Physical Review Fluids

Feeding by marine invertebrate larvae depends critically on the low-Reynolds-number fluid mechanics of the feeding currents they generate. However, the relative importance of gravity-driven and drag-driven mechanisms in shaping these feeding currents has remained unresolved. By combining μPIV measurements with an analytical Stokes-flow model, this study demonstrates that feeding currents of eastern oyster (Crassostrea virginica) veligers are gravity-dominated despite their submillimeter size, and quantifies how excess weight, swimming kinematics, and ciliary propulsion together determine larval clearance rates and flow structure.

The drafting–kissing–tumbling (DKT) interaction of settling particles is a canonical problem in multiphase flow research, yet its behavior in oscillatory environments remains largely unexplored. Using particle-resolved direct numerical simulations, we investigate how horizontal oscillations modify binary particle interactions over a wide frequency and amplitude range. We show that oscillatory effects become significant only when oscillation-induced inertia exceeds viscous forces (Re${}_p$ > 1), leading to changes in contact duration and a preferential particle alignment perpendicular to the oscillation direction. Pressure field analysis reveals the underlying hydrodynamic mechanism.

Vortex cavitation in submerged jets is usually associated with turbulent continuous flows, yet an impulsively expanding bubble in a confined tube can trigger cavitation in a remarkably organized vortex ring. Combining experiments, theory, and simulations, we show that the intense initial acceleration of the jet generates an over-pressure contribution to vortex ring circulation, producing unusually high incipient cavitation numbers. This mechanism also explains why larger nozzles promote cavitation, revealing a size-scale effect in impulsive jets.

Self-rewetting fluids that exhibit a quadratic dependence of surface tension with temperature result in thermocapillary flows that are markedly different from normal fluids. However, their behavior in the presence of an insoluble surfactant is not yet fully understood. We consider an inertialess surfactant-laden self-rewetting fluid film flowing down a heated cylinder under gravity, by deriving a reduced-order model under the lubrication approximation. A systematic linear stability analysis of our models indicates the stabilizing influence of surfactants at small wavenumbers, whereas conventional (anomalous) thermocapillarity is destabilizing (stabilizing) at mid-wavenumbers.

A mathematical model for the evolution of, and deposition from, a thin particle-laden droplet on a fully flooded porous substrate undergoing simultaneous evaporation and imbibition is formulated and analyzed. While the physical mechanisms driving evaporation and imbibition are rather different, it is found that there are several qualitative and quantitative similarities in the behavior of the droplet as it loses mass to its environment. Not only are these results of theoretical interest, but they are also relevant to a wide variety of practical applications that would benefit from an improved ability to predict and/or control the pattern of the final deposit left on the substrate.

What if we could learn not just which mode structures dominate a flow, but how they actually drive fluid-particle transport? This paper bridges Eulerian modal analysis and Lagrangian coherent structures by introducing modal-trajectory uncertainty, a sensitivity-based framework that reveals where specific modes influence particle trajectories and finite-time Lyapunov exponent (FTLE) structures. Across cylinder wakes, an oscillating-foil wake, and turbulent channel flow, the approach reveals interactions spanning vortex shedding, shear-layer instabilities, and large-scale turbulent motions.

Fractal flocs occur in sediments, aerosols, and many particulate flows, yet their unsteady hydrodynamic inertia is often approximated using equivalent-sphere or free-draining models. We compute the added-mass and added-moment-of-inertia tensors of large ensembles of porous flocs over a range of sizes and fractal dimensions. The results reveal morphology-dependent anisotropy, systematic departures from simple approximations, and decreasing floc-to-floc variability with increasing size. We provide compact stochastic closures that can be used in large-scale simulations.

Extreme events in turbulent flows are rare, abrupt bursts in the system observable, posing significant challenges for prediction and control due to their nonlinearity and high dimensionality. Here, we present a predominantly data-driven framework for their suppression, combining dimensionality reduction through symmetry-aware autoencoders and data-driven clustering in the latent space of the former for the identification of precursors. A control law defined in this latent space substantially reduces the frequency and intensity of extreme events, by up to 99.4% in a canonical chaotic flow, demonstrating scalability to higher-Reynolds-number regimes and practical control limitations.

