We find that the speed of the axial migration of a capsule can be accelerated by flow pulsation at a specific frequency. Our numerical results form a fundamental basis for further studies on cellular flow mechanics, since pulsatile flows are physiologically relevant in human circulation, potentially affecting the dynamics of deformable particles and red blood cells, and can also be potentially exploited in cell focusing techniques.

Understanding and regulating the Leidenfrost temperature is of great importance for practical applications such as spray cooling. In this study, nonnegligible cooling effects of Leidenfrost drops on surfaces with small thermal diffusivities are observed with an IR camera. The influence of thermal properties and surface roughness on the Leidenfrost temperature is investigated experimentally and explained by a simple theoretical model.

We study the complex orientation dynamics of a settling body with concave and convex surfaces using particle-resolving direct numerical simulations, and find multiple bifurcations of the dynamics as the Reynolds number of the body is varied from O(1) to O(20). The dynamics of irregularly shaped solid bodies in this range of Reynolds numbers is relevant in such varied phenomena as the tumbling of ice particles in atmospheric clouds, and of plant and animal microorganisms in the ocean.

In this work, we show that the collective motion of driven particles above a fluid-fluid interface is controlled by the viscosity ratio between the two fluids. Combining theory, large scale simulations, and mean-field models, we show that this parameter does not only control the speed and direction of motion of the particles, but also the growth of a fingering instability and the emergence of motile clusters that can entrain other particles or pump fluid. These results suggest new approaches to control the macroscopic response of driven suspensions, but also offer new strategies for guided particle transport, fluid pumping, and mixing.

The potential approach has been considered a reliable common practice for computing wave directional spectra from instrumental measurements in the field and in laboratories. These spectra present the main source of oceanographic data for advanced maritime design and are extensively used in calibration and validation of oceanic meteorological forecast models. However, according to its definition, the potential approach neglects ambient shearing currents. Our work, along with a proof that disregarding these currents may lead to significant deviations in the evaluated wave directional spectra, suggest a novel methodology based on rotational theory to correct these inherent flaws.

The work presents a previously unknown transition between two forms of chaotic organization in active matter as defect-free and defect-laden active turbulence, and identifies universal statistical features of active turbulence in polar active matter.

The propulsive performance of an aquatic vehicle propelled by a flapping foil can be greatly enhanced when the foil is elastically mounted to springs and dampers working under resonant conditions. The present results show that the propulsive efficiency can be further increased by selecting appropriate stiffness of the foil associated to that resonance.

The shape and deformability of red blood cells (RBCs) change with age, increasing their likelihood of becoming trapped in the splenic interendothelial slits (IESs) and being removed from the circulation. Existing evidence suggests that cell size and viscoelasticity vary with age, which impedes understanding of the mechanical retention of aged RBCs in the spleen. We present a computational study of the flow dynamics, shape changes, and mechanical retention of age-associated RBCs in the splenic IES. The results show that increasing RBC membrane viscosity slows the flow process early in aging, while increased sphericity due to cell size reduction eventually leads to RBC retention.

Could you have survived Boston’s Great Molasses Flood of 1919? Probably not, according to Purcell’s Scallop Theorem, swimming in molasses is nearly impossible. The theorem tells us that, at zero Reynolds number, no swimmer who deforms their body in a time symmetric fashion is able to move. In this paper we consider such a swimmer, but we introduce symmetry breaking by considering an inhomogeneous environment of variable viscosity. We prove that the swimmer is still unable to move, showing that the Scallop Theorem is stronger than we thought.

The propagation and wall scattering of localized two-dimensional thermodynamic disturbances in a rarefied gas are studied, showing fundamental differences between the continuum and free-molecular limits. At highly rarefied conditions, a late-time peculiar attraction force is detected between the gas and the boundary.

We report the mechanism and modeling for the formation of cavity-like structures on a planar interface subjected to a perturbed shock wave. The cavity is distinguished from bubbles and spikes formed in the standard Richtmyer-Meshkov instability. The cavity width is determined by the Mach-stem height when the shock accelerates the interface.

We observe propagation and interference of two coherent thermal waves in the highly turbulent core of convective Rayleigh-Bénard flow of cryogenic helium gas generated by top and bottom boundaries with harmonically oscillating temperatures. When the same amplitude of modulation is applied to both plates at slightly different frequencies, the resulting temperature interference pattern in the turbulent core shows a periodic increase and decrease in amplitude. If the observed time record of thermal wave interference were converted to corresponding pressure oscillations and shifted to higher frequencies, one would hear the acoustic beats often used, e.g., in tuning stringed musical instruments.

