In this paper, we present results on the effect of viscosity and surface tension on the collapse of large cavities produced by overdriving Faraday waves in a cylindrical container. The forcing amplitude of the container excitation has been increased at a rate such that the last stable wave amplitude $b$ was close to $b_s$, referred to as the singular wave amplitude. The collapse of the wave-depression cavity that follows $b⪅b_s$ gives rise to the largest surface jet velocities; when $b>b_s$, cavity collapse occurs with a bubble pinch-off. Viscosity has been varied by two orders of magnitude using water and glycerine-water (GW) solutions. Surface tension effects are highlighted by comparing with previously obtained results with FC 72, a low-surface-tension and low-viscosity liquid. The main objective has been to clarify how these fluid properties affect the cavity shape and cavity collapse dynamics. It is shown that the initial cavity depth depends only weakly on fluid properties, whereas the initial radius decreases with increasing viscosity and increases with decreasing surface tension. The collapse of the cavity is initially inertial with minimum cavity radius $r_m$ varying with time in the form $r_m\sim {\tau }^{\alpha }$, with $\alpha \simeq 0.5$, where $\tau =(t_0-t)$, with $t_0$ being the time at singular collapse. In high-viscosity fluids, there is an inertial-viscous transition to $\alpha =1$, whereas in water the transition is to $\alpha >1/2$ and is close to $2/3$, indicating an inertial-capillary transition. In low-viscosity and low-surface-tension fluid (FC 72), collapse remains inertial up to singular collapse. The transitions are characterized by the evolution of the relevant dimensionless flow parameters. It is shown that inertial-viscous transition occurs when the capillary number, $\mathrm{Ca}=U_r\mu /\sigma$, defined with the local radial velocity, $U_r$, changes from $\mathrm{Ca}<1$ to $\mathrm{Ca}>1$, while the local Ohnesorge number is large, $\mathrm{Oh}=\mu /\sqrt{\rho \sigma r_m}\gtrsim 0.1$. The local Reynolds number at transition remains large and decreases with decreasing $\tau$ to $\mathrm{Re}\sim 1$. The velocity of the jet, emerging from the free surface following singular collapse, increases with viscosity, and reaches a maximum in GW. Numerical simulations give an indication of the increase and localization of the pressure that drives the liquid jet with a high-speed precursor air jet.

We study the formation and inward propagation of a convective layer when a stably-stratified fluid with a composition gradient is cooled from above. We perform a series of two-dimensional simulations using the Bousinessq approximation with Prandtl number ranging from $\text{Pr}=0.1$ to 7, extending previous work on salty water to low Pr. We show that the evolution of the convection zone is well described by an entrainment prescription in which a fixed fraction of the kinetic energy of convective motions is used to mix fluid at the interface with the stable layer. We measure the entrainment efficiency and find that it grows with decreasing Prandtl number or increased applied heat flux. The kinetic energy flux that determines the entrainment rate is a small fraction of the thermal energy flux carried by convective motions. In this time-dependent situation, the density ratio at the interface is driven to a narrow range that depends on the value of Pr, and with low enough values that advection dominates the interfacial transport. We characterize the interfacial flux ratio and how it depends on the interface stability. We present an analytic model that accounts for the growth of the convective layer with two parameters, the entrainment efficiency and the interfacial heat transport, both of which can be measured from the simulations.

We investigate numerically the influence of a weak current on wind-generated surface deformations for wind velocity below the onset of regular waves. In that regime, the liquid surface is populated by small disorganised deformations elongated in the wind direction, referred to as wrinkles. These wrinkles are the superposition of incoherent wakes generated by the pressure fluctuations traveling in the turbulent boundary layer in the air. In this work, we account for the effect of a weak sheared current in the liquid, either longitudinal or transverse, by introducing a modified Doppler-shifted dispersion relation to lowest order in viscosity and current in the spectral theory previously derived by Perrard et al. [J. Fluid Mech. 873, 1020 (2019)]. This theory describes the simplified one-way problem of surface deformations excited by a prescribed turbulent forcing, thereby neglecting the retroaction of waves on turbulence in the air. The forcing is taken from a set of direct numerical simulations of a turbulent channel flow. We determine the wrinkle properties (size and amplitude) as a function of the liquid viscosity and current properties (surface velocity, thickness, and orientation). We find significant modifications of the wrinkle geometry by the currents: the wrinkles are tilted for a transverse current and show finer scales for a longitudinal current. However, their characteristic size is weakly affected, and their amplitude remains independent of the current. We discuss the implications of these results on the onset of regular waves at larger wind velocity. In this work, we introduce a spectral interpolation method to evaluate the surface deformation fields, based on a refined meshing close to the dispersion relation of the waves. This method, which can be extended to any dispersive system excited by a random forcing, strongly reduces the discretization effects at a low computational cost.

