When two bubbles submerged in a liquid are brought closely together, the intermediate liquid film separating the bubbles begins to drain. Once the film ruptures, the bubbles coalesce and form a neck that expands with time. The dynamics of the neck growth are well understood in the context of pure, Newtonian liquids. Yet much less is known about the dynamics of this singularity when the surrounding liquid contains long flexible polymers, which provide viscoelastic characteristics to the liquid's properties. Here we experimentally study the coalescence of bubbles surrounded by polymer solutions. In contrast to drop coalescence, and in spite of the singular stretching of polymers, we find that the presence of the dissolved polymers does not at all affect the coalescence dynamics at early times. The polymer elasticity is found to slow down the flow only during the later stages of coalescence. These observations are interpreted using an asymptotic solution of the Oldroyd-B model, which predicts a strong stress singularity near the extremity of the neck. However, the polymer stress turns out to diverge only in the azimuthal direction, which can explain why elastic effects remain subdominant during the initial stages of coalescence.

The dispersion of rigid elongated particles in turbulent channel flow bounded by rough walls is examined. In particular, the interplay between particle inertia, particle length, and wall roughness characteristics in determining particle spatial distribution, preferential orientation, and alignment within the flow is analyzed. The dispersion process is investigated by performing direct numerical simulations of the turbulent flow coupled with the Lagrangian tracking of the particles, modeled as prolate ellipsoids with varying aspect ratio and inertia, under dilute flow conditions. Simulations are carried out at friction Reynolds number ${\text{Re}}_{\tau }=180$ based on the channel half height in a domain confined by walls with different two-dimensional roughness characteristics. Particle elongation is varied considering different aspect ratios (ranging from $\lambda =1$ for the reference case of spherical particles to $\lambda =10$ for the longest particle set) while particle inertia is varied considering different Stokes numbers (ranging from ${\text{St}}^{+}=1$ for the least inertial particles to ${\text{St}}^{+}=100$ for the most inertial ones), for a total of 12 different particle sets. Our results show that particles are affected by the turbulent structures that form near the rough walls in a way that is biased both by elongation, namely length, and inertia. This is not observed with smooth walls: In this case, increasing particle inertia always leads to a stronger turbophoretic particle drift to the walls. Elongation is observed to have a quantitative effect on the slip velocity statistics, particularly for particles with small Stokes number. Differences induced by a change of the aspect ratio tend to vanish as particle inertia increases, since the relative translational motion between the particles and the surrounding fluid appears to be closely connected to preferential concentration phenomena. Wall roughness modifies the overall dynamics of the particles, as well as their distribution, orientation, and alignment with flow direction, especially in the near-wall region. Roughness hinders long-term wall deposition of particles and determines a more uniform spatial distribution across the channel. In the near-wall region, elongated particles tend to align with the streamwise direction even in the presence of large-scale roughness, albeit to a lesser extent compared to the case of smooth walls. A stronger effect of roughness is noticed on the alignment along the spanwise and wall-normal directions. Such alignment, as well as preferential orientation, is hampered by the strong velocity fluctuations induced by roughness and the behavior of the ellipsoidal particles resembles again that of spherical particles.

The present study reports an original physical model for simulating pulsating heat pipes (PHPs). Their high heat performance is due to the phase change over thin liquid films. The simulation of physically correct film behavior is thus crucial. The model adopts the one-dimensional approach, which is computationally efficient yet still capable of capturing major physical phenomena. The model assumes a spatially uniform film thickness, whereas both the film thickness and length can vary over time; therefore, we call it the oscillating film thickness model. It is based on the physical analysis of liquid film deposition by the receding menisci of Taylor bubbles and of contact line dynamics. Three key phenomena are addressed: (1) film deposition, (2) contact line receding due to dewetting acceleration by evaporation, and (3) mass exchange over films and contact lines. The model is evaluated by simulating the simplest, single-branch PHP, for which detailed experimental data are available. A quantitative agreement is reached. As the model includes the wetting properties, their impact on oscillations is analyzed, and a qualitative agreement with the experiment is demonstrated.

