Latest papers in fluid mechanics

Highly rarefied gas flow through a right-angled micro-corner

Physical Review Fluids - Mon, 12/23/2024 - 10:00

Author(s): D. Ben-Adva and A. Manela

The two-dimensional steady flow of a highly rarefied gas through a right-angled corner element is studied, based on the Boltzmann kinetic model and the Maxwell wall conditions. Closed-form expressions for the mass flow rate through the corner are derived, indicating a decrease of more than 40% in its mass transfer permeability due to the bend, compared with a straight channel configuration.


[Phys. Rev. Fluids 9, 123401] Published Mon Dec 23, 2024

Correspondence between flow structures and heat transfer in regime transitions in turbulent rotating thermal convection

Physical Review Fluids - Mon, 12/23/2024 - 10:00

Author(s): Lin Sun, Yun-Bing Hu, Li-Qiu Wang, and Ke-Qing Xia

We experimentally investigate the relationship between heat transport and flow structures in rotating thermal convection. Our results, regarding the geometric and dynamic properties of columnar structures, demonstrate that the behaviors of heat transfer efficiency are intimately related to the changes in the coherent structures in the bulk flow. The sharper transitions of the flow field statistics suggest that, in future studies of regime transitions, flow field measurements may serve as a more definitive criterion than those based on heat transport behaviors.


[Phys. Rev. Fluids 9, 123501] Published Mon Dec 23, 2024

Dynamics of bubble deformation and breakup in decaying isotropic turbulence

Physical Review Fluids - Mon, 12/23/2024 - 10:00

Author(s): Andre Calado and Elias Balaras

Bubbly flows are present in a multitude of processes in both natural and industrial systems. One critical phenomenon is bubble fragmentation, which drives interfacial area and mass/momentum transfer. Direct Numerical Simulations (DNS) of turbulent two-phase bubbly flows allow for improved control of physical parameters and access to flow variables which are challenging to obtain from traditional experiments. By performing DNS of turbulent bubble fragmentation at a moderate Weber number and varying the bubble diameter around the integral turbulence length scale, we examine the exchange between turbulent kinetic energy (TKE) and surface energy, as well as other local quantities.


[Phys. Rev. Fluids 9, 123604] Published Mon Dec 23, 2024

Resonance and damping in drop-cantilever interactions

Physical Review Fluids - Mon, 12/23/2024 - 10:00

Author(s): Crystal Fowler, Rehan Marshall, Maeji Son, and Sunghwan Jung

Droplet and cantilever systems are often studied to further applications for energy-harvesting technologies and to model the leaf-raindrop dynamics. This paper examines the interplay between the droplet and cantilevers of varying length by measuring the oscillation frequency, phase shift, maximum displacement, and damping coefficients. There is a significant difference in the measured values when resonance happens between the droplet and cantilever of a certain length. At the cantilever resonance length, high damping coefficients are attributed to the opposing inertial forces of the droplet and cantilever.


[Phys. Rev. Fluids 9, 123605] Published Mon Dec 23, 2024

Parametrizing the probability density function of wall-shear stress in turbulent channel flows

Physical Review Fluids - Mon, 12/23/2024 - 10:00

Author(s): A. Lakshmi Srinivas, Jingxuan Zhang, and Ruifeng Hu

Parametrization of the probability density function (PDF) of streamwise wall-shear stress (WSS) in turbulent channel flows at the friction Reynolds number from 180 to 5200 is investigated. Lognormal parametrization is found to be more accurate than Gaussian for both the original and rescaled PDF of WSS. The original PDF of the inner WSS fluctuations can be well parametrized by a lognormal distribution. The rescaled PDF of the outer WSS fluctuations can be precisely parametrized by a Gaussian distribution.


