A model for stationary cohesive granular flows on inclines is proposed based on the assumption of an ideal Coulomb material supplemented with a linear $\mu \left(I\right)$ dependence. For assessing the reliability of the model, discrete numerical simulations are carried out in two dimensions, varying both contact adhesion and flow slope on a large range of values. Focusing both on the surface velocity and on the thickness of the surface plug flow, a systematic comparison between discrete simulations data and analytical predictions derived from the model allows for testing the latter. In particular, the ideal Coulomb material hypothesis, including the independence of friction and cohesion, is corroborated, together with the applicability of the linear approximation of the $\mu \left(I\right)$ flow law. Confronting model and discrete simulations discloses a linear relation between macroscopic cohesion stress and local contact adhesion, in agreement with Rumpf's prediction. The bottom velocity is shown to obey a Navier-Robin condition with a slipping length poorly dependent on adhesion properties. In addition, the analysis of the surface topography shows how increasing contact adhesion leads to a more craggy surface line. In the context of a simplified flow configuration, the present work brings clarification on the intricate roles of contact adhesion and flow dynamics on the cohesive behavior of granular flows. It allows for testing the relatively simple theoretical framework and hypothesis used to derive analytical solutions, thus showing that many salient features of cohesive flows are captured by basic ingredients. Based on relatively accessible quantities in laboratory measurements, it may also shed an interesting light on experimental work.

We outline a workflow for examining statistically transient fluid flows, which eludes most standard modal analysis techniques. As a challenging example, we investigate the forcing-induced transient between statistically stationary and cyclostationary states and discuss the underlying flow physics. The transient dynamics of a turbulent supersonic twin-rectangular jet flow, forced symmetrically at a Strouhal number of 0.9, are studied using synchronized large-eddy simulations. Under plasma-actuated control, the statistically stationary jet evolves towards a cyclostationary state over a transient phase. Forcing-induced perturbations of the natural jet are extracted using synchronized simulations of the natural and forced jets. A database is collected that captures an ensemble of realizations of the perturbations within the initial transient, which is characterized by fast energy growth. The initial pulse of the actuators produces large, impulsive perturbations to the flow field. The mean flow deformation transient reveals that shock cells are deflected by the forcing towards then away from the nozzle, resulting in a subtle but permanent contraction of shock spacings. Despite this, time–frequency analysis shows that jet screech persists throughout the transient. Streamwise wavelet-based filtering of the time-varying mean flow deformation effectively isolates small-scale shock contraction and shear-layer modal instability waves from large-scale but low-amplitude perturbations. The optimal eigenvector from space–time proper orthogonal decomposition is an antisymmetric—or flapping—mode. Applying an analogous filter to the optimal mode, we track the evolution of a large-scale, transient, and dispersive wave-packet structure and observe the coupling of widely disparate scales enabled by spatial inhomogeneity and temporal transience.

As the closest living relatives of animals, choanoflagellates exhibit remarkable diversity. Even their microvilli collar, used to filter and capture food, varies significantly among species. This diversity suggests either strong environmental adaptation or an insensitivity to the collar geometry. Previous hydrodynamic studies have suggested that the pressure change across the collar is similar across species. In this study, we show that hydrodynamics imposes additional geometric constraints on the choanoflagellate collar. We create a simplified, reduced-order model that neglects finite collar length to investigate how the microvillus radius and the gap between microvilli influence the flow. Comparing with biological data reveals significant variation in the pressure drop between species. Additionally, a ridge emerges in the microvilli radius-gap phase space, along which both effective flux and power dissipation are maximized. Notably, several species cluster near the flux ridge but lie away from the power dissipation ridge. These observations suggest that choanoflagellate collars do not necessarily share a similar pressure drop. Instead, their geometry is influenced by the competing demands of maximizing flux and minimizing power costs. The broad variation observed among species is made possible by these ridgelike structures.

