Chemically active drops are not neutrally buoyant and thus often swim along a rigid wall; yet the influence of such confinement on self-propulsion is generally overlooked in theoretical studies. Using a model system for the active drop, we solve here numerically the nonlinearly coupled hydrochemical problem for the chemical transport and flow field around the moving drop in order to unveil mechanisms governing droplet propulsion parallel to the wall. We show that proximity to a rigid wall increases the drop’s swimming speed by focusing the strongest interfacial flows to the thin gap between the drop and the wall.

New analysis techniques allow removing the smearing effect of capillary waves from molecular dynamics simulation data. The attained resolution is unprecedented and allows resolving the hydrodynamic fields down to the molecular size. Matching the continuum description obtained with finite elements allows distinguishing dissipation at the liquid-solid interface from that at the contact line.

A solid plate withdrawn from a wetting liquid bath entrains a thin liquid film, a process known as dip coating. We theoretically investigate the influence of a soft layer coated on the plate. The liquid-film thickness is found to be enhanced at low pulling velocity, when the elastic deformations of the soft layer are larger than the liquid-film thickness. A scaling relation of the film thickness with the velocity and the elastic modulus is analytically characterized using an asymptotic-matching method, and confirmed with numerical integration.

Reduced-order models for plane Couette flow are obtained by Galerkin projection of the Navier-Stokes equations onto the leading controllability modes of the linearized system. Nonlinear dynamical systems so constructed, with various degrees of truncation, are numerically stable and reproduce statistics of minimal turbulent Couette flow at Reynolds numbers 500 and 1200 for different choices of modal bases. A closure model was not included to the Galerkin system, suggesting that this is not essential for the stability of such models, at least for the flow at hand.

For supersonic/hypersonic propulsion systems, the shock train in the inlet/isolator play a dominant role in compressing the incoming flow and providing suitable airflow to the engine. In this study, a detailed wind tunnel experimental study is performed in a supersonic isolator to investigate the potential of a micro-ramp as a form of shock train control. The results show that the micro-ramp modifies the structure of the shock train, inhibits the oscillation and upstream propagation of the shock train, and thus improves the total pressure recovery coefficient and safe working range of the hypersonic inlet/isolator.

Sharp changes in solar radiation intensity generate lateral density gradients and lateral intrusions along the surface in lakes with temperatures below the temperature of maximum density. Parameter dependence of these intrusions and their effect on the enhancement of vertical heat flux are explored via a series of two-dimensional high resolution, nonhydrostatic simulations initially at constant temperature. Rayleigh-Taylor-like instabilities are allowed to develop which interact with the lateral intrusions.

For decades researchers have been studying the problem of thermal instabilities in rotating spheres or spherical shells. A key assumption is that the geometrical flattening due to centrifugal force can be neglected, which seems to be a good approximation for many planets and stars. However, this has never been tested. In this paper, the linear stability onset problem at low Prandtl number is considered in oblate spheroids, whose shapes are consistent with figure theory, and it is compared with the previously known spherical system solutions.

We study a differentially-heated square cavity with inlet and outlet ports at the top of the side walls, quantifying how the effective thermal conductivity changes with the temperature difference across the system. As the Richardson number, Ri, is increased, we observe the formation of a series of system-spanning vortices through the merging of Moffatt eddies growing from the bottom corners of the cavity. We find that the convection, which enhances the transport of heat, is strongly suppressed by increasing the temperature drop.

The widely accepted Epstein-Plesset theory (EPT) suggests that all gas bubbles are diffusively unstable in practical situations of accelerative shrinkage or unbounded growth. The stable nanobubbles confirmed by experiments go against EPT, which is verified by experiments on bubble radii down to micrometers. The theoretical gap between stable nanobubbles and unstable microbubbles requires a full-scale physical understanding. We present a model that describes the state transition from stable nanobubbles to unstable microbubbles on homogeneous surfaces, achieving diffusive equilibrium at the liquid-bubble interface and mechanical equilibrium at the triple-phase contact line simultaneously.

Molten sand droplets deposited on gas turbine engine components cause severe damage. To better understand the deposition process, we study the wetting dynamics of a highly viscous molten sand droplet on a smooth substrate. A universal contact line spreading, independent of the droplet size, is observed with the characteristic inertial and viscous spreading regimes. It is also shown that the dynamic contact angle is in excellent agreement with Cox’s theory.

