New Papers in Fluid Mechanics

Author(s): Emad Chaparian

In this study, a comprehensive Darcy-type law for viscoplastic fluids is proposed. The two extreme limits of a yield-stress fluid flow in a porous medium are addressed individually and then are combined to propose a Darcy-type law which is valid across the entire range of the Bingham numbers (i.e. the ratio of the fluid’s yield stress to the characteristic viscous stress). These two extreme limits are namely the viscous limit (infinitely large pressure gradient compared to the yield stress of the fluid – ultra low Bingham number) and the plastic/yield limit (infinitely large Bingham number).

[Phys. Rev. Fluids 10, 093301] Published Thu Sep 11, 2025

Author(s): Adrian Herrera-Amaya, Nils B. Tack, Zhipeng Lou, Chengyu Li, and Monica M. Wilhelmus

Shrimp thrive in a wide range of climates, from the tropics to polar waters and inland freshwater. Our research shows how their swimming style has evolved to be highly resilient to environmentally driven changes in water properties. Our findings highlight the ability of the large biomass of shrimp-like crustaceans to adapt to vastly different water temperatures and viscosities. It also shows the potential for crustacean-inspired underwater vehicles; such drones could navigate environments with variable viscosity, such as phytoplankton blooms or oil spills, without requiring modifications to their control algorithms.

[Phys. Rev. Fluids 10, 093101] Published Wed Sep 10, 2025

Author(s): Hao Ye, Zhenyu Ouyang, and Jianzhong Lin

Active fluids can influence the behavior of passive particles, with sedimentation being an important such process. We investigate how the velocity of a sphere varies when settling in a square tube filled with active nematic fluids. While the direct active forces exerted on the sphere are weak in our study, activity can still significantly modify settling velocity by altering the flow structure and nematic field. The results manifest mechanisms of activity-induced shear thinning and thickening for various anchoring conditions and activity. As activity increases, transitions between flow patterns further influence settling, with different effects observed in extensile and contractile fluids.

[Phys. Rev. Fluids 10, 093102] Published Wed Sep 10, 2025

Author(s): Charbel Habchi, Aurelien Joly, and Pierre Moussou

The evaluation of added and coupling mass is central to fluid-structure interaction studies. Revisiting Batchelor’s 1967 framework, this work introduces a refined approach based on superposition of acceleration fields and numerical evaluation of kinetic energy in potential flows. By distinguishing between added and coupling mass, the method clarifies inertial forces for diverse body geometries and highlights implications for seismic design, underwater dynamics, and multiphase flow modeling.

[Phys. Rev. Fluids 10, 094301] Published Wed Sep 10, 2025

Author(s): T. Zemach

Previous research on gravity currents, the flow of a denser fluid through a less dense one, has largely focused on channels with rectangular cross-sections. How do these currents flow through more complex, nonrectangular shapes found in nature, like river estuaries or valleys? A generalized model that accounts for crucial effects of entrainment and drag in channels of various shapes is presented. This model reveals how these factors significantly reduce the current’s propagation speed while increasing its height and volume, especially in nonrectangular geometries. This work provides enhanced understanding of these flows, with findings that can be applied to diverse natural and engineered systems.

[Phys. Rev. Fluids 10, 094401] Published Wed Sep 10, 2025

Author(s): Guangtao Li, Liangquan Zhang, Xin Wang, Wen-Li Chen, Hui Li, and Donglai Gao

The interaction of vortex rings with solid surfaces is a classic problem in fluid dynamics, yet their collision with concave hemicylindrical shells remains poorly understood. This experimental study investigates vortex ring impingement on concave surfaces with varying curvature ratios (Dm /De = 15,10,5,2,1) at Re=1500. Using planar laser-induced fluorescence and particle image velocimetry, we reveal how curvature controls secondary vortex formation, rotation, and three-dimensional evolution. Dynamic models are proposed to explain the transition from tertiary to secondary vortex dominance as curvature increases, offering new insights into confined vortex-wall interactions.

[Phys. Rev. Fluids 10, 094702] Published Wed Sep 10, 2025

Author(s): F. C. Martins and J. C. F. Pereira

We study the counter-rotating vortex pair in laminar jets in crossflow via a vorticity transport analysis at various jet-to-crossflow velocity ratios (R). At high R, most vorticity is generated in the pipe, but boundary layer vorticity is entrained into the pair at low R.

The vortex pair undergoes bubble vortex breakdown at low-to-intermediate R, due to the adverse pressure gradient through the vortex cores. Breakdown persists at the onset of hairpin vortex shedding with decreasing R, but disappears upon further growth in instability amplitude. A new low frequency and spanwise symmetry-breaking instability characterized by alternating breakdown of the vortex pair was also identified.

