As random close packing is approached in a granular medium of frictionless spheres, the pressure, shear stress, and second normal stress become singular with exponents 5/2, 5/2, and 7/4, as predicted by a kinetic theory and confirmed by prior discrete element modeling simulations.

Shearing motions in isotropic turbulence are studied with a triple decomposition of velocity gradient tensor. A mean flow around the shearing motions exhibits a thin shear-layer pattern sustained by a biaxial strain. The thickness of each shear layer is well predicted by Burgers’ vortex layer. Interplay between the shear and biaxial strain causes enstrophy production and strain self-amplification.

Analytic solutions for the forward enstrophy cascade in two-dimensional quantum turbulence are found using nonequilibrium Kosterlitz-Thouless renormalization. Vortex pairs of large separation are injected into a superfluid film at a constant rate, and cascade to smaller separation due to frictional forces on the vortex cores, annihilating at the same rate they are injected. From the cascade dynamics it is shown that the vortex decay in temperature-quenched superfluids proceeds via the turbulent cascade, and we speculate this may be a fundamental characteristic of phase-ordering transitions in general.

Sperm have evolved to perform a difficult but crucial task, swimming thousands of times their body length to the egg through highly viscous fluids. This is achieved through their beating tail, a beautiful structure consisting of sliding filaments, powered by the action of motor proteins. Scientists have spent decades studying sperm propulsion but have tended to ignore the end piece of the tail, characterizing it as a “ragged end” with no motor activity. Mathematical modeling shows that the end piece helps the sperm to perform a faster and more efficient swimming stroke.

Recent experimental and theoretical work have shown that active particles in Poiseuille flow exhibit interesting dynamics, including accumulation at the wall and upstream swimming. Furthermore, active particles are known to exhibit nonmonotonic Taylor dispersion as a function of the flow speed. An analytical and numerical investigation shows that the essential physics required to produce upstream swimming and nonmonotonic dispersion are captured by the simple active Brownian particle model.

Experiments show that shear-thinning viscosity experienced by rotating flagella is the major reason for the enhancement of bacterial swimming speed in a non-Newtonian fluid. Shear-induced normal stress plays an important role in promoting flagella bundling.

In flames, extinctions and subsequent re-ignitions occur continuously, with such small lifetime that they are difficult to perceive. Here, the light emitted by a vortex in reactive conditions is sampled in time and projected on a wavelet base. The result of the analysis is a statistical description of these shutdown and re-ignition phenomena in terms of the event duration, occurrence, amplitude, and shape. The proposed data analysis technique is applied for the first time in the field of combustion, but has had a widespread application in nonreactive fluid dynamics and aeroacoustics.

The sedimentation of a suspension of rigid spherical particles in a polymeric fluid is studied with experiments and numerical simulations. It is shown that settling in a viscoelastic Boger fluid is time-dependent and inhomogeneous; both experiments and simulations exhibit the formation of particle-rich, fast-settling regions and particle-depleted regions with back flow (shown). When a cross shear flow is imposed, the mean particle settling rate is drastically reduced, suggesting that both fluid elasticity and the particle volume fraction of the suspension have important effects.

Soft modulated phases are known to undergo complex morphological transitions through layer remodeling. A quasi-incompressible phase-field model for a layered phase in contact with an isotropic fluid is derived, which couples the layering order parameter to velocity and also to a varying density field function of the order parameter’s amplitude. Evolution and fluid flows on distorted smectics and focal conic defects are investigated through the proposed model.

The dynamic evolution of a highly underexpanded transient supersonic jet is investigated via high-resolution time-resolved schlieren and numerical simulations. Experimental evidence is provided for the presence of a second triple shock configuration along with a shocklet between the reflected shock and the slipstream. A model is developed and applied to the numerical simulations to reveal the mechanism leading to the formation of the second triple point.

From the jets and vortices on Jupiter to the winds in stars, systematic astrophysical flows can be generated by the interaction of convection with rotation and stratification. This complicated interplay is investigated in detail; the nature of the flows is controlled by the influence of rotation. Rotating, stratified flows are found to possess a net helicity which, in combination with the self-generated large-scale flows, could play a key role in the generation of magnetic fields through dynamo action.