Droplet production from bursting bubbles has been extensively studied for single bubbles but remains sparsely quantified in controlled collective settings. This article explores how the trajectories of droplets produced by interacting bursting bubbles can be used to track back the mode of production, for clean and contaminated water. Noticeably, it compares velocity-size relationships, reviewed for jet and film drops from individual bubbles, with the measurements made in collective experiments.

This paper investigates the rim fragmentation of a millimeter-sized methyl-ethyl-ketone droplet imposed by the impact of different millijoule nanosecond laser beams. The physical mechanisms that determine the droplet deformation, expansion, and the evolution of the detached fragments from the rim are demonstrated, considering the influence of the laser beam energy. The key finding of our numerical study is the quantification of the evolution of fragments during the rim fragmentation process using a physically consistent multiscale framework.

The influence of gravity on water droplet freezing on a cold solid surface has been investigated in this study. Experimental results over a range of Bond numbers, for both pendent and sessile droplets, show that gravity can significantly influence the freezing processes via shaping the initial droplet. Despite the significant difference in the initial droplet shape, some remarkable similarities have been found for pendent and sessile droplets at small and large Bond numbers. The final height of a frozen droplet is found to be proportional to its initial height.

We study the motion of liquid droplets moving along partially wettable fibers. Employing lattice Boltzmann simulations, we observe three dynamic regimes: compact droplet, droplet breakup, and droplet oscillation. The transitions between these regimes depend not only on the droplet Bond number but also on the fiber curvature. In particular, droplet breakup is promoted with increasing fiber curvature. Furthermore, we can identify different scaling laws for the droplet velocity when the droplet size is large or small compared to the fiber radius.

Most models of astrophysical flows employ a “sound-proof” approximation, such as the Boussinesq, anelastic, and pseudo-incompressible models, which remove acoustic waves from the governing equations. Here, we address the accuracy of each of these models for describing magnetic buoyancy in the solar interior. We introduce a general sound-proof model and compare its behavior to that of the fully compressible system within a range of astrophysically relevant parameter regimes. We determine the conditions under which a sound-proof model can correctly capture the linear behavior of magneto-buoyancy instability.

In this work, the effect of precession in a rotating sphere filled with fluid is investigated using direct numerical simulations. Incompressible hydrodynamics and magnetohydrodynamics scenarios are examined. In the latter case, a comparison between the effect of prograde and retrograde precession is presented. We show that retrograde precession is more efficient at generating self-sustaining dynamos due to the presence of a stronger turbulent regime. Finally, the behavior of the magnetic dipole moment and its reversals are studied.

The leading edges of hypersonic vehicles are blunted to mitigate surface heat transfer. A key challenge in their design is the estimation of the laminar-turbulent transition location in the presence of strong, curved shocks. In this study on blunted flat plates, we numerically characterize various aspects of disturbance evolution in the entropy layer. Through controlled forcing, we show that entropy layer disturbances can lead to turbulence without the conventional first and second modes.

Pour coffee into a mug, and you might notice that the water cascading from the jug resembles a chain: A series of “links” oriented at 90° to one another. Such oscillating jets occur frequently in our daily lives, but their origin is still not fully understood. In this study, we performed experiments and direct numerical simulations to show that the wavelength and amplitude of the jet’s oscillations and surfaces are directly linked to the flow rate and size of the opening through which the liquid flows.

In applications such as water treatment or oil extraction, oil drops suspended in an aqueous fluid are injected in a porous medium. If their surfaces are laden with solid particles, the drops may lose their integrity when they pass through narrow pores. We study the condition for the expulsion of solid particles in experiments in which Pickering drops are made to flow in a constricted capillary tube. We show the key parameters are both the thickness of the lubricating film between the drop and the tube and the geometrical constraints exerted by the tube on the drop.

Air film curvature ahead of the contact line drives fast-spreading dynamics during droplet impact and spreading on a smooth surface. The fast-spreading process could lead to wetting failure and bubble entrapment. Reducing the surface tension of the impacting drop increases the critical capillary number threshold for wetting failure and suppresses the subsequent bubble entrapment.

The baroclinic torque, generated at the gas-liquid interface due to the misalignment of density and pressure gradients, results in a flow-focusing effect that drives bubble pinch-off from the meniscus. High-speed synchrotron X-ray imaging was used to visualize the entire gas-liquid interface. Volume-of-Fluid numerical simulations were employed to unravel the intricate mechanism.