Liquid films flowing along fibers can exhibit distinct dripping and jetting regimes, yet the differences in bead-formation mechanisms and the transition between these regimes have received limited attention. By combining experiments with a theoretical model, we discuss the mechanisms governing bead formation in both regimes and introduce a method for identifying and predicting the transition range across different fiber-nozzle geometries. This work advances the fundamental understanding of bead formation and regime transitions in film flows on fibers.

This study introduces a novel approach for generating isolated vortex gusts in both computational and experimental settings. Utilizing a symmetric airfoil undergoing simultaneous heaving and pitching, this method delivers coherent vortices while minimizing persistent wake disruption downstream. The result is a customizable approach allowing precise, systematic control over vortex strength, orientation, and position.

Thermal diffusivity measurements in a sheared particle-fluid suspension are performed in a Taylor-Couette cell with outer cylinder rotation. The enhancement in diffusivity follows a power law with Peclet number with an exponent of 0.5 for Peclet numbers below 700 at all volume fractions studied (0.14, 0.22, 0.30 and 0.36). The data do not fit existing theory and are consistent with a model that assumes the formation of a particle-free fluid layer near the inner cylinder which causes deviation from an initially linear behavior at low particle Peclet numbers.

In granular films, where grains bridge the two interfaces of a soap film, the liquid pressure controls the transition from bursting to a jammed state. Even more, a single bursting event combines liquid- and solid-like mechanical responses. By identifying both an effective surface viscosity governing the early liquid-like regime and an internal dissipation controlling the late fracture dynamics, we provide a unified physical picture and highlight the differences between granular films and granular rafts. The concepts introduced here may prove relevant for a broad range of systems in which particles and interfaces interact, from particle-laden foams to biological or bio-inspired membranes.

This study investigates the formation of the helical vortex rope in a Francis hydropower turbine by treating it as an unstable vortex breakdown mode in a simplified laminar flow. The vortex rope emerges through a supercritical Hopf bifurcation from an axisymmetric base flow in the draft tube. Without wall friction, a central recirculation zone develops, revealing subcritical solutions and hysteresis under partial-load conditions. The work describes the cyclic formation and collapse of the recirculation bubble as the helical vortex evolves. As flow approaches nominal load, the steady solution branch undergoes a transcritical bifurcation at finite Reynolds number.

Surface tension is frequently measured with the pendant drop method, while the ambient humidity is usually an afterthought. With carefully calibrated experiments and detailed numerical simulations, we show that evaporative cooling lowers the drop temperature by up to 9.5 K which in turn raises the measured surface tension by more than 1 mN/m. Evaporation-driven and Marangoni-induced flows additionally deform the drop, but only marginally. A passive humidity control removes these artifacts entirely and reveals the shallow minimum in the surface tension of aqueous 1,2-hexanediol mixtures.

Lifting surfaces operating at aerodynamically low Reynolds numbers often have laminar separation bubbles (LSBs) whose characteristics can significantly affect performance. We investigate the effect of localized surface roughness, which might form due to manufacturing, local contamination, damage or icing, on LSBs. We find that the localized roughness eliminates downstream laminar separation and modifies the LSB topology and dynamics over a substantial spanwise region extending well beyond the roughness itself. The associated effect on aerodynamic performance is considerably greater than would be expected based solely on the relative spanwise extent of the localized roughness.

Compared with a static liquid film, gas jet impingement on a flowing liquid metal film produces a shallower cavity and a strongly asymmetric profile through upstream liquid accumulation. Counter-current coupling upstream and co-current coupling downstream cause the wave-amplitude response to jet momentum to weaken upstream but strengthen downstream as the liquid Reynolds number increases. Under pulsating jets, an attenuation coefficient correlates wave amplitude with jet momentum and characterizes a pronounced spatial asymmetry in momentum transfer. These findings clarify how gas jets influence the liquid metal film flow and provide a reference for the design of liquid metal divertors.

