Ionic liquids (ILs), molten salts at low temperatures, are nonionizing, thermally stable, and with low vapor pressure, thereby offering promising applications for lubrication, cooling, and combustion, where drop impact on a heated surface plays a vital role. Drop impact on heated surfaces has been extensively investigated with molecular liquid drops, such as water and ethanol, but rarely explored with ILs. We experimentally investigate the impact dynamics of three types of IL drops onto a heated flat surface under broad ranges of impact velocity ($0.18\le U\le 4.22$ m/s) and surface temperature ($18\le T_s\le 455{}^{\circ }\text{C}$). The impact events observed with the ILs include spreading, spreading with bubbling, and splashing with bubbling. However, the dynamic Leidenfrost effect with an insulating vapor film causing droplet rebound, typically recorded for molecular liquid drops under an initial impact velocity, is not observed for the IL liquids under similar Weber number ($\text{We}\approx 2)$ and surface temperature (at $T_s=350{}^{\circ }\text{C}$). This suppression is attributed to the low gas pressure underneath the IL droplet, induced by evaporation and thermal decomposition of ILs, and can significantly benefit various thermal applications such as cooling and coating. Finally, the maximum spreading factor of the IL drops is modeled using an energy conservation concept and is consistent with experimental results.

We study size segregation in the limit of a single large particle (intruder) embedded within a flowing granular layer. Using discrete-element-method-based computations, we calculate and characterize the drag, lift, and buoyancy forces experienced by the intruder in two systems: (1) a simple shear flow without gravity and (2) gravity-driven flow down an inclined plane. In agreement with previous studies, the drag force in both systems is seen to follow Stokes' Law with a slightly different constant. However, in contrast with previous work, we report that lift force does not follow the Saffman relation; instead, its variation is captured effectively using a size-corrected Stokes' Law. The buoyancy forces calculated in our work scale in a manner that compares well with previous studies. The computed effective volumes of the intruders were found to be close to previously reported values, and the buoyancy force calculated using this effective volume was found to be significantly lower than our computed buoyancy force, suggesting contributions from other factors. Analysis of the relative velocity and stress in the neighborhood of the intruder shows that the lift force is caused by a net upward collisional stress arising from a higher relative inward velocity in the lower part of the intruder due to a positive slip velocity. The buoyancy force is higher than that predicted by Archimedean buoyancy due to large fluctuations in the pressure near the intruder. A continuum model for the segregation flux is presented based on the computed drag, lift, and buoyancy forces.

When releasing a droplet onto a distinct nonmiscible liquid layer, for positive spreading parameter $S$, a surface tension driven spreading arises. Here, for positive $S$, we report the reverse after the initial spreading phase: a secondary receding of the deposited volatile droplet. This behavior is found to result from the competition between two opposite effects: solutal and thermal Marangoni stresses, with the latter built from droplet's evaporation. To clarify the effects of evaporation-induced Marangoni, accurate infrared thermography tests are performed. These tests reveal a maximal temperature gradient at the droplet edge near the recession time. Also, universal scaling laws are proposed for the spreading dynamic of a law-viscous insoluble drop onto a more viscous oil, with droplet volume variation resulting from the evaporation and typical receding time. These predictions are successfully compared with experiments.

This study aims to understand the impact of instabilities and turbulence arising from canopy mixing layers on wind-driven wildfire spread. Using an experimental flume (water) setup with model vegetation canopy and thermally buoyant plumes, we study the influence of canopy-induced shear and turbulence on the behavior of buoyant plume trajectories. Using the length of the canopy upstream of the plume source to vary the strength of the canopy turbulence, we observed behaviors of the plume trajectory under varying turbulence yet constant cross-flow conditions. Results indicate that increasing canopy turbulence corresponds to increased strength of vertical oscillatory motion and variability in the plume trajectory/position. Furthermore, we find that the canopy coherent structures characterized at the plume source set the intensity and frequency at which the plume oscillates. These perturbations then move longitudinally along the length of the plume at the speed of the free stream velocity. However, the buoyancy developed by the plume can resist this impact of the canopy structures. Due to these competing effects, the oscillatory behavior of plumes in canopy systems is observed more significantly in systems where the canopy turbulence is dominant. These effects also have an influence on the mixing and entrainment of the plumes. We offer scaling analyses to find flow regimes in which canopy induced turbulence would be relevant in plume dynamics.

