Phase-field modeling of drop dynamics and direct numerical simulation of drop-laden turbulent flow are used to examine the complex topological changes of the dispersed phase — breakage and coalescence — and to determine the evolving drop size distribution. Potentials and perspectives for further development of the phase-field method to model mass transfer, heat transfer, and Marangoni effects due to thermocapillarity and surfactants are also discussed.

Wedge water entry serves as a key model to understand phenomena like high-speed craft slamming, seaplane landings, and diving aquatic birds. In this paper, wedge water entry experiments and simulations are used to examine how hydrodynamic loads, structural deflection, water contact lines, and rigid body motions are influenced by changes in the flexural rigidity of the wedge’s bottom panels. Preliminary findings indicate that the nondimensionalized spray root position and velocity versus time collapse despite significant variations in the panel’s flexural rigidity values (see figure for velocity curves). The study provides insights for future research and model improvements in water entry dynamics.

We introduce similarity relations and symmetry groups for the analysis of laminar Bingham fluid flows, presenting a valuable new expression for calculating the friction factor, particularly when pressure gradient data are available. Notably, we highlight the significance of friction coordinates as the most suitable framework for unraveling the complexities of Bingham plastic fluids.

Drop breakup is often associated with boiling or violent impacts onto targets. We report on experiments where the decrease of ambient pressure triggers the growth of a bubble in a drop that sits on a textured hydrophobic surface. We find a transition from top-breakup to triple-line breakup depending on the initial contact angle of the drop, which is captured by a model based on inertial dynamics.

The rupture of thin liquid films on solid surfaces is conventionally assumed to be driven by intermolecular forces, in the so-called spinodal regime. Here, a theoretical framework is created for the experimentally observed thermal regime, in which fluctuation-induced nanowaves rupture films. Fluctuating hydrodynamics is able to capture this regime and its predictions are verified by molecular simulations. Rare-event theory is then applied to this field for the first time to reveal a novel picture of how and when rogue nanowaves create rupture.

Pick up a green foxtail stem (a plant weed) and slowly rotate it between your fingers. The stem initially bends outward and whirls owing to its rotational inertia. However, as the rotational speed increases, the stem abruptly returns to its straight configuration, rapidly spinning about its axis (twirling). We combine experiment, simulation, and theory to reveal the underlying physical mechanism in this seemingly simple system. We quantify the effects of high-speed flow on the shape dynamics of a slender body by suitably designing a particular geometry of a flexible body in the elastohydrodynamic problem.

Dissolution and mass transfer of carbon dioxide gas into liquid barriers plays a vital role in many environmental and industrial applications. We study the downward dissolution and propagation dynamics of CO${}_2$ into a vertical water barrier confined to a narrow vertical glass cylinder, using both experiments and direct numerical simulations (DNS). Tracking the CO${}_2$ front propagation in time leads to discovery of two distinct transport regimes: A purely diffusive and an enhanced diffusive regime. Using DNS, we explain the propagation dynamics of these two transport regimes, namely by disentangling the contributions of diffusion and convection to the propagation of the CO${}_2$ front.

We investigate the most turbulent simulations to date for rapidly rotating convection in a plane layer geometry. The results show that the flow remains viscously controlled across the investigated parameter space, and that the length scales present within the flow remain comparable to those predicted by linear theory.

We study force balances in two-dimensional Rayleigh-Benard simulations with a large range of imposed background magnetic fields. We find two main regimes, a magnetically constrained regime where the linear Lorentz force dominates, and a magnetically influenced regime where the nonlinear Lorentz force heavily influences dynamics, meaning that classic hydrodynamical flow patterns are not achieved at the highest Rayleigh number studied in this work. Despite the impact of the nonlinear Lorentz force on the dynamics, we find classical hydrodynamic scaling of the Nusselt number and also derive scaling laws for the magnetic field in both of these regimes.

Through high-resolution pore-scale simulations, we uncover that impermeable solid porous matrices significantly influence the heat and fluid flow within these materials. We discover that as porosity decreases, temperature fluctuations increase; unlike porous materials with matrices being permeable to heat flux, impermeable ones cause less organized flow patterns. Furthermore, intense thermal energy dissipation mainly occurs near the surfaces of the porous material, and exhibits more intermittency compared to traditional convection systems.

A special caterpillar like motion is reported for glycerin droplets sliding on a horizontal hydrophobic substrate under the influence of a shear flow. The droplet elongates in the flow direction adopting a rivulet shape with the development of waves resulting in a caterpillar like motion.