Large staggered roughness elements in the entrance region of a subcritical channel flow generate intense turbulent eddies that gradually break down as the flow progresses into the downstream smooth channel (SC) of the configuration studied here. This flow combines several sources of complexity: surface roughness, a rough-to-smooth step change and relaminarization. The characteristics of the turbulence decay along the SC centerline agree reasonably well with those of homogeneous isotropic turbulence. Owing to the slow rate of relaminarization, the Nusselt number averaged over large streamwise distances in the SC remains close to that of an equivalent fully developed turbulent channel flow.

This article presents a simple to implement algorithm for detecting strong cabbeling in stratified fluids with a nonlinear equation of state. The algorithm is used to identify the manner in which the arrested development of the Rayleigh-Taylor instability occurs in the strongly cabbeling regime.

We study flow past V-formations of cylinders, inspired by migratory birds’ flight, examining how formation angle affects the drag of both individuals and the group. Using particle image velocimetry in a water tunnel, we evaluate the impact of changing angles on complex wake-wake/wake-body interactions, quantifying mean and fluctuating quantities, and analyzing vortex dynamics. We find that members with streamwise overlap experience major reductions in the drag force compared to a solo cylinder. Our study establishes a baseline for probing flow past groups of complex bodies (like drones) and demonstrates how optimizing formations can achieve desired performance goals, such as minimum drag.

Vortex reconnections are fundamental to quantum fluids. They are reconnections of quantum vortices defined by a topological change and irreversible energy transfer given by universal scaling laws, leading to quantum turbulence. We generalize this mechanism by studying reconnections of extra-dimensional quantum vortices. We find a new range of vortex interactions depending on their initial orientation – a four-dimensional generalization to the known three-dimensional case; vortex interactions with no reconnections; and a new class reconnection that keeps the same scaling law with no energy transfer, suggesting a form of reversible reconnection not allowed in three-dimensional systems.

Vertical accelerations can amplify small free-surface disturbances into strongly nonlinear sloshing through parametric resonance, yet the resulting regimes remain poorly characterized in horizontal cylindrical tanks. While the classical Mathieu equation predicts the onset of parametric instability, here, high-speed experiments, combined with data-driven modal analysis and classification, reveal nonlinear responses that develop beyond the instability threshold. The resulting dimensionless regime maps organize stable, longitudinal, breaking, and mixed-mode dynamics across different fill levels, exposing the rich flow phenomenology surrounding the primary parametric-instability region.

We present a numerical study of shock-induced dispersal in dense, flexible fiber curtains using a coupled discrete element method–computational fluid dynamics (DEM–CFD) approach. The fiber curtains exhibit distinct particle dynamics compared to spherical particle curtains. Increasing the fiber aspect ratio and reducing flexibility enhance geometric interlocking and prolong clustering, thereby increasing resistance to shock propagation. Finally, we modify a scaling law using an effective fiber aspect ratio to predict the temporal evolution of curtain expansion, accounting for fiber elongation and significant bending deformation.

This paper uses direct numerical simulations to systematically investigate supersonic flows in a bent pipe with a developing turbulent boundary layer and a core flow region. The results reveals that boundary layers experience intricate flow patterns at different azimuthal angles: secondary flows that drives the streaks move from the lower to the upper side; separation triggered by the combined effects of the adverse pressure gradient and flow deceleration on the upper wall; and the increase of the Görtler instability on the lower wall inducing the clustering and uplift of low-momentum fluids, the presence of Görtler-like vortices, and the baroclinic effect of turbulent transport processes.

Metal-pad-roll instabilities limit the safe and efficient operation of aluminum reduction cells, yet laboratory-scale models fall outside the assumptions of most existing theories. We develop an analytical stability theory for small rectangular two-layer cells that captures finite-depth, viscous, and capillary effects, including a parameter-free description of interfacial-wave damping. Validated against direct numerical simulations and experiments, the theory provides quantitative benchmarks for designing and interpreting small-scale MHD experiments and multiphase simulations.

Turbulent transport in large-eddy simulations relies on subgrid-scale (SGS) closures, yet conventional models typically assume that SGS stresses are uniquely determined by the resolved flow. We introduce a conditional diffusion model that learns the conditional distribution of SGS stresses from high-resolution atmospheric boundary layer simulations. By representing conditional variability rather than a single deterministic mapping, the proposed framework accurately reproduces SGS stress statistics, generalizes across unseen stability regimes and grid resolutions, and provides a new probabilistic framework for SGS turbulence modeling.