[Phys. Rev. Fluids 9, 124604] Published Mon Dec 23, 2024

Cavity dynamics of vertical water entry by air jet

Physical Review Fluids - Fri, 12/20/2024 - 10:00

Author(s): Zhihui Zou, Yunhua Jiang, and Bin Wu

Spheres falling into water create fascinating phenomena, such as crown-like splashes and clear cavities that are subsequently pinched off. These phenomena are universal and are generally controlled by the properties of the sphere. In this study, we report a new cavity formed by an air jet that lacks a distinct splash and features a rough cavity interface. We investigate the cavity dynamics, including formation, development, and pinch-off events.


[Phys. Rev. Fluids 9, 124006] Published Fri Dec 20, 2024

Nonequilibrium wall model for large eddy simulations of complex flows exhibiting turbulent smooth body separation

Physical Review Fluids - Fri, 12/20/2024 - 10:00

Author(s): Rahul Agrawal, Sanjeeb T. Bose, and Parviz Moin

We propose a nonequilibrium wall model for improving the predictions of flow separation in complex, turbulent boundary layers. Improved predictability of smooth body separation at multiple Reynolds and Mach numbers in flows over the NASA/Boeing speed bump and the Bachalo-Johnson bumps is demonstrated at resolutions where the equilibrium model fails to separate. Scaling arguments, followed by a posteriori verification suggest a weaker scaling of the required resolutions to capture flow separation using the proposed model compared to standard equilibrium closures.


[Phys. Rev. Fluids 9, 124603] Published Fri Dec 20, 2024

Unification theory of instabilities of visco-diffusive swirling flows

Physical Review Fluids - Fri, 12/20/2024 - 10:00

Author(s): Oleg N. Kirillov and Innocent Mutabazi

By employing local geometrical optics stability analysis adapted to visco-diffusive flows, we derive novel explicit instability criteria for isothermal and non-isothermal swirling flows, induced by the combination of rotation and shear in orthogonal directions and ubiquitous in various natural phenomena, such as tornadoes and tropical cyclones. Our advance stems from an observation overlooked in previous research: the neutral stability curves in these problems possess an envelope, which we have analytically determined using the connection between envelopes and polynomial discriminants. Our analytical results offer a general theory of instabilities across a wide range of swirling flows.


[Phys. Rev. Fluids 9, 124802] Published Fri Dec 20, 2024

Lagrangian versus Eulerian view on the mean drift and streaming flows in orbital sloshing

Physical Review Fluids - Fri, 12/20/2024 - 10:00

Author(s): A. Bongarzone and F. Gallaire

Orbital sloshing, a common technique in fluid mixing for processes like cell cultivation and fermentation, generates complex wave dynamics at the interface and a hidden Lagrangian mean flow in the fluid bulk. Distinguishing between the Eulerian viscous streaming and Stokes drift contributions to the overall Lagrangian motion has remained challenging, particularly in highly viscous fluids. This study presents a weakly nonlinear analysis, revealing that Stokes drift and Eulerian viscous corrections can be equally important in the mean flow generation, offering new insights into orbital sloshing wave dynamics beyond traditional inviscid models.


[Phys. Rev. Fluids 9, 124803] Published Fri Dec 20, 2024

Simplified mathematical model for erosion and deposition in a porous medium

Physical Review Fluids - Thu, 12/19/2024 - 10:00

Author(s): Amy María Sims, Sai Kunnatha, Emeka Peter Mazi, Priyanka Joseph, Kulsum Saber, Daniel Hwang, and Pejman Sanaei

We develop a two-dimensional mathematical model that investigates the processes of erosion and deposition in an elastic porous medium. To simplify, we assume homogeneity and nondimensionalize the parameters, including Darcy velocity, particle concentration, and shear stress before reducing the continuum model via asymptotic analysis by exploiting its small aspect ratio. Our results illustrate the evolution of the medium under a prescribed constant flux of particles, wherein we draw conclusions on how total volume changes based on varying coefficients that dictate the tendency of particles to adhere to or be eroded from the walls of the medium at varying values of shear stress.


