Fire ants are a notorious invasive insect found all over the world. Like flocks of birds and schools of fish, fire ants can be found in large aggregations. Here, we show that fire ants avoid each other on the water surface. As a result, small rafts quickly separate. To aggregate together in larger numbers, they rely on surface tension forces, the same ones that bring Cheerios together in a bowl of milk, to overcome their exploratory instincts.

Viscous flows through compliant conduits apply forces at the solid-liquid interface, leading to deformation of the cross-section, which affects the flow rate–pressure drop relation. Conventionally, calculating this relation requires solving the coupled elastohydrodynamic problem of flow and deformation. Instead, we employ reciprocal theorems for Stokes flow and linear elasticity to derive a closed-form expression for the flow rate–pressure drop relation in deformable channels, using a domain perturbation expansion that only requires the fluid flow solution and the elastic deformation due to the fluid stress distribution in an undeformed channel.

A study of the light-induced evacuation of a suspension of microwimmers (the micro-alga Chlamydomonas reinhardtii) through a bottleneck-shaped constriction is presented. We show that a transition from a jammed phase to an uninterrupted phase occurs when varying either the door size or the swimming velocity. The survival function of exit times is then found to evolve from a power law to an exponential trend. Moreover, we found that the evacuation time increases with increasing velocity, a behavior reminiscent of the “faster-is-slower” paradox present in crowd dynamics systems.

A new framework to quantitatively describe the energy exchange in high-speed turbulent flows is presented. The routes of energy exchange in hypersonic boundary-layer flows are highlighted and quantified, with special attention paid to effects of wall cooling. We expect that the present study can help the development of physics-informed models for compressible turbulence in the class of RANS (Reynolds-averaged Navier-Stokes) and LES (large-eddy-simulation), which currently heavily hinge on variable-density extrapolation of their incompressible counterparts.

How do motor proteins and other biological nanomachines form aggregates in cell membranes and show coordinated activity, essential for so many living processes ? Membrane Fluid mechanics provides a natural explanation. In this work, we show how force dipole motors induce special flows in a spherical fluid membrane and form aggregates on the sphere due to their mutual hydrodynamic interactions. Bulk confinement and membrane curvature effects compete with each other and display novel regimes of coordinated motion.

Instabilities of an active fluid jet (Center) are investigated. Left: For an active fluid of pullers, the jet breaks up into droplets in a varicose manner reminiscent of a Newtonian fluid. Right: For pushers, on the other hand, the jet buckles and undergoes a waving instability.

A model of the bronchioles lined by the airway surface liquid is employed to investigate the Plateau-Rayleigh instability that can lead to the occlusion of the airways. We demonstrate that increasing the surfactant concentration and their strength, the airway closure gets slowed down and the wall stresses are reduced up to 20%. We further predict the generation at which airway closure will occur in human lungs depending on the liquid lining thickness and the initial surfactant concentration.

It has long been unknown why the hawkmoth’s maximum forward flying speed is much lower than the theoretical prediction based on its body mass. Our computational fluid dynamics study revealed that as a hawkmoth’s flight speed increases, its wings inevitably generate a significant amount of negative lift during the upstroke, which renders the hawkmoth incapable of sustaining steady forward flight. A similar trend has also been observed for other insects, including fruit flies and bumblebees. However, birds and other flying vertebrates are able to overcome this limitation by flexing their wings during the upstroke.

Using ideas drawn from the analysis of a single nonreacting scalar in the context of Taylor dispersion, we derive formulas for closures for the mean state of a turbulent binary reacting system. These nonlinear expressions, derived in the context of a simplified sandbox problem, explicitly show the effects of reaction parameters on unresolved transport. When translated to a turbulent flow setup, the resulting models provide improved predictions of mean scalar quantities and showcase some limits to purely local modeling.

When a fluid flows past a blob of hydrogel, how much of it goes through the gel and how much goes around? The boundary condition on a fluid-gel interface is a subtle question rooted in the mismatch between the pore size and the macroscopic flow dimension. We test four boundary conditions and find one to be superior to the rest.

Viscoelastic fluids exhibit hysteresis in response to time-dependent stimuli due to a finite relaxation time of the polymeric chains. We have investigated hysteresis in the context of viscoelastic flow instability, which has applications in industrial, geophysical, and biological processes. The topology of the polymeric stress field regulates distinct flow states in viscoelastic flows, requiring a finite time for the transformation of the polymeric stress topology and leading to hysteresis in the flow field.