Air vehicles of all scales are susceptible to large-amplitude gusts that may lead to vehicle damage and instability. Therefore, effective physics-based control strategies and an understanding of the dominant unsteady flow physics underpinning gust encounters are desired to improve safety and reliability of flight in these conditions. To this end, this paper develops and experimentally demonstrates a proportional output-feedback lift regulation strategy based on the classical unsteady aerodynamic theories of Wagner and Küssner for wings encountering large-amplitude transverse gusts without a priori knowledge of gust strength or onset time. The tested vertical gust velocities ranged between 25% and 71% of the freestream speed. This strategy is found to successfully generalize to gusts of different strengths and directions, as well as wings at pre- and post-stall angles of attack. In addition, this paper applies dynamical systems analysis to Wagner's aerodynamic model to reveal the effect of the pitch-axis location, pitch input, and closed-loop feedback gains on the stability and robustness of the system along with the flow physics responsible. The real-time output feedback control strategy is experimentally tested in a water tow tank facility equipped with a transverse gust generator. Time-resolved force and flow-field measurements are used to discover the salient flow physics during these encounters and illustrate how closed-loop actuation mitigates their lift transients.

In this Letter, we systematically investigate the Leidenfrost temperature for hot solid substrates with various thermal diffusivities and surface roughnesses. Based on the experimental results, we build a phenomenological model that considers the thermal diffusivity of a solid substrate and derive a relationship between the surface roughness and the resulting vapor film thickness. The generality of this model is supported by experimental data for different liquids and solid substrates. Our model thus allows for a theoretical prediction of the Leidenfrost temperature and develops a comprehensive understanding of the Leidenfrost effect.

This study concerns a turbulent jet at Mach number $M_j=0.9$, subject to a uniform external flow stream at $M_f=0.15$. The analysis combines experimental and numerical databases, spectral proper orthogonal decomposition (SPOD), and linear modeling. The experiments involve time-resolved, stereo PIV measurements at different cross-sections of the jet. A companion large-eddy simulation was performed with the same operating conditions using the “CharLES” solver by Cascade Technologies to obtain a complete and highly resolved three-dimensional database. We assess the mechanisms that underpin the reduction in fluctuation energy that is known to occur when a jet is surrounded by a flight stream. We show that this energy reduction is spread over a broad region of the frequency-wave-number space and involves, apart from the known stabilization of the modal Kelvin-Helmholtz (KH) instability, the attenuation of flow structures associated with the nonmodal Orr and lift-up mechanisms. Streaky structures, associated with helical azimuthal wave numbers and very slow timescales, are the most strongly affected by the flight stream, in terms of energy attenuation and spatial distortion. The energy reductions are accompanied by a weakening of the low-rank behavior of the jet dynamics. These trends are found to be consistent, to a great extent, with results of a local linear model based on the modified mean flow in the flight-stream case.