Fully resolved simulations are conducted to determine hydrodynamic forces on clean spherical bubbles translating near a flat rigid wall in a linear shear flow. Flows range from low-but-finite $Re$ to nearly inviscid situations. Based on simulation results, semi-empirical expressions for drag and lift forces at arbitrary $Re$, relative shear rate, and separation distance are found. These improve over current ‘point-particle’ models which ignore wall effects, and may be used to predict realistic bubble trajectories and distributions in wall-bounded flows.

The Taylor-Culick approximation for a filament is revisited using numerical simulations. When the inertia force balances the surface tension force, a spherical blob appears at the extremity of the filament. This feature has a key impact on the tip dynamics, which moves with an oscillating velocity that has a mean value close to the Taylor-Culick prediction. On the other hand, in the viscous dominated regime, the radius of the filament grows uniformly over time, and no blob forms, making the tip velocity decrease after a short transient.

Asymmetric droplet splashing is observed during oblique impacts and under variable ambient pressure. These two effects are viewed as symmetry breaking factors which lead to new forms of splashing, including upward-only splashing and wing splashing.

The equilibrium solutions of liquid lenses are studied for any combination of the three surface tensions involved in the configuration. The case without gravity is analytically solved. The effects of its inclusion are numerically analyzed, showing that two families of equilibrium solutions are possible for the same set of physical parameters. An energy analysis is then performed to determine which of them is more likely to be found in nature.

A combined experimental and numerical study shows that for a droplet impacting on a liquid-gas interface of lower surface tension, the merged liquid-gas interface may experience two breakups, and vortical mixing is also enhanced beneath the interface by the Marangoni effect.

Surfactants are ubiquitous in everyday life, and can dramatically change the behavior of droplets and bubbles, even when present in tiny amounts. We use numerical simulations to shed light on the complex interaction among turbulent flow, interfaces, and surfactant dynamics: surfactants increase interface deformability and modify the local flow topology. Although the presence of surfactant-laden droplets has a minor effect on the macroscopic flow, turbulence is topologically modified via the action of Marangoni stresses, which promote an elongational type of flow at the interface.

A 1-micron Brownian colloid on a random walk can take more than a month to traverse a 1 mm distance. We present a strategy for a drop-in additive to induce spontaneous migration of particles in a suspension at a rate orders of magnitude faster than simple diffusion. The additive, referred to as a solutoinertial beacon, releases a solute as it sediments within the suspension. This solute flux propels colloids to migrate via diffusiophoresis. Theoretical and scaling analyses capture the experimental observations well and reveal design parameters that govern the dynamics of particle motion.

The oceans’ interior is stratified in density and thus can sustain internal wave propagation. These waves, when nonlinear, can generate a state of wave turbulence and contribute significantly to the global energy dissipation of ocean circulation. However, a full theoretical description of the statistical properties of such stratified turbulence is still being sought. We performed very large scale experiments in the Coriolis facility in Grenoble, France and observed a state of wave turbulence of internal waves, which will enable comparisons with theory and numerical simulations.

Drag reduction of an actuated turbulent boundary layer at a momentum-thickness-based Reynolds number Re${}_{\theta }$ = 1000 is computed, modeled, and predicted. The drag reduction for the set of actuation parameters is modeled using 71 large-eddy simulations. This drag model allows extrapolation outside the actuation domain for larger wavelengths and amplitudes. The modeling novelty combines support vector regression for interpolation, a parametrized ridgeline leading out of the data domain, a scaling for the drag reduction, and a discovered self-similar structure of the actuation effect.

For sufficiently low interfacial tension, spinning drops develop dumbbell shapes, with two large heads connected by a thinner central body. The time evolution of such shapes is exploited to investigate the effective interfacial tension (EIT) at the boundary between miscible molecular fluids. The EIT is found to be as low as 250 nN/m in water-glycerol systems, in excellent agreement with values calculated via an existing phase field model.

Detailed numerical simulations have unexplored potential in the study of droplet impact on flat surfaces that feature spatially nonuniform wettability. We demonstrate the capability of a numerical model to accurately predict the three-dimensional dynamics and outcomes of droplets orthogonally striking surfaces of spatially distributed wettability. The model successfully predicts experimental events leading to droplet splitting and lateral vectoring, providing a method for studying droplet impact on wettability patterns of any design.

