Physical Review Fluids

Author(s): Marco Vona, Isabelle Eisenmann, Nicolas Desprat, Raphaël Jeanneret, Takuji Ishikawa, and Eric Lauga

A continuum model is developed to analyze the stability of finite-size coherent structures in suspensions of strongly aligned swimmers. For dilute active jets, pullers undergo pearling instabilities while pushers destabilize into helical structures. The long-term nonlinear evolution reveals spreading and interaction of puller clusters and wavelength coarsening of pusher helices. These results are in close agreement with experiments performed with photophobic micro-algae controlled by light and hydrodynamically interacting agents-based numerical simulations.

[Phys. Rev. Fluids 10, 113101] Published Thu Nov 06, 2025

Author(s): Margaux Kerdraon, Albane Théry, Marc Pascual, Stéphanie Descroix, and Marie-Caroline Jullien

We study the dynamics of a droplet subjected to a thickness indentation in a microchannel. The droplet either reaches an equilibrium shape or splits depending on geometry. We show that its deformation is self-similar but that scaling laws are not sufficient to describe its dynamics and the possible breakup. We propose a model based on surface energy minimization that reproduces our observations in a microfluidic device. We predict whether the drop splits and model the dynamics of the deformation up to breakup, in agreement with our experiments. With our setup, the droplet breakup can therefore be controlled on-demand in situ with an active indentation of the channel thickness.

[Phys. Rev. Fluids 10, 114201] Published Thu Nov 06, 2025

Author(s): Purnima Jain, Navdeep Rana, Roberto Benzi, and Prasad Perlekar

The ordered state of microswimmers can be destroyed by an instability created by their swimming stresses. This leads to chaotic flows that resemble turbulence characterized by the presence of topological defects, a phenomenon known as active turbulence. We show that for pushers, the defect turbulent state transitions to a novel concentration-wave turbulent state reported earlier, where defects coexist along with concentration waves. This state emerges from an instability where fluctuations in the concentration of swimmers play a dominant role. Our study aims to provide a comprehensive understanding of the instabilities and turbulence in weakly inertial suspensions of pushers.

[Phys. Rev. Fluids 10, 114602] Published Thu Nov 06, 2025

Author(s): Haecheon Choi, Chonghyuk Cho, Myunghwa Kim, and Jonghwan Park

The predictive accuracy of large eddy simulation (LES) largely depends on the subgrid-scale (SGS) model. Many machine-learning-based SGS models have been trained on a single flow at relatively low Reynolds numbers and then applied to same or similar flows at similar Reynolds numbers. But what happens when the Reynolds number is much higher? Or when the flow geometry is entirely different? In this perspective paper, we examine these pressing challenges such as extrapolation to high Reynolds numbers, generalization to unseen flow configurations, preserving physical consistency, and the trade-offs in computational cost.

[Phys. Rev. Fluids 10, 110701] Published Wed Nov 05, 2025

Author(s): Ronald du Puits

This paper reports highly resolved measurements of the three-dimensional velocity field close to a hot solid surface which is surrounded by a colder fluid. The results provide new insights into the specific structure of the boundary layer flow close to a rough surface and how roughness elements influence the transport of heat between the surface and the fluid. The main finding of our work is that, in the domain of Rayleigh and Prandtl number we investigated, roughness only changes the flow field in a passive manner. Contrary to previous assumptions, it does not introduce additional buoyancy forces that could enhance the local heat transfer.

[Phys. Rev. Fluids 10, 113501] Published Wed Nov 05, 2025

Author(s): Liyun Liu and Weipeng Li

Nonuniform inflow alters the linear noise of an underwater propeller by amplifying the overall sound pressure level (OASPL) and introducing asymmetry into the noise directivity patterns. To uncover the underlying mechanisms we develop an equivalent emission point (EEP) acoustic model, which provides an intuitive framework for investigating the propagation behavior and source distribution of the loading noise. Results show that the amplified blade passing frequency (BPF) tone under nonuniform inflow arises from components associated with different harmonics of the blade force, and interference among these components is the primary cause of asymmetric noise radiation in the near field.

