Physical Review Fluids

Several problems are related to the behavior of aerosol particles around a porous screen, from fog collectors to the limitation of virus transmission by a facemask. The droplet size distribution in fog or human respiratory flows is wide enough that it can become relevant to consider the inertia of the largest droplets at the scale of the screen. The purpose of this work is thus to propose a model for such an aerodynamic effect and quantify the collection efficiency depending on screen parameters such as its size, solidity, and local Reynolds number at pore scale. The present computation is performed for water droplets around a square textile composed of fibers, providing optimal parameters.

Collisions between inertial particles in turbulence are often treated as a sequence of uncorrelated events, yet coalescence fundamentally removes particles and their future collision trajectories. We show that the statistical differences between coalescing and noninteracting “ghost” particles can be traced to this finite-time collision memory. By filtering ghost-particle trajectories to remove particles with recent collision histories, we reproduce the collision rates and pair statistics of coalescing systems, revealing coalescence as a form of history filtering in turbulent particle dynamics.

This study presents a systematic investigation of the second-shock induced micro jetting from sinusoidally perturbed tin surfaces using a combined smoothed particle hydrodynamics (SPH) and molecular dynamics (MD) approach. The key contributions of this work are: 1) identification of three distinct bubble morphologies after the first shock, controlled by $k_0{h}_0$; 2) revealing that two dimensional unloading and weak Richtmyer-Meshkov instability at the bubble base drive the morphological transition; 3) development of a novel predictive model for second-shock ejecta mass by treating the post shock interface as a multi mode perturbation and incorporating explicit mode coupling effects.

Cavitation bubbles generated by focused ultrasound can mechanically destroy tissues, as in histotripsy, where tissue damage scales with maximum bubble size. We extend an energy budget analysis to include viscoelastic and compressibility effects and derive an analytical expression for the maximum radius of a spherical bubble. The maximum radius decreases with increasing shear modulus but increases with pulse amplitude and wavelength. Our model accurately predicts the maximum radius under explosive growth conditions, established from pulse amplitude and frequency. These findings clarify the physics governing bubble growth and may inform the design of ultrasound-based therapies.

Rheoscopic flow visualization is widely used to study turbulent decay in wall-bounded flows, although the physical interpretation of the measured decay times has received less attention. By comparing rheoscopic visualization with particle image velocimetry in plane Couette–Poiseuille flow, we show that the measured decay times are closely associated with the slower decay of streamwise streaks rather than the faster decay of vortical rolls. These findings improve our understanding of rheoscopic measurements in transition to turbulence in plane shear flows.

Classical shock-wave theory relies on ideal-gas assumptions that fail in dense and supercritical fluid regimes. By coupling the Navier–Stokes equations with a real-fluid thermophysical model for supercritical CO${}_2$, this study resolves the internal spatial structure and non-ideal jump relations across shocks. Surprisingly, while non-ideal thermodynamics strongly alters pressure–density jumps, the physical shock thickness remains primarily governed by the upstream Mach number. These results reveal how dense- fluid thermodynamics reshapes compressive discontinuities, defining the limits of classical shock theory.

We report on a suite of direct numerical simulations of rotating plumes interacting with a pre-existing density interface. For an experimentally realizable range of rotation rates and buoyancy source sizes we find evidence of long-lived plumes that bifurcate from the source region and twist around each other as they descend. These plumes are a novel example of coherent structure formation at intermediate Reynolds number.

Data-driven closure modeling of transport systems is limited by the difficulty of generating diverse training data. We develop the Generalized Forcing Method, which uses prescribed forcings to excite the dynamics in the reduced space while preserving the target closure relation. Applications to linear transport problems, from shear dispersion to passive-scalar transport in homogeneous isotropic turbulence, demonstrate that the framework can identify accurate and stable reduced models.

