Electrokinetic flows involving fluids with nonuniform conductivity can trigger instabilities known as electrokinetic instabilities (EKIs). Prior studies have established the theoretical framework of EKI in Newtonian fluids, where the electric Rayleigh number is identified to govern the onset of instabilities. We propose in this work a theoretical formulation of the electric Rayleigh number for EKI in viscoelastic fluids with conductivity gradients. Our scaling analysis in terms of the Oldroyd-B model predicts that fluid viscoelasticity suppresses EKI through the combined viscous and elastic effects. We also conduct experiments to investigate the effects of fluid viscoelasticity and viscoelasticity gradients on EKI in hyaluronic acid polymer solutions. Our observations verify the theoretical analysis and demonstrate that the presence of viscoelasticity gradients causes a further suppression of EKI with the extent strongly dependent on the direction of rheology gradients. Furthermore, we analyze the perturbation energy, spatiotemporal evolution, and power spectra to understand and compare the EKI dynamics among different fluid combinations.

The Graetz-Nusselt problem is the classical framework for laminar forced convection, describing the balance between axial convection and transverse heat conduction. However, this balance breaks down at the nanoscale, where axial conduction and viscous dissipation become comparable to bulk transport terms. In this work, we first examine this modified balance using molecular dynamics simulations of water convective heat transfer in graphene nanochannels. The simulations show that thermal development is not determined by the inlet Péclet and Brinkman numbers alone. Nanoscale effects become a leading contribution to the energy balance and can even drive the bulk temperature above the wall temperature in heating cases, causing heat flux reversal and an apparent singularity in the local $\mathit{Nu}$. We then develop a generalized extended Graetz model that incorporates all key nanoscale effects, including axial conduction, bulk and interfacial dissipation, velocity slip, temperature jump, and thermal conductivity anisotropy. The model accurately predicts the evolution of local $\mathit{Nu}$ from the developing to the fully developed state. Using this model, we further analyze the effects of key nanoscale parameters and find that velocity slip affects heat transfer mainly through dissipation, while temperature jump directly weakens wall-fluid heat transfer and lowers $\mathit{Nu}$. Furthermore, we propose a dimensionless resistance ratio $\mathrm{\Pi }=Nu{\mathrm{\Lambda }}_{\mathrm{k}}$ that quantifies the interfacial contribution to total thermal resistance and distinguishes bulk-controlled, transition, and interface-controlled regimes.

We present a theoretical analysis of frictional transitions along the Stribeck curve for rough elastic contacts lubricated by a Newtonian fluid. Building on the mean-field framework of Persson and Scaraggi [J. Phys.: Condens. Matter 21, 185002 (2009)], we formulate a minimal elastohydrodynamic model that couples contact mechanics and lubrication through a homogenized pressure decomposition. Dimensional analysis reveals three independent dimensionless parameters governing the frictional response, which correspond to a dimensionless speed, normal load, and surface roughness. Using asymptotic expansions, we first characterize the boundary- and hydrodynamic-lubrication regimes, which arise naturally as the quasistatic and smooth-surface limits of the model. In both limits, the contact morphology converges toward Hertzian contact in the regime of large elastic deformation, with boundary layers regularizing the separation profile at the edge of the contact zone. We then analyze the mixed-lubrication regime and derive asymptotic expressions for the friction coefficient in the low- and high-speed limits. At high speeds, friction decomposes into a viscous contribution and a residual contact term, leading to a roughness- and load-dependent criterion for the transition to hydrodynamic lubrication that departs from constant-$\mathrm{\Lambda }$ ratio theories. At low speeds, friction reduction results from the progressive redistribution of the applied load between asperity contact and hydrodynamic pressure, yielding a characteristic transition speed from boundary to mixed lubrication. These results are summarized in a phase diagram that generalizes the classical Stribeck curve to a multidimensional parameter space.