[Phys. Rev. Fluids 9, 124306] Published Thu Dec 19, 2024

Ruelle-Takens-Newhouse and degenerate period-doubling routes to chaos in a wavy-channel flow under mixed convection

Physical Review Fluids - Thu, 12/19/2024 - 10:00

Author(s): Mohammad Hossein Doranehgard, Iman Borazjani, Nader Karimi, and Larry K. B. Li

Wavy channels are widely used to enhance heat transfer in various applications, but their nonlinear dynamics under mixed convection remain poorly understood. Our numerical study reveals that mixed convection fundamentally alters the routes to chaos in wavy-channel flows: symmetric channels show both the Ruelle-Takens-Newhouse and degenerate period-doubling routes, asymmetric channels show only the latter route, and semi-wavy channels show no routes to chaos. The Pomeau-Manneville intermittency route, previously observed in isothermal conditions, is notably absent. These insights into the nonlinear dynamics of wavy-channel flows provide crucial guidance for optimizing heat transfer devices.


[Phys. Rev. Fluids 9, 124403] Published Thu Dec 19, 2024

Theoretical and experimental investigation of the shapes formed by floating droplets excited with Faraday waves

Physical Review Fluids - Thu, 12/19/2024 - 10:00

Author(s): L. Mazereeuw

When the Faraday instability is induced in floating droplets in a viscous bath, a wave radiation pressure is exerted on the droplet boundary, causing it to evolve until a new equilibrium shape is reached. Different shapes are obtained by varying the forcing amplitude and frequency, though the system is highly hysteretic. We develop a theoretical model for the time evolution of the droplet boundary through the separation of timescales, with a strong agreement between the predicted equilibrium profiles and experimental observations.


[Phys. Rev. Fluids 9, 124404] Published Thu Dec 19, 2024

Transition route to elastic and elasto-inertial turbulence in polymer channel flows

Physical Review Fluids - Wed, 12/18/2024 - 10:00

Author(s): M. Beneitez, J. Page, Y. Dubief, and R. R. Kerswell

In this work we show that the polymer diffusive instability is able to trigger viscoelastic turbulence with and without inertia through a secondary linear instability providing a generic supercritical route to viscoelastic turbulence. The reported secondary instability resembles center or wall modes, establishing connections with previous results in the literature.


[Phys. Rev. Fluids 9, 123302] Published Wed Dec 18, 2024

Motion and deformation of a bubble in a Hele-Shaw cell

Physical Review Fluids - Wed, 12/18/2024 - 10:00

Author(s): K. Wu, D. J. Booth, I. M. Griffiths, P. D. Howell, J. K. Nunes, and H. A. Stone

We theoretically and experimentally study the propagation of approximately circular pancake-shaped bubbles in a Hele-Shaw cell under a uniform background flow at low Reynolds number. Bubble motion and deformation are determined by an interplay between the Hele-Shaw viscous pressure, the pressure drop due to the thin films surrounding the bubble, and the capillary pressure due to the in-plane curvature of the bubble boundary. Numerical, asymptotic and experimental results indicate that, with all other parameters constant, the in-plane aspect ratio of the bubble varies nonmonotonically with its size. The model is also extended for buoyancy-driven bubbles in inclined or vertical channels.


[Phys. Rev. Fluids 9, 123603] Published Wed Dec 18, 2024

Experimental investigation of the exit dynamics of a horizontal circular cylinder out of water and silicone oil

Physical Review Fluids - Wed, 12/18/2024 - 10:00

Author(s): Intesaaf Ashraf, Lionel Vincent, Romain Falla, Vincent E. Terrapon, Benoit Scheid, and Stéphane Dorbolo

A dolphin leaping out of the water. A piece of bread pulled from Swiss fondue. A car emerging from a bath of anti-corrosion fluid. More somberly, a missile launched from below sea level. The question is: how much liquid is carried along? We study the entrainment around a horizontal cylinder and observe that, whether in water or in oil (50 times more viscous than water), higher speeds result in greater liquid entrainment. By tracking the thickness of the liquid film at the top of the cylinder, we measure the drainage process down to a few microns. Remarkably, shortly after crossing the interface (typically within 1 second), the drainage becomes independent of the withdrawal speed.