[Phys. Rev. Fluids 10, 114802] Published Wed Nov 05, 2025

Author(s): Sijie Wang, Linfeng Zhang, Zeng Liu, Jianglong Sun, Xiaoyan Yang, and Guangyao Wang

This work establishes a nonlinear phase-resolved wave simulation framework that assimilates observations through the ensemble Kalman filter - pseudospectral Fourier-Legendre (EnKF-PFL) approach. The key image shows that it consistently suppresses the error growth of the PFL-only model and achieves close agreement with reference wave profiles for both regular and irregular waves. It further demonstrates robust performance under highly nonlinear conditions and strong disturbances, where conventional models deteriorate. A consistent set of optimal assimilation parameters is also identified, enabling a practical and predictive strategy for accurate ocean wave forecasting.

[Phys. Rev. Fluids 10, 114901] Published Wed Nov 05, 2025

Author(s): Thibault Leduque, Maxime Kaczmarek, Hervé Michallet, Eric Barthélemy, and Nicolas Mordant

This article reports an investigation into the statistical properties of an ensemble of random nonlinear water waves propagating in two dimensions in a large scale wave tank (27m x30m, 35 cm deep). By varying the peak frequency of the wave spectrum, we modify the wave dispersion and observe a transition in the system’s behavior. As the frequency decreases, the dynamics evolve from weak wave turbulence to a soliton gas in the shallow water regime. This transition is striking as these two theoretical frameworks are extremely different on fundamental grounds, with the former supporting an energy cascade while the latter is integrable.

[Phys. Rev. Fluids 10, 114801] Published Tue Nov 04, 2025

Author(s): Dilip Kumar Maity, Christopher Wagstaff, Sandip Dighe, and Tadd Truscott

A simple tilt transforms the dynamics of a liquid thread flowing along a wire. At a fixed flow rate of 350 mL/h, the system transitions between Rayleigh–Plateau, convective, and immediate droplet drop-off detachment by varying the inclination angle of the wire. Even within the classical Rayleigh–Plateau regime, both the droplet spacing and velocity change significantly with angle, revealing how geometry alone can tune the instability.

[Phys. Rev. Fluids 10, 113901] Published Mon Nov 03, 2025

Author(s): Julien Fontchastagner, Jean-François Scheid, Jean-Régis Angilella, and Jean-Pierre Brancher

We demonstrate the possibility of experimentally obtaining a steady chaotic flow in a closed box without external mechanical forcing. We study how a weakly conductive viscous fluid moves in this cubic domain when subjected to the Lorentz force created by two pairs of magnets and a small electric current. The flow pattern consists of a large vortex created by the first pair of magnets and a double vortex created by the other pair placed perpendicularly. Although each vortex taken separately has poor mixing properties, the combination of the two creates chaotic advection, leading to effective fluid mixing.

[Phys. Rev. Fluids 10, 114101] Published Mon Nov 03, 2025

Author(s): Sanjay Shukla

A system of self-gravitating bosons can form massive condensates, such as dark matter halos around galaxies. Studying such systems can help constrain the nature of dark matter. Yet, the role of turbulence and vortex dynamics within these structures remains elusive. Using direct numerical simulations of the Gross-Pitaevskii–Poisson equation, we show that halos like structures form through a sequential collapse — from sheets to cylinders to spheres. The resulting tangled vortical state alters energy transfer across scales, revealing a pathway for the emergence of large-scale cosmic structures.

[Phys. Rev. Fluids 10, 114601] Published Mon Nov 03, 2025

Author(s): Mohammad Javad Akbari, Hadis Edrisnia, Mohammad Hossein Sarkhosh, Mohammad Ali Bijarchi, and Mohammad Behshad Shafii

Liquid marbles, droplets encapsulated by hydrophobic particles, exhibit rich post-impact dynamics, yet their oscillatory behavior remains poorly understood compared to pure droplets. This study introduces a mass-spring-damper model validated against experiments to describe two distinct oscillation phases: free oscillation during bouncing and oscillation after the final bounce. By linking damping ratios and bounce numbers to dimensionless parameters (Oh, Bo, We), we uncover scaling laws and propose a proof-of-concept method for extracting liquid core properties, advancing both the fundamental physics and applications of liquid marbles.

[Phys. Rev. Fluids 10, 103604] Published Fri Oct 31, 2025

Author(s): Zhi Zhou, Petia M. Vlahovska, and Michael J. Miksis

The hydrodynamic force and torque exerted on a moving spherical particle with surface slip and a three-phase contact angle on a gas-liquid interface is investigated. Perturbation theory is employed to estimate the drag and torque on the particle in the limit of small capillary number and small deviations of the contact angle from 90 degrees. The interactions between two translating and rotating particles at a large separation distance are also examined.