Collective locomotion of swimming and flying animals is fascinating in terms of individual-level fluid mechanics and group-level structure and dynamics. Here we bridge and relate these scales through a model of formation flight that views the collective as a material whose properties arise from the flow-mediated interactions among its members. We build on and revise an aerodynamic model describing how flapping flyers produce vortex wakes and how they are forced by others' wakes. While simplistic, the model faithfully reproduces a series of physical experiments carried out over the past decade on pairwise interactions of flapping foils. By studying longer in-line arrays, we show that the group behaves as a soft “crystal” with regularly spaced member “atoms” whose positioning is, however, susceptible to deformations and dynamical instabilities. Poking or wiggling a member excites longitudinal waves (flow-mediated phonons, or “flonons”) that pass down the group while growing in amplitude, and indeed the internal excitation from flapping is sufficient to trigger instabilities. Linear analysis of the model explains the aerodynamic origin of the lattice spacing, the springiness of the “bonds” between flyers, and the tendency for disturbances to resonantly amplify. Other properties, such as the timescales for instability growth and wave propagation, seem to involve the full nonlinear behavior. These findings suggest intriguing analogies with physical materials that could be generally useful for understanding and analyzing animal groups. Several properties displayed by our system seem particularly relevant to biological collectives, namely, group cohesion and organization, sensitive detection of and response to perturbations, and transmission of information through traveling waves.

The interest in spontaneous charging and electrokinetic transport at immiscible liquid-liquid interfaces originated from early studies on biological membrane functions and elementary charge measurements. In recent years, with the rapid advancement of micro- and nanofabrication and measurement technologies, the regulation of multiphase flow and transport at micro- and nanoscales based on electrokinetic transport at liquid-liquid interfaces has become feasible. Among these, slippery liquid-infused surfaces (SLIS) have garnered significant attention, while several practical issues and potential mechanisms of electrokinetic transport regulation with SLIS remain unclear. This paper employs direct numerical simulations based on the diffuse interface model to explore the impact of groove oil depletion effects on electroosmosis and the influence of two-sided ion partition effects on streaming potential at SLIS. The study reveals the electroosmotic velocity reversal effect induced by geometric configurations, as well as the two-sided streaming potential effect induced by mixing and partitioning-induced charging. These findings provide foundational insights for the rational design of quantitative electrokinetic regulation strategies for multiphase flow at micro- and nanoscales.

Numerical simulations and proper orthogonal decomposition are used to identify the fundamental mechanism of magnetic field reversals in the von Kármán sodium experiment with ferromagnetic impellers close to contrarotation. Two numerical solvers are used and agree well on the resulting mechanism. The reversal dynamics rely on the interaction of four dominant axisymmetric modes: the magnetic dipole mode $D$ and quadrupole mode $Q$, as well as the zonal velocity mode $V$ and the $R_{\pi }$-symmetric mode $U$. This hierarchy leads to a reduced-order $DQVU$ Galerkin model derived from the magnetohydrodynamic equations. The model improves upon previous three-variable versions by adding a velocity variable $U$ and related interactions, which are shown to be essential for reproducing the main features of magnetic field dynamics.

Near-critical fluids are known to exhibit anomalous behavior in their thermophysical properties, such as diverging compressibility and vanishing surface tension on approaching the critical point. These fluids undergo high-density stratification very close to the critical point. The Navier-Stokes equations coupled to a phase field model are solved with a van der Waals equation of state. The pressure is expanded around the critical point and shows a cubic variation with the reduced density. Density-stratified base states are computed numerically. The supercritical case is treated through the linear stability analysis. The spectrum, the eigenmodes/eigenfunctions, and sound waves are then computed and plotted for temperatures close to the critical point (CP). Very close to CP, the speed of sound significantly changes as a function of the vertical direction, and the frequency bands strongly differ from the situation of an ideal gas. The same process of linearization has been performed for the subcritical state. Here, the Rayleigh-Taylor instability (RTI) is considered. Finally, numerical simulations confirm the propagation of acoustic waves in the supercritical case. In the subcritical case, spinodal decomposition is found for a randomly distributed initial density close to the critical one. Time series in the Rayleigh-Taylor unstable domain show interesting features. The time evolution of the total kinetic energy exhibits three distinct phases: high-frequency sound waves appear at early times, then RTI manifests, and finally, at longer times, surface waves appear with a much smaller frequency.

