Patient specific intracranial aneurysm flows are often assumed insensitive to head orientation, yet gravity can reshape secondary motion in complex geometries. Using time-resolved volumetric particle tracking velocimetry (PTV) in patient-specific basilar tip and internal carotid artery models, we compare vertical and horizontal orientations under matched physiological inflow. Orientation altered streamline topology, vortex coherence, and wall shear parameters. These results quantify when orientation can (and cannot) be neglected in aneurysm hemodynamics.

From inkjet printers to irrigation lines, particle-laden flows can fail abruptly by clogging. This Perspective reviews recent work on particulate suspensions in confined geometries and the key control parameters behind clogging. Some general guidelines are provided: particles can be too large (sieving), too crowded (bridging), or too sticky (aggregation). We highlight recent efforts to characterize, model, and delay clogs, and suggest some future research questions.

Inspired by stiffness modulation in biological fish and advances in smart materials, we study the propulsion of a self-propelled swimmer with spatially nonuniform and dynamically varying stiffness. We demonstrate how different stiffness modulation strategies affect swimming performance and internal actuation requirements. By introducing a cycle-averaged equivalent stiffness, the required actuation strength can be consistently scaled across different modulation patterns.

How do planar beating microswimmers discover efficient swimming gaits under local energetic limits? Using reinforcement learning on a discretized bead–bend–spring filamentous microswimmer with locally constrained torques, we reveal emergent waveforms (frequency, amplitude, wavelength) set by a three-way competition between active torques, elastic stiffness, and the action-update interval. Our work offers a new framework for how local constraints determine optimum swimming patterns for undulatory locomotion.

The atomization of airway mucus during speech is a primary mechanism for airborne disease transmission, yet the multiphase dynamics within the vocal folds remain largely uncharacterized. This study presents a Volume-of-Fluid CFD model to simulate the stretching and rupture of mucus bridges during phonation. Results show that small-scale surface perturbations seed realistic breakup patterns and that the bridge ruptures at a dynamic aspect ratio of approximately 20, far exceeding the quasi-static Rayleigh-Plateau stability limit. Higher transglottal pressures accelerate rupture, linking phonation intensity to aerosol generation.

This study investigates the characteristics of detonation structure and droplet behavior in $n$-heptane spray detonations where droplets experience a thermodynamic critical event, using the Eulerian-Lagrangian method. The droplet behavior at the critical state in the post-detonation area is captured and the resulting influence on the detonation structure and propagation is emphasized. The contribution of the droplet at the critical state to detonation is quantitatively estimated by the critical droplet fraction.

With experiment and theory we consider the branching morphodynamics of injecting a shear-thinning liquid from a point source to a point sink in a Hele-Shaw cell filled with a yield-stress fluid which has a sudden transition in its response as the local stress crosses a threshold. As the injection rate is increased an abrupt transition occurs, from a direct path connecting source to sink at low flow rates, to a rapid branching morphology at high flow rates, eventually converging to the sink. We show that global constraints imposed by boundary conditions, including source-sink separation, injection rate, and plate properties, shape branching morphodynamics and determine the transition point.

Despite the burst in studies on shear-thickening (ST) suspensions over the past decade, a simple and yet fundamental question has remained unanswered: what is the the drag on a solid object moving in such medium? By addressing this problem, we reveal a novel stress-focusing phenomenology in discontinuous ST suspensions, which we term jamming rays. Key features of these anisotropic and intermittent structures, such as their propagation direction, trigger force, and characteristic width, cannot be reconciled within the current understanding of ST suspensions. This work therefore opens a new avenue in the field by questioning the dynamics of these suspensions in nonuniform and unconfined flows.

We study how the presence of a moderate sweep angle affects the behavior of shock/boundary layer interactions by means of wall-resolved Large Eddy Simulations. Several sweep angles up to 40$\mathrm{deg}$ are investigated. The mean properties of the flow are first reported, before analyzing the unsteady dynamics of the interaction. Intermediate frequencies at the separation location appear when sweep is present. They are related to spanwise-travelling structures detected around the interaction region.