We study the orientation dynamics of two-dimensional concavo-convex solid bodies that are denser than the fluid through which they fall under gravity. We show that the orientation dynamics of the body, quantified in terms of the angle $\varphi$ relative to the horizontal, undergoes a transcritical bifurcation at a Reynolds number $R{e}_c^{\left(1\right)}$ and a subcritical pitchfork bifurcation at a Reynolds number $R{e}_c^{\left(2\right)}$. For $Re<R{e}_c^{\left(1\right)}$, the concave-downwards orientation of $\varphi =0$ is unstable and bodies overturn into the $\varphi =\pi$ orientation. For $R{e}_c^{\left(1\right)}<Re<R{e}_c^{\left(2\right)}$, the falling body has two stable equilibria at $\varphi =0\text{and}\varphi =\pi$ for steady descent. For $Re>R{e}_c^{\left(2\right)}$, the concave-downwards orientation of $\varphi =0$ is again unstable and bodies that start concave-downwards exhibit overstable oscillations about the unstable fixed point, eventually tumbling into the stable $\varphi =\pi$ orientation. The $R{e}_c^{\left(2\right)}\approx 15$ at which the subcritical pitchfork bifurcation occurs is distinct from the $Re$ for the onset of vortex shedding, which causes the $\varphi =\pi$ equilibrium to also become unstable, with bodies fluttering about $\varphi =\pi$. The complex orientation dynamics of irregularly shaped bodies evidenced here are relevant in a wide range of settings, from the tumbling of hydrometeors to the settling of mollusk shells.

The impact of drops on a thin liquid film is a phenomenon encountered in industrial applications, but also of particular interest in nature. Examples include the growth of stalagmites in karstic caves, a case where the drop feeds the film with ions that will subsequently precipitate. The mixing upon impact, which is witnessed both in the film and in the ejections, remains poorly understood. In the case of stalagmites, this short-term mixing directly affects the ion distribution in the film between impacts. In this work we investigate the mixing and ejection processes occurring during the impact of a free-falling drop on a thin, horizontal film of miscible liquid. We perform laboratory experiments and record side and top views of high-speed movies of impacts in a range of parameters close to impacts observed on stalagmites in caves. We observe that several outcomes arise from these impacts depending on the initial film thickness and Weber number. We relate the geometry of the splashing crown growth to the four scenarios observed. Additionally, the postimpact mixing patterns and film thickness variations are analyzed through an original colorimetry-based technique. From there we infer the size of the stain and quantity of water left by the drop in the film, as well as the total volume ejected away during the impact.

Bubbles and droplets are ubiquitous in many areas of engineering, including microfluidics where they can serve as microreactors for screening of chemical reactions. They are often formed out of a constriction (a microfluidic channel or a cylindrical tube) by blowing a given volume of gas into a liquid phase. It is obviously crucial to be able to control their size, which is not always easy due to the coupling between the volume of the bubble and the gas pressure induced by the Laplace law. In this paper, we examine the size and formation dynamics of soap bubbles blown from a cylindrical tube, which is the paradigm geometry for bubble and droplet formation. To do so, one end of the tube is closed by a soap film, while the other end is connected to a large reservoir of variable volume filled with gas. To inflate the bubble, we reduce the volume of the reservoir, which mimics air inflation through the lung diaphragm or the flow-rate-driven bubble formation in microfluidics geometry such as flow-focusing. As the volume of the reservoir decreases, the soap film curves and takes the form of a spherical cap with a smaller and smaller radius of curvature, which leads to the increase of the gas pressure in the reservoir, according to Laplace's law. This quasistatic process continues until a critical pressure is reached for which the bubble is quasihemispherical. Beyond this pressure, the film undergoes a rapid topological transformation and swells very rapidly (in less than 100 ms) until it reaches its final volume. We describe this instability in particular by showing that this unstable regime appears when a dimensionless number, which depends on the volume of the reservoir, the radius of the tube, surface tension, and external pressure, reaches a critical value. Using a quasistatic model that we solve analytically, we predict the bubble growth dynamics and the amplitude of the unstable height increase for any reservoir volume and constriction size.