This study investigates scalar (smoke) dispersion in the turbulent near-wake of an Ahmed body. The effects of rear slant angles ($\varphi ={15}^{\circ }, {25}^{\circ }, {35}^{\circ }$, and ${40}^{\circ }$) on the flow structures, scalar concentration, and dispersion are investigated. Smoke was essentially a passive scalar, with a typical Stokes number of $3.62\times {10}^{-5}$. Smoke particle concentration fields were measured (at ${\text{Re}}_l=1.9\times {10}^5$) using a nonintrusive technique, quantitative smoke visualization, based on laser Mie-scattering. Smoke dispersion was quantified using nondimensional dispersion ($\mathcal{D}$) and dispersion length scales (${\mathcal{L}}_y, {\mathcal{L}}_z$). The predominant turbulent wake structures were obtained by particle image velocimetry. The connection between wake structures and the dispersion phenomenon was explored. It is found that the existence, strength, and size of wake structures (spanwise and trailing vortices) depend on $\varphi$, and these structures play a key role in the dispersion phenomenon. Two distinct flow regimes and their implications on dispersion are explored. For $\varphi ={15}^{\circ },{25}^{\circ }$, there is no separation over the rear slant, and the trailing vortices are generated, which dominate and determine the dispersion scales beyond the recirculation region ($x^{*}>1$). The entrainment of flow due to trailing vortices increases the lateral dispersion (${\mathcal{L}}_z$) of smoke rapidly. For $\varphi ={35}^{\circ }, {40}^{\circ }$, a massive separation on the rear slant is observed, and the trailing vortices do not exist. The spanwise vortices dominate the dispersion phenomenon in the recirculation region ($x^{*}⩽1$) and determine the vertical dispersion (${\mathcal{L}}_y$) of the smoke. Based on the observed smoke dispersion for both the flow regimes, the characteristics of the near-wake are proposed.

This study shows that the turbulent velocities most strongly correlated with outer-scaled ($\delta$-scaled) wall-pressure fluctuations beneath a zero-pressure-gradient boundary layer reside within the logarithmic region. Even though contributions from the wake region are present, they are found to be statistically less dominant than those from the logarithmic region. The findings are based on bespoke measurements using an array of 63 microphones spanning $5\delta$ in the streamwise direction (where $\delta$ is the boundary layer thickness), which synchronously captures space-time $p_w$ data alongside streamwise velocity fluctuations $\left(u\right)$ from a single hotwire probe at the array's downstream end. The array is designed to spatially filter $p_w$ signals to uncover outer-scale contributions, by accurately resolving the large-scale portion of the frequency-wave-number $p_w$ spectrum while avoiding aliasing of small-scale energy. This design, and its effectiveness in anti-aliasing, is validated against previously published low-Reynolds-number simulation datasets of turbulent boundary layer flow. Present experiments span a friction Reynolds number range of $1400\lesssim {\mathrm{Re}}_{\tau }\lesssim 5200$, over which the large-scale energy in the boundary layer grows significantly. This growth is reflected in both the frequency-wave-number $p_w$ spectrum and the space-time $p_w$ correlations, both of which show scaling trends reflective of the large-scale pressure field convecting at an outer-scaled velocity of $0.75U_{\infty }$, where $U_{\infty }$ is the freestream velocity. The linear coherence between streamwise velocity and large-scale $p_w$ is directly quantified through space-time $p_w\text{–}u$ correlations, which show increasing magnitudes across the inner region with rising ${\mathrm{Re}}_{\tau }$. At the top of the logarithmic region, the correlation contours resemble outer-scaled coherent structures akin to large- and very-large-scale motions. A clear Reynolds number trend is also evident in the average convection velocities inferred from $p_w\text{–}u$ correlations, which increasingly deviate from the local mean velocity towards the outer-scaled convection velocity across the inner region. These insights provide a critical foundation for leveraging wall-pressure fluctuations in modeling and control of high-${\mathrm{Re}}_{\tau }$ boundary layers, where large-scale motions increasingly dominate the turbulence dynamics.

