Supercooled water drops move spontaneously while freezing in vacuum. This self-propulsion phenomenon is caused by an enhanced evaporation rate from the frozen regions of the drops. The evaporating molecules carry momentum, and the drops acquire an opposite momentum, the same as in rocket propulsion.

Soft particles display a rich zoology of dynamics with bifurcation dynamics depending on the interfacial mechanical response. One intriguing issue is the origin of motions along helices, a dynamic only observed with active particles. Numerical simulations of a vesicle, a passive particle in Poiseuille flow, show that this dynamic called swirling is in fact more ubiquitous and emerges spontaneously. Swirling is the ultimate state of the symmetry loss sequence parachute → croissant → slipper → swirling, where the last symmetry lost is the mirror plane of the slipper.

Thermally driven flows are ubiquitous in nature. An important and longstanding question is how heat transfer depends on the control parameters of the flow. In this Letter, the author presents a theoretical analysis that answers such question for large aspect-ratio turbulent thermal convection in a fluid between two vertical walls maintained at different temperatures. This analysis yields the dependence of heat flux and wall shear stress on the Rayleigh and Prandtl numbers in the high Rayleigh-number limit.

In the common phenomena of turbulent boundary layers, the large range of scales present in the flow gives birth to many questions. One of the topics addressed in our work is the distribution of energy across different scales, the size of these structures and their location in the boundary layer. Through numerical simulations and spectral analysis, we can finally study how the position and size of these scales evolve, at different Reynolds numbers and adverse-pressure gradient intensities.

High-resolution wind detection plays a crucial role in aviation safety and aerodynamics. In this work, the fine wind field within 700 meters is continuously detected at a resolution of 0.9 meters/0.5 seconds by a coherent Doppler wind LiDAR. A meter-scale perturbation by an electric fan is remotely sensed, and atmospheric turbulence with length scale down to 3 meters can be observed.

The collective motion of dense suspensions of swimming bacteria is typical of a broad class of active materials, which serve an array of important biological and ecological functions. This work combines microfluidic experiments with modal analysis, typically reserved for inertial turbulence, to quantify the active turbulence of bacterial suspensions. Our results unveil the underlying constituent flow structures responsible for the interactions of chaotic bacterial motion with solid boundaries and external flows, and establish an analysis framework to facilitate new experimental and modeling approaches in active matter systems.

We present the first study of irreversibility of bacterial turbulence, an exciting new field. We use Lagrangian statistics to show that irreversibility in bacterial turbulence leads, on average, to particles gaining energy faster than they lose it, the exact opposite of what occurs in fluid turbulence.

Capturing high temperature effects in hypersonic flow simulations relies upon expensive thermochemical gas models. We present here a novel model-agnostic machine-learning technique to extract a reduced thermochemical model of a gas mixture from a library. Combining techniques of dimensionality reduction, spectral clustering, and radial basis functions, an accurate and more efficient alternative to the original model is obtained.

This work presents the statistics of flame-self interaction during boundary layer flashback of a hydrogen-rich premixed flame. It is shown that the flame topology is significantly affected by the turbulent boundary layer and the heat loss to the wall. Furthermore, it is shown that the choice of the species used to define the progress variable has a significant influence on the distribution of flame-self interaction topologies.

A microfluidic canopy flow device, formed from a large array of slender polymeric pillars within a glass microchannel, is subjected to viscoelastic flow in the regime of elastic turbulence. The system results in the spontaneous emergence of waves in the form of propagating regions of low flow velocity compared to the bulk, also inducing Monami-like waves in the canopy if the pillars are flexible. Due to the analogies with classical (inertial) canopy turbulence, this new phenomenon is named “canopy elastic turbulence”.

Active mechanisms such as a magnetic torque, motion of a solid or liquid boundary, and changes in pressure induce motion of chiral particles perpendicular to the plane of the flow of a base liquid endowed with rotational degrees of freedom (such as a ferrofluid).

Numerical simulations of viscoelastic Rayleigh-Bénard convection (VRBC) are conducted to investigate time-dependent oscillating convection leading to the formation of reverse flowing cells. A viscoelastic kinetic-energy budget method is used to analyze the energy transport and a parametric analysis for the reversal flow dynamics. The emergence of the reversal convection can be explained by potential energy transfer between flow and fluid elasticity. The image shows periodic total kinetic energy ($E_{total}$), energy exchange rate (${\mathrm{\Phi }}_G$), and buoyancy flux (${\mathrm{\Phi }}_F$) time evolution in VRBC from the local view, during a half-period of a reverse process for Phan-Thien-Tanner (PTT) fluid.