Vibrational relaxation of single-component carbon dioxide is studied using the full and reduced state-to-state models and two multi-temperature approaches. The full kinetic scheme including all vibrational states and different kinds of vibrational energy transitions within and between ${\mathrm{CO}}_2$ modes is proposed and implemented to the 0-D code for spatially homogeneous relaxation. Contributions of various energy transitions are evaluated, and dominating relaxation mechanisms are identified for two generic test cases corresponding to compression (excitation) and expansion (deactivation) regimes. It is shown that the main relaxation channels are vibrational-translation (VT) transitions in the symmetric and bending modes and two intermode vibrational-vibrational (VV) exchanges. Reduced-order models are assessed by comparisons with the results of full state-to-state simulations. The commonly used two-temperature model with the single vibrational temperature fails to describe the relaxation for all considered initial conditions. The three-temperature model provides a good agreement with the state-to-state simulations for the excitation regime, but yields a considerable discrepancy for the deactivation mode. The sources of the discrepancies are detected and several ways for the improvement of numerically efficient multi-temperature models are proposed.

We investigate the dynamics of films of partially wetting liquids falling down a vertical plane in the presence of air. We focus on the little-studied case where the film is not in contact with lateral walls and thus has two side contact lines. By way of 3D direct numerical simulations, we study the fingering instability and the distribution of rivulets. Our results show that the instability develops as a cascade: two twin rivulets first develop at the free sides, followed by a series of twin rivulets growing closer to the center. This self-sustained fall is caused by the formation of stagnation zones at the front contact line as the liquid behind the ridge escapes toward a newly created rivulet. We also show that the established rivulet distance follows the prediction of linear instability, despite the cascade. The dynamics of the cascade is modified by inertial and film thickness effects, as well as by initial shape perturbations of the contact line. On the contrary, varying the contact angle does not affect the development of the cascade, apart from the average wavelength.

A remarkable variety of organisms and wet materials are able to endure temperatures far below the freezing point of bulk water. Cryotolerance in biology is usually attributed to “antifreeze” proteins, and yet massive supercooling ($<-40{}^{\circ }\mathrm{C}$) is also possible in porous media containing only simple aqueous electrolytes. For concrete pavements, the common wisdom is that freeze-thaw (FT) damage results from the expansion of water upon freezing, but this cannot explain the high pressures ($>10$ MPa) required to damage concrete, the observed correlation between pavement damage and deicing salts, or the FT damage of cement paste loaded with benzene (which contracts upon freezing). In this work, we propose a different mechanism—nanofluidic salt trapping—which can explain the observations, using simple mathematical models of dissolved ions confined between growing ice and charged pore surfaces. When the transport time scale for ions through charged pore space is prolonged, ice formation in confined pores causes enormous disjoining pressures via the ions rejected from the ice core, until their removal by precipitation or surface adsorption at lower temperatures releases the pressure and allows complete freezing. The theory is able to predict the nonmonotonic salt-concentration dependence of FT damage in concrete and provides some hint to better understand the origins of cryotolerance from a physical chemistry perspective.

The coupling effect of the isotropic wall slip and the spanwise oscillation boundary conditions on the drag reduction and turbulence properties are studied by direct numerical simulations. As the slip length increases, the drag reduction gradually changes from the oscillation dominated to the slip dominated. The increase of slip length will decrease the maximum spanwise velocity of the fluid on the wall, which is responsible for the decreased ability of the oscillatory wall motion to reduce the skin-friction drag. The drag reduction decomposition shows that the contribution from the modifications of turbulent dynamics will undergo a shift from drag increase to drag reduction as the isotropic slip length increases. Compared with the respective no-slip reference flow, the drag reduction of wall slip in laminar and turbulent channel flows can be expressed in a unified form versus the outer scale slip length. Furthermore, many aspects of the turbulence properties are influenced by the coupling effect. First, the wall slip condenses the envelope range of the phase fluctuations caused by the oscillatory wall motion, as well as the magnitude of the periodic fluctuation of the phase-averaged friction coefficient. Second, an unexpected property of the coupled boundary condition is found that the existence of the Stoke layer delays the relaminarization process caused by the large slip length. Third, the wall slip would narrow the periodic inclination of the streaks and then inhibit the energy transfer process in the horizontal direction. Fourth, in terms of phase, the Stokes strain and the shear angle have the same lag phase with the spanwise velocity, while the hysteresis of turbulent dynamics leads to the larger lag phase of the streaks and phase-averaged friction coefficient. These new features are valuable for increasing knowledge on this topic.