Experimental high-speed visualization techniques are evolving rapidly and provide valuable tools for learning about ultrafast bubble dynamics that cause unwanted but also desirable damage, such as cavitation and acoustically driven bubbles and droplets relevant for biomedical applications. This article offers my personal perspective on the quest to illuminate the hidden physics of externally stimulated bubbles that have a remarkable ability to focus energy. The focus is given to advanced experimental techniques including ultrafast videomicroscopy and synchrotron x-ray imaging to characterize bubble jetting, vapor bubble nucleation and shape deformations of periodically driven bubbles.

A chain of ascending bubbles can occasionally form stable, regular, and aesthetically striking patterns on a liquid’s free surface: a captivating phenomenon that also poses a fundamental challenge in fluid mechanics. This work establishes a framework linking the pattern morphology to the local dynamic interplays between the bubbles, the free surface, and the surrounding liquid flow. A heuristic model is proposed and validated against simulations and existing experimental data. The present study provides insights into understanding, designing, and controlling collective behaviors in broader self-organized systems.

Bubble dynamics is critical for mass, momentum, and heat transport in two-phase flows. We investigated the effects of surface wettability on bubble formation, departure, and the surrounding liquid flow. Two-phase particle image velocimetry (PIV) captured the transient liquid-phase velocity field and instantaneous bubble shape. The experiments revealed strong wettability dependence in bubble size, departure frequency, and thus liquid phase flow dynamics. Applying finite difference analysis to PIV data, we further reconstructed time-resolved pressure, viscous stresses, and wall velocity gradients, enabling understanding of how wettability modulates the bubble-liquid interaction.

Altough many studies have focused on the aerobreakup of an isolated droplet, the coupling dynamics of multiple droplets remain less understood. By investigating the aerobreakup of parallel-arranged droplets under shock impact, this work systematically reveals how the decrease of droplet spacing induces breakup modes transition, i.e., from bag breakup to trailing and shuttlecock modes at low Weber numbers, and from open to closed configurations at high Weber numbers. Quantitative and theoretical analyses demonstrate that a reduced spacing accelerates the gas velocity between droplets, thus facilitating the homodromous bending and filament formation.

The dynamics of a spherical particle undergoing Quincke electro-rotation in the vicinity of a planar electrode are investigated. Increasing the electric field induces a transition from steady rolling to periodic and then chaotic oscillations, with the onset threshold depending on the particle–surface gap and particle inertia. When allowing for normal motion of the particle the electrostatic attraction reduces the gap, which in turn suppresses chaotic behavior and reestablishes a steady rolling state.

We investigate the influences of fluid shear thinning and shear thinning gradients on electrokinetic instability (EKI) in microchannel flows with conductivity gradients via the addition of xanthan gum (XG) polymer. We also perform a scaling analysis to account for the fluid shear thinning effect on the electric Rayleigh number in terms of a power-law model. The critical values of this dimensionless number for the onset of EKI exhibit similar variations to the threshold electric field across fluid configurations (i.e., shear thinning (ST)/Newtonian (N), ST/ST, N/ST) and XG concentrations.

Thermal convection under a horizontal magnetic field is known to promote quasi-two-dimensionalization, yet its impact on transport in strongly confined geometries is incompletely understood. Using three-dimensional simulations of low-Prandtl-number convection in a small-aspect-ratio cell, we reveal a transition from 3D cellular structures to quasi-2D rolls accompanied by regular/irregular flow reversals. Unlike classical roll breakup and reconnection, the reversals originate from mode competition within vertically stacked vortices. Unified transport scaling and an extension of elliptical instability theory quantitatively explain the observed dimensional transition and flow-state selection.

In mesoscale modeling, wind turbines are commonly parameterized as single-column momentum sinks and sources of turbulence kinetic energy, which can lead to substantial errors when turbines are located near grid boundaries. This study proposes a Gaussian-based multi-column spatial distribution method that analytically allocates sink and source terms across multiple grid columns. When implemented within the Fitch wind farm parametrization, the proposed method significantly improves the modeling accuracy of wind farms, particularly when turbine rotors span multiple grid columns. The method provides a robust and physically consistent improvement for future mesoscale wind-farm simulations.