Aggregation and disaggregation of clusters of attractive particles under flow are studied and highlight the growth of the average steady-state cluster size as a power law of the adhesion number, a dimensionless number that quantifies the ratio of attractive forces to shear stress. Such a power-law scaling results from the competition between aggregation and disaggregation processes, as previously reported. Here we rationalize this behavior through a model based on an energy function, for which minimization yields the power-law exponent in terms of the cluster fractal dimension, in good agreement with the present simulations and with previous works.

Coherent structures in thermal turbulence are highly efficient in the global transport of heat. However, it is notoriously difficult to manipulate these structures due to the random motions of the turbulent background and the dissipative nature of turbulence. In this study we identify a new mechanism for achieving heat transfer enhancement in turbulent thermal convection, which is to promote both the plume coherency and flow strength via manipulation of the coherent structures in the bulk of the flow.

Planning energy-efficient trajectories can be beneficial for unmanned aerial vehicles to increase their endurance. We designed a smart self-propelling agent that can migrate long distances in a thermal turbulent environment by utilizing the updrafts of thermals. We choose Rayleigh-Bénard convection, which is a paradigmatic turbulent convection system, as the flow environment. Using a reinforcement learning algorithm, we train the agent in the Rayleigh-Bénard convection cell with an aspect ratio between cell length and cell height of 2, and then test the smart agent migrating in the convection cell with an aspect ratio up to 32.

Carbon dioxide (CO${}_2$) can be stored in huge quantities in deep saline reservoirs, thus providing a key contribution to the fight against global warming. A complete understanding of the convective dissolution process can help optimize CO${}_2$ storage. Observing in a different direction, i.e. parallel to gravity, highlights the 3-dimensional structure of convection dominated, at an early stage, by falling sheets. That allows an accurate measurement of the onset time of convection, verifying the predicted power-law master curve independent of the liquid phase composition. This demonstrates that adding salt simply shifts the thermodynamic state by to a different Rayleigh number.

Damage from repeated bubble collapse to neighboring rigid objects is a consequence of cavitation. Few studies exist on the dynamics of bubbles collapsing near a corner, i.e., two flat rigid surfaces making a right angle. Here we solve the three-dimensional compressible Navier-Stokes equations for gas and liquid flows to quantify the pressure fields. The second wall affects the collapse by (i) causing the re-entrant jet to no longer point in the direction normal to the closest wall but at an angle toward the corner and (ii) causing the maximum wall pressure to be observed in the corner when the bubble is sufficiently close to equidistant from each boundary due to shock reflections.

A compressible two-phase flow solver is developed for the simulation of a collapsing bubble pair. With the adaptive mesh and high-resolution scheme, the discontinuities and shock waves due to bubble collapse are well captured. Numerical results show that the collapse of two bubbles horizontally arranged ($\varphi =0^{\circ }$) produces the highest wall pressure peak among the oblique angles ($0^{\circ }\le \varphi \le {90}^{\circ }$) while the perpendicular arrangement ( $\varphi ={90}^{\circ }$) leads to the lowest wall pressure peak, even weaker than that of the single bubble collapse.

The destabilizing effect of anisotropic velocity slip on shear flows was discovered in recent years. We study the linear instability of channel flow subject to the anisotropic slip caused by microgrooves and present the slip setting that maximizes the destabilizing effect. The results are informative for designing superhydrophobic walls to introduce flow instability in low-Reynolds-number channel flow.

The effect of low-frequency oscillation on a separated shear layer has experimentally been investigated. We find that the intense low-frequency oscillation of the separated shear layer may change the regular vortex shedding from the separated shear layer to an intermittent vortex shedding. An effort has been made to explain the mechanism of this intermittent vortex shedding. A nondimensional parameter has also been proposed to determine the possible existence of the intermittent vortex shedding in a separated shear layer.

Thermal plumes arising from localized buoyancy sources have been studied extensively, mostly focusing on entrainment and mixing in fully developed turbulence. Here, we focus on the primary instabilities of laminar plumes and their transitions as the strength of the buoyancy source is increased. Many prior studies in this transitional regime have reported a myriad of disparate spatiotemporal plume characteristics. We show that this wide variety is tied to various ways an axisymmetric system can undergo symmetry breaking, that the nature of the ambient stratification plays an important role in the symmetry breaking, and this determines the details of the plume spatiotemporal characteristics.

We develop a thermodynamically consistent phase-field model for investigating coupling effects of temperature and surfactant concentration on droplet migration under a fully developed Poiseuille flow. The surface tension in our model contains the classical linear part for thermocapillary effects, but also a nonlinear coupling term of temperature and surfactant concentration that recovers the Langmuir equation of state. Using numerical simulations, we find that this coupling introduces extra thermal-induced and surfactant-induced Marangoni forces to droplet migration, leading to a competition between the two, especially at high surfactant concentration.