It is now recognized that aerosol transport contributes to the transmission of the SARS-CoV-2 virus. Here, we improve existing social distancing guidelines for airborne pathogens, which are typically given in terms of distance with vague statements about contact times. Also, estimates of inhalation of virus in a contaminated space usually assume a well-mixed environment, which is realistic for some, but not all, situations. In particular, we consider a local casual interaction of an infected individual and a susceptible individual, both maskless, account for the air flow and aerosol transport characteristics of speaking and breathing in a poorly ventilated space, and propose social distancing guidelines that involve both space and contact time, based on a conservative model of fluid dynamics of the interactions.

Permeable porous coatings on a flat solid support significantly impact its electrostatic and electrokinetic properties. Existing work has employed the mean-field Poisson-Boltzmann theory by focusing on simplified cases, such as weakly charged and/or thick porous films compared to the extension of an electrostatic diffuse layer. In this paper, we obtain a closed-form analytical solution for electrostatic potential profiles by lifting the assumptions of both a small volume charge density and a thick film. Our analysis provides a framework for interpreting and predicting superproperties specific to porous films of an arbitrary thickness, from an enhanced ion absorption to a consequent amplification of electro-osmotic flows due to emergence of slip velocity at an interface with an outer electrolyte leading to a large zeta potential. The latter can be tuned by varying the amount of added salt and remains finite at even high concentrations. Our theory is valid for systems obeying the nonlinear Poisson-Boltzmann equation and the results are relevant for hydrogel coatings, porous carbon and silica, polyelectrolyte brushes, and more.

We numerically study the dynamic behavior under a symmetric shear flow of selected examples of concentrated phase emulsions with multicore morphology confined within a microfluidic channel. A variety of nonequilibrium steady states is reported. Under low shear rates, the emulsion is found to exhibit a solidlike behavior, in which cores display a periodic planetarylike motion with approximately equal angular velocity. At higher shear rates, two steady states emerge, one in which all inner cores align along the flow and become essentially motionless and a further one in which some cores accumulate near the outer interface and produce a dynamical elliptical-shaped ring chain, reminiscent of a treadmillinglike structure, while others occupy the center of the emulsion. A quantitative description in terms of the (i) motion of the cores, (ii) rate of deformation of the emulsion, and (iii) structure of the fluid flow within the channel is also provided.

We study dynamo action using numerical simulations of planar Boussinesq convection at rapid rotation (low Ekman numbers, $\text{Ek}$), focusing on subcritical dynamo action in which the dynamo is sustained for Rayleigh numbers, $\text{Ra}$, below the critical Rayleigh number for the onset of non-magnetic convection, ${\text{Ra}}_c$. These solutions are found by first investigating the supercritical regime, in which the dynamo is able to generate a large-scale magnetic field that significantly influences the convective motions, with an associated Elsasser number of order ${\text{Ek}}^{1/3}$. Subcritical solutions are then found by tracking this solution branch into the subcritical regime, taking a supercritical solution and then gradually lowering the corresponding Rayleigh number. We show that decreasing the Ekman number leads to an extension of the subcritical range of $\text{Ra}/{\text{Ra}}_c$, down to an optimal value of $\text{Ek}={10}^{-5}$. For magnetic Prandtl numbers of order unity, subcriticality is then hampered by the emergence of a large-scale mode at lower Ekman numbers when the dynamo driven by the smaller scale convection generates relatively stronger large-scale magnetic field. The inability of the large-scale mode to sustain dynamo action leads to an intermittent behavior that appears to inhibit subcriticality. The subcritical solutions are also sensitive to the value of the magnetic Reynolds number (or equivalently, the magnetic Prandtl number, $\text{Pm}$), as values of the magnetic Reynolds number greater than 70 are required to produce dynamo action, but large values lead to fluctuations that are able to push the system too far from the subcritical branch and towards the trivial conducting state.