Interactions between fluids and granular materials are prevalent on the Earth's surface. In the case of fluid flow over a sediment bed, the fluid imparts a shear stress to the granular materials. When the applied shear stress is above a critical value, the grains become entrained in the fluid flow. Prior experimental studies have shown that granular beds subjected to a subcritical fluid flow can strengthen in the same direction as the subcritical flow. In contrast, granular beds can become weaker in the direction opposite to the subcritical fluid flow. To investigate the grain-scale mechanisms that control directional strengthening and weakening, we perform discrete element method simulations of granular beds subjected to model fluid flows in two and three dimensions with varied interparticle static friction coefficients and conditioning flow speeds. In these studies, the subcritical grain motion does not cause significant bed compaction. Instead, we find that the strength of a granular bed in a particular direction is highly correlated with the fraction of surface grains that can be dislodged by a fluid force applied in that direction. Further, the anisotropic bed strength only persists over a finite timescale that is set by the Shields number. We also show that interparticle static friction is not required for bed strength anisotropy, but varying the friction affects the magnitude of the anisotropy. This research enhances the grain-scale understanding of erosion of granular beds caused by fluid flows and underscores the importance of tracking the history of the fabric of the bed surface since it couples strongly to bed strength.

Direct numerical simulation (DNS) is employed to investigate the wake dynamics of a circular cylinder positioned near a plane wall, subject to an unsteady approaching flow with an in-line oscillating component. This study is performed across three gap-to-diameter ratios ($G/D=1.0$, 0.5, and 0.2) at a Reynolds number of $R{e}_D=500$, an oscillation amplitude of $A/D=0.2$, and frequency ratios spanning $0<f_d/f_{st*}<3.0$. The resulting flow states are highly dependent on both wall proximity and forcing frequency. For lower gap ratios ($G/D=0.2$), vortex shedding is entirely suppressed at low frequencies, yielding to a single-row mode at higher frequencies. For larger gap ratios ($G/D=0.5$ and 1.0), low-frequency forcing induces a “desynchronized” regime where vortex shedding remains independent of the inflow oscillation. At elevated frequencies, the flow enters a “locked-in” regime, synchronizing with the external forcing frequency $f_d$. The specific transitional pathways within this locked-in state are dictated by the gap size: the $G/D=0.5$ configuration features a direct transition from the A mode to the S mode, whereas the $G/D=1.0$ case undergoes intermediate mode competition, transitioning sequentially through the A mode, the bimodal regime, and the S mode. The A mode exhibits an asymmetric topology with a $1/2$ subharmonic frequency response, where shedding vortices form clusters that convect and dissipate downstream. The S mode presents a quasisymmetric pattern synchronized with the oscillation. The bimodal regime serves as a transitional state where vortex shedding alternates between A and S modes. Furthermore, the results indicate that high-frequency oscillations effectively suppress the development of three-dimensional flow instabilities. Quantitative analysis using time-averaged spanwise kinetic energy spectra demonstrates that increasing the oscillation frequency effectively suppresses the three-dimensional instabilities in the near wake.