Foams, films, and bubbles are ubiquitous in our everyday lives and in processes such as textile dyeing and fermentation. The rupture dynamics of Newtonian soap films is well-studied, especially in the inviscid case. Here we explore the rupture of soap films formed from a non-Newtonian fluid, a colloidal suspension. When these films are thick compared to the length scale of the colloidal particles they are composed of, they rupture in a manner similar to a Newtonian fluid, even at quite high colloidal volume fractions. However, when allowed to thin before rupture to a thickness comparable to the colloidal size, they exhibit exotic instabilities such as wrinkling and folding like a fabric.

Particles can aggregate on a fluid surface through capillary interactions, a phenomenon famously known as the Cheerios effect. Despite their ubiquity, few studies have considered the dynamics of the rafts comprising heavy granular particles. We experimentally test the collapse of granular rafts upon biaxial compression and observe two unexpected failure modes: individual particle expulsion for small particles and collective creasing for large particles. By combining potential energies of the raft and liquids, we calculate the shape of the particle-laden interface, which quantitatively matches our experiments.

The wake instabilities associated with a streamwise and transversely rotating sphere at very high rotational speeds are not clearly understood. This article attempts to address typical wake instability characteristics when the sphere rotates along the streamwise and transverse directions through direct numerical simulations. We have found that a larger streamwise rotation makes the wake bifurcate into large-scale shear-layer structures. In contrast, the azimuthal stretching during transverse rotation splits the wake into a twin jet-type flow pattern, resulting in the transverse expansion of the wake.

Plumes of particles falling in still water transfer kinetic energy to the fluid and set it in motion. This article presents experimental results of simultaneous particle tracking and fluid flow measurements of dilute particles plumes with varying particle size and density. While the settling speed of the plumes is found to be enhanced by an increase of the particle concentration, their motion relative to the fluid is entirely unaffected. The energy transfer from particles to the fluid scales linearly with their concentration and is more effective for smaller and lighter particles.

Our measurements show that liquid permeability of foam-filled granular packings is fully controlled by the microstructure of the foam. The main parameter is the bubble-to-grain size ratio, the increase of which leads to a very sharp decrease of the liquid permeability. The decrease of the liquid saturation also contributes to decrease the permeability. The choice made for the surfactant is shown to be also important.

We phrase boundary-value problems for a dense, steady, fully developed, gravitational flow of identical inelastic spheres and water over an inclined erodible bed in the presence of sidewalls. We obtain numerical solutions for the profiles of the solid volume fraction, the strength of the particle velocity fluctuations, and the mean velocities of the particles and fluid. We compare these with those measured in experiments.

A generalized dynamic resuspension model for particles rolling on a surface with fractal-like multiscale roughness elements is introduced and compared with experimental measurements. Second, nonGaussian stochastic models for the flow velocity seen by a particle are compared with a Gaussian stochastic model. Third, particle resuspension is predicted using large-eddy simulations with a stochastic model for the subgrid-scale flow velocity. The present results confirm the importance of including the multiscale asperity model, nonGaussian flow velocity model, and particle subgrid-scale model to accurately predict resuspension dynamics.

Caustic singularities of the spatial distribution of particles in turbulent aerosols enhance collision rates and accelerate coagulation. Using a Gaussian statistical model in three spatial dimensions, we show that caustics form by an optimal fluctuation of the fluid-velocity gradients at weak particle inertia. This optimal gradient fluctuation must reach a large threshold for a caustic to form. To do so, it propagates along the Vieillefosse line, which distinguishes rotational and extensional flow regions in the $Q$-$R$ plane of invariants of the fluid-velocity gradient matrix.

The shape of flexible fibers in a suspension varies depending on the hydrodynamic interaction of each fiber with the flow and with the neighboring fibers. By examining the total fiber forces from direct numerical simulations, this study proposes a reduced-order stochastic model that predicts the three-dimensional shape of fibers settling inertially in a suspension of different concentrations. While at low concentrations gravity is the main contributor to the planar deformation of the fibers, at higher concentrations, the interaction of fibers with each other creates large out of plane deformations.

We take inspiration from the turbulent Rayleigh-Bènard flow of the Pi Chamber and consider a particle-laden direct numerical simulation (DNS). The aim of our study is to develop a simple stochastic model that can accurately describe the residence times of the particles in the flow. The underlying conceptual picture is based on the repeated trips that particles take between the top and bottom boundaries, driven by the convective cells that occur in Rayleigh-Bènard turbulence. Beyond demonstrating the model’s efficacy, we use the framework to shed light on the independent roles that gravity and inertia play in governing residence times.

High-amplitude localized waves in a narrow channel can juggle large-sized droplets for tens of thousands of cycles without coalescing. The wave launches and softly catches the drop at every cycle, and laterally traps it. This system can be considered the gravity-wave analog of optical tweezers.