[Phys. Rev. Fluids 10, 094703] Published Wed Sep 10, 2025

Author(s): Sijie Wang, Yuxiao Cheng, Zifei Yin, Paul Durbin, and Weipeng Li

A new hybrid Reynolds-Averaged-Navier-Stokes/Large Eddy Simulation (RANS/LES) model is proposed. With the discontinuous Galerkin spectral element method (DGSEM), it gives a robust and efficient approach for simulating high-speed and high-Reynolds-number turbulent flows. A change of variable from omega in the k-omega model to g, gives the k-g model, which avoids the singularity near the wall and makes the model work in DGSEM. The model is tested in various cases, including wall-bounded turbulence, wall-modeled LES, flow separation, and compressible flows, with good performance.

[Phys. Rev. Fluids 10, 094902] Published Wed Sep 10, 2025

Author(s): Lorenzo Piro, Massimo Cencini, and Roberto Benzi

Extreme Lagrangian acceleration in turbulence is often attributed to particle trapping in coherent vortex structures. Here, we show that shell models, despite lacking vortex filaments, reproduce the extreme intermittent fluctuations of Lagrangian acceleration seen in real flows. Within the multifractal framework, we accurately predict acceleration moments and full probability distribution functions (PDFs) across a wide range of Reynolds numbers. This shows extreme Lagrangian fluctuations can arise from inertial-range dynamics alone, supporting the universality of multifractal statistics and highlighting shell models as effective tools to capture key features of turbulence.

[Phys. Rev. Fluids 10, L092601] Published Wed Sep 10, 2025

Author(s): Fabio Guglietta, Diego Taglienti, and Mauro Sbragaglia

We study the statistics of deformation of neutrally buoyant droplets in homogeneous isotropic turbulence (HIT), wherein the characteristic droplet size R is smaller than the characteristic Kolmogorov scale η of the turbulent flow. We systematically focus on the characterization of droplet deformatio…

[Phys. Rev. Fluids 10, 093601] Published Tue Sep 09, 2025

Author(s): Chenyue Wang, Jihui Ou, and Jie Chen

Near-space hypersonic vehicles at 40–60 km altitude encounter local rarefaction effects that influence the boundary-layer stability and the laminar–turbulent transition. By incorporating slip boundary conditions and nonlinear constitutive relations into the Navier–Stokes equations and linear stability theory, we systematically investigate hypersonic boundary-layer stability over a blunt cone under near-continuum conditions. The results show that rarefaction thins the boundary layer and stabilizes second-mode instabilities, particularly at high wall temperatures or with small bluntness. These results offer new insights into hypersonic stability and transition in the near-continuum regime.

[Phys. Rev. Fluids 10, 093901] Published Thu Sep 04, 2025

Author(s): Eric Parish and Matthew Barone

The wall-normal Reynolds stress has a negligible contribution to the wall-normal momentum equation in low-Mach, zero pressure gradient boundary layers, and conservation of momentum indicates that pressure is uniform throughout the boundary layer. In hypersonic boundary layers analysis suggests the wall-normal Reynolds stress becomes significant compared to mean pressure and that it must balance with pressure. We validate this and show that the wall-normal Reynolds stress results in non-negligible pressure deficits. We show that errors in standard RANS models for the wall-normal Reynolds stress have a noticeable impact (5% for a Mach 14 cold-wall flow) on wall quantities of interest.

[Phys. Rev. Fluids 10, 094601] Published Thu Sep 04, 2025

Author(s): Yu Qiu, Luis Cueto-Felgueroso, Amir A. Pahlavan, Bauyrzhan K. Primkulov, and Ruben Juanes

Contact lines, where fluid interfaces meet solid surfaces, pose a fundamental challenge to modeling fluid-fluid displacement in confined geometries, as they violate the classical no-slip boundary condition. Recent experiments reveal that contact-line motion in a capillary tube produces compact displacement at low flow rates and unstable fingering at high flow rates. We present a phase-field model with a novel formulation of the boundary wetting conditions. Our model captures the equilibrium configurations at arbitrary wettability, and also predicts dynamic configurations, including wetting transitions, thin-film formation and interface pinch-off, in quantitative agreement with experiments.