We investigate the wake structure and the three-dimensional stability of the two-dimensional von Kármán vortex street developing in the wake of a permeable circular cylinder. The flow through the porous medium, assumed homogenous and isotropic, is described by the Darcy law, with a Navier slip coupling condition at the interface with the pure fluid region. The two-dimensional and steady flow past the cylinder is initially considered. Permeability induces a downstream displacement of the recirculation region, which reduces its dimensions until it eventually disappears. Linear stability analysis shows that the flow is progressively stabilized as permeability increases. We identify a critical value of permeability beyond which the steady wake is linearly stable independently of the Reynolds number. Two-dimensional, time-dependent simulations are then carried out. A progressive downstream displacement of the region of onset of the vortex shedding is observed, together with a decrease in the oscillation frequency. Oscillations of aerodynamic forces are progressively quenched with permeability owing to the downstream displacement of the onset region of the vortex shedding. At the same time, traveling vortices are observed far downstream of the body, in opposition with the impervious case, characterized instead by the formation of two shear layers of opposite vorticity, at very large distances from the body. We perform linearized simulations for the evolution of three-dimensional perturbations on the two-dimensional von Kármán vortex street. The growth rate and the spatial structure of the perturbations are extracted from such linearized dynamics by employing a sparsity-promoting dynamic mode decomposition (SP-DMD). As permeability increases, the unsteady vortex street past the cylinder is progressively stabilized with respect to three-dimensional perturbations until the transition to three-dimensionality is prevented. We identify a critical value of the permeability beyond which the vortex shedding preserves its two-dimensionality, at least in the considered parameters space.

Diffusion-driven flow is a boundary layer flow that results from the combined influence of gravity and diffusion, which exists in density-stratified fluids whenever a gravitational field is not parallel to the solid boundary. In this paper, we investigate the unsteady diffusion-driven flows that emerge in a parallel-plate channel domain with a linear density stratification. We first compute the time-dependent diffusion-driven flows and perturbed density field using eigenfunction expansions under the Boussinesq approximation. In channel domain, the unsteady flow converges to a steady-state solution either monotonically or nonmonotonically (highly oscillatory), depending on the relation between the Schmidt number and the nondimensionalized stratified scalar diffusivity, while the flow in the half-space inclined plane problem exhibits oscillatory convergence for all parameters. To validate the Boussinesq approximation, we propose the quasi-Boussinesq approximation, which includes transverse density variation in the inertial term. Numerical solutions show that the relative difference between the Boussinesq and quasi-Boussinesq approximations is uniformly small. We also study the mixing of a passive tracer induced by the advection of the unsteady diffusion-driven flow and present the series representation of the time-dependent effective diffusion coefficient. For small Schmidt numbers, the effective diffusion coefficient induced by the unsteady flow solution can oscillate with an amplitude larger than the effective diffusion coefficient induced by the long-time-limiting steady-state flow. Interestingly, the unsteady flow solution can reduce the time-dependent effective diffusion coefficient temporally in some parameter regimes, below even that produced by pure molecular diffusion in the absence of a flow. However, at long times, the effective diffusion is significantly enhanced for large Péclet numbers.

We combine experiments and numerical computations to examine underlying fluid mechanical processes associated with bioaerosol generation during violent respiratory maneuvers, such as coughing or sneezing. Analogous experiments performed in a cough machine—consisting of a strong shearing airflow over a thin liquid film—allow us to illustrate the changes in film topology as it disintegrates into small droplets. We identify that aerosol generation during the shearing of the liquid film is mediated by the formation of inflated baglike structures. The breakup of these bags is triggered by the appearance of retracting holes that puncture the bag surface. Consequently, the cascade from inflated bags to droplets is primarily controlled by the dynamics and stability of liquid rims bounding these retracting holes. We also reveal the key role of fluid viscosity in the overall fragmentation process. It is shown that more viscous films when sheared produce smaller droplets.