Enstrophy dynamics is investigated in homogeneous isotropic turbulence, revealing the interplay between vortex stretching, diffusion, and interscale transport. Using direct numerical simulations and a two-point statistical framework, the interscale enstrophy transfer is shown to simultaneously transfer longitudinal vorticity increments towards larger scales and transverse vorticity increments towards smaller scales, resulting in a net transfer towards smaller scales. Conditional correlation coefficients are used to provide further insights into the role of pressure in energy and enstrophy interscale transport events.

Population-balance models offer a route to connect dispersed vapor bubbles with macroscopic cavitation, but their closures require experimental constraints. Here, point-by-point optical probe measurements reveal self-similar void-fraction and bubble-velocity profiles, yielding an effective Schmidt number for the bubbly/non-bubbly transition. The measurements further show that bubble-size distributions and bubble velocities depend strongly on the local flow region, providing new guidance for region-dependent population-balance closures in cavitating flows.

The inertial layer is a cornerstone of wall-turbulence theory and is characterized by a balance dominated by Reynolds shear stress. Using direct numerical simulation of turbulent channel flow at Re${}_{\tau }\approx 16000$, we identify a wall-normal region where the Reynolds shear stress approaches its expected constant value and where additional inertial-layer predictions for velocity fluctuations and dissipation are simultaneously satisfied. These observations provide strong evidence for the emergence of inertial-layer dynamics at this Reynolds number.

Can a floating ring protect offshore structures from impactful ocean waves? This study presents how a floating annular viscoelastic plate can significantly reduce wave-induced forces and wave run up on a vertical cylinder by combining elastic deformation with viscous energy dissipation. The results reveal that tuning the plate’s flexibility and damping can provide appreciable protection to the central structure. These findings provide a promising passive strategy for enhancing the resilience of offshore structures.

In the late 1970s, J.S. and S.E. Tsakadze carefully monitored the long-term spin down of a vessel containing superfluid helium initially set into rotation. They occasionally observed short spontaneous spin-up events followed by relaxation. These strange events are still a mystery, but it is thought that they are due to the quantum nature of the rotating superfluid (the presence of quantized vortex lines). If so, Tsakadzes’ experiments shed light onto another mystery: the observed glitches of the rotation of neutron stars. We revisit Tsakadzes’ experiments and discuss them in view of the current understanding of quantized vortex lines and quantum turbulence.

The free surface of thin film of a yield stress fluid has been shown to develop undulations when subject to uniaxial compression. In this paper, using a novel experimental setup, we characterize this surface instability for the first time. We show that the amplitude of the undulations depends linearly on the Bingham number, the non-dimensional yield stress, whereas the wavelength scales with the initial thickness of the compressed layer. These experiments enhance our understanding of surface instabilities in related systems, such as recent experimental work scraping a thin layer of a viscoplastic fluid.

Why do bubbles moving through a viscous fluid in a Hele-Shaw cell select a particular velocity, even in the limit of vanishing surface tension? We address this longstanding problem using exact time-dependent solutions for bubble dynamics in a Hele-Shaw channel, including a new class of rational solutions. Our results show that the experimentally observed velocity, twice that of the background flow, emerges as the unique dynamically stable attractor, revealing an intrinsic selection mechanism that does not rely on surface tension.

Dynamic mode decomposition (DMD) is a powerful tool for uncovering coherent flow dynamics, yet the need for time-resolved data often limits its experimental use. We show that the evolving gaps between tracer particles in standard, non-time-resolved particle image velocimetry (PIV) images contain enough dynamical information to recover velocity-associated DMD modes. The proposed gap-based DMD approach extends modal analysis to measurements that would otherwise be considered temporally under-resolved.

The amplification of large-scale magnetic fields via the Tayler-Spruit dynamo has been invoked to explain the slow rotation of stellar cores and the formation of magnetars. The existence of this mechanism has recently been proved via 3D MHD numerical simulations of a differentially rotating stably stratified fluid. Using the same setup, we show that the dynamo can produce a wide variety of magnetic field dynamics, including reversals and hemisphericity, by varying thermal stratification. In a fiducial simulation, we interpret the appearance of reversals as the result of the coupling of two large-scale magnetic modes with opposite equatorial symmetries by the symmetry breaking of the flow.