The Burgers-Rayleigh-Bénard system is a one-dimensional toy model of a convective cell that is amenable to a thorough theoretical analysis. Despite its simplicity, it has a lot in common with realistic Rayleigh-Bénard thermal convection: a supercritical pitchfork bifurcation at the onset of convection; the organization of the flow into boundary layers and a bulk zone; Nusselt and Reynolds numbers scaling laws obeying the ultimate regime in the asymptotic high Rayleigh limit. However, differences with realistic natural convection are also present: the persistence of flow stationarity, the absence of turbulence and the possibility of thermally insulating shock states.

By subjecting a ferrofluid emulsion to an external magnetic field, the flow at the droplet scale is altered, resulting in significant changes in the emulsion’s rheological behavior. We present the first study of the rheological response of a dilute ferrofluid emulsion under extensional flow. Our findings reveal that at the microscopic level, the droplets experience a field-induced internal torque that leads to a complex non-symmetric stress tensor. We identify the remarkable presence of shear stresses within a material experiencing a pure extensional flow.

Bubbles suspended in a sheared fluid can form chains parallel to the flow. To understand when and why these intriguing chains form, we study experimentally and numerically two-bubble interactions. Our study shows that the interaction is governed by the deformation and relative position of the bubbles, and is reduced by fluid elasticity.

When an impacting drop is given a sufficiently high velocity, it is expected to splash shortly after impact, due to take-off of the spreading lamella. In this study, we show that the splashing threshold is decreased when the surface is cooled below the liquid freezing point, due to the interplay between solidification and spreading which affects the apparent dynamic wettability of the surface. This effect is such that below -60°C, the splashing behavior becomes identical to that on superhydrophobic surfaces.

We created a new publicly available data set of high-fidelity numerical simulations for two obstacles in tandem invested by a fully resolved turbulent boundary layer. In the three configurations of skimming flow, wake interference, and isolated roughness, we identify the regions of intense turbulent fluctuations, production and transport of turbulent kinetic energy, and examine the anisotropy-invariants map.

The absolute instability of jets and wake flows have often been studied when either unconfined in the flow normal direction, or when confined by solid, impermeable walls. In this study, we confine the flow by compliant, or flexible, walls that are only able to move in the flow normal direction. By using a local instability analysis, we find that wall compliance can have a significant effect on existing flow instabilities and can even induce new absolutely unstable modes.

We study a turbulent jet in a uniform external flow stream. This flow configuration is of interest in aeroacoustics and aeronautics, as it mimics the effect of forward flight in real aircraft. We perform a thorough characterization of the effect of the flight stream, combining experimental and numerical data, signal processing, and linear modeling. We show that the flight stream reduces fluctuation energy in a broad region of the frequency-wavenumber space and stabilizes flow structures underpinned by modal and non-modal instability mechanisms. Streaky structures, associated with helical azimuthal wavenumbers and very slow timescales, are the most strongly affected by the flight stream.

Nanofilms are susceptible to surfactant contamination. This work investigates surfactant effects on nanowave dynamics on a bounded film with a finite depth, using both molecular dynamics simulations and analytical theories. It is shown that the presence of surfactants leads to a slower decay of surface mode temporal correlations, which, however, cannot be predicted by only considering the decreased surface tension due to the added surfactants. Based on the Boussinesq–Scriven model for surface viscosity, a linear stability analysis of Stokes flow for films with arbitrary depth is conducted. The obtained dispersion relation considering surface viscosity can justify the simulation results.

We investigate the stability of a thin film flow on a compliant substrate with broken time-reversal symmetry. The broken time-reversal symmetry induces odd viscosity in the liquid. Using coupled equations encompassing the film and substrate, we ascertain that inclusion of a compliant substrate induces instability within the system. Nonetheless, the introduction of odd viscosity notably enhances its stability. Furthermore, under conditions of weak nonlinearity, the incorporation of odd viscosity can effectively mitigate the occurrence of chaotic behavior arising from compliant substrates. Numerical simulations confirm that odd viscosity has a substantial impact on substrate deflection.

Based on Darcy’s law, two-fluid flow is dependent on a relative permeability function of saturation only that is process/path dependent with an underlying dependency on pore structure and wettability. We develop an Artificial Neural Network (ANN) that relies on fundamental geometrical measures to determine relative permeability. The developed ANN is based on a prescribed set of state variables based on physical insights that predicts the relative permeability of 4,500 unseen pore-scale geometrical states with $R^2$ = 0.98.

In this work, we show experimentally a method for reducing impact force. We report experimental obvservations of different cavity regimes and impact modes formed by a gas jet under different conditions and examine their impact force. The jet cavity of interest can achieve 90% reduction of impact force. The mechanism of impact force reduction is considered to be the result of the reduction of the added mass and the coupling of the jet momentum.