In Dynamic Closure Theory, the coarse-grained energy dissipation rate ${\varepsilon }_r$ is equipped with a two-state coherence closure fraction $C\left(\varepsilon \right)\in (0,1)$—the fraction that is cascade committed (forward flux) at dissipation level $\varepsilon$—and its complementary open branch decays as a power law, $1-C\left(\varepsilon \right)\sim {(\varepsilon /{\varepsilon }_0)}^{-\lambda }$, with a singled-out, untuned default $\lambda =1$. Crucially, $C$ is neither the dissipation density $p\left(\varepsilon \right)$ nor a deterministic function of $\varepsilon$: we give it an operational meaning as the dissipation-conditioned backscatter fraction, $1-C\left(\varepsilon \right)=P({\mathrm{\Pi }}_r<0\mid {\varepsilon }_r=\varepsilon )$, with ${\mathrm{\Pi }}_r$ the signed filtered interscale energy flux, so that by construction, the one-point $\varepsilon$-statistics carry no information about it. This observable is measured directly on three Johns Hopkins Turbulence Database forced isotropic fields: the ${8192}^3$ simulation at ${\mathrm{Re}}_{\lambda }\approx 1300$ (five snapshots, six disjoint ${512}^3$ subvolumes, $k_{max}\eta \approx 1.9$), its finely resolved ${\mathrm{Re}}_{\lambda }=613$ snapshot ($k_{max}\eta =5.3$), and the $32{768}^3$ simulation at ${\mathrm{Re}}_{\lambda }\approx 2556$ ($k_{max}\eta \approx 2.8$), using Gaussian filter widths $\mathrm{\Delta }/\eta \approx 4–100$, together with a hierarchical (subvolume and spatial-block) bootstrap, a phase-randomized surrogate control, and controlled resolution-degradation experiments. All three datasets show the same directional association: the backscatter fraction falls monotonically with ${\varepsilon }_r$, from $\approx \frac{1}{2}$ (random flux sign) in quiescent regions to a few percent in the most intensely dissipating ones, and at fixed Reynolds number, the conditional curve is scale invariant—written in absolute units of ${\varepsilon }_r$, it collapses across filter widths, which is the sense the term carries throughout. The functional form, however, is not the one written down: the data display a plateau near $\frac{1}{2}$ below $\langle \varepsilon \rangle$ that the logistic closure (which plateaus at unity) cannot capture, so the global fit rules out $\lambda =1$ at every scale, and at high Reynolds number, the curvature of the knee and the steepness of the tail emerge as separate exponents. The estimated upper-branch steepness varies across the three Reynolds-number datasets and is not consistent with a Reynolds-number-independent unit exponent. Across 20 disjoint subvolumes at ${\mathrm{Re}}_{\lambda }=613$ (each binned relative to its own mean), the upper branch (${\varepsilon }_r\ge \langle \varepsilon \rangle$) has fitted steepness 0.48–0.74 (95% intervals excluding unity), with a wide between-region scatter; the four subvolumes of the full analysis lie in the steep half of that distribution, so once between-region variance is included, the default $\lambda =1$ is not supported. Before the physical size of the analysis units is matched, the ${\mathrm{Re}}_{\lambda }=613$ upper branch appears two or more times steeper than at the higher Reynolds numbers, where the fitted $\lambda$ is 0.20–0.32 at ${\mathrm{Re}}_{\lambda }\approx 2556$, excluding both unity and zero, and $-0.02–0.18$ at ${\mathrm{Re}}_{\lambda }\approx 1300$. The steepness is, however, sensitive to the size of the region used to normalize ${\varepsilon }_r$ (the analyzed cores span about $200\eta , 390\eta$, and $560\eta$ in the three fields); after size matching, the remaining difference is only $10–20\%$ and is not resolved by the present intervals, so the apparent Reynolds-number dependence of the tail steepness is dominated by analysis-unit size. Two resolution controls—degrading the resolved ${\mathrm{Re}}_{\lambda }=613$ field to the resolution of the ${\mathrm{Re}}_{\lambda }\approx 1300$ run, and low-passing the ${\mathrm{Re}}_{\lambda }\approx 2556$ field on its own grid—shift the upper-branch steepness by less than 0.1 when compared on the same subvolumes, so neither the missing dissipative-range modes nor the coarse-grid gradient error tested here reproduces the flatter branch at high Reynolds number on the subvolumes as analyzed. The steepness of the intense-dissipation branch thus depends on the scale of the region against which the dissipation is normalized and, beyond that, its Reynolds-number or inertial-range-width dependence is, at most, $10–20\%$ within the sampled range, while the shoulder of the curve stays approximately unchanged across the three fields. A $Q$-criterion coherence proxy computed on the same fields shows no tail whatsoever (slope $\approx 0$) since a positive-definite gradient invariant does not encode the flux sign and is not an interchangeable proxy for flux-direction occupancy. We explain why one-point dissipation statistics cannot test the closure, and we state precisely what the measurement does and does not establish.