Micron particles rotate and their trajectories deviate from the impact direction when impacting the liquid surface obliquely; thus their impact dynamics is largely different from that of vertical impact, which remains to be studied in depth. In this study we established a three-dimensional simulation method by solving the coupled equations of particle motion and fluid flow and adopting a dynamic meshing technique and slip velocity boundary condition that accurately reproduced the particle motion and gas-liquid interface evolution during oblique impact. Our results, based on detailed flow field and acting forces and moments analyses, demonstrate that the particle's nonaxisymmetric wetting leads to the nonaxisymmetric distribution of fluid pressure and shear stress along the particle surface, and thus deviates the dominating forces at different impact stage from its direction of motion, which makes the particle trajectory deviate from the impact direction and also generates a viscous moment that rotates the particle. We also illustrate how the impact angle and Weber number modulate the flow field around the particle as well as the acting forces and finally influence the particle motions. These findings provide a deep understanding of the dynamics of the nonaxisymmetric impact of the micron particle on liquid surfaces, which is a ubiquitous scenery in nature and industry.

The present theoretical study is concerned with evaporation-induced apparent contact angles for a perfectly wetting one-component liquid placed on a flat solid substrate and undergoing diffusion-limited evaporation into ambient air. The analysis pertains to a distinguished small vicinity of the contact line (the “microregion”), where such angles are established and where various microscopic effects typically enable relaxing the well-known evaporation-flux singularity. We proceed from a Joanny-Hervet-de Gennes-type approach, involving the spreading coefficient, disjoining pressure in the form of an inverse cubic law, and a truncated microfilm (precursor film) starting abruptly at a solid surface. A more classical regime with an (infinitely) extended adsorbed microfilm is recovered therefrom in the limit of large spreading coefficients upon additional incorporation of the Kelvin effect (dew point shift due to the liquid–gas pressure difference). The latter regime is critically revisited with a view to clarifying the scaling prefactor known in the literature. The influence of the kinetic resistance to evaporation is analyzed as well.

We present experiments on rigid, nylon fiber translation, orientation, and rotation dynamics (one-way coupled) in a fully developed turbulent channel flow (Friction Reynolds number, ${\mathrm{Re}}_{\tau }=435$). The experiments were performed using two-orthogonal view, digital inline Fraunhofer holographic cinematography that allowed us to track individual fibers and determine their three-dimensional (3D) position and orientation along tracks. The research focused on fiber length effects, and two fiber types having similar Stokes numbers but different mean lengths, ${\overline{L}}^{+}$ (=27.7 and 50.8), were investigated. Time-resolved data were acquired in the buffer layer, the log layer, and the wake region of the turbulent boundary layer. In the buffer layer irrespective of length, fibers moved faster than the fluid, presumably as a result of fiber accumulation in high-speed streaks. Beyond the buffer layer fibers lagged the fluid, more so for the longest fibers due to increased drag. Probability density functions of instantaneous components of fiber velocities showed that the longest fibers exhibit increased probabilities of “extreme” transverse and wall-normal velocities, mostly in the log layer and the wake region. This is attributed to their interaction with larger, more energetic turbulence structures. Significant fiber-wall interactions were absent, even for the longest fibers due to fiber preferential alignment with the streamwise direction resulting in limited fiber-wall interaction even when the ratio of fiber length to wall-normal distance is smaller than unity. Upon approaching the wall, fiber rotation rates strongly increased. In the wall-normal plane, in-plane fiber rotation rates as a function of wall-normal position were the same for both fiber types. However in wall-parallel planes, in-plane rotation rates of the shorter fibers were higher than those of the longer ones in the buffer layer and vice versa in the wake region. Measured mean-squared fiber tumbling rates strongly increased in the buffer layer for both fiber types, while they remained nearly constant in the log layer and the wake region. A clear length effect was apparent, and the longest fibers consistently tumbled at a higher rate than the shorter ones, surmised to be the result of their interaction with more energetic, larger turbulence structures.