Molecular dynamics simulations show how the Rayleigh-Plateau instability of a nanoscale liquid filament evolves with and without imposed interfacial perturbations between two immiscible liquids of equal viscosity. For a single-mode perturbation, the growth rate deviates from classical theory at small radii but converges to macroscopic predictions for thicker filaments. Without imposed perturbations, thermally induced breakup has a power-law dependence of breakup time on minimum filament radius. These results demonstrate that continuum theory remains valid down to scales of about 15 molecular diameters, while thermal fluctuations are increasingly important in thinner liquid filaments.

Controlling fluid flows to induce laminarization is a challenging task due to the chaotic nature of turbulent flows. We introduce the “minimal seed for relaminarization”: the smallest perturbation of a turbulent state that triggers laminarization without a chaotic transient. This minimal seed and its trajectory provide an efficient laminarization pathway out of the turbulent region of the state space. Using a nonlinear optimization framework in a nine-mode shear flow model, we compute the minimal seed for relaminarization, analyze the associated dynamical structures, and show that it provides a useful reference for developing a control to trigger relaminarization.

Particle motion at fluid interfaces is commonly governed by static capillary interactions, limiting active control. Here, we introduce a geometry-controlled capillary well formed by a neck-shaped meniscus around a vertically actuated rod, enabling reversible trapping and guided migration of particles. Elliptical rods generate anisotropic curvature landscapes that focus particles toward regions of maximum curvature, revealing an optimal aspect ratio for trapping efficiency. Theoretical predictions are validated through experiments and simulations.

In fresh water between temperatures of 0 and 4 degrees Celsius, the equation of state (EOS) for density is nonlinear and the differences in density are extremely small. We call this the “cold water regime”. Using three-dimensional direct numerical simulations, we study the impact of the nonlinearity of the EOS on the development of the Kelvin-Helmholtz instability. We find that the nonlinear EOS displaces the pycnocline vertically above the shear layer which leads to differences in the onset of three-dimensional flow — both in terms of timing and scale. We further comment on how the extent of these differences changes under varying Prandtl and Reynolds numbers.

Tip streaming in a coflowing device is probably the simplest way to generate quasi‑monodisperse droplets that are much smaller than the device’s fluid passages. We show that flow stability cannot be determined from the linear stability analysis of the steady microjetting mode but from the linear superposition of decaying eigenmodes triggered by an initial perturbation. The red line in the image corresponds to a direct numerical simulation of an asymptotically stable microjetting. The experiment and simulation show the perturbation growth leading to jet breakup. These results call into question the validity of linear stability analysis applied to coflowing and similar configurations.

Air–water interfaces are often assumed to be either mobile or rigid, yet in practice their mechanical response is governed by a subtle interplay between hydrodynamic stresses, trace surfactants, and capillary deformation. Using dynamic colloidal-probe AFM, we probe the frequency-dependent viscoelastic response of an air–water interface at micrometer to nanometer separations. We show how surfactant transport induces a transition from mobile to immobilized behavior, while hydrodynamic pressure generates an additional capillary stiffness at small gaps. Together, these effects reveal a unified visco-capillary framework for interfacial rheology beyond purely viscous descriptions.

Buoyancy-driven instabilities strongly control mixing and reaction rates in stratified reactive fluids, yet how chemical reactions reshape the density field that drives these flows remains unclear. This work shows that a bimolecular reaction at a miscible interface can fundamentally alter density profiles, triggering convection even around initially stable stratifications and amplifying mixing in Rayleigh–Taylor unstable cases. By mapping flow regimes in terms of reactant and product density contributions, the study reveals how reactions govern instability onset, plume directionality, and overall reaction yield.

Predicting and controlling the liquid dynamics in a groove or grooved systems is of importance for various technological applications. Solutions for the flow of a wetting liquid in a channel of flattened triangular cross section are studied considering the combined effects of capillary imbibition and evaporation. Two main regimes are identified: the pure corner flow regime and the regime with partial bulk invasion. Situations where the mass transfer rate is independent of the external air relative humidity are exhibited for both regimes.