[Phys. Rev. Fluids 10, 094004] Published Wed Sep 03, 2025

Author(s): Clément Bret, Pantxo Diribarne, Jérôme Duplat, and Bernard Rousset

Quantum vortex lines are angstrom-scale tubes with quantized circulation ±κ, the Feynman–Onsager constant, that form the backbone of the superfluid velocity field in He-II flows. When forced to turbulence, they organize into a tangle characterized by its mean intervortex spacing. Combining second-sound resonant cavity measurements with 3D Lagrangian tracking for the first time, we observe that the mean intervortex spacing scales with the κ based Reynolds number following a Kolmogorov-like –3/4 power law. We then show both this scaling and the prefactor value can be explained with classical turbulence formalism due to the quantum restricted depth of the superfluid component energy cascade.

[Phys. Rev. Fluids 10, 094701] Published Wed Sep 03, 2025

Author(s): Min Gao, Zhihao Li, and Xinpeng Xu

We propose the Diffuse-Resistance-Domain (DRD) approach — a thermodynamically consistent DNS method for microscale fluid-structure interactions (mFSI) in multicomponent multiphase flows. By unifying interfacial dynamics through Onsager’s variational principle and smooth interpolation of resistance coefficients, DRD naturally enforces complex boundary conditions — like dynamic contact angles — without ad hoc assumptions. It avoids evolving solid-phase fields, cutting computational cost while enabling high-fidelity simulations of moving or deforming boundaries. DRD accurately captures interfacial phenomena in microfluidics, active matter, and porous media, showing strong agreement with experiments.

[Phys. Rev. Fluids 10, 094001] Published Tue Sep 02, 2025

Author(s): Swarnaditya Hazra and Jason R. Picardo

When do mucus films plug lung airways? We show that the answer is not determined by just the film’s volume. While very thin films always stay open and very thick films always plug, we find a range of intermediate films for which plugging is uncertain. The fastest-growing linear mode of the Rayleigh-Plateau instability ensures that the film’s volume is divided among multiple humps. However, the nonlinear growth of these humps can occur unevenly, due to spontaneous axial sliding—a hump that slides rapidly can sweep up a disproportionate share of the film’s volume and so form a plug. This sliding-induced plugging is robust and prevails with or without gravitational and ciliary transport.

[Phys. Rev. Fluids 10, 094002] Published Tue Sep 02, 2025

Author(s): Menghua Zhao, Julien Dervaux, Tetsuharu Narita, François Lequeux, Laurent Limat, and Matthieu Roché

Liquid droplets sliding under gravity slow down markedly on viscoelastic substrates.However the influence of substrate thickness has remained elusive. Focusing on droplets with Bond numbers below one, we uncover a strong dependence of sliding velocity on thickness when the latter varies from microns to millimeters. Building upon these results, we establish a scaling relation linking velocity, droplet size, and layer thickness that capture the experimental data.

[Phys. Rev. Fluids 10, 094003] Published Tue Sep 02, 2025

Author(s): Charlotte Pheasey, Loïc Chagot, Panagiota Angeli, Lyes Kahouadji, and Omar K. Matar

Understanding the flow topology of Taylor (plug) flow in small channels and microchannels is imperative for mass and heat transfer applications, yet comprehensive analysis remains limited, particularly when viscous continuous phases are employed. This study investigates liquid-liquid flow in microchannels, examining internal vortices within Taylor plugs through experimental and computational methods. The results reveal new insights into vorticity and geometry diminishment, advancing understanding of liquid-liquid Taylor flow in microchannels and system mass transfer optimization.

[Phys. Rev. Fluids 10, 094201] Published Tue Sep 02, 2025

Author(s): Svenja Ehlers, Norbert Hoffmann, Tianning Tang, Adrian H. Callaghan, Rui Cao, Enrique M. Padilla, Yuxin Fang, and Merten Stender

This work presents a novel Physics-Informed Neural Network (PINN) approach for nonlinear ocean wave data assimilation and prediction. The method leverages potential flow theory to parameterize wave dynamics and integrates physical constraints into the loss function, enabling accurate reconstruction and prediction of phase-resolved, nonlinear, and dispersive wave fields from sparse measurements. Validated against analytical solutions and laboratory experiments, this PINN framework infers full spatiotemporal wave and velocity potential fields efficiently and thus advances the use of physics-informed machine learning for ocean wave modeling and understanding.

[Phys. Rev. Fluids 10, 094901] Published Tue Sep 02, 2025

Author(s): Yu-Chen Zang (臧雨宸), Hai-Feng Jiang (蒋海峰), Di-Chao Chen (陈帝超), Xing-Feng Zhu (朱兴凤), and Da-Jian Wu (吴大建)

A rigorous formalism is presented for the time-averaged acoustic radiation force on a spherical particle near an infinite planar boundary subjected to a plane wave. The background medium is assumed to be a weakly thermoviscous fluid, with the viscous and thermal boundary layers much smaller than eit…

[Phys. Rev. E 112, 035101] Published Tue Sep 02, 2025