We study experimentally the dynamics of non-wetting blobs flowing simultaneously with a wetting fluid in a quasi-two-dimensional porous medium consisting of random obstacles. The blobs continuously merge forming larger ones (coalescence) and breakup into smaller ones (fragmentation) leading to an overall dynamic equilibrium between the two processes. In this work, we analyze the probability functions for coalescence and breakup as a function of blob sizes and total flow rate.

Tiny Leidenfrost droplets exhibit spontaneous motion on a flat, heated plate. Despite their lack of contact with the plate or exposure to external forces, the author discovered that these droplets move in random zigzag patterns. Through thermographic analysis, it was observed that a vertical “hot belt” appears on the droplet’s surface and it spins as if the droplet is a spinning top. This rotational behavior was attributed to internal flows generated within the droplet, originating from buoyancy and the Marangoni force. Finally, the rate of spin was quantitatively characterized in relation to the Marangoni relaxation time and the temperature of the substrate.

Dendrite formation due to morphological instability in cathodic electrodeposition of a metal and related fluid flows are investigated. We show that the electric double layer fine structure and the finite electrode reaction rate regularize the electrodeposition front short-wave singularity and select a range of unstable perturbation modes. The critical wavelength corresponds to the fastest growing mode and scales with the electric double layer width and the reaction-diffusion length. The emerging electroconvective (electroosmotic) flow in the nonequilibrium regime selects the cathodic diffusion layer width as the dominant length scale for morphological instability and emerging dendrites.

Mechanical structures in nature and in applications are often very flexible and also, are exposed to vortex-laden flows. While vortex-induced-vibration is one of the most commonly studied fluid-structure interactions, we explore what can be called vortex-forced-vibrations of a flexible sheet to provide physical insights on the average reconfiguration, vibration amplitude, and frequency response of a thin sheet forced by von Karman vortices.

An air finger is injected with constant volumetric flow rate into a bifurcating liquid-filled Hele-Shaw channel topped with an elastic sheet, to study airway reopening. Imposing collapsed channel configurations, we observe finger propagation in straight parts of the channel that are reflected in finger morphologies. The mechanics involved also controls recovery of steady states post-bifurcation, and, in some regimes, can lead to multiple reopening scenarios downstream from the bifurcation for experimentally indistinguishable fingers in the main channel. Our results suggest that steady state propagation is unlikely to be recovered between bifurcations in practical networks.

The interplay between viscosity ratio and contact angle for viscous-dominated flows in a two-dimensional porous medium is studied with Direct Numerical Simulations. Viscous fingering, intermediate flow, and compact displacement regimes were examined for various viscosity ratios at the sample scale. Even at the pore-scale, various pore filling mechanisms were found. For imbibition and drainage, cooperative filling occurs at large viscosity ratios. At low viscosity ratios, film flow (strong imbibition), and unstable bursts (strong drainage) were seen. Changes in pore filling mechanisms occurring at the same contact angle indicates the viscosity ratio impact on effective wettability variation.

During the Leidenfrost effect, a stable vapor film separates a hot solid from an evaporating liquid. As the solid cools, the vapor layer is metastable, and can undergo a violent collapse accompanied by explosive boiling of the liquid. We provide insight into this failure mechanism using a computational model with hydrodynamics, thermodynamics, and rapid evaporation of the liquid phase. By varying both liquid and solid properties, we find that inertial forces, which are typically ignored in theoretical treatments of the vapor layer, are likely responsible for initiating the instability leading to failure.

One of the main flow geometries into which bubbles break in turbulence are uniaxial straining flows. In this work we investigate bubble deformations and breakup in these flows, as a function of the Reynolds number, the Weber number (We, ratio of inertia and capillarity) and the initial bubble shape. We demonstrate that, even though linear stability analysis predicts the existence of a stable position for some We smaller than a critical value, bubbles can break due to their initial shape. Finally, we propose a reduced nonlinear model based on oblate-prolate bubble oscillations which quantitatively captures the deformations dynamics and bubble lifetime.

Lubrication flow in a soft Hele-Shaw cell formed by the narrow gap between a rigid boundary and a confined elastic solid can choke itself: The viscous pressure gradient squeezes the soft material which bulges into the flow path near the cell outlet, ultimately interrupting the flow if the flow rate is too large. Here we show that if the lubrication flow is driven by injected gas displacing the viscous liquid, then the critical injection flow rate above which choking occurs is increased by both the proximity to the cell rim of the gas-liquid interface and by the compression of the gas, resulting in a reduced tendency of the cell to choke.

