocity for advected tracer particles are singular continuous (fractal). At higher amplitudes of the force, the flow pattern periodically oscillates, ensuring the onset of Lagrangian chaos in tracer dynamics. We illustrate the transformation of the power spectrum from the fractal object to the conventional smooth curve (subplots (i-iv)), and describe the accompanying changes in the pattern of autocorrelation for the tracer velocity.


[Phys. Rev. Fluids 9, 014401] Published Thu Jan 18, 2024

Role of vorticity distribution in the rise and fall of lift during a transverse gust encounter

Physical Review Fluids - Thu, 01/18/2024 - 10:00

Author(s): Antonios Gementzopoulos, Girguis Sedky, and Anya Jones

In this paper, we experimentally investigate the flow fields and lift transients associated with transverse gust encounters. We highlight the key differences between inviscid and viscous gust encounters and discuss how contrasting shed vorticity distributions lead to dissimilar lift behavior during the gust exit. Our findings contribute to a refined understanding of the utility of inviscid models in the prediction of loads during atmospheric gust encounters.


[Phys. Rev. Fluids 9, 014701] Published Thu Jan 18, 2024

Absolute and convective instability of a round jet emerging into an ambient medium of different viscosity

Physical Review Fluids - Wed, 01/17/2024 - 10:00

Author(s): Jinwei Yang and Vinod Srinivasan

The case of a round jet of one liquid emerging into an ambient medium of a different viscosity is relevant to many industrial mixing situations. A linear spatiotemporal analysis of such a configuration shows that for sufficiently high viscosity contrast, the flow is absolutely unstable. Both axisymmetric and helical modes are unstable, with the axisymmetric mode being slightly dominant. The absolute/convective instability transition boundary defined in terms of viscosity ratio and jet Reynolds number is compared to the results of recent experiments and good agreement is found.


[Phys. Rev. Fluids 9, 013903] Published Wed Jan 17, 2024

Pore-corner networks unveiled: Extraction and interactions in porous media

Physical Review Fluids - Wed, 01/17/2024 - 10:00

Author(s): Ninghua Zhan, Yiping Wang, Xiang Lu, Rui Wu, and Abdolreza Kharaghani

A novel pore-corner network extraction method is proposed. To validate our proposed extraction method, a generalized network model is developed to simulate evaporation in a porous medium composed of packed spherical beads. The modeling results are in good agreement with the experimental data, particularly in terms of the variation of liquid distribution over time. Our proposed extraction method not only contributes to disclose the structures of pores and corners in real porous media but also benefits the development of generalized network models that can be employed to understand in detail the multiphase transport in porous media from the pore scale perspective.


[Phys. Rev. Fluids 9, 014303] Published Wed Jan 17, 2024

Level crossings reveal organized coherent structures in a turbulent time series

Physical Review Fluids - Wed, 01/17/2024 - 10:00

Author(s): Subharthi Chowdhuri and Tirtha Banerjee

Detection of coherent structures in experiments, when using single-point temporal measurements, poses challenges due to their three-dimensional nature. Past methods, relying on ad hoc thresholds, lacked consistency across studies. To address this, the level-crossing method has been introduced and applied to two datasets of wall-bounded turbulent flows. This method allows the identification of coherent structures in a more objective manner, avoiding the need for arbitrary thresholds. An interesting discovery is that coherent structures influence near-wall turbulence through nonlinear interactions, a phenomenon not discernible through traditional spectral analysis.


[Phys. Rev. Fluids 9, 014601] Published Wed Jan 17, 2024

Undular bore theory for the modified Korteweg–de Vries–Burgers equation

Physical Review E - Tue, 01/16/2024 - 10:00

Author(s): L. F. Calazans de Brito and A. M. Kamchatnov

We consider nonlinear wave structures described by the modified Korteweg–de Vries equation, taking into account a small Burgers viscosity for the case of steplike initial conditions. The Whitham modulation equations are derived, which include the small viscosity as a perturbation. It is shown that f…


[Phys. Rev. E 109, 015102] Published Tue Jan 16, 2024

Electro-poroelastohydrodynamics of the endothelial glycocalyx layer and streaming potential in wavy-wall microvessels

Physical Review Fluids - Tue, 01/16/2024 - 10:00

Author(s): C. Teodoro, J. Arcos, O. Bautista, and F. Méndez

The study of the endothelial glycocalyx layer (EGL) has been of great interest in recent years due to its direct relationship with human health. Some works have modeled the EGL under the triphasic mixture theory (TMT), considering an electrically charged porous medium interacting with the electrolyte under a pressure gradient. Our study is based on a model that couples the mechanical and electrical interaction, considering TMT, a non-Newtonian electrolyte, and wavy walls using the domain perturbation method, focusing on the shear stresses in the EGL and on the induced streaming potential that can be used as a biocompatible energy source.