The technique of semidefinite programming is used to replace direct numerical simulations and time integrations of turbulent flows with an optimization problem which is easier to solve but which returns only upper bounds to the time averages the direct numerical simulation would have produced. This idea is applied to rotating Rayleigh-Bénard convection at infinite Prandtl number. The obtained bounds are not sharp but they reproduce the qualitative behavior of the results known from direct simulations and experiments.

The classical Fourier law, which typically provides a very accurate description of heat transport, is, nonetheless, only an approximation, since it allows for propagation of information at infinite speed. Corrections to the Fourier law, which typically are very small, are referred to as the Maxwell-Cattaneo (M-C) effect. Here we study the onset of rotating convection, a problem of geophysical and astrophysical relevance, incorporating the M-C effect. We show how, at high rotation rates (which are readily achieved in rotating astrophysical bodies), the M-C effect (although nominally small) can come into play, causing fundamental changes to the nature and scale of the preferred mode of convection.

We assess experimentally the existence of a superlinear scaling for the growth of the mixing region in a confined porous medium. We employ an optical method to obtain high-resolution measurements of the density fields in Hele-Shaw flows, and we perform experiments for large values of the Rayleigh-Darcy number. We can confirm that the growth of the mixing length during the convection-dominated phase follows the scaling predicted by previous two-dimensional simulations.

Direct numerical simulation of a rising skirt bubble reveals for the first time two toroidal vortices in the wake (left) as well as vorticity concentrated inside the skirt film (right).

In this study, we used a three-dimensional biconcave capsule with a residual stress field to numerically simulate the dynamics of a single red blood cell (RBC) in both confined and unconfined Poiseuille flow. We confirm that taking the stress-free configuration of the RBC to be an oblate near-sphere leads to results which are most consistent with the experiments in both shear flow and Poiseuille flow settings.

This simulation study shows that the occurrence of multilobe shapes for red blood cells is robust and does not require cytoskeleton elasticity in the model, in a marked contrast with previous claims. Multilobes are caused by membrane tension due to hydrodynamic stress. This result advances considerably our understanding of multilobe shape revelation of red blood cells under flow.

A bouncing droplet with its associated wavefield forms a hydrodynamic analog of de Broglie’s pilot-wave. Two bouncing droplets are trapped in their respective cavities and become correlated oscillators at a distance. Then a wide barrier suddenly isolates the two oscillators which transition to new and identical distributions, as a consequence of the previously established correlations. These distributions are different from what they would have been had the oscillators been isolated at the onset.

During the evaporation of a water drop on a salt substrate, the inner flows experience a reversal, their inward motion being progressively replaced by an outward one. The transition between the two flow directions proceeds via the migration of a stagnation line, where the radial velocity is zero, from the periphery to the center of the drop. This reversal is a consequence of the change of flow regime inside the drop, from a Marangoni flow to a coffee-stain capillary flow, due to the progressive homogenization of the concentration.

We analyze the splash transition of millimetric drops of liquids of varying viscosities impacting normally over different types of superhydrophobic substrates in which the amplitude of random surface asperities is varied from hundreds of nanometers to tens of microns. Our experiments indicate that the amount of air entrapped between the substrate and the drop increases when the value of the relative roughness is increased, thus reducing friction. We are able to predict the spreading-splashing transition using a previous model once the effect of the surface asperities is taken into account in the expression of the shear stress at the wall.

Polymer injection in trip-wire induced laminar-to-turbulent transition regions in flat-plate boundary layers is studied experimentally. It is shown that the transition process which is initiated by the amplification of perturbations in a separated shear layer downstream of the trip wire is accelerated by the injection of polymer leading to an earlier breakdown to turbulence. However, significant levels of polymer induced drag reduction are noted in the mid-to-late transitioning regions highlighting a critical shear stress based onset criteria for activation of drag reduction.

A Mach 1.55 shock and subsequent reshock interacting with predominantly single-mode and multi-mode interfaces between nitrogen and carbon dioxide is studied through three-dimensional adaptive mesh refinement simulations using the FLASH code. The simulations are directly compared with the PLIF/PIV experimental measurements to validate the computational code. Development of instability after first shock and chaotic turbulent mixing after reshock are discussed for the entire shock tube domain.

Supersonic flow over a blunt fin generates a large, three-dimensional, separated region which oscillates even in the absence of upstream disturbances. Previous studies of the flow control of shock-wave/boundary-layer interactions showed that upstream forcing of the separation motion is possible for weaker interactions, where the separation unsteadiness is directly driven by incoming turbulence. In this work, we demonstrate that time-periodic forcing of the incoming boundary layer can modulate the separation unsteadiness of the strong interaction induced by a blunt fin. The results suggest opportunities for flow control to mitigate the adverse thermomechanical loads of separation unsteadiness.