Band-ensemble Spectral Proper Orthogonal Decomposition (SPOD) computes modes from bands of neighboring Fourier modes of the full-length Fourier transform. Compared with the commonly applied Welch-based SPOD algorithm, this allows the number of modes to vary with frequency, enabling an adjustable bias–variance trade-off across the spectrum. Additionally, the resulting expansion coefficients allow precise frequency estimates to be attributed to individual modes, enabling improved detection of tonal modes in broadband–tonal flow cases.

A circular cylinder undergoes drag crisis beyond the critical Reynolds number via formation of a laminar separation bubble (LSB) downstream of each shoulder. We investigate the dynamics of these LSBs at low intensity free stream turbulence (FST). We report, for the first time, a synchronization between formation of LSB and the Karman vortex shedding for a stationary cylinder, albeit only at the lowest FST investigated. Increasing the FST to mimic the background FST in some of the past studies completely alters the transition dynamics, as several subregimes observed at the lowest FST disappear at higher FST. The findings provide a valuable benchmark of unperturbed transition dynamics.

Unstable periodic orbits are thought to form the backbone of turbulence, yet their computation in thermal convection remains scarce near the onset of chaos. We compute a steady state and three families of periodic orbits in two-dimensional Rayleigh-Benárd across nearly two decades of Rayleigh number, and use Floquet analysis to characterize every bifurcation in the route to chaos. Additionally we find that all symmetries are broken before this onset, and the heat-transport scaling is unaffected throughout.

Particles and drag-reducing polymers can each strongly alter turbulent transport, but what happens when the two act together? Experiments in Taylor-Couette turbulence uncover a transition from particle-dominated to polymer-dominated dynamics as flow strength increases. The coupling is also shape dependent, with spherical particles producing a stronger drag response than oblate and prolate particles.

Turbulent flows contain a wide range of vortical structures, from the tiny to the large. One of the central tasks of turbulence theory is to provide a simple yet reliable means of identifying how much kinetic energy is, on average, held at each scale. Traditionally, Fourier analysis is used for this purpose. However, a decomposition into sine waves is somewhat artificial. Here we provide a real-space diagnostic, quite unrelated to Fourier space, which measures the scale-by scale distribution of kinetic energy.

This study presents a numerical investigation of flow in a curved channel with a low height-to-width aspect ratio. By independently varying the Reynolds number, channel curvature and aspect ratio, we identify two distinct scaling laws for the intensity of secondary flows, each corresponding to a different flow regime: one governed by a balance between viscous and curvature forces, and the other by an equilibrium between inertial and centrifugal forces.

What governs the formation of scalar gradients essential for mixing in turbulent flows? Using high-resolution Direct Numerical Simulations (DNS) at high Reynolds and Schmidt numbers, we show that intense scalar gradients arise primarily from near-optimal alignment with the most compressive strain direction, rather than unusually strong strain alone. These gradients organize into thin sheets within shear layers between vortex tubes, where their amplification is ultimately arrested by molecular diffusion. Remarkably, their structure and dynamics exhibit near-universal behavior across Reynolds and Schmidt numbers, revealing a robust picture of small-scale turbulent mixing.

Horizontal convection is often modeled with a linear temperature-density relation. Two-dimensional simulations show that a density maximum, such as that of water near 4°C, can generate full-depth mixing plumes, reorganize the circulation, and enhance heat transport. An energy-budget analysis suggests that plume height controls an additional potential-energy transfer associated with this enhancement.

Motivated by the underground storage of hydrogen and CO₂, we use analytical approximations to study the evolution of a gas current injected into porous, axisymmetric, dome-shaped channels. Unlike flow in flat channels, where injection dominates, flow in bell-shaped channels can remain influenced by buoyancy. In parabolic-shaped channels, slopes steepen with distance from the injection point, leading to distinct stages of spreading with different flow patterns along the channel. These results show how geological curvature can alter gas spreading and suggest that, under suitable conditions, parabolic-shaped aquifers may offer more efficient use of storage volume than flat aquifers.