A study of velocity scaling and breakup criteria for jets formed as a result of an acceleration and deceleration process is presented. The process is characterized by five dimensionless numbers: Weber number, Ohnesorge number, Bond number, and dimensionless acceleration and deceleration times.

Machine learning is integrated into low-Reynolds-number locomotion to enable a class of self-learning, adaptive (smart), micro-swimmers. Instead of specifying locomotory gaits in advance, a self-learning swimmer develops and adapts its propulsion strategy based on interactions with the surroundings via reinforcement learning. Without requiring prior knowledge, the swimmer can recover previously known propulsion strategies, and improve and adapt in different media. This development can enable the design of smart micro-robots with robust locomotive capabilities in complex environments

Molecular dynamics is employed to investigate the impact of nanodroplets on superhydrophobic surfaces decorated with nanoridges. It is concluded that the decorated nanoridges can significantly promote the bouncing performance of nanodroplets. Five distinct bounce modes are identified, and they determine the variation laws of contact time and bounce velocity of nanodroplets.

Singularity representations of the Stokes flow due to point torques and sources in a spherical geometry are derived. Both rigid spheres with no slip and spherical bubbles are considered. Surprisingly, for an axisymmetric torque with no slip the solution consists of a single point image torque, similar to point charges in electrostatics.

The flow field in the sphere wake may be affected by roughness patterns such as those found on pine cones. These may enhance pollination by generating favorable flow patterns that capture conspecific pollen. A combination of time-resolved particle image velocimetry (PIV) and tomographic PIV is used to study the instantaneous three-dimensional vortex shedding, as well as mean velocities and turbulent stresses, in the near wake of a smooth and a rough (mimicking a pine cone) sphere, at intermediate Reynolds numbers (from 200 to 5000), where little quantitative flow field data are available.

Dynamics behavior of rising microbubbles in homogeneous stratified turbulence is investigated by direct numerical simulation. Stratification creates a predominantly horizontal oscillatory motion in a fluid, inducing bubbles’ zigzag pattern. Despite this oscillating motion, the horizontal dispersion of a single bubble is rather suppressed. The horizontal separation of paired bubbles displays a power-law growth in the Batchelor and Richardson regimes.

From nutrient uptake by aquatic microorganisms to droplet condensation in clouds, scalar absorption is a crucial mechanism, often enhanced by turbulence. Mean-field approaches offer good predictions of this increase but cannot describe the fluctuations, which can be important. Numerical simulations are used to study the effects of turbulent fluctuations on the Lagrangian statistics of absorption of a scalar field by tracer particles. Statistics of the uptake rate are found to be closely related to statistics of the local shear rate experienced by the particles along their trajectories.

We simulated particle-laden flow in a vertical channel with a Reynolds-averaged Navier–Stokes (RANS) two-fluid model including a Reynolds-stress model (RSM). Primary and turbulent statistics from counterpart Eulerian–Lagrangian (EL) and Eulerian–Eulerian Anisotropic-Gaussian (EE-AG) simulations were used to inform parameters and closures in the RSM. Flow in the channel center, including transitions, compared well with other simulations as mass loading increased. For small-Stokes particles, the RSM at greater mass loadings reproduced the transient clustering observed in the other models.

Liquid metal batteries can be more rapidly charged and discharged when the bottom layer alloy is being efficiently mixed by flows. A numerical investigation into how solutal buoyancy and electrovortex flows interact and influence the alloy composition is presented. Theoretical estimates for the minimal flow magnitude needed to enhance mixing during discharge are derived, and a novel scaling law for the intensity of the solutal convection flow that is observed during charge is proposed.

How turbulence is modified in the presence of a network of finite-size slender rigid fibers is investigated as a minimal model to study flows within canopies. Direct numerical simulations are performed, complemented by a state-of-the-art immersed boundary method to fully resolve the multiscale fluid-structure interaction and directly access and analyze both the small- and the large-scale dynamics. In particular, it is shown that the large-scale dynamics can be effectively modeled by means of a Darcy’s friction term for which a phenomenological expression is proposed.