It is well known that Goncharov’s theory based on Layzer’s approach for the Richtmyer-Meshkov instability provides good predictions for bubbles, but could give qualitatively incorrect predictions for spikes. This leads to the belief that Layzer’s approach is not applicable to spikes. We show that, by incorporating the distinctive behaviors of spikes and bubbles properly, Layzer’s approach is actually applicable to both spikes and bubbles, and can provide accurate predictions for both spikes and bubbles, for systems with arbitrary density ratios, and for the entire evolution process of unstable fingers.

The coalescence of a droplet on a curved liquid-air interface owing to the presence of an inclined bottom wall is investigated experimentally. In addition to the partial and complete coalescence, two new outcomes, namely migrating partial coalescence and multidrop pinch-off, are also observed. Regime maps demarcating different coalescence outcomes are shown.

We investigate the performance of the vertical-averaging approximation for solute concentration in an evaporating binary thin film under three common evaporation models: constant, one sided, and diffusion limited. While this approximation overperforms its formal assumptions, there are significant discrepancies in predicted film-height and solute-concentration profile outside of the region of validity. We elucidate the mechanisms giving rise to these discrepancies and derive scaling relations describing the region of validity of the approximation under each evaporation model.

The influence of the surrounding gas on cavity formation behind a sphere impacting a body of liquid is studied. Contrary to the classical picture, it is found that, in a range of parameters, cavity formation can be suppressed by lowering the density of the ambient gas. This is attributed to the gas slowing sealing of the thin crown sheet behind the sphere.

Recently it was observed that when interfacial tension is lower by 2-3 orders of magnitude than normal liquids, pinch-off dynamics is dominated by bulk diffusion, with a scaling behavior unlike that of Stokes flow. We use the Cahn-Hilliard–Navier-Stokes model to investigate the pinch-off dynamics of a liquid thread surrounded by a viscous external fluid. A length scale is introduced to characterize the competition between diffusion and viscous flow. The crossover between Stokes and diffusion-dominated regimes is examined numerically and analytically for the change in scaling behaviors of similarity solutions as the pinching neck of the interface accesses the characteristic length scale.

The rupture of thin free films of Newtonian fluids are analyzed theoretically and computationally when the two free surfaces are covered with insoluble surfactant and surface rheological (viscous) effects are important. For highly viscous fluids in the Stokes limit, we show that the dynamics near the rupture singularity exhibits self-similarity of the second kind and the dominant balance involves van der Waals pressure and bulk and surface viscous stresses. For moderately viscous fluids inertia is also important and self-similarity is of the first kind. Closed-form expressions for the sheet’s thinning rate are found and can be used to determine values of surface viscosity from experiments.

The dissolution of vertically aligned pairs of droplets - consisting of either the same or different long-chained alcohols - is experimentally and numerically investigated. The dissolution process creates both density gradients in the bulk water and surface tension differences along the interfaces, leading to the competition of buoyancy forces and Marangoni forces. While the configuration is simple, the interaction results in nonmonotonic dissolution behavior and complex dynamics, which however can be understood and modeled.

Inspired by the ability of lotus leaves to repel water, super hydrophobic coatings are engineered and used to reduce drag. The prospect of drag reduction depends on the surface geometry and flow conditions. In this work, we suggest a new mechanism incorporating the resistance induced by the flow inside the lubricating layer which could be responsible for the decrease of drag-reduction performance in the laminar flow regime. A simple model combining Stokes’ flow theory around a sphere and a numerically correlated slip velocity, which takes into account the tortuosity and the porosity of the surface texture, is presented and validated against laboratory experiments.

This work is motivated by the fact that the thinner-anguilliform and thicker-carangiform types of fish adapt to each other’s kinematics under certain conditions. Our two-dimensional hydrofoil model-based numerical simulations demonstrate that the adaptive kinematics increases thrust force for the thinner-fish while it increases propulsive efficiency for the thicker-fish; as found in nature. Further, hydrodynamic reasons are discussed for such adaptive behavior. This bioinspired and biomimetics study may assist need-based efficient design of underwater-vehicles.