Predicting particle collisions in turbulence is critical for applications from cloud formation to industry, yet a model for mean radial relative velocity (MRV) remains elusive. Using direct numerical simulations, we investigate the motion of colliding particles and establish the relationship between MRV at collisional distances, and the particle Stokes and flow Reynolds numbers. A resonant length scale, the spatial scale where inertial particles capture momentum from turbulent flow, is introduced. By finding the relationship between this scale and particle and flow parameters, we provide inputs for a robust framework to predict particle MRV at collisional scale across a range of conditions.

Kelvin–Helmholtz (KH) instability can dramatically reshape how particles settle and spread in stratified shear flows. Using direct numerical simulations with Lagrangian tracking, we show that KH billows can either slow, trap, or strongly accelerate particle settling depending on particle size, with small particles settling up to seven times faster than their Stokes velocity. The results reveal how preferential sampling of coherent flow structures and limited encounter times with turbulence fundamentally alter particle dispersion and sediment transport in mixed layers.

Cavitation around marine propellers operating in nonuniform wake flows spans multiple spatial and temporal scales, yet its relationship to pressure pulsations is poorly understood. This study integrates pressure measurements with high-speed imaging and digital inline holography to resolve the evolution from large-scale sheet cavities and vortex tubes to intermediate-scale cloud clusters and microbubbles. We show that distinct cavitation regimes have characteristic spectra, with cloud cavitation producing the strongest multiscale coupling. By reconstructing phase-resolved microbubble statistics, a bubble model is developed that accurately reproduces mid- and high-frequency pressure components.

Collisions between grains in dense granular flows give rise to diffusive-like particle trajectories. Here, we extend this framework beyond spherical grains by experimentally investigating the dynamics of individual semi-rigid fibers immersed in an index-matched granular flow. We systematically examine the effects of fiber length, diameter, grain size, and shear rate on fiber motion. The fiber center of mass undergoes a diffusive dynamics, with a diffusion coefficient that increases as the fiber length decreases relative to the grain size. Finally, we propose an empirical relation linking the fiber diffusion coefficient to that of the surrounding grains and to fiber geometrical properties.

Granular materials composed of elongated particles exhibit morphological inhomogeneity under shear, driven by the interplay between particle alignment and dilatancy. Using discrete-element simulations in a linear split-bottom shear cell, we show how friction, particle shape, and initial packing conditions influence the steady-state surface morphology. Our results reveal that particle aspect ratio is the primary factor governing depression formation on the free surface, while friction localizes deformation within the shear band.

Unlike common cavitation flows, Venturi cavitation exhibits unique scaling due to its longitudinal pressure gradient. This study experimentally reveals four distinct cavitation patterns and their transitions with cavitation number and pressure recovery. A piecewise model combining Short and Extended Cavity theories, smoothed logarithmically, successfully predicts the cavity length. The established σ–κ mapping offers a practical tool for pattern identification and length prediction across Venturi geometries.

Free-surface flows of dense suspensions made of contactless silica microparticles are studied in microfluidic rotating drums experiments. We show that sedimented piles at rest exhibit aging: longer waiting times before tilting delay flow onset and reduce flow speed. This effect is not caused by compaction, crystallization, nor particle contacts, but likely arises from microscale relaxation dynamics driven by thermal agitation and time evolution of electrostatic inter-particles repulsive forces.

The heat transfer in a turbulent thermal mixing layer of a fluid laden with inertial particles with finite heat capacity is investigated using direct numerical simulations. A reduced phase-space analysis, combined with a moment-of-total-enthalpy formulation, reveals how particle inertia and finite thermal response organize velocity–temperature correlations. These mechanisms lead to a self-similar regime in which particles dominate enthalpy transport, with maximum enhancement near a unity Stokes number. The results connect phase-space dynamics to global heat-transfer scaling in inhomogeneous turbulence.