We investigate the thermocapillary instability of a three-dimensional shear-imposed viscous falling film for infinitesimal disturbances of arbitrary wavenumbers. The results show that the H-mode and the thermocapillary S-mode instabilities intensify, but the thermocapillary P-mode instability attenuates if a constant shear stress is applied in the co-flow direction. However, a completely opposite event occurs if it is applied in the counter-flow direction. Interestingly, as the imposed shear stress rises, the onset of instability for the H-mode and the onset of stability for the S-mode merge with each other and produce a single onset for the primary stability.

Fluid from a circular jet impacting onto a moving wall can spread out into a distinctive, thin, U-shaped lamella over a range of Reynolds numbers and wall-to-jet velocity ratios. The lamella travels a distance in the direction counter to the motion of the wall that is about one third of the width that is eventually reached far downstream. Scaling with either of these lengths collapses the footprint of the lamella on the moving surface onto a universal curve.

The planar boundary affects the flow physics of the wake behind a rotationally oscillating cylinder about its axis and has potential practical engineering applications in areas dealing with fluid mixing and offshore and marine engineering. Wake modes in the transition regime of the flow behind a circular cylinder have been a topic of interest for a long time. In this study, various wake modes in the cross-plane and the spanwise plane are discussed for various gap distances (distance from the planar boundary to the cylinder) and cylinder forcing parameters (oscillation amplitude and frequency) of a rotationally oscillating cylinder.

A novel low-Reynolds-number fluid pump architecture is proposed wherein the offset interdigitated teeth of a hot comb and a cold comb in contact with a liquid cause its unidirectional motion due to thermocapillary stresses active along the unequal menisci between the teeth. The pump involves no moving parts, requires no external driving pressure, and pumps a continuous stream of liquid rather than a series of discrete droplets.

Turbulent flows laden with liquid droplets are known as a member of turbulent dispersed multiphase flows (TDMF) that occurs in many natural, chemical, and industrial processes. In this research, we utilize a phase/Doppler particle analyzer to experimentally evaluate the impact of droplets generated from air-assisted atomization on the turbulent modulation of continuous gas-phase flow. The presence of liquid droplets affects gas-phase turbulence in three distinct manners: increasing fluctuation velocity, expanding the region with steep fluctuation velocity gradients, and causing axial stretching within the shear region.

Numerical simulations using the dispersed-phase continuum (DPC) and the particle-in-cell (PinC) models were performed to characterize the onset of instability where the airflow in the print gap of inkjet printers becomes nonuniform in the spanwise direction. The results indicated that the baseflow shifts from a uniform flow field to a standing wave regime at a specific number density, which characterizes a supercritical pitchfork bifurcation. Furthermore, the comparison between both models showed that the DPC technique effectively computed the mean flow field with a reduction of up to 1000x in processing time.

The collapse of a granular column is a benchmark case for the study of granular materials. In this work, the collapse dynamics of two-dimensional columns of elongated grains are numerically investigated in dry and immersed conditions. The focus is set on the orientation of the grains and its influence on the behavior of the column. An energy-based approach suggests the influence of grain shape on mobility is limited to the prefactor of a power-law.

We investigate the dynamics of magnetoelastic robots as they move through a water-saturated granular bed with transverse undulation of the body being actuated by an oscillating magnetic field. The robots create dynamic burrows as they fluidize the medium and move with speeds that depend on the field frequency and strength and the sediment depth. A transition from nearly rigid to anguilliform body motion is observed because of a subtle balance of thrust, drag, and elastic forces. The overall trends in the robot’s speed are generally consistent with Lighthill’s theory of elongated body swimming.

Dynamical systems with extreme events are difficult to capture with data-driven modeling, due to the relative scarcity of data within extreme events. A recently developed technique called CANDyMan decomposes the system into separate charts containing extreme and non-extreme states, learning dynamical models in each chart via time-mapping neural networks, then stitching the charts into a global dynamical model. We apply CANDyMan to a low-dimensional model of turbulent shear flow which undergoes extreme intermittent quasi-laminarization events. We show that the multi-chart model better forecasts the dynamical system evolution than either a standard single-chart model or Koopman-based model.

We derive a scaling prediction for the typical core radius of the isolated vortices that span baroclinic turbulence. This refinement of the vortex-gas theory does not impact the scaling behavior of the eddy diffusivity for linear bottom drag. By contrast, it induces a new scaling-law for the eddy diffusivity in the ultra-low-quadratic-drag regime. We validate these predictions numerically.

