Latest papers in fluid mechanics

Author(s): Wen-Yi Chang and George Constantinescu

Surface mounted cylinders in combined wave-current flow generate a wide range of coherent structures that include vortex tubes (VT), wake billow vortices (VW), and horseshoe vortices observed in steady flow. Additionally, horizontal near-bed vortices are sometimes generated on the wake side of the cylinder. The formation of these vortices and the forces acting on the cylinder are a function of the ratio between the steady current velocity and the oscillatory velocity (0≤Us/Um≤1.4) and of the Keulegan-Carpenter number, KC=UmT/D (1.5≤KC≤30.8), where T is the period of the oscillatory flow.

[Phys. Rev. Fluids 10, 024804] Published Thu Feb 20, 2025

Author(s): Wen-Yi Chang and George Constantinescu

Surface mounted cylinders in combined wave-current flow generate a wide range of coherent structures that include vortex tubes (VT), wake billow vortices (VW), and horseshoe vortices observed in steady flow. Additionally, horizontal near-bed vortices are sometimes generated on the wake side of the cylinder. The formation of these vortices and the forces acting on the cylinder are a function of the ratio between the steady current velocity and the oscillatory velocity (0≤Us/Um≤1.4) and of the Keulegan-Carpenter number, KC=UmT/D (1.5≤KC≤30.8), where T is the period of the oscillatory flow.

[Phys. Rev. Fluids 10, 024804] Published Thu Feb 20, 2025

Author(s): Ruoqing Gao, Cheng Liu, Yuxiao Yang, and Changhong Hu

When a rising bubble impacts the free surface, the interface deforms, and the bubble rebounds multiple times before ultimately rupturing. This study investigates the kinematic characteristics of bubble motion influenced by the Morton number (liquid viscosity) and the Weber number (surface tension) through numerical simulations. The results reveal that the parameters associated with bubble bouncing exhibit a distinct logarithmic-linear decay with the Morton number and a linear decrease with the Weber number. Furthermore, the first rebound of the bubble is predominantly governed by the approach velocity, while subsequent rebounds are primarily influenced by the bubble’s shape.

[Phys. Rev. Fluids 10, 023604] Published Wed Feb 19, 2025

Author(s): Xiaohang Shi, Qiulin Qu, Peiqing Liu, Tianxiang Hu, Yunlong Zheng, and Peizhe Zhou

When a flat plate impacts a water surface, bubbles of different shapes can be entrapped depending on impact velocity. This work numerically investigates the morphology of air bubbles and the underlying physics. In a wide range of impact velocities, two typical air bubble patterns are found: small-bubble pattern and large-bubble pattern. Importantly, their evolutions are shown to be dictated by two physical factors: the compression-expansion intensity of trapped air during initial impact stages (before the plate edge contacts the water surface) and wind-driven waves below the plate.

[Phys. Rev. Fluids 10, 024803] Published Wed Feb 19, 2025

Author(s): Xiaohang Shi, Qiulin Qu, Peiqing Liu, Tianxiang Hu, Yunlong Zheng, and Peizhe Zhou

When a flat plate impacts a water surface, bubbles of different shapes can be entrapped depending on impact velocity. This work numerically investigates the morphology of air bubbles and the underlying physics. In a wide range of impact velocities, two typical air bubble patterns are found: small-bubble pattern and large-bubble pattern. Importantly, their evolutions are shown to be dictated by two physical factors: the compression-expansion intensity of trapped air during initial impact stages (before the plate edge contacts the water surface) and wind-driven waves below the plate.

[Phys. Rev. Fluids 10, 024803] Published Wed Feb 19, 2025

Author(s): Mateus C. Guimarães, Fernando T. Pinho, and Carlos B. da Silva

The figures show passive scalar contours in Newtonian (top) and viscoelastic (bottom) turbulent jets generated by direct numerical simulations (DNS). Initially, the depletion of small scale perturbations by the polymers suppresses the small scale mixing, but later allows the appearance of very large coherent structures that promote the stirring and thus enhance large and intermediate scale stirring.

[Phys. Rev. Fluids 10, 023303] Published Tue Feb 18, 2025

Author(s): Titouan Moriniere and Thierry Schuller

Understanding the interaction between an acoustic field and a dispersed cloud of droplets in particle-laden flows is critical, particularly for thermoacoustic instabilities in spray flame combustors. This study develops analytical expressions for particle velocity and number density, leading to insights into the particle clustering mechanism. While individual droplets may remain largely unaffected by acoustic perturbations, the droplet population spatial distribution can still undergo significant disturbances. An evanescent convective wave in the particle velocity response is identified as the primary driver of clustering, which is predominantly a convective phenomenon at low Mach numbers.