Wind tunnel experiments were conducted on six multiscale rough walls immersed in deep turbulent boundary layers. The multiscale features are introduced by fractal iteration of two reference cuboid models of aspect ratio 1 (Standard) and 3 (Tall), systematically adding smaller model length scales. Through these iterations, the frontal and plan solidities, ${\lambda }_f$ and ${\lambda }_p$, respectively, and the roughness mean height are kept the same. Spatially averaged three-dimensional velocity and pressure fields allow for a direct and indirect estimation of the friction velocity and aerodynamic parameters. Results show that the boundary layer height and roughness aerodynamic parameters experience significant changes across fractal iterations, despite the fixed solidities and average height of the arrays; this is in disagreement with many morphometric methods. Mean velocity profiles collapse well when plotted in velocity defect form, supporting Townsend's similarity hypothesis. However, a departure from outer layer similarity for the streamwise fluctuating velocity is observed. Changes are recorded in both roughness and inertial sublayer depths with the additional model length scales. Dispersive stresses are found to be contributing up to 20% of the overall momentum flux for the reference cuboid models; however, these contributions decrease with each fractal iteration down to below 10%. Finally, quadrant and spectral analysis show that the additional length scales act to dampen turbulent mixing below the canopy layer, while they enhance it above the canopy height.

We present a strategy for direct numerical simulations of nucleate boiling on three-dimensional unstructured grids, enabling the representation of complex geometries. The method relies on a ghost fluid formulation with two temperature fields and two velocity fields, one for each phase. Sharp discontinuities are enforced at the interface for pressure, temperature, and velocity. Numerical methods with third-order spatial accuracy are used for the temperature field and for interface-curvature evaluation. We also introduce a new level-set reinitialization procedure accounting for the presence of a contact line, which allows the accurate imposition of the contact angle for bubbles and droplets on a surface. The phase-change solver is first assessed against reference cases of rising bubbles and film boiling. Three-dimensional simulations of nucleate boiling of water are then performed and compared with the results of Huber et al. [Int. J. Heat Mass Transf. 113, 662 (2017) and Long et al. [J. Comput. Phys. 513, 113159 (2024)], obtained on structured two-dimensional axisymmetric grids. Finally, nucleate boiling on a complex geometry, namely an artificial nucleation site, is simulated. The results compare favorably with the experiments of Siedel [Exp. Therm. Fluid Sci. 32, 1504 (2008)]. Overall, this strategy enables three-dimensional direct numerical simulations of nucleate boiling on unstructured grids and complex geometries.

This paper is aimed at understanding and predicting the statistics of coalescing particles in turbulent flow, and their relation to the physics of the simpler and better-understood system of collisionless ghost particles in turbulence. We perform three distinct families of direct numerical simulations under identical flow conditions. The first family involves ghost particles that do not mutually interact; the second contains particles (monomers) that coalesce upon collision, with lost monomers replenished by injection of new particles at random positions to maintain a constant monomer density (CR); and the third family involves particles with the same collision-coalescence kinetics but without replenishment of lost monomers. All analyses are monodisperse; only the statistics and physics of the monomers are considered. Across the range of Stokes number studied ($\mathrm{St}=0.01–3.0$), we find that the collision kernel ($K$) and the values of radial distribution function [RDF, $g\left(r\right)$] near particle contact ($r\approx d$, where $d$ is the particle diameter) of the ghost-particle (GP) system are higher than those of the coalescing systems. We show that a velocity-filtered version of the RDF $g_{\mathrm{G}}^{(-)}\left(r\right)$ is a reasonable proxy for estimating the CR system's RDF value at contact $g_{\mathrm{CR}}\left(d\right)$. We systematically report on the residual discrepancy between $g_{\mathrm{G}}^{(-)}\left(d\right)$ and $g_{\text{CR}}\left(d\right)$ as well as between the corresponding kernels $K_{\text{G}}$ and $K_{\text{CR}}$. We show that the discrepancy is due to correlation among successive collisions within the GP system by showing that a history-filtered version of the GP collision kernel, which is void of such correlations, reproduces the corresponding kernel of the coalescing systems. We introduce a collision-correlation time, ${\tau }_{\text{cc}}$, to quantify how long current collisions influence future collision events and show that it is finite and has a narrow range across the range of $\mathrm{St}$ studied. We show that when particles with past collisions within a period comparable to ${\tau }_{\text{cc}}$ are filtered out in the GP system, the resulting history-filtered RDF ${\tilde{g}}_{\text{G}}(r;\tau \approx {\tau }_{\text{cc}})$ reproduces the RDF of the coalescing systems ($g_{\text{CR}}\left(r\right)$ and $g_{\text{CN}}\left(r\right)$). Finally, we show that among all history-bearing collisions in the GP system, the fraction of repeated collisions involving identical particles is an exponentially decaying function of $\mathrm{St}$. This work unifies the coalescing and GP systems under a single framework and affords an elegant interpretation of the former as a history filtered equivalent of the latter.