[Phys. Rev. Fluids 9, 013101] Published Tue Jan 16, 2024

Onset of Rayleigh-Bénard convection in dielectric liquids with electric conduction

Physical Review Fluids - Tue, 01/16/2024 - 10:00

Author(s): Yuxing Peng, R. Deepak Selvakumar, and Jian Wu

The convective flow of dielectric liquids with finite electric conductivity subjected to the simultaneous action of an electric field and a destabilizing temperature gradient is investigated. The instability of the system is studied through modal stability analysis, and it is found that the onset of the flow is delayed when an electric field is applied. Numerical simulations are performed to show the flow patterns and heat transfer characteristics. The dual solutions are highlighted in the bifurcation diagram.


[Phys. Rev. Fluids 9, 013902] Published Tue Jan 16, 2024

Flow stability in shallow droplets subject to localized heating of the bottom plate

Physical Review Fluids - Tue, 01/16/2024 - 10:00

Author(s): Khang Ee Pang, Charles Cuvillier, Yutaku Kita, and Lennon Ó Náraigh

When the surface tension of a droplet or a film varies inhomogeneously, a surface-tension gradient occurs, which induces a flow inside the fluid. Thermocapillary flows induced by localized heating have been observed experimentally in millimeter-sized water droplets. In particular, these experiments reveal that when such droplets are heated from below by a localized heat source targeted at the droplet center, a twin vortex pair perpendicular to the substrate is observed. This work aims to obtain some theoretical understanding to explain the onset of such vortices.


[Phys. Rev. Fluids 9, 014003] Published Tue Jan 16, 2024

Spectral analysis for elastica dynamics in a shear flow

Physical Review Fluids - Tue, 01/16/2024 - 10:00

Author(s): Lujia Liu, Paweł Sznajder, and Maria L. Ekiel-Jeżewska

Equations are derived for the evolution of a three-dimensional perturbation of a straight slender elastic fiber at an arbitrary orientation in shear flow at low-Reynolds-number. For the orientation in the plane of the shear flow and the flow gradient, the spectral analysis is performed for in-plane and out-of-plane perturbations. The most unstable eigenfunctions and eigenvalues are analyzed and compared to the previous results for the in-plane perturbations in the shear flow, and for arbitrary perturbations in the compressional flow.


[Phys. Rev. Fluids 9, 014101] Published Tue Jan 16, 2024

Simple generalization of kinetic theory for granular flows of nonspherical, oriented particles

Physical Review Fluids - Tue, 01/16/2024 - 10:00

Author(s): Dalila Vescovi, Ben Nadler, and Diego Berzi

The collective motion of nonspherical particles displaying preferential alignment can be modeled by extending the kinetic theory of granular gases. A linear dependency on the orientational tensor into the constitutive relations for the stresses, and a balance law for the orientational tensor itself, in which a key role is played by the randomizing effect of collisions, permit to reproduce the discrete simulations of homogeneous shearing flows of cylinders at different aspect ratios.


[Phys. Rev. Fluids 9, L012301] Published Tue Jan 16, 2024

Dispersion and deformation of molecular patterns written in turbulent air

Physical Review Fluids - Fri, 01/12/2024 - 10:00

Author(s): Willem van de Water, Nico Dam, and Enrico Calzavarini

This letter “H” is written in turbulent air by tagging molecules in the focus of intense laser beams that cross in space. In the course of 40 microseconds, turbulence deforms and disperses the pattern. It could be used as a way to measure the velocity of the turbulent eddies. However, more importantly, it reveals the intricate interplay between molecular diffusion and turbulent dispersion.


[Phys. Rev. Fluids 9, 014502] Published Fri Jan 12, 2024

Forced and natural dynamics of a clamped flexible fiber in wall turbulence

Physical Review Fluids - Fri, 01/12/2024 - 10:00

Author(s): Giulio Foggi Rota, Morie Koseki, Riya Agrawal, Stefano Olivieri, and Marco Edoardo Rosti

In turbulent flows, slender flexible bodies exhibit complex motions such as the swaying of seagrass or the oscillations of a pylon in the wind. Here we characterize the dynamical behavior of a clamped flexible fiber immersed in wall turbulence over a wide range of natural frequencies by DNS. Only two flapping states are possible: one where the fiber oscillates at the characteristic frequency of the largest turbulent eddies and another where the natural structural response dominates. We observe for the first time that in the turbulence dominated regime the fiber always sways at a frequency proportional to the largest scale of the flow, regardless of its structural parameters.