The free fall of a sphere is a seminal problem in fluid mechanics that becomes remarkably complex as its velocity approaches the “drag crisis” regime. This study demonstrates that intermittency in the drag coefficient at this critical stage renders the temporal evolution of the velocity intrinsically unpredictable. By combining numerical simulations with a stochastic two-state model, we reveal huge velocity dispersions. These findings highlight the necessity of statistical descriptions for bodies falling at high Reynolds numbers, with direct implications for ballistics and sports aerodynamics.

In our rapidly urbanizing world, an arms race has emerged between increasingly numerous polluting agents and the urban planners who model the behavior of these harmful gases. Physics-Informed Neural Networks (PINNs) have recently entered the space of data-driven fluids research as a tool for inferring physical fields from sparse and/or noisy measurements. This study aims to expand the existing use cases of PINNs by introducing them to the world of passive scalar transport through a low Reynolds number cylinder flow. The PINN is required to close the governing equations with limited data and unknown boundary conditions, thus demonstrating inference of previously unknown physical fields.

Which flow patterns are most effective at cooling hot boundaries? We address this question by optimizing incompressible two-dimensional fluid flows in a straight channel to maximize heat transfer under a fixed input power budget. Using an adjoint-based gradient framework combined with the Broyden–Fletcher–Goldfarb–Shanno (BFGS) optimization algorithm, we find that optimal flows remain predominantly unidirectional up to a critical Péclet number. Beyond this threshold, our improved numerical scheme uncovers a new class of wavy optimal flows. These flows are characterized by finger-like structures extending from the channel walls, which reorganize transport pathways and enhance thermal exchange.

A machine-learning-based surrogate model is proposed for the large-eddy simulation of three-dimensional turbulent flows over curved boundaries with strong flow separation. The model, termed as hybrid U-Net and Fourier neural operator (HUFNO), is based on an integrated framework of convolutional neural networks and Fourier neural operators, tailored for problems involving mixed periodic and non-periodic boundary conditions. The HUFNO model is validated in the fast prediction of turbulent dynamics of periodic-hill flow, with transferable accuracy to unseen initial conditions, Reynolds numbers, and hill shapes.

We study homogeneous isotropic turbulence laden with a very large number of small bubbles. Using an efficient four-way coupled point-particle method, this work quantifies when and how bubble–bubble interactions and feedback on the fluid become relevant. While the back-reaction of microbubbles leaves the turbulent energy budget largely unchanged under typical circumstances, excluded-volume interactions can significantly modify Lagrangian bubble statistics, suppressing preferential concentration once vortex filaments are filled up. These results identify limits of validity for one-way coupling and reveal new ways bubbles can probe coherent structures in turbulence.

Compressible turbulence adds further mechanisms of anomalous energy dissipation in comparison to its incompressible counterpart. They are caused by pre-shocks and shocks. To quantify these contributions, we extend the framework of Duchon and Robert to the compressible flow case and analyze anomalous dissipation for one-dimensional gas dynamics examples.

Intermingling between buoyancy induced by a density gradient and externally imposed turbulence on propagation of a reactive interface is investigated through an autocatalytic reaction forming a thin front in an aqueous incompressible medium. Turbulence generated by oscillating grids, either spatially decaying or nearly homogeneous, allows flow effects to be isolated. Measurements with velocimetry and fluorescence reveal both the classical Huygens-type propagation and a reactive mixing regime where turbulent advection ignites dispersed reactions. Minute density variations are shown to influence front dynamics, highlighting a tight coupling between chemical kinetics and turbulent transport.

How a dispersed bubble phase reshapes turbulence remains a long-standing question, especially at moderate void fractions where coupling spans many scales. Using high-resolution lattice-Boltzmann simulations of forced homogeneous isotropic turbulence, we find that the global energy cascade stays close to Kolmogorov behavior up to 24% gas volume fraction. Phase-conditioned spectra, however, show a distinct gas-phase regime: a near-flat low-$k$ range followed by a $k^{-3}$ scaling at intermediate scales, consistent with localized bursts between two finite wavelengths. Our results separate universal transfer from phase-specific small-scale modifications in bubble-laden flows.