We derive the general solution of the unsteady Stokes equations in unbounded viscous fluid exterior to a sphere. The solution is an expansion in vector spherical harmonics and it uses the decomposition originally introduced by Lamb. The coefficients of the expansion are uniquely determined by the arbitrary velocity distribution at the spherical boundary. The proposed solution can be applied, for example, to construct transient flow around a spherical “squirmer”, to study particle-particle and particle-wall interactions in oscillatory flows and in other applications.

The turbulence anisotropy in moderately dense turbulent fluidization is characterized with the aid of the barycentric anisotropy map (BAM). Therein, the phase-filtered anisotropy Reynolds stress tensor is considered to classify the possible states of turbulence into: 1C (one-dimensional), 2C (two-dimensional isotropic) and 3C (three-dimensional isotropic). The turbulence trajectories on the gas phase, as highlighted in the side Figure, have revealed a prevalent 2-D ellipse-like turbulence in the dilute regions, changing to 1-D turbulence in transition areas, and tend towards 3-D turbulence in elongated pancake-like at the interface and inside the clusters.

Bubbly turbulent flows involve complex interactions between the water and the bubbles in which bubble deformation is known to play an important role. We experimentally investigate an idealization, namely, the interaction of a deforming air bubble and a rigid buoyant particle (non-deforming bubble) with a single vortex ring. On the bubble/particle dynamics side, after capture by the low-pressure vortex core, the bubble undergoes large azimuthal expansion within the ring, unlike the particle. This difference in the shape of the bubble compared to the particle, within the ring, leads to distinct differences in the ring time evolution, which brings us insight into the role of deformability in these interactions.

This work reports that the frequency spectra can exhibit different behavior at the same flow condition, which constitutes the hysteresis phenomenon. Results are analyzed from the perspective of modern dynamical systems. The boundary layer is viewed as the forced oscillator, with the acoustic waves being the forcing mechanism. Both the bifurcation theory and the Poincaré section are introduced to investigate the dynamic process when freestream velocity changes.

We investigate the performance of active swimmers in a strongly heterogeneous two-dimensional weakly turbulent flow. We demonstrate that there are three regimes of preferential transport for rod-like swimmers as the swimmers’ intrinsic speed increases. We further reveal that the three regimes are due to the relative strengths of three different effects: the intrinsic speed of the swimmers, the reorientation ability of the shear layer at the interface of two flow regions, and the attracting Lagrangian Coherent Structures of the flow field.

How different is three-dimensional turbulence from two-dimensional turbulence? Is the only difference the absence of vortex stretching in 2D? In that case 3D turbulence without vortex stretching should behave as 2D turbulence. We show that this is not the case.

Shock-boundary layer interactions in a supersonic turbine cascade are investigated using a wall-resolved large eddy simulation. Results reveal that near-wall streaky structures (red color) drive the motion of the suction side separation bubble (blue color), which in turn promotes oscillations of the reattachment shock (black color) and shear layer flapping.

Wind tunnel measurements over commercially available foams have been performed to characterize the boundary-layer flow over realistic permeable rough surfaces. The mean flow indicates that the flow over these surfaces can be considered similar to flow over rough-walls in the log region. The near-wall region appears to be under the combined influence of roughness and permeability. An analysis framework based on the data is presented where future studies can examine the combined influence of roughness and permeability.

A wing submerged in the wake of an upstream airfoil experiences periodic forces. The frequency and the amplitude of the airfoil oscillations produce vortex streets of varying wavelength and circulation. The amplitude of the lift coefficient of the wing depends on a single wake parameter, which is the Strouhal number based on the amplitude of the upstream airfoil.

Sudden large-scale geophysical flows such as cliff collapses, rockfalls, or massive landslides are known to be tsunamigenic and constitute a significant hazard for coastal populations and infrastructures. A key challenge is to be able to predict a priori the amplitude of the tsunami wave that would be engendered by a given cliff collapse. In this study, we introduce a comprehensive model of shallow water waves generated by the subaerial collapse of a granular column. The proposed model is able to capture the role of the geometry of the collapse as well as the water depth and is successfully compared to a large dataset of experiments.

The Garrett and Munk spectrum describes the turbulence of the ocean interior due to internal gravity waves. We use the unique large scale Coriolis facility and force turbulence by large scale waves to reproduce the main features of wave turbulence in a stratified fluid in the laboratory. A transition from weakly nonlinear waves to strongly nonlinear stratified turbulence was indeed observed when increasing the buoyancy Reynolds number.

The transition from a purely inertial to a purely viscous liquid slumping induced by a dam-break flow over a horizontal surface is highlighted and characterized as a specific adaptive regime, during which the viscous regime progressively invades the entire current.