[Phys. Rev. Fluids 10, 104007] Published Fri Oct 31, 2025

Author(s): Shilong Li, Zhideng Zhou, Xiaolei Yang, Guowei He, and Haitao Chen

The role of surface roughness on flow separation is yet to be fully understood. Our high-fidelity simulations of flows over periodic hills reveal that roughness systematically enlarges the separation bubble and shifts its position. A key finding is a universal geometric similarity across all rough surfaces, where bubble outlines collapse under a single coordinate transformation. Furthermore, we identify a dual role of roughness: it simultaneously depletes near-wall momentum and counteracts the adverse pressure induced by the hill slope, with momentum loss becoming the dominant driver of flow separation at high roughness.

[Phys. Rev. Fluids 10, 104608] Published Fri Oct 31, 2025

Author(s): Adnan Morshed, Ricardo Cortez, and Lisa Fauci

We present a novel framework using regularized Stokeslet surfaces and regularized Stokeslet segments to model long, filiform swimmers inside tubular confinements of arbitrary geometry. Swimmer motion results from the dynamic interaction between time-varying preferred curvatures and elastoviscous forces, which depend on properties of the fluid and flagellum, and the confinement geometry. The image demonstrates that the no-slip condition at the tube wall is maintained while the swimmer moves downward.

[Phys. Rev. Fluids 10, 104903] Published Thu Oct 30, 2025

Author(s): Yu Jun Loo and Silas Alben

We present a numerical method for thin plates falling in inviscid fluid that incorporates leading-edge vortex shedding. Including leading-edge vortex shedding restores physical dynamics to inviscid vortex sheet simulations, enabling large-amplitude fluttering and tumbling.

[Phys. Rev. Fluids 10, 104701] Published Wed Oct 29, 2025

Author(s): Daniel M. Harris and Jack-William Barotta

When a floating body is internally or externally vibrated, its self-generated wavefield can lead to steady propulsion along the interface. In this article, we review several related and recently discovered systems that leverage this propulsion mechanism and interact hydrodynamically with one another via these surface waves. These accessible, tunable, and visually appealing systems motivate future investigations into a number of outstanding questions in fundamental fluid mechanics, while potentially also informing advances in the fields of active matter, hydrodynamic quantum analogs, and robotics.

[Phys. Rev. Fluids 10, 100503] Published Mon Oct 27, 2025

Author(s): Javier Jiménez

We review the efforts during the past 50 years to characterize turbulent flows in terms of coherent structures, but remark that, in the same way as the cycle of water on Earth cannot be fully described by ‘coherent’ rivers or storms, 80% of the volume of turbulent flows cannot yet be represented in terms of structures. The objects in the accompanying figure (originally from A. Lozano-Durán, 2011) are Reynolds-stress structures and vortices in a turbulent channel, but most of the volume is empty. We point to specific problem areas, and discuss what the future role of new analysis techniques could be.

[Phys. Rev. Fluids 10, 100504] Published Mon Oct 27, 2025

Author(s): Yee Li (Ellis) Fan, Bernardo Palacios Muñiz, Nayoung Kim, and Devaraj van der Meer

Upon the impact of a flat disk on a boiling liquid, i.e., a liquid that is in thermal equilibrium with its vapor, a thin vapor pocket is entrapped under the disk. We experimentally investigate the dynamics of the entrapped vapor pocket, focusing on its time evolution and its subsequent influence on the hydrodynamic loads under various conditions. We found that the dynamics of the entrapped vapor pocket is primarily governed by the phase change process, where condensation (vaporization) will induce (frustrate) its rapid collapse, impairing the cushioning. This differs significantly with that of a non-condensable air pocket, which is known to always provide a load reducing cushioning effect.

[Phys. Rev. Fluids 10, 100505] Published Mon Oct 27, 2025

Author(s): Ján Šimkanin and Juraj Kyselica

The impact of different buoyancy sources on convection within the Earth’s core and the resulting magnetic field is numerically investigated. To explore this, three models — thermally driven, chemically driven, and thermochemically driven dynamos — are gradually analyzed. These analyses confirm that both the magnetic field and the convective velocity field depend on the sources of buoyancy. Furthermore, the study finds that dipole polarity reversals and the super-rotation of the Earth’s inner core are influenced by different buoyancy sources. Note, as the viscosity decreases, this dependence weakens.

[Phys. Rev. Fluids 10, 103705] Published Mon Oct 27, 2025