[Phys. Rev. Fluids 10, 024302] Published Tue Feb 18, 2025

Author(s): Fabiola Trujano-Jiménez, Varvara E. Zemskova, and Nicolas Grisouard

The upper ocean is home to several hydrodynamic instabilities which are known to generate strong vertical flows crucial for the transport of physical and biochemical properties. Here we study the two-dimensional Ekman-Inertial Instability (EII), the analytical description of which is currently limited to one dimension, and compare its effects with those of Inertial Instability (InI). We found that EII grows significantly faster and creates stronger vertical flows than InI. Moreover, we explore the sensitivity of its growth rate to variations in the Rossby number of the initial flow. We found that EII and InI radiate near-inertial waves that propagate in regions of anticyclonic vorticity.

[Phys. Rev. Fluids 10, 024802] Published Tue Feb 18, 2025

Author(s): Thomas Burton, Sean Symon, Ati S. Sharma, and Davide Lasagna

We propose a framework for approximating the statistical properties of turbulent flows by combining variational methods for the search of unstable periodic orbits with resolvent analysis for dimensionality reduction. Traditional approaches relying on identifying all short, fundamental unstable perio…

[Phys. Rev. E 111, 025104] Published Tue Feb 18, 2025

Author(s): Vivaswan ChandraShekar, Guillaume Maîtrejean, and Hugues Bodiguel

The development of preferential pathways in viscoelastic flow through a porous medium is a widely researched topic. We recognize a gap in the works that use simple geometries to model flow through such complex media. These geometries are often symmetric, like the archetypal confined cylinder. Therefore, in this work, we explore the flow of viscoelastic and shear-thinning viscoelastic fluids using finite-volume numerical simulations in three simple yet asymmetrical geometries involving a flow bifurcation. We show a robust elastic phenomenon of enhanced flow asymmetry above a Weissenberg number (Wi) of unity, irrespective of the geometry with even the slightest asymmetry in the pathways.

[Phys. Rev. Fluids 10, 023302] Published Fri Feb 14, 2025

Author(s): Vivaswan ChandraShekar, Guillaume Maîtrejean, and Hugues Bodiguel

The development of preferential pathways in viscoelastic flow through a porous medium is a widely researched topic. We recognize a gap in the works that use simple geometries to model flow through such complex media. These geometries are often symmetric, like the archetypal confined cylinder. Therefore, in this work, we explore the flow of viscoelastic and shear-thinning viscoelastic fluids using finite-volume numerical simulations in three simple yet asymmetrical geometries involving a flow bifurcation. We show a robust elastic phenomenon of enhanced flow asymmetry above a Weissenberg number (Wi) of unity, irrespective of the geometry with even the slightest asymmetry in the pathways.

[Phys. Rev. Fluids 10, 023302] Published Fri Feb 14, 2025

Author(s): Jordi Casacuberta, Sven Westerbeek, Juan Alberto Franco, Koen J. Groot, Stefan Hickel, Stefan Hein, and Marios Kotsonis

Stationary velocity-perturbation streaks are found to be inherent to laminar swept-wing boundary layers interacting with a forward-facing step, often promoting premature laminar-turbulent transition. Using Direct Numerical Simulations, this study reveals that these streaks emerge as a linear response of the step flow to incoming three-dimensional (3D) perturbations via the lift-up effect, further amplified by base-flow deceleration, i.e., a streamwise analogous “push-forward effect.” By elucidating steak formation and stability, this work contributes to the predictive understanding of the transition of 3D boundary layers with surface features, with implications for aerodynamic design.

[Phys. Rev. Fluids 10, 023902] Published Fri Feb 14, 2025

Author(s): Lorenzo Betti, Jordy Queiros Campos, Amandine Lechantre, Léa Cailly-Brandstater, Sarra Nouma, Jérôme Fresnais, Etienne Barthel, Yann Bouret, Xavier Noblin, and Céline Cohen

Because of their practical importance in applications like self-cleaning and drag reduction, superhydrophobic surfaces have been widely studied. However, the link between microscopic surface properties and macroscopic dynamic contact angles remains an open question. This study systematically examines dynamic contact angles on superhydrophobic micropillar surfaces across a wide range of velocities, analyzing their dependence on solid surface fraction. We compare existing models to identify dissipation sources and propose a new mechanism based on droplet detachment from pillars, as observed in our experiments.