Supersaturation in clouds was modeled as a passive scalar with the phase-relaxation time under mean uniform vertical gradient. The theoretical prediction that there exist two $-5/3$ power-law ranges in the supersaturation variance spectrum was examined using direct numerical simulation (DNS) and large-eddy simulation (LES). Although DNS at a Taylor microscale Reynolds number of 946 suggested the existence of two $-5/3$ ranges, the results were not conclusive because of the limited extent of the inertial-convective range. To achieve a wider inertial-convective range, LES was employed, and a modified eddy diffusivity model for a passive scalar without a phase-relaxation term was developed to yield a $k^{-5/3}$-spectrum across a broad range of wave numbers. LES results obtained by varying the phase-relaxation time demonstrated the coexistence of two $k^{-5/3}$ spectral ranges with different amplitudes. As the phase-relaxation time decreased, the transition between the two ranges shifted toward higher wave numbers, which is consistent with theoretical predictions. Furthermore, the probability density function (PDF) of the band-pass-filtered supersaturation indicated that the PDF tails at small scales became increasingly elongated depending on the ratio of the turbulence timescale to the phase-relaxation time.

The accurate characterization of the three-dimensional deformations of slender fibers and thin sheets in flow is a key experimental challenge in the study of particle-laden flows. We propose a high-resolution, single-camera method to visualize nonintrusively the shape of a transparent crumpled sheet, as it translates, rotates, and deforms. We perform periodic scans of the crumpled shape by illuminating it with a sequence of stacked light sheets at a rate much faster than its deformation and image the scattered light signal in a plane near orthogonal to the plane of lighting. Processing of the data using a pinhole camera model yields a noisy spatiotemporal dataset of the strongly deformed time-evolving surface of the sheet, which we reconstruct in three dimensions using a neural autoencoder. We validate the robustness of the shape reconstruction algorithm to noise using synthetic data sets, and demonstrate the accurate reconstruction of laboratory sedimentation experiments with elastic disks. We find that the inclusion of isometricity-enforcing penalties into the cost function of the autoencoder enables us to robustly reconstruct highly folded shapes, where different regions of the sheet overlap.

Despite their widespread use, mesoscopic models for nonideal fluids have rarely been systematically validated against microscopic simulations. In this work, molecular dynamics (MD) simulations of confined fluids are mapped onto a mesoscopic framework, enabling direct comparison with lattice Boltzmann (LBM) formulations. By analyzing the moments of the distribution function, we identify a force formulation that consistently reproduces the microscopic statistics and macroscopic force balance. The results show that a hybrid formulation combining pseudopotential and free-energy approaches provides the most consistent description. These findings establish a direct link between microscopic particle dynamics and mesoscopic modeling, offering practical guidance for the development and selection of LBM models for nonideal and multiphase flows.

Mosquitoes exhibit unique motion patterns that have attracted growing interest from researchers aiming to understand their underlying aerodynamic mechanisms. However, previous studies have primarily focused on hovering flight. In the present study, we measured for the first time the kinematic and morphological data of mosquitoes Aedes togoi in forward flight using three orthogonally aligned and synchronized high-speed cameras, and numerically computed the forces acting on the wing and the surrounding flow. The results show that mosquito wing kinematics in forward flight exhibit two distinctive features that differ from those observed in most other insects. One feature is that the mosquito's stroke plane remains nearly horizontal; the other is that its wing pitching motion is remarkably asymmetric between the downstroke and upstroke. More specifically, the pitch-down acceleration at the beginning of the stroke and the pitch-up amplitude in the middle of the stroke are highly asymmetric. By establishing the kinematic-fluid-aerodynamic link, two novel thrust-generation mechanisms in mosquito forward flight are revealed: asymmetric pitch-down acceleration mechanism and asymmetric pitch-up amplitude mechanism. The mass-specific power of mosquitoes in forward flight is found to be comparable to that of hovering, and approximately 27% lower than that of the smaller mosquitoes Aedes aegypti, potentially offering larger mosquitoes an energetic advantage in seeking and feeding on hosts. These findings not only deepen our understanding of mosquito flight mechanics but also provide new insights for the kinematic design of micro flapping-wing vehicles in forward flight.