We use lubrication theory to analyze the shape of the bow-wave ahead of a squeegee moving through a layer of viscous fluid like a bulldozer being used to clear up a large pool of spilt molasses or slurry. For a relatively long squeegee, the key is to find quasi-steady cross-sections normal to the squeegee, and then to determine their slow variation along the squeegee by considering the lateral flux required to divert the flow around the ends. Our simple flux-balance arguments can be straightforwardly adapted to squeegees of arbitrary shape and to other rheologies such as viscoplastic fluids.

We describe a method to numerically model flows through porous media by representing the porous medium as a randomly distributed collection of regularized Stokeslets. We establish a correspondence between our model and the Brinkman model description of porous media. The method provides a flexible way to treat porous media with complex geometries and deforming boundaries. We demonstrate it using examples of microscale swimmers.

Microchannel flow of dense bidispersed suspensions containing particles of two different sizes, both in high concentrations, can lead to selective enrichment of larger particles along the channel center due to shear-induced particle migration. Here, we show experimentally that subjecting such a flow to an acoustic standing wave field hastens the particle migration and segregation process, and also design a numerical model that explains the observations.

The effect of wall-normal external force and particle inertia on the deposition of particles in wall-bounded turbulence has been investigated by a point-particle Lagrange simulation. The preferential distribution of particles in the near-wall regions and the insufficient deceleration in the viscous sublayer jointly cause the enhancement of deposition velocity. The external forces strongly affect the clustering of particles in coherent flow structures. The increasing of the wall-normal force inhibits the clustering of particles, which further affects the deposition velocity.

Lofting characteristics and sedimentation patterns in reversing buoyancy particle-bearing jets are investigated experimentally and theoretically. The presence of particles and buoyancy show notable differences in the jet dynamics and shape of the sediment deposit. The suction force significantly influences the lofting location of the jet. The sedimentation dynamics are affected by the bottom drag and entrainment of the ambient fluid.

Phase reduction is a reduced-order modeling technique that can express the high-dimensional periodic dynamics with a single scalar phase variable. We develop an adjoint-based phase reduction framework for incompressible periodic flows. This adjoint-based analysis reveals the high-fidelity spatial sensitivity fields with respect to a perturbation over the limit cycle of a periodic flow in a computationally efficient manner.

Extreme events are sudden and violent changes in the state of a nonlinear system, which may have adverse consequences on the system’s components. We use recurrent neural networks to time accurately and statistically predict extreme events in a reduced-order model of chaotic shear flow at multiple Reynolds numbers. Through the predictions of the networks, we control the flow and reduce the occurrence of the events, therefore improving the operability of the system.

Observations and computer models are almost always resolution-limited. Is it possible to infer the sub-resolution flow from that we already resolve? This is indeed possible by performing an operation analogous to a Taylor series expansion, but in scale rather than in space. What makes this inference possible is a property called “scale-locality,” which gives us a powerful handle on how resolved and unresolved structures are coupled.

The inertial lag in particles’ response to turbulent motions drives clustering, but differential sedimentation attenuates it when sizes are unequal. We accurately model the radial distribution function of polydisperse sedimenting particles in turbulence by building on previous theory and direct numerical simulations and validate it against available experimental data. Our model suggests that inertial clustering of micron size drops in clouds promotes like-sized drop collision. This can break the bottleneck of small nearly monodisperse droplets formed by condensation and facilitate a transition to rapid differential-sedimentation-driven coalescence which, in turn, leads to precipitation.

Spatiotemporal patterns in the streamwise velocity probability density function (PDF) are examined in a variety of wall bounded flow datasets. Experimental data spanning a range of Reynolds numbers, with very long temporal and spatial domains, suggest that the rate of observed temporal variations scales in inner units, while the flow geometry (internal versus external) and the use of a convection velocity have a marginal effect. Results from synthetic databases generated from the resolvent framework and the attached eddy model compare well with experiments further supporting the robustness of these features and highlighting the need for their inclusion in uniform momentum zone modeling.

We investigate the intermittent behavior of velocity circulation statistics in quantum turbulence. We perform grid turbulence experiments in superfluid helium and numerical simulations of the two-fluid Hall-Vinen-Bekarevich-Khalatnikov (HVBK) model to show that the scaling properties of circulation moments are compatible with classical turbulence. We also show that there is no apparent temperature dependence on the intermittent behavior of the turbulent superflow.