We investigate the trajectories of small particles in a three-dimensional free flow around and through a porous screen. Our model is based on potential flows at different scales coupled to a viscous particle dynamics. We compute numerically the proportion of the particles deviated around a square screen, and thus the section of interception for which particles are impacting or crossing the screen; the three-dimensionality of the flow leads to a particular geometry of this section of interception that cannot be deduced from a two-dimensional approach. We further take into account the effect of finite-size particles and viscous effects at the pore scale. These results are applied to determine the collection efficiency, i.e., the number of particles that impact the fibers. We show in particular how the optimal screen solidity is sensitive to the local Reynolds number based on the pore length scale. Finally, we propose a formula of the collection efficiency of porous screens valid in a wide range of the parameters and discuss the aerodynamic contribution to the collection.

We employ combined smoothed particle hydrodynamics and molecular dynamics simulations to systematically investigate the evolution of microjetting from tin targets with sinusoidal surface perturbations under twice-shock loading. Results show that the initial scaled amplitude $k_0{h}_0$ critically controls the bubble morphology after the first shock and the subsequent coupling behavior of second-shock ejecta: a smooth single bubble forms at $k_0{h}_0=0.25$; for $0.50\le k_0{h}_0\le 1.20$, a single microspike splits the bubble into two sub-bubbles, leading to two second jets that first diverge and later merge under strong mode coupling; for $1.40\le k_0{h}_0\le 2.00$, two microspikes divide the interface into three sub-bubbles, producing three independently growing microjetting with markedly weaker coupling. To predict the second-shock ejecta mass across this wide $k_0{h}_0$ range, an approach is introduced that treats the postshock bubble interface as a multimode perturbation. Through Fourier modal decomposition and incorporation of intermode coupling, a predictive model is developed that shows good agreement with simulation results. The effectiveness of the computational approach is further validated against existing second-shock microjetting experiments. This work elucidates the physical linkage between initial surface conditions, shock-induced interface morphology, and jet-coupling dynamics, providing a quantitative framework for the prediction of ejection under multiple-shock loading.

Cavitation can be exploited to create local and precise destruction of biological tissues. In applications such as histotripsy, cavitation bubbles are created by focused megahertz and megapascal pressure waves, grow, and inertially collapse, thereby inducing stresses that mechanically homogenize tissue. As the size of the damaged region has been shown to scale with the maximum size of the bubbles, we seek to determine the dependence of the maximum size of a bubble on the initial conditions, namely properties of the ultrasound and the tissue. For this purpose, the energy budget analysis for an oscillating bubble is extended to include viscoelasticity while also accounting for compressibility. By relating the bubble energy at maximum size to the initial energy, an analytical expression for the maximum radius can be obtained. The maximum radius is shown to decrease with increasing shear modulus of the medium, and to increase with increasing amplitude and wavelength of the wave. The proposed analytical model accurately predicts the maximum radius as long as the growth is explosive, i.e., the bubble velocity reaches an asymptotic constant value. Conditions for explosive growth are derived in terms of the pressure waveform amplitude and frequency.

Shock waves have historically been modeled assuming ideal-gas behavior, yielding universal jump relations and a viscous structure that depends almost exclusively on the Mach number. However, many fluids of physical interest operate in dense or supercritical regimes where ideal-gas thermodynamics no longer applies. Therefore, this work investigates the structure of shock waves at supercritical-fluid conditions without invoking ideal-gas assumptions. Starting from the Navier-Stokes equations coupled with a real-fluid thermophysical model, the governing relations are derived for the spatial evolution of thermodynamic variables across the shock. Using supercritical carbon dioxide as a representative fluid, it is shown that nonideal thermodynamics strongly modifies the pressure-density jump relations and internal profiles of shocks, producing substantial deviations from classical Rankine-Hugoniot predictions. Interestingly, despite these pronounced thermodynamic effects, the physical thickness of the shock remains primarily controlled by the upstream Mach number. Taking both the structural and thermodynamic observations together, these results ultimately reveal how nonideal fluid behavior fundamentally alters compressive discontinuities, thereby establishing the strict limits of classical shock theory in dense and supercritical fluids.