The transient response of Langmuir turbulence to an abrupt onset of surface heating is investigated using large eddy simulations. We show that the transient response of Langmuir turbulence is substantially different from wind-driven shear turbulence, with more gradual decay of turbulence intensity near the surface and much quicker response at depth. This is related to the more coherent downwelling plumes of Langmuir turbulence that extend throughout the surface boundary layer. The results have implications for improving Langmuir turbulence parameterizations that assume an equilibrium turbulence state with the surface forcing, which may fail in the early morning phase of a diurnal cycle.

We investigate the effects of compressibility and geometry on turbulent/nonturbulent mixing in the absence of a mean shear. Focusing on initially homogeneous isotropic turbulence adjacent to a quiescent fluid in planar and cylindrical geometries, we theoretically predict the evolution of the mixing region width and turbulent kinetic energy and validate these predictions using direct numerical simulation. Compared to decaying homogeneous isotropic turbulence, we find that the decay rate is enhanced by dilatation due to energy transport to the nonturbulent region and by diverging geometries.

The Josephson-Anderson relation was originally conceived to understand drag in quantum fluids in which vorticity is quantized. Surprisingly, it also explains drag in classical fluids when vorticity is constantly being generated. Drag ensues when vortices cross the streamlines of the background potential flow.

The present reserch investigates two-dimensional numerical simulations of vortex-induced vibrations (VIV) of initially tandem circular cylinders with two degrees of freedom in both the streamwise and transverse directions, undergoing rigid collisions for varying mass ratios. The cylinders exhibit a natural tendency to reconfigure their mean positions into a side-by-side arrangement. As the mass ratio increases, the VIV dynamics progressively evolve from chaotic behavior toward more organized, periodic-like states.

Wave turbulence describes the dynamical properties of random nonlinear wavefields in infinite systems. Here, we experimentally investigate finite-size effects on gravity-capillary wave turbulence, using local magnetic forcing to generate a random, homogeneous wavefield – unlike previous studies that relied on oscillating tanks where global forcing dominates. We observe a smooth transition from continuous to discrete wave turbulence with increasing confinement, as finite-size effects weaken three-wave resonant interactions. This study bridges the gap between idealized infinite systems and confined environments, offering new insights into the role of boundaries in wave turbulence.

Inertial waves transport energy and momentum in rotating fluids, impacting mixing and tidal dissipation in Earth’s oceans, gaseous planets, and stellar interiors. This study examines the linear stability and nonlinear breakdown of finite-amplitude propagating plane inertial waves. We use numerical simulations to validate the frequency-dependent anisotropy of the most unstable perturbations predicted by linear Floquet theory and explore how the wave energy is partitioned between being dissipated in a cascade and accumulated in long-lived geostrophic modes.

Experiments show that the presence of a buoyant particle layer on a liquid bath markedly changes the characteristics of an impulse wave generated by the subaerial collapse of a granular column. Relative to a clean interface, buoyant particles delay the transition from non-breaking to breaking waves with two distinct particle-accumulation regions emerging in the wave form: a static buildup adjacent to the collapsed grains that buttresses the pile, and a dynamic concentration zone traveling with the wave front that suppresses breaking. These results can provide potential insights into wave propagation in proglacial fjords laden with ice mélange or floating microplastic accumulations on the ocean.

The behavior of waves on a water surface is usually classified in terms of the different modes of oscillation that are present. However, for water that is flowing, this description alone will miss a vital part of the physics.

Fluid flows are usually described from a fixed point in space, while Lagrangian methods that follow the fluid often rely on particle trajectories embedded in a prescribed geometry. We develop a geometric formulation in which the observer moves with the fluid and the geometry of space itself evolves with the flow. This perspective naturally separates physical dynamics from observer effects, introduces geometric generalizations of inertial forces, and yields exact solutions and new stability results, including a proof of Couette flow stability at all Reynolds numbers.

A fully meshless approach enhances flow fields from sparse, randomly-positioned particle measurements. By merging information from locally similar snapshots over time, high-resolution, physically consistent velocity fields are reconstructed directly from scattered data without relying on grids at any step. Validated on experimental three-dimensional jet flow, the method reveals subtle structures and velocity derivatives that remain hidden to conventional techniques, providing a clearer, more faithful view of complex fluid dynamics.