Asymmetric shear instabilities combine the intense mixing of a Kelvin-Helmholtz instability with the interface-preserving property of a Holmboe instability. We show that even a small amount of asymmetry can produce a different flow evolution than would be expected for symmetric shear instabilities. These effects can be seen in everything from the underlying linear instability to the turbulent flow statistics.

In many subsurfaces, such as soils, the detrimental transport of contaminants can be remedied via the introduction of adsorbing matter, such as biochar. We investigate, via numerical simulations, the transport mechanisms governing adsorption in such chemically heterogeneous porous media, and we show that molecular diffusion and advection determine the effective medium adsorption at high and low adsorbing volume fractions, respectively.

We derive a relation for the correlation between pressure Hessian and velocity gradient in homogeneous flows. This relation provides restrictions to the parameters in the closure models of pressure hessian in velocity gradient dynamics, and together with the Poisson equation, we could obtain an integral expression for the fourth-order moment of velocity gradient in isotropic flows. Furthermore, this relation could be generalized to shear flows, and it is approximately satisfied even in the presence of a shear and of a wall, as occurs in turbulent channel flows.

The flow past isolated and sheltered two-dimensional (2D) bar roughness elements is investigated via 2D and 3D flow measurements for a wide range of configurations. The presented results highlight the effects of sheltering by an upstream 2D bar on the wake of a downstream 2D bar, including flow reattachment, turbulence, vortical structures, and other flow quantities.

In the study of dispersed multiphase turbulence, using a jet array has shown promise in creating intense homogeneous, nearly isotropic turbulence with high dissipation rates in water and wind tunnels. However, the scaling of turbulent characteristics with jet nozzle diameter, velocity, and downstream location has not been fully explored. Our research, combined with previous experiments focusing on near-field measurements, offers a comprehensive understanding of the decay of kinetic energy and energy dissipation rate, as well as the flow inhomogeneity and anisotropy generated by a jet array.

Deep reinforcement learning was applied to turbulence control for drag reduction in direct numerical simulation of turbulent channel flow. The learning determines the optimal distribution of wall blowing and suction based on the wall shear stress information. From an investigation of the optimal actuation fields, two distinct drag reduction mechanisms were identified. One of them, which had not previously been recognized, attempts to cancel the near-wall sweep and ejection events.

High-pressure transcritical turbulence with strong density fluctuations are encountered in many technical applications. Understanding of the turbulent energy transport modulated by real-fluid thermodynamic effects at transcritical conditions remains an open issue for developing reliable computational models. We clarify the effects of real-fluid thermodynamics through budget analyses of turbulent kinetic energy and dissipation rate from DNS of transcritical channel flows. From this physical underpinning, new scaling laws for turbulent length scales and turbulent kinetic energy budgets are proposed, thereby contributing to improvements of wall-models for LES and RANS simulations.

Physics-based uncertainty quantification is applied to a boundary layer over a flat plate with a statistically stationary separation bubble to assess sources and dynamics of model error (ME) in two-equation Reynolds-Averaged Navier-Stokes turbulence models in separated flows. Two distinct ME modes are found that correspond to the qualitative behavior of ME in a turbulent wall-bounded flow (upstream of the separation bubble) and turbulent free-shear flow (within the separation bubble), with a superposition of these ME modes in intermediate regions. A complex understanding of ME results as it changes throughout the flow, but is ultimately comprised of the elements of canonical flows.

Can the inverse cascade of two-dimensional turbulence display intermittency? This work shows that if forced in a fractal set of dimension smaller than two, a two-dimensional turbulent flow develops clusters of different size vortices not uniformly spread in space as the ones shown in the figure, that display intermittent properties.

Changes in flow caused by flow reattachment on the side surface of a freestream-aligned circular cylinder depend not only on fineness ratio, but also on angle of attack (AoA), which significantly affects the aerodynamics. In this study, the aerodynamic forces on the cylinder and flow fields are investigated in wind tunnel tests under the flow without support interference using a magnetic suspension and balance system (MSBS) and PIV measurements. The AoA where flow reattachment starts to appear is identified, and two flow regimes are observed in the 0-15 degree range. The difference in the time-averaged and fluctuating aerodynamic force due to the flow regime change also appears.

The long-time dynamics of rotating elliptical vortices raise a question: Do they eventually become axisymmetric, or can they evolve to a nonaxisymmetric tripole state? This article explores the question using an adaptive vortex method to compute the vortex filamentation and formation of crescent-shaped lobes and a spiral halo around the core. The answer may depend on the initial vorticity profile, and a third possibility is that the vortex oscillates between axisymmetric and tripole states due to repeated exchange of fluid between the halo and lobes.