We present laboratory experiments on turbulence in a linearly stratified fluid driven by an ensemble of internal gravity waves which approaches statistical homogeneity and axisymmetry. In a way similar to several recent experimental works, nonlinearities develop through the establishment of a set of internal wave modes at discrete frequencies, when the forcing amplitude is increased. We show that the most energetic of these modes are resonant eigenmodes of the fluid domain. The discretization of the energy in frequency and wave number associated to the emergence of these modes prevents the flow from approaching a regime described by the weak/wave turbulence theory, in which a forward cascade carried by a statistical ensemble of weakly nonlinear waves in an infinite domain forms an energy continuum in the frequency and wave-number spaces. We then show that the introduction of slightly tilted panels at the top and at the bottom of the fluid domain allows to inhibit the emergence of the discrete wave modes. In this new configuration, the nonlinear regime results in a continuum of energy over one decade of frequencies which is mainly carried by internal gravity waves verifying the dispersion relation. We therefore achieved a turbulent flow approaching a three-dimensional internal wave turbulence regime with no discretization of the energy in the frequency and wave-number domains. These results constitute a significant step forward in the search of the laboratory observation of a fully developed weakly nonlinear internal-gravity-wave turbulence.

Intermittency is one of the central obstacles for understanding small-scale dynamics in the fully developed hydrodynamic turbulence. The modern approach is largely based on the multifractal theory of Parisi and Frisch which is, however, phenomenological. It was shown recently that the intermittency can be related to the hidden scale invariance. The latter is a statistical scaling symmetry unbroken in a rescaled (projected) formulation of equations of motion. In the present paper, we consider a shell model of turbulence and describe how the hidden symmetry manifests itself through all scales, both in the inertial interval and in the transition to forcing and dissipation ranges. In the inertial interval, we derive anomalous scaling laws from the hidden symmetry. Then we show how a complicated form of the dissipation range is controlled by intermittent rescaled Reynolds numbers within a large range of dissipation scales. This dissipative intermittency can be removed by using a special class of dissipation models. For such models, the hidden scale invariance is restored both in the inertial interval and the dissipation range. Overall, the presented approach deduces the multifractal theory and some of its basic conclusions from the hidden scaling symmetry of equations of motion.

The enhancement of skin-friction drag and surface heat flux by the transition to turbulence is a crucial physical phenomenon for the design of high-speed vehicles. An interpretable mapping of how various flow phenomena such as turbulence and the streamwise growth of the boundary layer (BL) thickness influence these key surface quantities is desirable for advancing our understanding of fundamental flow physics, as well as informing engineering design analysis. To accomplish such a mapping, an angular momentum integral (AMI) equation, originally derived for incompressible flows by Elnahhas and Johnson [J. Fluid Mech. 940, A36 (2022)], is here introduced for compressible turbulent BL flows of a calorically perfect gas. By isolating the laminar BL friction at the same Reynolds number, the AMI equation obtains a straightforward interpretation for skin-friction alteration by other flow phenomena relative to an equivalent laminar BL. Similarly, a moment of total enthalpy integral (MTEI) equation is proposed to quantitatively map the effects of turbulence and other physical flow phenomena to the Stanton number relative to an equivalent laminar BL. Direct numerical simulations (DNSs) of zero pressure gradient isothermal flat plate BLs up to edge Mach number of 7 are used to demonstrate the utility of the AMI and MTEI equations. The DNS cases span a variety of wall-temperature boundary conditions ranging from nominally adiabatic to strongly cooled walls. The derivation of the AMI equation itself suggests the use of shear stress-weighted average viscosity, which collapses the relative influence of turbulence on compressible BL skin friction over the wide range of edge Mach numbers and temperature boundary conditions considered. The AMI results quantify how the variation of mean density inside a turbulent BL impacts the ability of correlated velocity fluctuations to transport momentum and enhance the skin friction coefficient relative to the equivalent laminar BL. From an alternative viewpoint, the results demonstrate how the effect of compressibility on laminar BLs can be used to gain insight into turbulent BLs. The MTEI results similarly demonstrate how the mean density alters the impact of turbulence on the transport of total enthalpy and Stanton number. In doing so, the MTEI results highlight the relative role of turbulent fluxes of enthalpy and mean kinetic energy on the Stanton number. In light of these results, the AMI and MTEI equations provide a quantitative perspective on the fundamental physics of skin friction and surface heat flux in high-speed BLs, which may prove useful for evaluating flow control schemes and complementing existing analysis tools.