[Phys. Rev. Fluids 10, 024003] Published Fri Feb 14, 2025

Author(s): Saif Farhat, Diogo Bolster, and Guillem Sole-Mari

Pore-scale concentration fluctuations play a crucial role in mixing-limited reactions in porous media. We mathematically establish a direct link between mixing interface deformation and pore-scale concentration gradients. Contrary to the classical assumption that these fluctuations eventually get washed out, we show that for Peclet numbers above a critical threshold, advection sustains them indefinitely. Our analytical model quantifies the elongation of the mixing interface and accurately predicts reaction product formation in three-dimensional porous media, offering new insights into transport and reaction kinetics at the pore scale.

[Phys. Rev. Fluids 10, 024501] Published Fri Feb 14, 2025

Author(s): Fan Xu, Jun Zhong, Jinghong Su, Bidan Zhao, Yurong He, Chao Sun, and Junwu Wang

The interaction between wall shear and roughness leads to distinct heat transfer behavior in different regimes in an annular centrifugal Rayleigh-Bénard convection (ACRBC) system. In the buoyancy-dominant regime, an increase in the non-dimensional angular velocity difference (Ω) significantly enhances heat transfer. However, as Ω continues to rise, a sharp reduction in heat transfer is observed in the transitional regime. Beyond a critical value of Ω, the flow enters a shear-dominant regime, where heat transfer remains unchanged despite further increases in Ω.

[Phys. Rev. Fluids 10, 024604] Published Fri Feb 14, 2025

Author(s): Xinru Wang (王新茹), Jianxun Zhu (朱建勋), Lars Erik Holmedal, Dag Myrhaug, and Hong Wang (王红)

Oscillatory flow past an elliptic cylinder with an aspect ratio of 0.4 has been investigated for low Keulegan-Carpenter (KC) and Reynolds (Re) numbers by conducting two-dimensional numerical simulations. Four flow regimes A, C, F’ (a newfound one) and F are identified and mapped out, and the physics mechanisms underpinning the transition between the top- and bottom-dominated vortex shedding in flow regime C, the one-sided vortex pair shedding in a new flow regime F’, and the diagonal two-sided vortex pairs shedding in flow regime F have been investigated in detail. The resulting hydrodynamic forces acting on the cylinder are also explained in light of the vortex dynamics around the cylinder.

[Phys. Rev. Fluids 10, 024702] Published Fri Feb 14, 2025

Author(s): Emanuele Zuccoli, Edward J. Brambley, and Dwight Barkley

This paper proposes a novel set of two-dimensional governing equations to describe surface waves propagation on top of vortical flows, such as those easily observable in a swimming pool (left image). The model presented here overcomes three limitations of existing models, namely: it is not restricted to potential base flows; it does not assume the base flow to have a flat free surface; and it does not require the use of infinite-order differential operators. The model can be also applied in the case of rapid swirl where the base free surface is substantially deformed, as shown in the right image.

[Phys. Rev. Fluids 10, 024801] Published Fri Feb 14, 2025

Author(s): Emanuele Zuccoli, Edward J. Brambley, and Dwight Barkley

This paper proposes a novel set of two-dimensional governing equations to describe surface waves propagation on top of vortical flows, such as those easily observable in a swimming pool (left image). The model presented here overcomes three limitations of existing models, namely: it is not restricted to potential base flows; it does not assume the base flow to have a flat free surface; and it does not require the use of infinite-order differential operators. The model can be also applied in the case of rapid swirl where the base free surface is substantially deformed, as shown in the right image.

[Phys. Rev. Fluids 10, 024801] Published Fri Feb 14, 2025

Author(s): L. Manfredini and Ö. D. Gürcan

A class of shell models is proposed where the shell variables are defined on a recurrent sequence of integer wave numbers such as the Fibonacci or the Padovan series or their variations, including a sequence made of square roots of Fibonacci numbers rounded to the nearest integer. Considering the si…

[Phys. Rev. E 111, 025103] Published Fri Feb 14, 2025

Author(s): Raghavendra Naidu S., Kamal Poddar, and Sanjay Kumar

The dynamics of liquid drop impact on a liquid surface is studied experimentally. The interplay between the inertia forces and surface tension forces during the expansion and contraction of the cavity determines the shape of the cavity. In shallow water, the cavity expansion and retraction are dominated by surface tension forces but in deep water the inertia forces and gravity forces dominate the cavity dynamics. The cavity expansion resembles a source of the potential flow below the surface of the liquid. Time resolved Particle Image Velocimetry (PIV) measurements enable estimates of time variation of the source strength.

[Phys. Rev. Fluids 10, 023603] Published Thu Feb 13, 2025