Quench experiments where the flow passes from a fully turbulent state to a laminar state by an abrupt decrease in the flow Reynolds number ($\text{Re}$) have been extensively studied in the literature to quantify the turbulent-laminar transition process in wall-bounded flows. Measurements have been classically made using rheoscopic fluid visualizations, which make turbulent coherent structures easily identifiable, allowing for quantification of the evolution of a turbulent fraction—the percentage of a given observation window where turbulence is deemed active by the presence of coherent structures, such as streamwise vortices called rolls, and modulations of the streamwise velocity fluctuations called streaks. Decay characteristic times of these structures have therefore been extensively measured. However, owing to the nature of visualization based techniques, only a single decay time is typically extracted, whereas measurements of the velocity field can reveal distinct decay times associated with different velocity or kinetic energy components. As a result, the physical meaning of the decay time inferred from visualization alone is not straightforward. The goal of the present paper is to perform such a comparison quantitatively, using particle image velocimetry (PIV) measurements and rheoscopic fluid visualizations in the same setup: a plane Couette-Poiseuille experiment. We observe via PIV different characteristic times of decay for streamwise (streaks) and spanwise (rolls) velocity fluctuations. We show that the characteristic time of decay of the turbulent fraction observed by visualization is close to the decay of the streaks.

The bubbly-nonbubbly transition region characterizes the edge of cavitation and involves active dynamics of dispersed vapor bubbles. Experimental data in this region are essential for developing numerical models that bridge dispersed bubbles and macroscopic vapor cavities, with the population balance model being a representative example. In this study we perform point-by-point measurements of cavitating flow around a twisted hydrofoil using an optical probe system, obtaining local void fraction, bubble size, and velocity. The void-fraction and streamwise bubble-velocity profiles both exhibit self-similar behavior. The void-fraction profile is well fitted by a Gaussian function, which closely resembles the Gaussian form of the vertical gradient of the streamwise bubble velocity. This gradient arises from the error-function distribution of the bubble-velocity profile. The similarity between these two Gaussian profiles suggests a connection between high vapor concentration and strong shear in the bubble-velocity field. Nevertheless, their characteristic widths reveal distinct spreading scales. This scale separation gives an effective bubble-momentum-based Schmidt number of ${\mathrm{Sc}}_{\mathrm{eff}}\approx 1.63$, indicating that bubble-velocity recovery extends over a broader region than vapor-concentration spreading. The bubble size distributions exhibit distinct characteristics depending on the local flow conditions: the inner region features high void fraction and strong shear, and the outer region shows low void fraction and weak shear. The typical $-4/3$ scaling for small bubbles is observed in both regions, whereas the $-10/3$ power law scaling is observed only in the outer region, where the flow approaches isotropic turbulence. Within the strong shear region, large bubbles are abundant and thus the size distribution exhibits a flatter slope. This finding suggests that the choice of a presumed distribution function in population-balance modeling should be region-dependent within the cavitation zone. The experimental data further demonstrate that bubble velocities are strongly size-dependent and vary across different flow regions, offering a potential basis for modeling the slip velocity of bubbles that do not strictly follow the liquid phase. Overall, this study provides experimental constraints for closing the population balance model, which governs the transport and diffusion of dispersed vapor bubbles in cavitating flows.