This work analyzes the characteristics of turbulent transport of mass and momentum in the tilted rocket rig configuration using large eddy simulation. The tilted rocket rig is a configuration that results in inclined Rayleigh-Taylor mixing with a two-dimensional mean flow. Analysis of the spatial distributions and budgets of turbulent quantities is analyzed, and a term frequently neglected in RANS modeling is identified as a dominant contribution in the transverse turbulent mass flux budget.

In forced two-dimensional flows, the energy input is not only balanced by viscous dissipation within the fluid, but also by friction along the boundaries. In this study, we investigate the classic experimental realization of quasi-2D turbulence in electromagnetically driven layers of conducting fluids and confirm that friction dominates over viscous dissipation in such systems. The emergence of fully developed turbulence is governed by the frictional Reynolds number (Re${}_{\alpha }$) rather than the commonly used viscous Reynolds number (Re), as evidenced by the fact that descriptors from 58 cases of different forcing in five layer configurations collapse onto master curves when plotted against Re${}_{\alpha }$.

The present study investigates the momentum and heat fluxes in subsonic/supersonic channel flows. By employing the spectral linear stochastic estimation, the near-wall fluxes are decomposed into large and small-scale components, and the logarithmic-region fluxes are decomposed into active and inactive parts, respectively. The findings of the present study demonstrate that the generation of the extreme events of the heat and momentum transfers in the near-wall region should be ascribed to the near-wall small-scale flow, and both the inactive and active components contribute to the mean fluxes in the logarithmic region.

In this paper, we investigate the forces which arise between two plates in turbulent flow. We confirm previous studies which find an attractive force between plates. By simulating several plate sizes, we unravel the causes of the force, linking them primarily to vorticity.

Describing the coarse-grained velocity gradients over the inertial range scales in turbulence is a challenging but essential part of turbulence modeling. Most models assume, based on dimensional arguments, that the inertial range dynamics involve only the eddy-turnover time associated with the length scale. By analyzing the velocity gradient perceived by four fluid tracers initially forming a regular tetrahedron, however, we find that due to the fast dynamics of the pressure gradient, the perceived velocity gradients at inertial-range scales involve also the Kolmogorov time, the smallest time scale of turbulence. Our finding calls for a re-examination of the dynamics of turbulence models.

Extreme events in the near-wall region of turbulent flows play a central role in regulating the energy budget. These events are nonlinear in both space and time, and they are comprised of bursting near-wall streaks and coherent fluid packets being ejected away from/swept towards the wall. We demonstrate that Convolutional Neural Networks, which are capable of autonomously extracting three-dimensional spatial features, can identify such nonlinear coherent structures in a completely data-driven manner, with no a-priori knowledge of the underlying physics.

We systematically analyze the breaking of Oberbeck-Boussinesq symmetry in Rayleigh-Bénard convection at high turbulence intensity due to the temperature dependence of material properties. As unique signature of the symmetry breaking, we take the shift of bulk temperatures measured in the center of the convection cell from values expected for symmetrical temperature profiles. We render maps of the shift measured in the phase diagram of the fluid-cold helium gas in this case using nonlinear regression by deep neural networks. This can be considered as a first step in disentangling the longstanding problem of experimentally identifying the ultimate regime of heat transport in turbulent flows.

We show that laminar vortex rings can be generated by impulsive body forces having particular spatial and temporal characteristics. Numerical simulations and analytical models show that the strength of these rings can be accurately predicted by considering diffusion alone, despite the nonlinear nature of the generation process. It is found that when the duration of the force is less than a time scale based on the force radius and fluid viscosity, the ring circulation can be predicted accurately using an inviscid model.

Mini-tsunamis are generated when a ship travels through a depth change and can cause significant coastline erosion. It is found that the amplitude of mini-tsunami and wavelength are increased by the higher order bottom and free-surface nonlinearities, and those effects are more significant for a fast-traveling ship in a narrow channel. Our work can shed light on nonlinear effects in the generation of mini-tsunamis, and advance the development of warning systems for coastal erosion owing to the ship-generated waves.

We examined interactions in water channels between swimmer-shaped objects moving at different speeds. Results showed that the ideal place for drafting is at the hip of a neighboring swimmer or just behind a lead swimmer. The former reduces drag by 30% and the latter reduces drag by 40%. Results were confirmed by CFD simulations.

We investigate the aerodynamic and aeroacoustic characteristics of a small-scale drone rotor operating in axial descending flight. The integrated aerodynamic forces and the far-field noise are measured by wind tunnel experiments. Then, near-field flow structures and corresponding noise source analysis are presented based on computational aeroacoustics (CAA) simulations. We found the unique acoustic feature of descending flight is the haystacking-like spectral humps, possibly caused by the interaction between the blade leading edge and the rotor wake.