New Papers in Fluid Mechanics

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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