In the present work we investigate the multiscale dynamics of enstrophy in homogeneous isotropic turbulence by exploiting the two-point formalism provided by the Kármán-Howarth-Monin-Hill approach extended to the enstrophy governing equation. The study is conducted on direct numerical simulations with a Taylor-based Reynolds number in the range of $140\lesssim {\mathrm{Re}}_{\lambda }\lesssim 400$. The two-point enstrophy budget at scales $r>10\eta$ appears to be entirely determined by production via vortex stretching, which balances enstrophy destruction, and to be dominated by the diffusive transport at smaller scales, thus preventing the emergence of a range dominated by the inertial transport of enstrophy. Consequently, the second-order structure function of vorticity only shows a viscous scaling $\sim r^2$ at very small scales, followed by a constant behavior at larger scales. This plateau, together with the statistical isotropy, implies that the structure function is equally distributed among its longitudinal and transverse contributions at such scales. The decomposition in longitudinal and transverse contributions also highlights a dual nature of the inertial enstrophy flux. In particular, enstrophy appears to be transferred across scales through a nontrivial combination of direct and reverse interscale transfer. It is shown that the dual nature of this transfer is strictly related to the vortex stretching mechanism, which, in addition to producing enstrophy through vorticity amplification, also transfers longitudinal vorticity towards larger scales (by stretching the vortical elements) and transverse vorticity towards smaller scales (by contracting these vortical elements in the radial direction). The sum of these two contributions results in an overall transfer of enstrophy from large towards small scales. We propose the use of the pressure transport term as a proxy to obtain some information on the dynamics of relevant events of inertial energy and enstrophy transport. The new findings highlight the relevance of inertial compression events in longitudinal energy transport. At the same time, a good correlation between transverse energy transport events and the radial contraction of vortical elements due to vortex stretching mechanisms is also found.

Most existing studies of the effects of rotation on the Rayleigh–Taylor instability consider stratified layers. However, in nature and in the laboratory, localized sources of unstable buoyancy are also commonly observed. We report on laboratory-scale direct numerical simulations of isolated buoyancy sources interacting with a preexisting pycnocline. We demonstrate that for a readily accessible parameter range, long-lived, coherent structures that share characteristics of plumes and eddies spontaneously develop. These plumes interact with the pycnocline, but never fully penetrate it. In contrast, strong rotation leads to a surface-trapped response appearing as sheets in laboratory experiments. Finally, we demonstrate that temperature-induced changes in buoyancy behave differently than salt-induced changes, due to the lower diffusivity in the latter that reduces plume dilution and increases penetration into and through the stratified layer. This distinction is important because the majority of experimental literature involves salt-induced stratification.

Data-driven closure modeling for transport partial differential equations requires training data that are accurate, affordable, diverse, and directly tailored to the target closure fields. We develop the Generalized Forcing Method (GFM), a data-generation framework for training linear transport closure models. GFM generates such data by running simulations with a zero initial condition and an extra body force that is constructed compatibly with the reduced dynamics. This framework leads to implicit GFM, which prescribes resolved trajectories, and explicit GFM (eGFM), which constructs a basis of admissible forcings. We apply eGFM to linear transport closure problems involving parallel and spatially inhomogeneous flows, flows with random coefficients, and passive-scalar transport in homogeneous isotropic turbulence. The results show that eGFM can identify accurate and stable reduced models when the reduced variables and model form are consistent with the underlying closure relation.

We characterize a surface instability that occurs when a yield stress fluid is subject to uniaxial compression. We describe experiments using a prestretched elastic sheet which, upon controlled release, uniaxially compresses a layer of petroleum jelly creating surface undulations perpendicular to the direction of compression. We measure experimentally the amplitude and wavelength of the surface undulations, and their dependence on the thickness of the material and the Bingham number, the nondimensional yield stress. Results show that the amplitude of the undulations depends linearly on the Bingham number, 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 experiments scraping a thin layer of a viscoplastic fluid. Characterizing these surface undulations is integral for geophysical applications in understanding topographic features in accretionary wedges.

In view of the growing interest in the pioneering experiments of J. S. Tsakadze and S. E. Tsakadze [J. Low Temp. Phys. 39, 649 (1980)] relating the behavior of nonstationary rotating superfluid helium to neutron stars' glitches (sudden jumps in angular velocity followed by a relaxation), we revisit these experiments. We comment on the main results through the lens of our current understanding of superfluid hydrodynamics. In particular, we consider the possibility of a state of quantum turbulence following the initial spin-up of the vessel, and analyze the rotational decay observed by Tsakadze and Tsakadze in analogy with recent studies of quantum turbulence in the two-fluid regime of helium II.