In confined rotating convection, a strong zonal flow can develop close to the side wall with a modal structure that precesses anticyclonically (counter to the applied rotation) along the side wall. It is surmised that this is a robust nonlinear evolution of the wall modes observed before the onset of bulk convection. Here, we perform direct numerical simulations of cylindrically confined rotating convection at high rotation rates and strong turbulent forcing. Through comparison with earlier work, we find a fit-parameter-free relation that links the angular drift frequency of the robust wall mode observed far into the turbulent regime with the critical wall mode frequency at onset, firmly substantiating the connection between the observed boundary zonal flow and the wall modes. Deviations from this relation at stronger turbulent forcing suggest early signs of the bulk turbulence starting to hamper the development of the wall mode. Furthermore, by studying the interactive flow between the robust wall mode and the bulk turbulence, we identify radial jets penetrating from the wall mode into the bulk. These jets induce a large-scale multipolar vortex structure in the bulk turbulence, dependent on the wave number of the wall mode. In a narrow cylinder the entire bulk flow is dominated by a quadrupolar vortex driven by the radial jets, while in a wider cylinder the jets are found to have a finite penetration length and the vortices do not cover the entire bulk. We also identify the role of Reynolds stresses in the generation of zonal flows in the region near the sidewall.

The leading-edge vortex (LEV) formation on the caudal fin (CF) has been identified as playing a key role in efficient lift-based thrust production of fish-like propulsion. The enhancement of the CF LEV through its interaction with vortices formed upstream due to a median fin with a distinct shape is the focus of this paper. High-speed, high-fidelity videos and particle imaging velocimetry (PIV) were obtained from rainbow trout during steady forward swimming to visualize the undulatory kinematics and two-dimensional flow behavior. Body kinematics are quantified using a traveling-wave formulation that is used to prescribe the motion of a high-fidelity three-dimensional surface model of the fish body for a computational fluid dynamics (CFD) study. The pressure field of the CFD result is compared and validated with the PIV result from the experiment. Using CFD, the vortex forming and shedding behaviors of the anal fin (AF) and their capturing and interaction with the trunk (TK) and the CF are visualized and examined. Coherent AF-bound LEVs are found to form periodically, leading to thrust production of the AF. The vortices subsequently shed from the AF are found to help stabilize and reinforce the LEV formation on the CF by aiding LEV initiation at stroke reversal and enhancing LEV during a tail stroke, which leads to enhancement of lift-based thrust production. The CF is found to shed vortex tubes (VTs) that create backward-facing jets, and the ventral-side VT and the associated backward jets are both strengthened by vortices shed by the AF. An additional benefit of the AF is found to be reduction of body drag by reducing the lateral crossflow that leads to loss of beneficial pressure gradient across the body. Through varying AF-CF spacing and AF height, we find that CF thrust enhancement and TK drag reduction due to the AF are both affected by the position and size of the AF. The position and area of the AF that led to the most hydrodynamic benefit are found to be the original, anatomically accurate position and size. In this paper, we demonstrate the important effect of vortex interaction among propulsive surfaces in fish-like propulsion.

When a fluid in turbulent motion is tagged by a nonuniform concentration of ideal tracers, the mean velocity of the tracers may not match with the mean velocity of the fluid flow. This implies that conventional particle tracking velocimetry will not produce the mean flow of a turbulent flow unless the particle seeding is homogeneous. In this work, we consider the problem of mean flow estimation from a set of particle tracks obtained in a situation of nonhomogeneous seeding. To compensate the bias caused by the nonhomogeneous particle seeding, we propose a modified particle tracking velocimetry method. This method is called a time-delayed velocity and considers the velocity trajectory of a given particle shifted in time with respect to its position. We first introduce our method for an ideal advection–diffusion model and then we implement it for a turbulent channel and a turbulent jet. For both situations, we find that the velocity bias caused by the nonhomogeneous tracer concentration is reduced with a time delay introduced between position and velocity of the tracer trajectories. For the turbulent channel, the error on the mean flow estimation monotonically decreases for increasing time delays. For the turbulent jet, the error on the mean flow estimation also reduces with positive time delays but the time delay should not be too large. We interpret this limitation as a consequence of the spatial dependence of the mean flow. For the turbulent channel, this limitation does not appear because the velocity for the mean flow streamlines is constant. For both flows, the optimal time delay for the velocity bias compensation is consistent with the Lagrangian timescales of the flow. This method gives promising elements to take into account inhomogeneous seedings in velocity fields measurements for all kinds of turbulent flows and interesting perspectives to understand how Lagrangian trajectories from various sources build an Eulerian mean field.

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.