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%).

We report direct experimental observations of self-sustaining CH4–air flames rotating along the edge of an unheated circular Hele-Shaw cell, extending beyond previous studies in which such flames were observed only under external heating. Formed under fuel-rich conditions, these flames exhibit stable traveling-wave behavior, with edge velocities exceeding nominal laminar flame speeds and a structure comprising both premixed and diffusion branches. The rotation arises from a balance between local flame speed, flow, and wall heat loss. Parametric studies have identified regimes of single and multiple rotating waves, as well as transitions to ring flames or extinction under extreme conditions.

We investigate subcritical turbulence in plane Couette flow using a finite-amplitude columnar vortex instead of the conventional Lundbladh-type perturbation. We show that, although the perturbation modifies the turbulent structure, the key characteristics of subcritical turbulence remain unchanged. The figure illustrates the breakdown of the columnar vortex into tripolar and quadrupolar structures in unstratified and stably stratified flows. The columnar vortex breakup provides a new mechanism for streamwise streak formation.

We revisit the classical hoop-stress mode (HSM) instability in viscoelastic Taylor-Couette flow using the FENE-P model. We show how finite polymer extensibility modifies the onset of the hoop-stress mode for a range of gap width ratios. By systematically comparing linear stability predictions with experiments and direct numerical simulations, we identify where the model succeeds and where its limitations emerge and what are the possible reasons for the discrepancies. We also compare our linear stability results with an augmented Pakdel–McKinley criterion that incorporates finite extensibility and finite gap effects.

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.

The ability of red blood cells to deform under shear flow is critical to cardiovascular function, yet capturing their complex fluid-structure interactions remains challenging. We extend a perturbative approach to capture cellular enucleation, and probe the emergence of shape instabilities under shear flow. This method isolates how a cell’s initial orientation, membrane mechanics, and viscosity contrast drive morphological instabilities, such as stomatocytes and trilobes. Ultimately, this framework provides a baseline for understanding how disorders like sickle cell disease and malaria compromise cells dynamics.

This study investigates nonaxisymmetric shape oscillations of an inviscid droplet. A highly accurate Galerkin framework is extended to fully three-dimensional droplet oscillations, and modal coupling is examined for different initial configurations, beginning with four nonaxisymmetric initial shapes (see Figure). The results show that the droplet keeps the symmetries of its initial shape and velocity throughout the oscillation. Only those modes that exhibit all symmetries of the initial configuration are excited. Conversely, modes which violate the symmetries are not excited.

In preparation for the DRESDYN precession dynamo experiment, we numerically investigate how wall material properties impact the growth of magnetic modes driven by a precessing flow. We identify two oscillating magnetic modes - dipolar and quadrupolar – whose competition explains the drastic changes in dynamo thresholds for different types of boundaries. For given hydrodynamic parameters, these modes are paramount for a comprehensive understanding of all electromagnetic configurations, including the realistic modeling of the upcoming first liquid sodium campaign.

When gravity currents converge inward, such as sea breezes closing in around an island or lake, their collision unfolds very differently from the situation when two gravity currents meet head-on. Using high-resolution three-dimensional simulations, this study reveals, for the first time, how such collisions generate turbulence: vorticity arises primarily through tilting of azimuthal vorticity rather than through the stretching mechanism as seen in planar collisions. The resulting eddy diffusivity data support existing mesoscale-model parameterizations used to represent gravity current collision events in atmospheric and oceanic models.

Rayleigh-Taylor mixing is important in natural phenomena and engineering applications, but limited spatial resolution in practical experiments and simulations constrains studies of this mixing process. Resolution-limited fields preserve only large-scale features such as mean profiles and mixing widths, but fail to resolve small-scale structures, resulting in lower fluctuation variance and higher apparent mixedness. Convolutional neural network (CNN) based superresolution reconstructs multiscale flow fields and recovers mixedness evolution, with robust generalization across unseen phases and flow parameters.

Low-flow-rate instabilities in vaneless diffusers remain a long-standing challenge in turbomachinery, with the roles of core-flow dynamics and boundary layers still debated. Using a hierarchy of linear stability analysis and two- and three-dimensional URANS simulations, we identify two distinct instability mechanisms and trace them back to their physical origins. The low-flow-rate instability is shown to arise from a two-dimensional inviscid core-flow mechanism, while a new instability, associated with inlet modulation and three-dimensional flow features, is reported at high flow rates.

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.

