Physical Review Fluids

Author(s): F. Serafini, F. Battista, P. Gualtieri, and C. M. Casciola

Turbulent wall-bounded flows of dilute polymer solutions achieve a universal state known as Maximum Drag Reduction (MDR). At MDR, elongated polymers primarily sustain velocity fluctuations, destroy the classical path of turbulent kinetic energy of wall-bounded Newtonian turbulence, and induce a mean linear effective viscosity, whose slope defines a new inner length scale for the. system. Analogously to Newtonian turbulence, the mean velocity shows a universal logarithmic behavior (Virk’s law) in the case of a large separation between the inner and the outer scale of the system.

[Phys. Rev. Fluids 10, L111301] Published Fri Nov 14, 2025

Author(s): C. Cura, A. Hanifi, A. V. G. Cavalieri, and J. Weiss

Turbulent separation bubbles (TSBs) are known to exhibit broadband low-frequency unsteadiness; however, the origin of this phenomenon remains disputed. This work demonstrates that the low-frequency dynamics of a family of TSBs with varying separation extent arise from a forced response to a stationary global mode, rather than from self-sustained oscillations. The forced response remains robust even when the TSB vanishes in the time average or when linear global instability arises. These findings reconcile previous ambiguities regarding the origin of low-frequency unsteadiness in TSBs and further provide guidance for future flow control strategies.

[Phys. Rev. Fluids 10, 114607] Published Thu Nov 13, 2025

Author(s): Huixin Li, Mohammad Mehdi Zamani Asl, Bastian Bäuerlein, Kerstin Avila, Duo Xu, and Marc Avila

T-shaped mixers are workhorses for rapid mixing across scales, yet turbulent regimes remain underexplored experimentally. We scale up T-shaped mixers from sub-millimeters to centimeters, and implement flow measurements using techniques of particle image velocimetry and planar laser-induced fluorescence across laminar to turbulent regimes, validated against direct numerical simulations. We successfully replicate the flow characteristics in low-Reynolds-number regimes from micro-scale devices in literature, and also reveal enhanced turbulent mixing in the outlet channel, offering new insights into mixing dynamics at multiple scales.

[Phys. Rev. Fluids 10, 114502] Published Wed Nov 12, 2025

Author(s): Shane Nicholas, Mohammad Omidyeganeh, Alfredo Pinelli, Alessandro Monti, Giulio Foggi Rota, and Marco E. Rosti

When flexible filaments are exposed to flow, they naturally reconfigure into streamlined shapes—but how filament inclination alone alters turbulence remains unclear. Using large-eddy simulations of inclined filament canopies, we show that tilting the filaments transforms the flow from a canopy-turbulence regime to one where the canopy is largely sheltered from the outer flow, even yielding net drag reduction. A unified virtual-origin framework explains this transition, linking geometry, turbulence penetration, and drag.

[Phys. Rev. Fluids 10, 114605] Published Wed Nov 12, 2025

Author(s): S. Midya and F. Thomas

In this study, the skewness in both canonical and actuated zero-pressure-gradient turbulent boundary layers (Reθ =1770) is decomposed using the real part of the bispectrum, revealing the triadic interactions contributing to skewness. The bispectra of the canonical TBL are compared with those from a case where plasma actuation introduces large-scale spanwise vortices in the outer layer. Actuation serves to examine how imposed outer-layer structures influence near-wall dynamics. Results indicate that linear inner–outer interactions dominate: in the actuated TBL, outer-layer structures modulate near-wall vortex strength but do not trigger their formation.

[Phys. Rev. Fluids 10, 114606] Published Wed Nov 12, 2025

Author(s): Ismaël Zighed, Nicolas Thome, Patrick Gallinari, and Taraneh Sayadi

This uncertainty-aware Reduced Order Model (ROM) demonstrates enhanced robustness and generalization across varying dynamical regimes of unsteady flow. It provides systematic and reliable predictions by leveraging a Variational Autoencoder (VAE) to construct a suitable latent manifold, and attention mechanisms in the latent space to capture temporal and parametric dependencies.

[Phys. Rev. Fluids 10, 114902] Published Wed Nov 12, 2025

Author(s): Aimen Laalam and Parisa Bazazi

When a droplet falls through another liquid, it usually oscillates between oblate and prolate shapes, but what if its interface could resist those oscillations? In this study, researchers demonstrate how an in situ–formed viscoelastic “skin” at the droplet surface suppresses oscillations entirely, transforming falling droplets into stable oblate bodies. By coupling high-speed imaging with interfacial rheology, the study reveals that nanoparticle, surfactant assemblies can tune interfacial elasticity and damping in real time, shedding new light on how interfacial viscoelasticity governs droplet dynamics across multiphase flows.

