Recent experimental results have shown that binary granular materials fluidized by combined vibration and gas flow exhibit Rayleigh-Taylor-like instabilities that manifest themselves in rising plumes, rising bubbles, and the sinking and splitting of granular droplets. This work explores the physics behind the splitting of a granular droplet that is composed of smaller and denser particles in a bed of larger and lighter particles. During its sinking motion, a granular droplet undergoes a series of binary splits resembling the fragmentation of a liquid droplet falling in a miscible fluid. However, different physical mechanisms cause a granular droplet to split. By applying particle image velocimetry and numerical simulations, we demonstrate that the droplet of high-density particles causes the formation of an immobilized zone underneath the droplet. This zone obstructs the downwards motion of the droplet and causes the droplet to spread and ultimately to split. The resulting fragments sink at inclined trajectories around the immobilized zone until another splitting event is initiated. The occurrence of consecutive splitting events is explained by the reformation of an immobilized zone underneath the droplet fragments. Our investigations identified three requirements for a granular droplet to split: (1) frictional interparticle contacts, (2) a higher density of the particles composing the granular droplet compared to the bulk particles, and (3) a minimal granular droplet diameter.

Soap bubbles are by essence fragile and ephemeral. Depending on their composition and environment, bubble bursting can be triggered by gravity-induced drainage and/or the evaporation of the liquid and/or the presence of nuclei. They can also shrink due to the diffusion of the inner gas in the outside atmosphere induced by Laplace overpressure. In this Letter, we design bubbles made of a composite liquid film able to neutralize all these effects and keep their integrity for more than 1 year in a standard atmosphere. The unique properties of this composite film are rationalized with a nonlinear model and used to design complex objects.

A new direct numerical simulation of a Poiseuille channel flow has been conducted for a friction Reynolds number of $10000$, using the pseudospectral code LISO. The mean streamwise velocity presents a long logarithmic layer, extending from 400 to 2500 wall units, longer than it was thought. The maximum of the intensity of the streamwise velocity increases with the Reynolds number, as expected. Also, the elusive second maximum of this intensity has not appeared yet. In case it exists, its location will be around $y^{+}\approx 120$, for a friction Reynolds number extrapolated to approximately 13 500. The small differences in the near-wall gradient of this intensity for several Reynolds numbers are related to the scaling failure of the dissipation, confirming this hypothesis. The scaling of the turbulent budgets in the center of the channel is almost perfect above 1000 wall units. Finally, the peak of the pressure intensity grows with the Reynolds number and does not scale in wall units. If the pressure at the wall is modeled as an inverse quadratic power of ${\text{Re}}_{\tau }$, then $p_{\infty }^{\prime +}\approx 4.7$ at the limit of infinite Reynolds number.

The focus of this study is to investigate primary and secondary bifurcations to weakly nonlinear flows (weak branch) in convective rotating spheres in a regime where only strongly nonlinear oscillatory sub- and supercritical flows (strong branch) were previously found [E. J. Kaplan, N. Schaeffer, J. Vidal, and P. Cardin, Phys. Rev. Lett. 119, 094501 (2017)]. The relevant regime corresponds to low Prandtl and Ekman numbers, indicating a predominance of Coriolis forces and thermal diffusion in the system. We provide the bifurcation diagrams for rotating waves (RWs) computed by means of continuation methods and the corresponding stability analysis of these periodic flows to detect secondary bifurcations giving rise to quasiperiodic modulated rotating waves (MRWs). Additional direct numerical simulations (DNS) are performed for the analysis of these quasiperiodic flows for which Poincaré sections and kinetic energy spectra are presented. The diffusion timescales are investigated as well. Our study reveals very large initial transients (more than 30 diffusion time units) for the nonlinear saturation of solutions on the weak branch, either RWs or MRWs, when DNS are employed. In addition, we demonstrate that MRWs have multimodal nature involving resonant triads. The modes can be located in the bulk of the fluid or attached to the outer sphere and exhibit multicellular structures. The different resonant modes forming the nonlinear quasiperiodic flows can be predicted with the stability analysis of RWs, close to the Hopf bifurcation point, by analyzing the leading unstable Floquet eigenmode.