Reactive fronts in porous media are often destabilized by concentration dependent viscosity of the constituent fluids, leading to viscous fingering. These processes are known to be strongly influenced by hydrodynamic dispersion, originating from incomplete mixing at the pore scale. Yet, their impact and the resulting instabilities remain poorly understood. This study combines transient and quasi-steady-state linear stability analysis to reveal how hydrodynamic dispersion and diffusivity contrasts govern the growth and onset of instabilities, providing new physical insights, with potential applications in contaminant remediation, enhanced oil recovery, and carbon sequestration.

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.

Capillary attachment is a fundamental phenomenon in microengineering, yet the lack of comprehensive theories makes it difficult to predict a pendant drop’s particle-lifting capability. We investigate the critical roles of stability in this process, specifically addressing how to maximize lifting capacity. Our work establishes the theoretical framework required to successfully lift particles in practical applications.

A millimetric droplet may walk across the surface of a vibrating liquid bath, self-propelled by its own wave field. This pilot-wave hydrodynamic system has provided the basis for the field of hydrodynamic quantum analogs, the goal of which is to redefine the boundaries between classical and quantum systems. We here deepen our understanding of pilot-wave hydrodynamics through consideration of its energetics, evaluation of the partitioning between droplet and wave energies. Photo credit: Pedro Nachbin.

Interfacial Hele-Shaw flows often involve fingering instabilities, tip-splitting phenomena and striking pattern formations. This study revisits a simplified model that characterizes the complexity of these fingering patterns and identifies some universal features. We test this model using fully nonlinear numerical simulations and find that it is very effective at predicting near-universal properties of the fingering patterns for one-phase flows with a sufficiently small surface tension parameter. For larger values of this parameter and for two-phase flows, there are discrepancies between the model and our observations, which we explain by studying the morphology of the advancing fingers.

Gravity-driven thin films laden with soluble surfactants play a central role in coating technologies and microfluidic systems, where reduced-order models are widely used to predict interfacial dynamics. Some weighted-residual formulations, however, introduce spurious interfacial mass growth, violating total surfactant conservation. We resolve this inconsistency by developing a conservative weighted-residual model that consistently couples wall slip, adsorption-desorption, and the Marangoni effect while rigorously preserving total surfactant mass. The resulting framework provides physically consistent predictions of instability onset, nonlinear wave dynamics, and surfactant transport.

Classical similarity theory describes the far field of a laminar jet, but not how a finite Poiseuille inlet loses its memory and reaches that state. We develop a self-consistent two-mode analytical model that predicts this pre-asymptotic evolution without empirical fitting. The theory uncovers a universal Reynolds-scaled centerline decay and shows that the transition length grows linearly with Reynolds number, while reproducing full velocity profiles for both round and slot jets in close agreement with Navier–Stokes simulations.

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.

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.

Reactive solute transport in open-channel flows is central to understanding contaminant migration in rivers, wetlands, and engineered waterways, but the combined influence of wind-induced shear, porous-bed resistance, and wall absorption has remained unresolved. This study introduces a generalized dispersion model that captures the coupled transient effects of those three influences through analytical transport coefficients and concentration distributions, validated by Brownian dynamics simulations. The findings reveal how wind direction governs pollutant dispersion, retention, and removal, and provides a predictive framework for layered environmental flows.

Previous theoretical works showed that, in double-diffusion regimes, the onset time and mixing velocity of convective fingers are controlled by a dynamic density jump across stratified layers. Here, we experimentally validate these scalings by analyzing differential diffusion-induced convection in horizontal stratifications within a Hele-Shaw cell. Using a wide range of solute combinations and concentrations, we vary the two key parameters of the problem: the diffusion coefficient and buoyancy ratios. We show that, in both double-diffusive and diffusive-layer-convection regimes, convective dynamics are governed by dynamically generated local adverse density jumps.

Ocean Alkalinity Enhancement (OAE) is a promising marine carbon dioxide removal strategy involving the release of alkaline particles into the ocean. However, its effectiveness depends on how these particles disperse. This study uses high-resolution simulations to show that collective convective instabilities, rather than individual Stokes settling, fundamentally govern particle settling. These instabilities can accelerate vertical particle transport by up to two orders of magnitude, significantly limiting horizontal spread and surface residence time. These fundamental fluid-dynamic constraints provide essential guidance for optimizing future OAE deployments.