While understanding breaking waves is crucial for the development of parametrizations used in ocean wave modeling for both deep and shallow water, the complete process of wave breaking is not well understood. Here we present direct numerical simulations of two-dimensional solitary waves that shoal and break on a uniform beach in shallow water, with the presence of storm surge represented by an inshore region. The storm surge depth, beach slope, and wave amplitude are varied to study the dependence of energy dissipation in the breaker on wave and bathymetric parameters. We classify wave breaker types and find a separation between plunging and spilling breakers when scaled by breaking amplitude and depth. We compare energy dissipation during the breaking process with results from the literature without storm surge. A representation of energy dissipation in this solitary wave breaker data is also compared with prior experiments and simulations of breakers in deep water, and possibilities of a unifying model are explored. We conclude that a previously developed shallow-water inertial dissipation model for wave breaking on a uniform slope can be extended to this storm surge environment with good data collapse, and we further discuss possibilities for a general parametrization of wave breaking valid across different depth regimes.

Bubbles appear when a carbonated drink is poured in a glass. Very stable bubble chains are clearly observed in champagne, showing an almost straight line from microscopic nucleation sites from which they are continuously formed. In some other drinks such as soda, such chains are not straight (not stable). Considering pair interactions for spherical clean bubbles, bubble chains should not be stable, which contradicts these observations. The aim of this work is to explain the conditions for bubble chain stability. For this purpose, experiments and direct numerical simulation are conducted. The bubble size as well as the level of interface contamination are varied to match the range of parameters in typical drinks. Both factors are shown to affect the bubble chain stability. The transition from stable to unstable behavior results from the reversal of the lift force, which is induced by the bubble wake. A criteria based on the production of vorticity at the bubble surface is proposed to identify the conditions of transition from stable to unstable bubble chains. Beyond carbonated drinks, understanding bubble clustering has impact in many two-phase problems of current importance.

Cleaning surfaces with air bubbles in an aqueous medium has been a topic of discussion in recent years due to the growing interest in sustainable methods for cleaning biological surfaces such as agricultural produce. Specifically, in a bubble-injection method, inclined surfaces are targeted by many millimetric air bubbles that collide with and slide along the surface. The collision and subsequent sliding of these air bubbles exert shear stress on the surface, causing contaminants to be removed. The shear stress is proportional to the tangential speed of the bubble with respect to the surface divided by the thickness of the thin film of liquid between the bubble and the solid surface. In this study, we conduct experiments to test the cleaning efficacy at different angles of inclination of a contaminated surface. We use two different types of surface coated with either a protein solution or a bacterial biofilm. Our experimental results indicate that bubbles exhibit the highest cleaning efficacy at the surface angle of $\theta \approx {20}^{\mathrm{o}}$ with respect to the horizontal plane for polydisperse bubbles in the range of 0.3–2 mm and with an average radius of 0.6 mm. To gain a better understanding of the underlying mechanism, we perform a numerical analysis of a single air bubble colliding with a clean surface at various angles. Our numerical and theoretical results show that the average shear force that the bubble exerts on the surface reaches a maximum at $\theta \approx 22.5^{\mathrm{o}}$ which agrees well with the experiments. We also compare the maximum shear stress in sliding phase with the shear stress required for removing different types of bacteria as a fluid-mechanics-based guideline for geometrical design of bubble cleaning devices.