Large aperture telescopes are pivotal for exploring the universe, yet even with state-of-the-art manufacturing and launch technology, their size is limited to several meters. As we aim to build larger telescopes—extending tens of meters—designs in which the main mirror is based on liquid deployment in space are emerging as promising candidates. However, alongside their enormous potential advantages, liquid-based surfaces present new challenges in material science, mechanics, and fluid dynamics. One of the fundamental questions is whether it is possible for such surfaces to maintain their precise optical shape over long durations, and in particular under the forces induced by the telescope's accelerations. In this paper, we present a model and a closed-form analytical solution for the non-self-adjoint problem of the dynamics of a thin liquid film pinned within a finite circular domain. We use the 50-m Fluidic Telescope concept as the case study and examine the liquid dynamics of the telescope under both slewing actuation and relaxation regimes, elucidating the role of geometrical parameters and liquid properties. The solutions reveal a maneuvering “budget” wherein the degradation of the mirror surface is directly linked to the choice of maneuvers and their sequence. By simulating 10 years of typical operation, we show that, while the maximal deformation might reach several microns, the spatial distribution and propagation rate of the deformation allow the telescope to maintain its optical functionality for years, with at least a substantial portion of the aperture remaining suitable for astronomical observations. The model provides valuable insights and guidelines into the performance of liquid-film space telescopes, marking a crucial step toward realizing the potential of this innovative concept.

In many natural and industrial environments, small droplets are irradiated with visible and infrared light, affecting how they evaporate. Without external heating, droplets usually shrink faster as they become smaller. The simplest description of this is the $D^2$ law, where the shrinking rate $dR/dt$ is proportional to $t^{-1/2}$ or $R^{-1}$. However, modeling this becomes more complicated when irradiation-driven volumetric heating is considered, and only few studies have investigated the evaporation behavior of levitating droplets under strong irradiation. Here we show the effects of weak and strong irradiation on single, optically levitated, micrometer-sized water droplets. We find that the droplets shrink following distinct accelerating and decelerating regimes, that we characterize through two power laws. Under weak irradiation, the shrinking rate accelerates, similar to the $D^2$ law. Under strong irradiation, however, the shrinking decelerates following the power law $dR/dt\sim R$, i.e., inverse of the $D^2$ law. Between these two regimes, at radii of 3–5 $\mu \mathrm{m}$, a turnover occurs from decelerating shrinking back to accelerating shrinking. Our findings shed new light on droplet evaporation under weak and strong irradiative heating, such as mists under sunshine or fuel droplets irradiated by flames in rocket engines or internal combustion.

The interaction between low-order roughness and large-scale topography is experimentally investigated using high-resolution particle image velocimetry in a refractive-index-matched channel at two bulk Reynolds numbers differing by an order of magnitude, $\text{Re}=4.0\times {10}^3$ and $4.0\times {10}^4$. A two-dimensional sinusoidal roughness is superimposed on a two-dimensional wavy wall to isolate the effects of multiscale surface modulation on near-wall turbulence. The added roughness reorganizes the flow by inducing larger coherent structures near the surface. Two-point correlations and integral length scales confirm enhanced near-wall coherence, while quadrant-hole analysis reveals a marked suppression of ejection events. This suppression weakens the local turbulence production term $-\langle u^{\prime }{v}^{\prime }\rangle dU/dy$, leading to attenuation of the separated shear layer that develops downstream from the topographic crest within the adverse-pressure-gradient region. The resulting flow exhibits up to 25% lower peak turbulent kinetic energy and Reynolds shear stress relative to the smooth-wavy configuration. Even modest low-order roughness can therefore reorganize the coupling between near-wall and outer-layer motions, a mechanism that persists across an order of magnitude in Reynolds number and offers valuable insight for modeling transport and drag over natural and engineered surfaces.