We present experimental observations of the spatial distribution of large inertial particles suspended in a turbulent swirling flow at high Reynolds number. The plastic particles are heavier than the working fluid so that their dynamics results from a competition between gravitation and turbulent agitation. Two suspension regimes are observed. At low rotation rate particles are confined near the bottom for any size or density. At high rotation rate, particles are loosely confined: small particles are nearly homogeneously distributed and large ones found near the top as if gravity was reversed. We discuss these observations using a minimal random walk model accounting for particle inertia.

A filter membrane may be frequently used during its lifetime, with filtration and drying processes occurring in the porous medium for several cycles. As a consequence, the filter performance ultimately deteriorates after several cycles. In this work, we develop a mathematical model that provides insights to the overall porous medium evolution over cycles of filtration and drying processes and predicts the timeline to discard the filter based on its optimum performance.

Roughening particles decreases the shear-induced diffusivity of concentrated suspensions. This experimental observation defies expectations based upon results from low concentration, where roughness causes larger, irreversible displacements and a consequent increase in diffusivity. Simulations at high concentrations reveal that rougher particles organize into layers along the flow direction, which lowers the collision frequency and diffusivity relative to smoother particles.

We investigate the unbounded inertial flow of a Newtonian fluid past a fixed regular tetrahedron, the Platonic polyhedron with the lowest sphericity. A new symmetric double-hairpin vortex shedding regime is observed and analyzed in the edge facing the flow (TE) case due to its unique angular position. The dominant frequency of this double-hairpin vortex shedding is twice that of the single-hairpin vortex shedding in the flow past a sphere, a cube or a tetrahedron at other angular positions. Our study highlights the significant effects of the particle shape and particle angular position on the flow dynamics and regime transitions.

We compute the unbounded flow past a fixed Platonic polyhedron at a Reynolds number ranging from 100 to 500 and investigate the effects of angularity and angular position of the Platonic polyhedron on the flow field. In the steady regime, the flow symmetry in the particle wake is determined by the particle front surface. Our study reveals the intrinsic relationship between angular edges and hairpin vortex generation, clarifies the effects of particle angularity on the flow regime transitions, and highlights the crucial role of the flow recirculation region in determining the vortex shedding mode.

We numerically investigated the rheology of concentrated suspensions of fibers by including hydrodynamic, noncontact conservative, and inter-fiber contact interactions. In this work, we provided the first quantitative explanation of the origin of yield stress, shear thinning, and normal stress differences in fiber suspensions by focusing on contact and noncontact contributions.

A versatile and highly customizable space-time-frequency pattern recognition network is extended to complex flow analysis. Through synthetic and realistic examples, the potential of using kernel-mode decomposition as a modal identification tool is proven. Being amenable and interpretable, there is an expansive possibility for adopting a user-defined physics-representing mode bank and extracting relevant dynamics using this regression network.

Buoyant convective plumes have a strong impact on the behavior and spread of wildfires. This experimental study investigates the role of turbulent coherent structures rising from canopy mixing layers on the trajectory and behavior of buoyant plumes. The turbulent structures give rise to unsteady behavior of the buoyant plume that is observed as strong vertical oscillatory motions. This oscillatory motion is found to fluctuate at the dominant frequency of the canopy-induced instability. The analysis also looks at mixing and transport rates through the impacted plume.

The predictions of resolvent analysis with and without eddy viscosity are evaluated for a friction Reynolds number of 550. The low-rank behavior of the standard resolvent identifies energetic regions of the flow whereas the eddy resolvent is low rank when the resulting projection of the leading eddy resolvent mode onto the leading mode from spectral proper orthogonal decomposition is maximum. The energy dissipation and transport that are introduced by the eddy viscosity are examined to explain the success or failure of predictions from the eddy resolvent.

Turbulent channel flow with underlying rough and smooth surfaces present abreast is investigated using direct numerical simulations. Secondary roll cells, exhibiting a strong updraft in the smooth half of the channel, are observed. The roll cells have significantly attenuated the bulk flow and turbulence in the smooth half of the channel.

This survey addresses two primary concerns within the domain of machine learning for wall modeling in large-eddy simulations: the adherence to physical laws and the perceived opacity of the prediction process, often termed as the “black box” problem. Three machine-learning wall models are tested against the law of the wall and their predictions are analyzed and explained. This research marks an initial step towards gaining wider acceptance of machine-learning wall models in the scientific computing community.

A generalized quasilinear (GQL) approximation is applied to homogeneous shear turbulence following previous work on the quasilinear (QL) model. For the GQL approximation, the velocity fluctuations are decomposed into low- and high-wavenumber groups, the former of which is solved by considering the full nonlinear equations whereas the latter is obtained from the linearized equations around the former.

The wall modeling that accounts for the nonequilibrium effects of boundary layers, such as the pressure-gradient and convective terms, is desirable for predicting separated and reattached flows at high Reynolds numbers. However, typical nonequilibrium wall models solve partial differential equations on wall-layer meshes, which can complicate utilization of the model for complex geometries. In this study, we propose a nonequilibrium wall model that solves ordinary differential equations in the wall-normal direction, eliminating the need for wall-layer meshes.