When a partially filled glass is gently tilted, the liquid can go over its brim without spilling. This happens because of the surface tension and has a direct indication when studying the emptying of a partially filled capillary by modifying the present mathematical criteria of critical emptying. Here, we present an experimental study of the edge effect on emptying a finite-lengthed horizontal capillary, a condition that has never been studied before. We observed that due to the edge meniscus from the finite length of the capillary, critical emptying conditions are significantly altered. Furthermore, by optimizing the edge parameters and the brim wetting effect, water can hold onto much wider capillary brims. We performed numerical simulations using realistic geometries and found excellent agreements with our experimental results. Our results generalize the capillary emptying criterion so that they can be used in realistic geometries and provide guidance in understanding a wide range of phenomena in microfluidics and optofluidics.

On designed heterogeneous surfaces, the lateral motion of a droplet can be controlled without requiring an external energy input. However, it is still a challenge to elaborately manipulate the self-migration behaviors of an impinging droplet because of a lack of fundamental understanding of these behaviors. Through direct numerical simulation, a wide range of parametric studies are conducted to investigate the effect of the Weber number ($\mathrm{We}$), wettability difference, and offset impinging on the four phases—asymmetric spreading, directional retracting, detaching, and migrating—of droplets impacting heterogeneous surfaces. The results indicate that asymmetric spreading and directional retracting toward the hydrophilic area are induced by the unbalanced net forces acting on the three-phase contact line. The increase in the wettability difference leads to the increase in the migration distance from the borderline, the decrease in the droplet bouncing height, and the decrease in the contact time between the liquid and the nonwetting area. An increasing $\mathrm{We}$ influences the spreading diameter and the migration distance, but no significant difference can be observed during the retracting and detaching stages for moderate $\mathrm{We}$ owing to the combined effect of the spreading rate and the surface tension force. In addition, offset impinging plays a major role in droplet spreading and deformation. These physical insights provide developing guidelines for the design of surfaces to manipulate the behaviors of impinging droplets.

One of the primary challenges in the study of gust encounters lies in isolating, defining, and parametrizing a specific problem in a highly unsteady, three-dimensional, and multiscale flow. Recent efforts have decomposed the complex gusty environment typical of atmospheric turbulence and wakes into three canonical gust types: transverse gusts, vortex gusts, and streamwise gusts. By applying analytical, experimental, and numerical methods, a comprehensive picture of the flow fields and force histories typical of gust encounters has been achieved, shedding light on how the defining parameters in the problem affect unsteady forcing. The focus of the current work is on vortex formation and lift production on rigid two-dimensional wings. Despite these simplifications, analytical and low-order modeling of gust encounters resulting in massively separated flows remains a challenge, largely because the force response of the wing is highly sensitive to the growth and motion of vorticity in the flow. It also remains to be seen how well canonical gusts represent real-world gust encounters, and whether there is some region of the parameter space where linear superposition of these inherently nonlinear flows results in a reasonable approximation of the true problem. This article provides an overview of a limited selection of recent and classic work on large-amplitude gust encounters to motivate a discussion of current challenges in parametrizing and modeling these flows, as well as thoughts on working toward a long-term goal of mitigating gust responses.

High Reynolds number wind tunnels are essential tools for testing theories of turbulence. This need has led to the construction of tunnels that use compressed gases as the working fluid, and such facilities have given new insights into the behavior of turbulence in wall-bounded flows. Here, we focus on results obtained at Princeton using the Superpipe and the High Reynolds number Testing Facility that have given us new insights into the behavior of wall-bounded flows, in particular, fully developed pipe flow, and turbulent boundary layers in zero pressure gradients.

As fluid dynamicists, good communication is key to our success in academia, industry, and research. Yet our training in this critical skill is often lacking. In this article, I present a framework for the science communication process and techniques for integrating communication training into everyday practices. With some forethought and habit-building, preparing engaging conference presentations, writing journal articles, or interacting with journalists does not have to be a painful, last-minute scramble. Instead, these activities can be just another part of your communication lifestyle.

In this perspective we provide a brief overview of the state of knowledge and recent progress in the area of multiphase flow through deformable granular media. We show, with many examples, that the interplay between viscous, capillary, and frictional forces at the pore scale determines the mode of fluid invasion. We pay particular attention to the central role of wettability on the morphology of granular-pack deformation and failure. Beyond their intrinsic interest as processes that give rise to spectacular pattern formation, these coupled phenomena in granular media can control continental-scale fluxes like methane venting from the seafloor and geohazards like earthquakes and landslides. We conclude this perspective by pointing to fundamental knowledge gaps and exciting avenues of research.