[Phys. Rev. Fluids 9, 124005] Published Wed Dec 18, 2024

Engelund bedload transport formula for sparsely vegetated channels

Physical Review Fluids - Wed, 12/18/2024 - 10:00

Author(s): Yihan Qu, Zhiheng Ye, Qingwei Lin, and Limo Tang

Bedload transport in vegetated channels, especially sparsely vegetated channels, is highly complex. As flow intensity varies, bedload transport can be divided into two stages: local and global bedload transport. In this study, the Engelund formula, which is typically used for bedload transport in bare beds, has been extended to calculate the bedload transport rate in sparsely vegetated channels. The results show that the bedload formula based on bed shear stress is more appropriate for medium and high bedload transport rates (global bedload transport), while the bedload formula based on grain shear stress is more suitable for weak bedload transport rates (local bedload transport).


[Phys. Rev. Fluids 9, 124502] Published Wed Dec 18, 2024

Global stability analysis of falling jets with surfactants

Physical Review Fluids - Tue, 12/17/2024 - 10:00

Author(s): Hu Sun, Qiyou Liu, Dingwei Zhang, Bingqiang Ji, Lijun Yang, and Qingfei Fu

Liquid jets with insoluble surfactants undergo thinning and acceleration due to gravitational stretching. The evolution of the jet’s basic flow and surface perturbations is influenced by a combination of inertia, surface tension, viscosity, gravity, and Marangoni stress. Global stability analysis results show that the Marangoni effects caused by the surfactants promote thinning of the basic flow and inhibit the growth of perturbations. Gravity, on the other hand, not only suppresses the perturbation development but also increases the optimal forcing frequency in forced falling jets.


[Phys. Rev. Fluids 9, 124004] Published Tue Dec 17, 2024

Surfing droplets on nanoscopic films driven by surface acoustic waves

Physical Review E - Tue, 12/17/2024 - 10:00

Author(s): N. S. Satpathi, L. Malik, S. Nandy, T. Sujith, L. Y. Yeo, and A. K. Sen

Formation of micron-sized droplets on open surfaces continues to remain a challenge in microfluidics. The problem is even stiffer for highly spreading liquids. Here, we report the formation of microdroplets from a nanoscopically thick film of low surface tension and low-viscosity liquid following it…


[Phys. Rev. E 110, 065108] Published Tue Dec 17, 2024

Numerical study of the oscillatory boundary layer over wall-mounted flexible filaments

Physical Review Fluids - Mon, 12/16/2024 - 10:00

Author(s): Yu-Hang Xiong, An-Kang Gao, Xi-Yun Lu, and Shaohua Chen

This study explores the flow-structure interaction between the oscillatory boundary layer flow and an array of wall-mounted flexible filaments using a penalty-immersed boundary method. The key finding is that the presence of filaments lifts the boundary layer by the average filament height. A partial slip boundary condition with a complex-valued slip length is introduced to quantify the velocity-velocity gradient relation. It indicates the slip length reaches a stable value at the upper edge of the filaments. This study provides insight into modeling the effective boundary condition of the filament-attached wall.


[Phys. Rev. Fluids 9, 124101] Published Mon Dec 16, 2024

Heat transport and flow structures in inclined circular enclosures

Physical Review Fluids - Mon, 12/16/2024 - 10:00

Author(s): Snehal Sunil Patil, V. R. Krishna Priya, and Rajaram Lakkaraju

Long-term seasonal weather variations can arise from a misalignment between sunlight and the direction of a planet’s gravitational pull, though plausible reasons are yet unclear. Inspired by such events, we have carried out numerical simulations and unmasked large-scale circulations and their reversals in canonical convection. At optimal orientations, the large-scale motions and core mixing events work against each other to achieve maximum heat transport. Our research findings have potential implications for climate studies and the development of thermal control strategies for device applications.


[Phys. Rev. Fluids 9, 124305] Published Mon Dec 16, 2024

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