Physical Review Fluids

Author(s): Valerio Lupi, Daniele Massaro, Adam Peplinski, and Philipp Schlatter

Swirl switching is the temporal rotation of the plane of symmetry of the cross-sectional vortices about the equatorial plane of a curved pipe and can induce considerable structural vibrations. We investigate the effect of bending angle and inflow conditions by performing high-fidelity direct numerical simulations of spatially developing bent pipe flows and extracting spatially coherent structures through proper orthogonal decomposition. Our results show that upstream turbulence is not the primary cause of swirl switching. Instead, the phenomenon likely arises because of a symmetry-breaking instability of the shear layer originating within the curved section.

[Phys. Rev. Fluids 10, 114608] Published Fri Nov 14, 2025

Author(s): Dhananjay Singh, Harshit Tiwari, Lekha Sharma, and Mahendra K. Verma

We present a novel mathematical framework to compute mode-to-mode energy transfers and fluxes for compressible flows. This formalism captures detailed energy conservation within triads and allows decomposition of transfers into rotational, compressive, and mixed components, providing a clear picture of energy exchange among velocity and internal energy modes. The key image shows the decomposed energy fluxes.

[Phys. Rev. Fluids 10, 114609] Published Fri Nov 14, 2025

Author(s): Lei Dong, Wenqiang Zhang, Dandan Xiao, and Xuerui Mao

The modulation frequency exerts a significant effect on the evolution of plasma-induced vortices, giving rise to three distinct flow structures: vortex-free evolution, leapfrogging, and coexistence of multiple vortex pairs. Among them, the formation of leapfrogging enhances the entrainment coefficient of the plasma jet, thereby potentially enabling more effective flow control.

[Phys. Rev. Fluids 10, 114701] Published Fri Nov 14, 2025

Author(s): Patrice Meunier

Precessing flows in cylinders are highly effective for mixing a passive scalar, as illustrated here with the thin streaks of fluorescent dye. The stretching and folding of these streaks results from chaotic advection by the flow which becomes resonant at specific cylinder heights. This paper reviews theoretical, experimental, and numerical studies of the resonances and the instabilities of a precessional flow, as well as their implications for efficient mixing.

[Phys. Rev. Fluids 10, 114803] Published Fri Nov 14, 2025

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