In this study, we propose closure models for the subgrid turbulent heat flux and drift temperature components. These terms arise as unresolved contributions in the filtered two-fluid model (TFM) for heat transport equations of turbulent gas-particle flows. In the context of moderately dense turbulent fluidization, the drift temperature appears as a correction to the filtered resolved interphase heat transfer component and expressed by a covariance of gas phase temperature and solid concentration. The derived models are mainly motivated by our findings showing that the most commonly used subgrid heat flux models associated linearly with the gradient resolved temperature are invalid in this kind of multiphase turbulence. They reveal a clear misalignment with the actual gas and solid phase turbulent heat flux vectors. Thus, the proposed models are principally based on Taylor development of the subfilter terms, identified as Clark's gradient model [Clark et al., J. Fluid Mech. 91, 1 (1979)]. They differ only in the dynamic formation of model coefficient using the test filters. We find that the conventional dynamic Smagorinsky-type model leads to large errors in predicting the subfilter turbulent heat flux and solid concentration variance, and our dynamic procedure based on a Taylor series expansion of test-filtered resolved terms (Leonard term) [Balarac et al., Phys. Fluids 20, 035114 (2008)] substantially improves the predictive accuracy. This last approach was originally assessed based on the optimal estimator concept, i.e., conditional expectations and statistical minimization of modeling errors; thus we use this terminology in referring to all of our models adopting this dynamic procedure. Two dynamic drift temperature closures are accordingly presented based on optimal estimator dynamic procedure coefficient. They basically approximate the covariance of gas phase temperature and solid concentration by the product of its variances or the gradient Clark's expansion. All of our models are examined in a framework of an a priori study, where filtering fine-grid TFM simulation data of laboratory-scale wall-bounded turbulent fluidization of Geldart B group particles is employed. The presented models provide more accurate and robust tools for heat transfer TFM simulations of industrial-scale fluidized bed reactors.

Under unstable stratification, the Reynolds shear stress $-\overline{u^{\prime }{w}^{\prime }}$ in the atmospheric surface layer (ASL) reflects a multiscale superposition in which large-scale motions can contribute appreciably, motivating a scale-resolved quantification beyond standard similarity scaling. Using multiheight SLTEST velocity measurements under unstable conditions, we obtain an additive scale decomposition of $-\overline{u^{\prime }{w}^{\prime }}$ from momentum-flux cospectra and quantify cumulative contributions with an ogive $C\left(k\right)$, where $k$ is the streamwise wave number. Interpreting $C\left(k\right)$ as an empirical cumulative distribution in $lnk$, we parametrize it with a lognormal cumulative distribution function, yielding a compact two-parameter description $(\mu ,\sigma )$ of the dominant contributing scale and the breadth of scale allocation. Two indicators isolate the key mechanisms: the cumulative contribution from large scales and a counterstress fraction ${\eta }_{\mathrm{neg},1}$ that measures cospectral contributions opposite in sign to the net stress. Under weakly unstable conditions, normalized premultiplied cospectra collapse when plotted against the nondimensional wave number $kz$, where $z$ is the measurement height, with a peak at $kz\approx 0.5$, and the large-scale contribution increases with relative height, indicating enhanced large-scale influence aloft. For weak-cancellation records (${\eta }_{\mathrm{neg},1}<0.1$), ogives are accurately captured by the lognormal form, and $(\mu ,\sigma )$ reproduce the observed cospectral morphology across heights. With increasing instability, the dominant contribution shifts to larger wavelengths, the large-scale flux fraction increases, and scale-domain cancellation strengthens, with larger magnitudes and stronger sensitivity at higher relative heights. A variable-interval time-averaging-based burst analysis provides a consistent time-domain counterpart: events become sparser while the stress magnitude concentrates within fewer intervals, and a time-domain cancellation metric increases concurrently. These results establish a low-dimensional height-stability parametrization of momentum-flux scale allocation and show that unstable ASL transport is organized by coupled large-scale enhancement, strengthened cancellation, and increased intermittency.