A spinning rotor can drive a free neighbor to turn either against it, like a gear, or with it, like a belt, as a recent quasi-two-dimensional experiment showed. Using direct numerical simulations of strictly planar flow, we compute the torque-free spin of the passive rotor across gaps, Reynolds numbers, and corral sizes. The planar model reproduces the observed gap-driven sequence of vortex rearrangements and the order of magnitude of the coupling, and it contains an inertia-driven spin reversal, but at a narrower gap than in the experiment, leaving finite-depth effects as candidate contributors to the difference.

Sand-laden cavitation spans solid particles, macroscopic vapor cavities, and microbubbles, yet existing simulations rarely resolve their coupled multiscale dynamics. We develop a two-way coupled Eulerian-Lagrangian framework that simultaneously captures cavity evolution and discrete particle-bubble motion. The simulations reveal that sediment particles accelerate cavity shedding and weaken the re-entrant jet, while increasing the Hinze scale by reducing turbulent dissipation and suppressing the breakup cascade from large to small bubbles.

Standing Alfvén waves develop in a poloidal channel of a liquid-metal blanket of a fusion reactor during the first few milliseconds of a plasma disruption event. The waves are characterized by high-frequency oscillations of velocity, electric current, and induced magnetic field, with Lorentz forces exceeding, in some cases, ${10}^9{\text{ N/m}}^3$.

Vortex stretching is regarded as the engine of three-dimensional turbulence, yet the separate roles of its two components, vorticity amplification and vortex tilting, remain unclear. By selectively suppressing vorticity amplification in the Navier–Stokes equations, we show that it is essential for the forward energy cascade and the dissipative anomaly: without it, energy dissipation vanishes at high Reynolds number and the flow switches to a forward enstrophy cascade with anomalous enstrophy dissipation. Yet intermittency and multifractality persist, revealing that the geometric reorientation of vortex lines by tilting alone is sufficient to sustain them.

In Newtonian turbulence, velocity fluctuations exhibit a complex, multifractal hierarchy of scaling laws. Here in polymeric turbulence, experiments reveal a simpler, bifractal-like scaling for longitudinal velocity structure functions in the polymer-dominated elastic range, with distinct branches for low- and high-order moments. The results suggest that sheetlike flow structures govern the rare, intense fluctuations, providing insight into how polymer elasticity reshapes turbulent intermittency.

Flow-induced vibrations are often classified by frequency synchronization, but this alone can obscure changes in the coupling between fluid forces and structural motion. Simulations of an inclined D-section cylinder at low Reynolds number reveal a finite interval in which lift and motion separate in frequency and their phase difference wanders. This transitional response connects lock-in to low-amplitude, nearly antiphase motion without developing into sustained galloping.

Viscous leaky-dielectric drops in electric fields can deform, translate, and spontaneously rotate through the Quincke rotation instability. We develop a three-dimensional nonlinear model coupling charge relaxation, convection, deformation, and electrohydrodynamic interactions to study the dynamics of a pair of drops. The model predicts spiraling trajectories, attraction, repulsion, and sustained pair translation, and shows how interactions can qualitatively alter the onset of Quincke rotation.

This study uses a nonlocal Gross-Pitaevskii model to probe how pressure and obstacle size govern roton emission and vortex nucleation in superfluid ⁴He — the first single-framework analysis of these critical velocities across pressures up to solidification (~25 bar). By tuning interaction parameters to match experimentally measured dispersion curves at each pressure, the authors compute both critical velocities for moving disks, mapping their dependence on pressure and size. The results show that the gap between the two critical velocities widens with increasing pressure, suggesting that roton emission should be easier to observe experimentally at higher pressures.

Time-resolved spiral magnetic resonance velocimetry is used to characterize turbulence in a Schwarz Diamond triply periodic minimal surface monolith. Turbulent energy spectra show that the pore geometry limits the maximum eddy size, and that Kolmogorov -5/3 scaling emerges for Reynolds numbers above 2000. Reynolds stress tensor maps reveal that local turbulence anisotropy is imposed by the channel architecture.