Air curtains restrict buoyancy-driven exchange through building doorways, but sealing effectiveness saturates near about 80 to 85% and the transport pathways fluid takes to leak through have remained unclear. We combine large-eddy simulation with Lagrangian particle tracking to follow individual fluid parcels, classifying their trajectories into three pathways: a gravity current active only before the curtain establishes, and two turbulent-entrainment routes that dominate afterward. Once the curtain is established, the residual leakage is shown to be governed by entrainment at the jet interface rather than by buoyancy.

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.

The turbulent flow beneath a water surface is difficult to measure directly, yet it governs important processes such as mixing and air–water gas exchange. We show that the SHallow REcurrent Decoder (SHRED), a lightweight recurrent neural network, can reconstruct subsurface turbulent flow fields using only three sparse measurements of the surface height. The method performs well on both numerical simulations and laboratory experiments, highlighting its potential for future remote sensing of rivers and other free-surface flows.

Direct numerical simulations reveal the orientation and transport of spherical, spheroidal, and rod-like gyrotactic microswimmers in three-dimensional turbulence. Strongly gyrotactic swimmers preferentially align with the vertical, while weakly gyrotactic swimmers are nearly isotropic. Rod-like swimmers are more strongly affected by fluid shear and align with the principal strain direction. Orientation correlations depend on the gyrotactic response time, and all shapes transition from ballistic to diffusive transport. A reduced two-dimensional model for spherical swimmers reproduces the key statistics.

In this work, we analyze two-dimensional turbulence in a Galerkin-truncated system with pseudo-logarithmic and random mode selection in Fourier space. The results confirm the presence of the double energy cascade ($k^{-5/3}$ and $k^{-3}$) and show how anisotropy influences the organization of vorticity without altering universal scaling laws. Finally, particle pair dispersion follows Richardson superdiffusion (${\ell }^2(t)\sim t^3$), demonstrating that preserved spectral interactions sustain turbulent transport.

Classical shell models of turbulence fail to reproduce the dual energy–enstrophy cascade of two-dimensional flows because they predict the wrong equilibrium spectra. Introducing a hierarchical spatial organization across scales restores the correct equilibrium scaling and leads to a statistically stationary dual cascade. The resulting model consistently captures both the equilibrium properties and the nonequilibrium cascade dynamics of two-dimensional turbulence.

Small-scale intermittency is conventionally modeled by power laws of higher-order moments, which include both kinematic and turbulence-driven effects. By using Kullback-Leibler divergence relative to a Gaussian random field, we isolate purely turbulence-induced intermittency, which scales logarithmically with the Taylor Reynolds number. This reveals an emergent symmetry between the turbulence intermittency of dissipation rate and enstrophy, showing that enstrophy appears more intermittent for purely kinematic reasons. The work establishes an information-theoretic framework for characterizing the intermittency and uncertainty of small-scale turbulence.

Mixed convection for heavy liquid metals is still $terra$ $incognita$ in many aspects, because of the complex flow mechanism, strong thermal diffusivity, and optical opacity. Understanding the flow organization is key to turbulence research and practical engineering. We use high-fidelity scale-resolving numerical simulations to explore the unique large-scale longitudinal roller structures in mixed convection, while also considering the low Prandtl number effect of heavy liquid metals. A new physical interpretation for the slope modulation phenomenon of the mean velocity profile is established, which is expected to give new insights into wall function modeling in mixed convection.

Most studies of turbulent wall pressure reduce its dynamics to wavenumber-frequency spectral descriptions, leaving the instantaneous, spatially evolving pressure field largely inaccessible to experiments and prohibitively expensive simulations. We introduce a probabilistic generative approach that reconstructs full spatiotemporal wall-pressure fields from sparse surface measurements and a low-cost mean-pressure descriptor. Combining patchwise neural representations with latent diffusion, the model adapts to unseen sensor layouts and pressure-gradient regimes without retraining. It recovers coherent structures, temporal evolution, and key turbulence statistics.

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.

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.

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.

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.

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.

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.

Wind-driven wave evolution is commonly viewed as a two-stage process: initial wave growth followed by frequency downshift. Here, we reveal a previously unrecognized intermediate stage in the evolution of capillary–gravity waves under wind forcing and dissipation. This stage is characterized by coupled three- and four-wave near-resonant interactions that generate new wave components, broaden the spectrum, and may reduce total wave energy. These findings constitute a major step forward in understanding the evolution of wind waves from small ripples (2.5 cm) to larger-scale waves.

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.

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.

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.

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.