[Phys. Rev. Fluids 9, L012601] Published Fri Jan 12, 2024

Marangoni vortex rings in miscible spreading

Physical Review Fluids - Thu, 01/11/2024 - 10:00

Author(s): Anurag Pant and Baburaj. A Puthenveettil

This work investigates the dynamics of a unique, radially expanding vortex ring in a water layer when a miscible, volatile drop of ethanol spreads as a film on the air-water interface. The study unravels the link between the dynamics at the interface and the generation of vorticity in the water layer below it. A novel scaling is proposed for the radius and velocity of such vortex rings, where they are shown to be dependent on time as well as the properties of the drop and the substrate.


[Phys. Rev. Fluids 9, L012701] Published Thu Jan 11, 2024

Air-cushioning below an impacting wave-structured disk: Free-surface deformation and slamming load

Physical Review Fluids - Wed, 01/10/2024 - 10:00

Author(s): Yee Li (Ellis) Fan, Utkarsh Jain, and Devaraj van der Meer

A radially symmetric sinusoidal wave structure is imprinted on an impacting circular disk to modulate the way the disk forces the free water surface. The experiments support the argument that the surface elevation around the disk edge prior to impact is an instability of the Kelvin-Helmholtz type, as the free surface resonates when the forcing wavelength on the disk is close to the most unstable wavelength predicted by theory. Besides, our wave-structured disk is also found to promote gradual inertial wetting of the impacting surface to effectively retain the entrapped air pocket (as shown in the figure), which, in turn, mitigates the peak impact force.


[Phys. Rev. Fluids 9, 010501] Published Wed Jan 10, 2024

Universal scaling law for electrified sessile droplets on a lyophilic surface

Physical Review E - Tue, 01/09/2024 - 10:00

Author(s): Dipin S. Pillai and Kirti Chandra Sahu

Electrified sessile droplets on solid surfaces are ubiquitous in nature as well as in several practical applications. Although the influence of electric field on pinned sessile droplets and soap bubbles has been investigated experimentally, the theoretical understanding of the stability limit of gen…


[Phys. Rev. E 109, L013101] Published Tue Jan 09, 2024

Molecular diffusion of mass and energy predicted by <i>ab initio</i> potential energy surfaces for air components at high temperatures

Physical Review Fluids - Tue, 01/09/2024 - 10:00

Author(s): Paolo Valentini, Maninder S. Grover, and Nicholas J. Bisek

The accurate characterization of molecular transport properties is essential for high-fidelity simulations of reactive, hypersonic flows. The correct prediction of energy and mass diffusion in the laminar, high-temperature, multicomponent boundary layer of a hyper-velocity flow has profound implications for the accurate modeling of gas-surface interactions and thermal loads on the aeroshell. In this work, molecular transport is investigated by solely using ab initio potential energy surfaces. Our approach removes the empiricism associated with simplified molecular interactions models used in previous studies and is applicable to arbitrary gas mixtures.


[Phys. Rev. Fluids 9, 013401] Published Tue Jan 09, 2024

Adjoint-based machine learning for active flow control

Physical Review Fluids - Tue, 01/09/2024 - 10:00

Author(s): Xuemin Liu and Jonathan F. MacArt

We develop neural-network active flow controllers through a deep learning PDE augmentation method (DPM). In two-dimensional, incompressible, confined cylinder flow with Re = 100, we compare drag-reduction performance and optimization cost of adjoint-based controllers and deep reinforcement learning (DRL)-based controllers. The DRL-based controller demands 4,229 times the model complexity of the DPM-based one. The DPM-based controller is 4.85 times more effective and 63.2 times less computationally intensive to train than the DRL-based counterpart. In laminar compressible flows, successful extrapolation of the controller to out-of-sample flows demonstrates the robustness of the learning approach.


[Phys. Rev. Fluids 9, 013901] Published Tue Jan 09, 2024

Front tracking simulation of droplet displacement on solid surfaces by soluble surfactant-driven flows

Physical Review Fluids - Tue, 01/09/2024 - 10:00

Author(s): Xinglong Shang, Zhengyuan Luo, Bofeng Bai, Long He, and Guoqing Hu

Comprehensive numerical investigations of droplet displacement in soluble surfactant driven flows using the front-tracking method are presented. Surfactant transport in the bulk and at interfaces shapes droplet displacement and determines the transition conditions between steady-state sliding and detachment. Detachment is highly dependent on surfactant replenishment at interfaces, especially at receding contact lines where the nonuniform concentration induced Marangoni flow impedes movement. The critical effective capillary number can be used as a criterion to evaluate the ability of the surfactant to detach the droplet, giving a unique logarithmic relationship with detachment time.


[Phys. Rev. Fluids 9, 014002] Published Tue Jan 09, 2024

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