Intense vorticity structures (IVS) are known to exist in virtually all turbulent flows, and consist of regions with particularly intense vorticity with (typically) a tubelike shape that keeps its coherence for a relatively long time. We investigated these structures at high Reynolds numbers (Re) using direct numerical simulations (DNS) of isotropic turbulence. The IVS aspect ratio (length to radius) is found to be the same for simulations with very different Re. Since the IVS radius scales with the Kolmogorov microscale, independent of Re, the IVS length at high Re also scales that way, and not with the integral scale or Taylor microscale, as has been suggested in the past.

Supersonic impinging jets resonate and produce loud acoustic tones that are well predicted by Powell’s feedback formula. However, Powell’s model does not account for tone modulation in a two-jet configuration. This work develops a new theory that incorporates an additional coupled acoustic feedback loop, dependent on jet impingement height and separation distance, to predict impinging tone frequencies augmented by coupled resonance. The model is successfully compared with simulation and experimental data.

Past studies have shown that decaying homogenous turbulence exhibits nonclassical properties near the initial conditions that gave rise to it (e.g. a turbulence grid in laboratory settings, or a numerical forcing in simulated turbulence). Here, we explore these nonclassical properties using a combination of numerical and theoretical work. A novel cascade picture for turbulence near initial conditions is also proposed.

In compressible Rayleigh–Taylor instability, flow compressibility plays an important role in the generation of large-scale kinetic energy, which mainly comes from the conversion of potential energy for small stratification parameter (Sr) and conversion of internal energy through pressure-dilatation work for large Sr. The latter leads to bubble heights increasing rapidly and bubbles that are bigger at large Sr. The overall statistics of normalized subgrid-scale (SGS) flux of kinetic energy is nearly independent of Sr, but the reverse SGS flux increases significantly with increase of Sr. The compression motions enhance direct SGS flux and the expansion motions strengthen the reverse SGS flux.

Roughness activates the role of Prandtl number in deciding mean global heat transfer rate.

We detect so far unknown complex singularities in the temporal domain of the inviscid Burgers equation in one space dimension. The (early!) loss of time-analyticity of the velocity is accompanied by the appearance of initially localized resonant behavior which, as we claim, is a temporal manifestation of the so-called tyger phenomenon, reported in Galerkin-truncated implementations of inviscid fluids. We test two methods that reduce the amplitude of these tygers significantly. Our techniques are straightforwardly adapted to higher dimensions and/or applied to other equations of hydrodynamics.

Resolvent analysis is a powerful tool in fluid mechanics, both for laminar and turbulent flows. Depending on application, computational cost is still a limiting factor. We develop a parabolic method for the calculation of low-frequency resolvent modes, associated with streaky structures, which can enable more than an order of magnitude speed up compared to usual global calculations. The method is applied to both laminar and turbulent boundary layers, unveiling physical mechanisms such as those associated with streak growth due to free-stream turbulence, in the laminar case, and the double peak in the frequency spectrum of velocity fluctuations, in a turbulent boundary layer.

Strong, long-lived vortices play a prominent role in two-dimensional (2D) turbulent flows, and understanding their statistics is key to unravelling 2D turbulence. Here it is shown that clustered same-sign vortices in forced 2D turbulence have a scale-invariant distribution of areas and are described by a mean-field theory. Larger clusters come apart more quickly, and the flow surrounding the remaining clusters becomes progressively more randomized and neutral, while vortex merger concentrates coherent enstrophy into a smaller and smaller fraction of the domain.

Stirring a stably stratified fluid can lead to the formation of a system of well-mixed layers separated by sharp interfaces. We present a one-dimensional mixing-length model for layering, and investigate trends in the solutions to late times. Over time, layers drift and merge together on an inverse logarithmic timescale.

A nonequilibrium, strongly stratified turbulent flow is examined with a numerically simulated data set. We find self-organization of the flow around a critical state with a local gradient Richardson number (Ri) of about 1/4. The turbulence spontaneously organizes into layered, anisotropic flow structures within which the local Ri probability density function peaks around 1/4 for regions with the most significant dissipation rates and eddy viscosities. The results suggest that self-organization of the flow around Ri = 1/4 could be a property of layered anisotropic stratified turbulence and may not require the specific external forcing mechanisms previously reported.