Using a large-scale wind tunnel equipped with an aerodynamic balance mounted on a rotating turntable, we first study the aerodynamic force exerted on a cyclist in time trial position from head to tailwind limits. A theoretical model for the aerodynamic force is developed to show the origin of the different experimental features. The theoretical analysis is then extended to the power dissipated in crosswinds and it is shown that the expression commonly used in the literature only holds in the pure head and tailwind limits but must be corrected in the general case of a crosswind. A new analytical expression for the total aerodynamic power is derived and used to determine the cycling speed in arbitrary crosswind conditions.

We analyze a set of bidirectional wave experiments in a linear wave flume of which some are conducive to integrable turbulence. In all experiments the wavemaker forcing is sinusoidal and the wave motion is recorded by seven high-resolution side-looking cameras. The periodic scattering transform is implemented and power spectral densities computed to discriminate linear wave motion states from integrable turbulence and soliton gas. Values of the wavemaker forcing Ursell number and relative amplitude are required to be above some threshold values for the integral turbulence to occur. Despite the unavoidable slow damping, soliton gases achieve stationary states because of the continuous energy input by the wavemaker. The statistical properties are given in terms of probability density distribution, skewness, and kurtosis. The route to integrable turbulence, by the disorganization of the wavemaker induced sinusoidal wave motion, depends on the nonlinearity of the waves but equally on the amplitude amplification and reduction due to the wavemaker feedback on the wave field.

Face covering, commonly known as a facemask, is considered to be one of the most effective pieces of personal protective equipment to reduce transmissions of pathogens through respiratory droplets—both large droplets and liquid aerosol particles. Facemasks inhibit the expulsion of such respiratory droplets from the user and protect the user from inhaling pathogen-laden potentially harmful droplets or their dried nuclei. While the efficacies of various dry face masks have been explored in the recent past, a comprehensive investigation of a wet mask is lacking. Yet users wear masks for an extended period and, consequently, due to the deposition of respiratory droplets released through multiple respiratory events, the mask matrix becomes wet. We, herein, present an experimental study on the dynamics of sequential impacts of droplets on masks to understand how wetness affects possible penetration and secondary atomization of the impacted droplet. Two different types of masks, hydrophobic and hydrophilic, were used in this study to evaluate the underlying physical mechanisms that control the penetration in each of them.

Face masks are used widely to mitigate the spread of infectious diseases. While their main purpose is to filter pathogenic droplets, masks also represent a porous barrier to exhaled and inhaled air flow. In this study, we characterize the aerodynamic effect of the presence of a mask by tracking the air exhaled by a person through a mask, using both infrared imaging and particle image velocimetry performed on illuminated fog droplets surrounding a subject. We show how a mask confines the exhaled flows within tens of centimeters in front of a person breathing or speaking. In addition, we show that the tissue of common surgical face masks has a low permeability, which efficiently transforms the jetlike flows of exhalation produced during breathing or speaking into quasivertical buoyancy-driven flows. Therefore, wearing a mask offers a strong mitigation of direct transport of infectious material in addition to providing a filtering function. By comparing results on human subjects and model experiments, we propose a model to rationalize how a mask changes the air flow, and thus we provide quantitative insights that are useful for descriptions of disease transmission.

Passive droplet transport mechanisms, in which continuous external energy input is not required for motion, have received significant attention in recent years. Experimental studies of such mechanisms often ignore, or use careful treatments to minimize, contact angle hysteresis, which can impede droplet motion, or even arrest it completely. Here, we consider the effect of contact angle hysteresis on bendotaxis, a mechanism in which droplets spontaneously deform an elastic channel via capillary pressure and thereby move. We seek to understand when contact angle hysteresis prevents bendotaxis. We supplement a previous mathematical model of the dynamics of bendotaxis with a simple model of contact angle hysteresis, and show that this model predicts droplet trapping when hysteresis is sufficiently strong. By identifying the equilibrium configurations adopted by these trapped droplets and assessing their linear stability, we uncover a sensitive dependence of bendotaxis on contact angle hysteresis and develop criteria to describe when droplets will be trapped.