We introduce a simple droplet generation technique that rapidly stretches a liquid ligament on both quiescent and vibrating baths. By systematically varying the stretching distance and the radius of the cylindrical probe used to form the ligament, we map the parameter space in which the ligament pinches off and collapses into a single droplet. The resulting droplet size follows the volume-conservation scaling $R\sim a^{2/3}{L}^{1/3}$ , with excellent reproducibility (radius variation below 5%).

Numerical studies of viscoelastic Stokes flow often regularize the stress singularities predicted by the Oldroyd-B model, obscuring their underlying structure. We introduce an efficient FFT-based projection solver for incompressible flow and validate it in the classical four-roll mill. By analyzing the polymeric stress directly in Fourier space, we quantify its power-law scaling with resolution and determine the critical Weissenberg number for steady-state convergence without adding artificial stress diffusion.

Granular materials dissipate energy through frictional rearrangement and dissipative collisions, yet these mechanisms are often entangled or partially neglected in continuum models. We develop a unified viscoelastic–viscoplastic framework that links the coefficient of restitution $e$ directly to continuum viscosities while preserving the established $\mu (I)$ rheology during plastic flow. Implemented within the material point method, the model reproduces the steady-state Bagnold solution, wave propagation, impact, dynamic reposing behavior, and vibration-induced pattern formation. These results emphasize that restitution must be embedded correctly to capture granular flow dynamics accurately.

Wall-attached motions leave an intermittent footprint on the near-wall flow, but conventional coherence analyses reveal only its time-averaged behavior. Using synchronized two-point hot-wire measurements in a high-Reynolds-number turbulent boundary layer, we develop a wavelet-based framework that resolves this wall imprint in time and scale while recovering established mean coherence scaling. The results show that the characteristic persistence scale grows with wall distance even as coherence at that scale weakens, and that strongly wall-imprinted events exhibit systematic wall-normal changes in scale-local energy transfer and energy distribution.

Polymer diffusive instability (PDI) provides a unique route to instability in viscoelastic flows at vanishing Reynolds numbers, yet how realistic wall conditions modify its behavior remains unclear. Here, we show that wall slip regulates PDI in viscoelastic Poiseuille flows by altering the dominant instability pathway. Increasing slip weakens the near-wall PDI-1 mechanism and favors a shear-driven PDI-2 regime. Combining linear stability analysis and direct numerical simulations, we reveal the energy-transfer processes underlying this slip-induced transition and provide new insight into controlling polymer-flow instabilities.

The velocity law of the wall is a well-known law for wall turbulence. In compressible turbulence, the temperature law of the wall is also of interest, but its overall accuracy is inferior to its velocity counterpart. We propose to construct the compressible law of the wall for heat energy using total enthalpy. A semilocal total enthalpy transformation of no fitted parameters is constructed for channel flows, which is structurally analogous and comparable in accuracy to the Trettel–Larsson velocity transformation. This new transformation can potentially serve as a wall model for energy equations to accurately predict the enthalpy or temperature in compressible turbulence.

Resolvent analysis is a cornerstone tool for predicting how flows respond to external forcing, but for unsteady flows the choice of linear operator is not obvious. The common mean-flow resolvent, linearized about the time-averaged flow, can misidentify the dominant receptivity mechanisms, whereas the statistically optimal mean resolvent correctly captures physically relevant phenomena such as the vortex pairing in jets presented here. This work introduces a projection method that enables mean resolvent analysis at low computational cost without resorting to adjoint equations about the unsteady attractor, thereby opening a path toward more complex flows.

The offshore wind industry is expanding in deep, seasonally stratified waters, where infrastructure-induced mixing may alter ocean dynamics and ecosystems. We present the first fully structure-resolved direct numerical simulations of stratified flow past a vertical cylinder as an idealized model for flow interactions with offshore wind foundations. We identify two distinct wake regimes: a weakly stratified regime characterized by a narrow turbulent wake, and a strongly stratified regime characterized by thermocline-spanning recirculation cells and stationary internal waves, providing a new mechanism for far-field energy transport and a possible explanation for wake persistence discrepancies.