[Phys. Rev. Fluids 10, 113601] Published Wed Nov 12, 2025

Author(s): Tetsuro Tsuji, Koichiro Takita, and Satoshi Taguchi

Thermo-osmosis is a nanoscale fluid flow along solid surfaces driven by temperature variation. In this paper, a model for thermo-osmosis is proposed within a slip-flow theory for molecular fluids. The key is to combine the generalized slip-flow theory for molecular gases with the effects of fluid–solid interaction potentials. By tuning the potentials, or molecular “affinity,” the theory reproduces the reversal of flow direction observed in molecular simulations: when the fluid–solid interaction is favorable (unfavorable), the flow is directed toward the hot (cold) region. This work provides a starting point toward a universal model of slip phenomena in gases and liquids at the nanoscale.

[Phys. Rev. Fluids 10, 114202] Published Wed Nov 12, 2025

Author(s): Charlie Lloyd and Robert Dorrell

Sediment-laden flows are inherently density stratified due to their vertical variation of particulate concentration. Stratification provides an inherent mechanism for flow-scale mixing processes. Here we investigate how this change in mixing mechanics impacts sediment transport using simulations of a thermally stratified turbulent channel flow with passively transported particulates. Flow-scale mixing structures (hairpin vortices) are shown to have a profound impact on sediment transport due to their coincidence with strong concentration gradients. As a result classical diffusive-based Fickian models, which assume small-scale mixing, underpredict the capability of flows to suspend sediment.

[Phys. Rev. Fluids 10, 114501] Published Wed Nov 12, 2025

Author(s): Mark S. Tachie, Wei-Jian Xiong, and Bing-Chen Wang

Turbulent flow through a longitudinally-rib-roughened square duct is studied using direct numerical simulation (DNS). To understand the rib effects on the velocity field, DNS of a smooth-wall duct flow is also performed which serves as a baseline case of comparison. The impacts of longitudinal ribs on the flow structures, statistical moments of the velocity field and turbulence kinetic energy budget balance are investigated. It is interesting to observe tertiary vortex structures, which significantly alter the distributions of viscous and turbulent stresses within a longitudinally-rib-roughened square duct.

[Phys. Rev. Fluids 10, 114604] Published Mon Nov 10, 2025

Author(s): T. J. J. M. van Overveld and V. Garbin

Colloidal particles at fluid interfaces stabilize drops and bubbles, yet their effect on mass transfer remains ambiguous, with experiments showing either strong hindrance or minimal effect, even at near-complete surface coverage. We resolve this ambiguity by modeling transient diffusion with the Fick-Jacobs equation, revealing that particle layers hinder diffusion only at short times due to reduced cross-sectional area. Our model provides a simple criterion for predicting hindered diffusion and captures prior experimental findings into a regime map, offering a unifying framework for diffusion in particle-stabilized multiphase systems.

[Phys. Rev. Fluids 10, L112501] Published Mon Nov 10, 2025

Author(s): Anson G. Thambi and William E. Uspal

For systems of interfacially driven microswimmers, breaking symmetries of the particle shape and interfacial actuation can lead to self-organization on multiple length scales. For instance, under certain conditions, there is an absorbing phase transition for discoidal swimmers with non-axisymmetric actuation. The particles initially form immotile ordered clusters, and on larger length scales, the clusters realize a spatial distribution characterized by class I disordered hyperuniformity.

[Phys. Rev. Fluids 10, 113102] Published Fri Nov 07, 2025

Author(s): Y. Zhu, A. Valance, and R. Delannay

Discrete simulations of granular flows on smooth inclines reveal that a simple law, based on a Froude-like number—the ratio of slip velocity to the square root of wall pressure—accurately describes local wall friction over various angles and mass loads, in both steady and unsteady regimes. A similar law governs wall packing fraction, providing general boundary conditions for flows between smooth walls. Interestingly, a rich variety of flow patterns emerges. The example below illustrates the temporal evolution of the packing fraction in a cross-section of the flow at an inclination of 65 degrees. The flow exhibits successive condensation and evaporation of a dense core.

[Phys. Rev. Fluids 10, 114301] Published Fri Nov 07, 2025

Author(s): Guillaume Costa, Amaury Barral, Adrien Lopez, Quentin Pikeroen, and Berengere Dubrulle

We explore how efficiency, a measure of the energy stored in a flow, governs the transition from smooth, viscous dynamics to singular, inviscid ones. Using fluids on log-lattices within a reversible framework, we reveal self-similar blow-ups and their continuation beyond blow-up through stochastic friction. These post-blow-up states connect non-dissipative and dissipative regimes, offering a dynamical route to construct singular solutions of the Euler equations.

[Phys. Rev. Fluids 10, 114603] Published Fri Nov 07, 2025

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