A temporally developing three-dimensional turbulent boundary layer (3DTBL) caused by applying a transverse pressure to an initially stationary turbulent channel flow is investigated by analyzing direct numerical simulation results at an initial Reynolds number Re${}_{\tau }$ between 180 and 930. It is shown that such a nonequilibrium 3DTBL can be interpreted as a turbulent-turbulent transition that is characterized by the development of a laminar boundary layer in a turbulent environment followed by transition.

This paper deals with discerning outer-layer universality in the mean velocity profiles of turbulent boundary layers. For that purpose we derive an objective criterion which generalizes the classical wall scaling, as well as the Rotta-Clauser and Zagarola-Smits scaling. Outer-layer universality in the present scaling clearly emerges at displacement thickness Reynolds number exceeding about 3000. We propose universal velocity distributions based on simple patched logarithmic/parabolic fitting functions, which are accurate to within less than 1%

Neighboring convection rolls are experimentally found to be stable in either counter-rotating or co-rotating states, with stochastic switching between the two states. These states are quantitatively described by analytic solutions of a low-dimensional stochastic model in which turbulent thermal diffusion between the rolls provides the forcing that stabilizes both states, using the same turbulent thermal diffusivity parameter that characterizes heat transport. This is presented within a generalized model framework that can account for the geometry of the convection cells and other forces.

Flow configurations that display a significant degree of order, characterized by a well-defined number of low-speed streaks along the walls and quasi-streamwise vortical structures, hold valuable insights into the dynamics of turbulent pipes. Using stereoscopic particle image velocimetry, we investigate transitions between these organizational states (OS) in a large pipe rig facility. We find that the OS modes and their transitions exhibit a recurring statistical self-similar pattern, up to time horizons related to the flow outer scales. We also demonstrate that this distinctive statistical behavior can be accurately described by a lower-level Markovian model of the OS mode transitions.

The interplay between viscoelasticity and inertia in dilute polymer solutions at high deformation rates can result in inertio-elastic instabilities. We show how fluid elasticity has a nonmonotonic effect on jet stability depending on magnitude, creating two distinct regimes. The nonlinear evolution of these instabilities generates a state of elasto-inertial turbulence (EIT) with different spatiotemporal features than Newtonian turbulence. We use high-speed digital schlieren imaging and dynamic mode decomposition to quantify EIT and identify two modes of instability which can lead to a transition to turbulence at a lower Reynolds number with flow-aligned structures in the turbulent region.

Scaling arguments for steady turbulence in a rotating fluid are set up. These are used to show that it displays dual scaling regimes: a rotation-dominated regime for the largest scales and a Kolmogorov scaling regime for intermediate scales. For decaying rotating turbulence, it is speculated that the energy spectrum, while carrying a decaying amplitude, contains a large-scale rotation-dominated regime that increases in size over time, and an intermediate-scale Kolmogorov scaling regime that shrinks in time.

We develop an efficient streaming framework to reconstruct turbulent velocity fluctuations from limited measurements with the potential for real-time implementation. This is achieved by introducing techniques to efficiently compute linear estimators and recursively update streaming measurements in the frequency domain. For flow in a minimal channel, the reconstructions capture dominant flow features and have computation times consistent with requirements for real-time experiments.

In this work, we experimentally demonstrate the utility of unsteady potential flow models in developing closed-loop control strategies for mitigating the lift transients on a wing experiencing large-amplitude transverse gusts. The developed closed-loop controller mitigates lift for gusts of various strengths and directions and for wings with pre- and post-stall angles of attack. Time-resolved force and flow field measurements are used to discover the salient flow physics during these encounters and illustrate how closed-loop actuation mitigates their lift transients.

As a daily life experience, when stirring a cup of tea or coffee, one may observe vortices induced by the starting and stopping motion of a spoon. This paper investigated the annular vortices induced by the unsteady motion of a disk in water from experimental measurements and numerical simulations. The circulation and core radius of the vortices are shown to scale with power laws of the disk diameter, stroke length, and time in agreement with inviscid theoretical predictions.

Periodic surface waves develop in a falling two-layer liquid film as the result of the long-wave gravitational instability at a finite Reynolds number. Nonlinear wave dynamics is dominated by an inelastic collision and merging of waves that travel at different speeds. The merging process becomes arrested when growing waves have reached a nearly homoclinic solution representing a solitary wave. A quintessentially two-layer dynamical regime is found, which corresponds to a ruptured second layer. In this regime, the first layer adjacent to the solid wall acts as a conveyor belt, transporting isolated rolling droplets made of the second fluid downstream.