The current pandemic has spurred a great deal of debate about the use of cloth masks as effective alternatives to medical masks for the general public. Despite an abundance of experimental studies on the filtration of aerosols by cloth masks, there is currently a lack of analytical understanding to predict mask performance a priori. In this study, we establish a quantitative model for the pressure drop across woven heterogeneous cloth masks and their filtration efficiencies for aerosols with aerodynamic diameters less than 1 $\mu \mathrm{m}$. To compare the intrinsic filtration capabilities of diverse materials, we introduce a filtration quality factor. Finally, we present a decision map to illustrate the tradeoffs between filtration efficiency and breathability and to provide practical guidance on the purchase of cloth masks. For the most commonly used cotton, polyester, and polypropylene masks, the mechanical filtration capability is fairly independent of the material, but it exhibits a high sensitivity to the porosity and fiber diameter. Additionally, keys to a high-quality cloth mask include a homogeneous structure, micro- or even nanofibrous materials, and a suitable mass per unit area of the textile.

We have studied vortex-induced vibrations (VIV) of a flexibly mounted cylinder with one degree of freedom for different angles that the direction of oscillations makes with the incoming flow, covering the entire range from a purely inline direction (α = 0°) to a purely crossflow (transverse) direction (α = 90°). Force and displacement measurements together with flow visualizations of the wake are used to characterize the response of the cylinder over a range of reduced velocities ($1<U^{*}<14$). It is shown that the two lock-in ranges that are observed in a purely inline VIV response of the cylinder persist to higher angles: The first lock-in range, which corresponds to a symmetric or alternating-symmetric wake, persists up to α = 40°, and the second one up to α = 50°. The response of the system follows that of a purely inline case both quantitatively and qualitatively up to α = 15°, a relatively large deviation from the pure inline direction. The lock-in range, which is typically observed in a purely crossflow VIV response, appears at angles as low as α = 25°. For α > 50°, this crossflow-induced VIV response is the only response that is observed. It is also shown that the symmetric and alternating-symmetric wakes that had been observed previously in a purely inline case are observed for angles up to α = 30° and α = 15°, respectively, indicating that their formation does not rely on a perfect alignment between the direction of oscillations and the flow direction. For larger angles, a skewed symmetric shedding is observed in which one of the two vortices that are shed simultaneously is larger than the other one.

The impact of droplets on dry and prewetted meshes was investigated using high-speed photography. The prewetting resulted from previous impacts. It was observed that an increase of the Weber number We resulted in a transition from no penetration of the mesh through an incomplete penetration to a complete penetration. It was found that both penetrations could be entirely suppressed by liquid trapped in the mesh (prewetted mesh), with the height $h$ of the sessile droplet being a key factor. The phase diagram in the $(\mathrm{We}, h)$ plane determining the expected type of impact has been determined. The transition Weber numbers for both types of penetration increased monotonically with $h$. A model for predicting the transition thresholds is proposed and compared with experimental results. It is suggested that the impact type is determined by the momentum exchange between the impacting droplet and the liquid trapped in the mesh.

Wind- and water-driven migrating sand dunes frequently interact with elevated natural and artificial topographical features. The dune-obstacle interaction can alter the migrating behavior of the dune and, depending on the nature of the obstacle, it may generate various societal and technological risks. Here we study the problem of dune-obstacle interaction in a paradigmatic quasi-two-dimensional domain realized in a subaqueous laboratory experiment. Generically, dunes interact with obstacles either by crossing over the obstacle or by being trapped. We describe how the selection of these two distinct dynamical behaviors depends on the size and shape of the obstacle, focusing in particular on the fluid flow in the immediate vicinity of the obstacle. Specifically, we perform a modal decomposition of the measured flow field and we discover that the outcome of the dune-obstacle interaction is closely related to the flow structure above the obstacle.