Simulations of decaying two-dimensional turbulence show a persistent trend in the statistical temperature, from “colder” states in which dipoles are prevalent, to “hotter” states dominated by clusters of like-signed vortices. The spontaneous heating effect is shown to be consistent with a decay law for the vortex number density that is faster than the $t^{-2/3}$ law deduced from similarity arguments.

Predictability of trailing-edge stall phenomena using wall-modeled large-eddy simulations (LES) is investigated with a wall-resolved LES database. An analysis based on the momentum integral relation shows that the skin friction accumulation effect near the leading edge to the mid chord dominates boundary layer development, and thus, affects flow separation prediction near the trailing edge. The results indicate that accurate wall modeling near the leading edge to the mid chord is essential for predicting stall phenomena, but not necessarily required near and downstream of the separation.

While it is known that the mean flow in a spanwise rotating channel follows a linear law at the pressure side with an additive constant $C$, the exact dependence of this additive constant on the Reynolds number and the rotation speed was not entirely clear. It is shown that this additive constant $C$ is a logarithmic function of a rotating induced length scale. After determining the mean-flow scaling, this knowledge is used for wall modeling and for relating the skin friction and the flow rate.

A forced plane wall jet (PWJ) is used as the model flow field to study the linear and nonlinear interactions within complex wall-bounded flows. Large-scale, large-amplitude forcing is used to isolate an energy-transfer pathway within the flow. The direction of this transfer is a forward or inverse cascade, depending on the streamwise location. It is also interpreted that the PWJ naturally transfers energy from the near-wall boundary-layer structures to the outer free-shear-layer structures.

Particle Image Velocimetry is used to measure the wall-shear stress (WSS) in a smooth-wall turbulent boundary layer at low-to-moderate Reynolds number. The fluctuating intensity of WSS is found to follow an empirical log-law with Re due to the effect of outer-layer large scale motions (LSMs) on the wall. A scale decomposition analysis suggests that the LSMs leave a mild footprint effect on the WSS in the present Re range. The amplitude modulation effect is inferred to play a more prominent role in affecting the characteristics of fluctuating WSS.

The occurrence of backflow events, defined as negative wall-shear stress, is rare in turbulent flows. It is found that these backflow events are further suppressed in a toroidal flow when compared to a straight pipe flow at a nominally similar Reynolds number. The reduction of backflow is due to the secondary flow of Prandtl’s first kind occurring in the toroidal flow.

A discrete vortex model is utilized to simulate the unsteady aerodynamics of a wing encountering a large-amplitude transverse gust. A closed-loop control law based on pitching input is selected to regulate lift during the gust encounter. The unsteady flow physics of the gust encounter under closed-loop pitch actuation is studied and the total lift force is decomposed into individual contributions to study the various mechanisms with which closed-loop control achieves lift regulation.

An extension of dynamic mode decomposition is presented to synthesize globally consistent modes from velocity fields measured from multiple partially overlapping spatial domains.

Buoyancy-induced flows in many industrial and natural processes are significantly affected by rotation. The effects of pipe axial rotation on flow patterns and front dynamics of buoyant miscible exchange flows are studied experimentally and numerically. With increasing rotation speed, the flow pattern is experimentally shown to transition from a slumping interface to complete transverse mixing. To complement experimental observations, specific simulations provide a cross-sectional view of the pipe, in which the front dynamics is shown to change as the rotation speed increases.

Using ultrafast x-ray imaging, downward jetting of a Leidenfrost drop is observed and studied for the first time. The downward jetting is caused by capillary waves propagating on the bottom liquid-vapor interface during retraction of the drop. The downward jetting can be inhibited in the case of viscous damping of capillary waves. A jetting criterion depending on the Ohnesorge and Weber numbers is suggested: (Oh)(We${}^2)\le 66\pm 10$.

The changing physics of internal hydraulic jumps in two-layer flows with increasing upstream shear is investigated. As shear increases, internal hydraulic jumps or super- to super-critical transitions can form, entrainment becomes important, mixing efficiency decreases, and the two-layer approximation has limited applicability.

Triadic resonance represents a mechanism by which internal wave energy is transferred to different frequencies and spatial scales. An experimental study of superharmonic internal wave excitation by temporal and spatial resonant interaction between internal wave modes is presented. The experimental observations are compared with theoretical amplitude evolution equations, following which, superharmonic generation near resonance is also studied.