We investigate the cleaning mechanism of an oil-impregnated porous surface by utilizing a method of vortex ring interaction. The kinetic energy of a vortex ring is exploited to expunge the trapped oil from the spherical-bead-based porous surface. Qualitative and quantitative measurements were made using high-speed shadowgraphy imaging, planar laser-induced fluorescence (PLIF) imaging, and particle image velocimetry (PIV) techniques. The interaction phenomenon is explored from the perspective of vortex ring dynamics followed by oil sheet dynamics. Five different strengths of a vortex ring [characterized by the circulation ($\mathrm{\Gamma }$) -based Reynolds number (${\mathrm{Re}}_{\mathrm{\Gamma }}=2550$–14 260: ${\mathrm{Re}}_{\mathrm{\Gamma }}=\mathrm{\Gamma }/\nu$, where $\nu$ is the kinematic viscosity)] and two types of surface porosity ($\varphi$) (open area ratio, 0.21 and 0.41) are considered for parametric analysis. The interaction process with oil is divided into three regimes: (i) penetration, (ii) bag formation, and (iii) bag breakup. Depending on the strength of the vortex ring, it is observed that surface cleaning takes place from both upstream and downstream regions of the porous surface. The vortex rings with higher ${\mathrm{Re}}_{\mathrm{\Gamma }}$ can expunge more oil through a complicated interaction process involving Rayleigh-Taylor- and Rayleigh-Plateau-type instabilities. However, along with ${\mathrm{Re}}_{\mathrm{\Gamma }}$, the interaction dynamics depend strongly on the shape and $\varphi$ value. The interaction process is characterized by vorticity cancellation, Kelvin-Helmholtz (KH) instabilities, and three-dimensional interactions.

We investigate the effect of a forest of pillars on a granular layer steadily flowing over a rough inclined plane. We quantify experimentally how the steady flow rate of grains is affected by the interpillars distance for different layer thicknesses and slope angles. We then propose a model based on a depth-average approximation associated with $\mu \left(I\right)$ rheology that considers the additional force exerted by the pillars on the granular layer. This model succeeds in accounting for most of the observed results when taking into account some inertia due to the nonvanishing Froude number of the flow.

Production, drainage, and stability of foam films, i.e., films in contact with their menisci, are fascinating problems that remain still unsolved. In this article, we propose to explore the regime of large velocities and large film sizes. This is not accessible in experiments classically conducted in the literature and allows us to study the regime of large extension and large extension rates. With our setup, we make soap films up to 2 m high by pulling a horizontal fishing line driven by belts out of a soapy solution at velocities ranging from 20 cm/s to 250 cm/s. We characterize the thickness profile of the central part of the film that behaves like a rubber band under tension. We show that its thickness profile is well described by a static model in which a homogeneous elastic film is stretched by its own weight. This leads to an exponential thickness profile with a characteristic length given by a competition between gravity and surface elasticity. The prefactor is fixed by the shape and area of the film, governed by the fishing line motion but also by a continuous extraction of foam film from the lateral menisci, thicker than the central part, and that progressively invades the film from its lateral boundaries. The model we propose captures the subtle interplay between gravity, film elasticity, and film extraction and leads to predictions in good agreement with our experimental data.

The flow-induced vibration of a whisker in the wake of a movable circular cylinder with time-varying streamwise gaps was experimentally investigated to understand how whiskers detect the variation of upstream target's swimming status. A time-resolved particle image velocimetry was used to characterize the vortex flow dynamics and vibrations of whisker across various initial gaps and gap growth rates. Results showed that despite the time-varying gaps, the evolution of vortex flow and whisker dynamics remained overall synchronized. Due to the nonlinear decaying rate of vortex strength along streamwise direction, the vibrations of whisker sensor were more sensitive to gap growth rates with smaller initial gaps. However, the sensitivity of whisker (i.e., the decaying rate of whisker vibration intensity over unit time) to gap growth rates gradually decreased when such growth rate became sufficiently high, regardless of initial gaps. Using a physical model based on the balance between whisker inertial force, system damping, restoring force, and flow loading with time-varying magnitudes, we well described the response of whisker sensor in the cylinder wake flow; the model revealed that the reduction of whisker's sensitivity under high gap growth rate was attributed to the sufficiently strong vortices initiated by the fast movement of upstream cylinder, which compensated the influence of vortex strength decay due to the growth of gap. In addition to cases with whisker vibrating in the wake centerline, our complementary measurements with the whisker in the cylinder edge highlighted the time-varying equilibrium vibration positions of whisker sensor due to the “wake-stiffness” effect.