A nonintrusive data assimilation methodology is developed to improve the statistical predictions of large-eddy simulations (LES). The ensemble-variational (EnVar) approach aims to minimize a cost function that is defined as the discrepancy between LES predictions and reference statistics from experiments or, in the present demonstration, independent direct numerical simulations (DNS). This methodology is applied to adjust the Smagorinsky subgrid model and obtain data assimilated LES (DA-LES) which accurately estimate the statistics of turbulent channel flow. To separately control the mean and fluctuations of the modeled subgrid tensor, and ultimately the first- and second-order flow statistics, two types of model corrections are considered. The first one optimizes the wall-normal profile of the Smagorinsky coefficient, while the second one introduces an adjustable steady forcing in the momentum equations to independently act on the mean flow. Using these two elements, the data assimilation procedure can satisfactorily modify the subgrid model and accurately recover reference flow statistics. In a posteriori testing, the retrieved subgrid model significantly outperforms more elaborate baseline models such as dynamic and mixed models. The robustness of the present data assimilation methodology is assessed by changing the Reynolds number and considering grid resolutions that are away from usual recommendations. Taking advantage of the stochastic formulation of EnVar, the developed framework also provides the uncertainty of the retrieved model.

In statistically homogeneous and isotropic turbulence, the average value of the velocity increment ${\delta }_{r}u=u(x+r)-u\left(x\right)$, where $x$ and $x+r$ are two positions in the flow and $u$ is the velocity in the direction of the separation distance $r$, is identically zero, and so to characterize the dynamics one often uses the Reynolds number based on $\sqrt{\langle {\left({\delta }_{r}u\right)}^2\rangle }$, which acts as the coupling constant for scale-to-scale interactions. This description can be generalized by introducing structure functions of order $n$, $S_n=\langle {\left({\delta }_{r}u\right)}^n\rangle$, which allow one to probe velocity increments including rare and extreme events, by considering ${\delta }_{r}u\left(n\right)=O\left(S_n^{1/n}\right)$ for large and small $n$. If $S_n\propto r^{{\zeta }_n}$, the theory for the anomalous exponents ${\zeta }_n$ in the entire allowable interval $-1<n<\infty$ is one of the long-standing challenges in turbulence (one takes absolute values of ${\delta }_{r}u$ for negative $n$), usually attacked by various qualitative cascade models. We accomplish two major tasks here. First, we show that the turbulent microscale Reynolds number $R_{\lambda }^T$ (based on a suitably defined turbulent viscosity) is 8.8 in the inertial range with anomalous scaling, when the standard microscale Reynolds number $R_{\lambda }$ (defined using normal viscosity) exceeds that same number (which in practice could be by a large factor). When the normal and turbulent microscale Reynolds numbers become equal to or fall below 8.8, the anomaly disappears in favor of Gaussian statistics. Conversely, if one starts with a Gaussian state and increases $R_{\lambda }$ beyond 8.8, one ushers in the anomalous scaling; the inference is that $R_{\lambda }^T\approx 8.8$ and remains constant at that value with further increase in $R_{\lambda }$. Second, we derive expressions for the anomalous scaling exponents of structure functions and moments of spatial derivatives, by analyzing the Navier-Stokes equations in the form developed by Hopf. We present a novel procedure to close the Hopf equation, resulting in expressions for ${\zeta }_n$ in the entire range of allowable moment order $n$, and demonstrate that accounting for the temporal dynamics changes the scaling from normal to anomalous. For large $n$, the theory predicts the saturation of ${\zeta }_n$ with $n$, leading to several inferences, two among which are (a) the smallest length scale ${\eta }_n=L{\text{Re}}^{-1}\ll L{\text{Re}}^{-3/4}$, where Re is the large-scale Reynolds number, and (b) that velocity excursions across even the smallest length scales can sometimes be as large as the large-scale velocity itself. Theoretical predictions for each of these aspects are shown to be in excellent quantitative agreement with available experimental and numerical data.

This study considered how the presence of current impacted wave dissipation within a meadow of flexible marsh plants. A wave-damping model was developed from a prediction of current- and wave-induced force on individual plants. The model was validated with laboratory experiments. Wave decay was measured over a meadow of flexible model plants geometrically similar to Spartina alterniflora with and without a following current. Consistent with previous observations, the wave energy dissipation depended on the ratio of current velocity ($U_c$) to wave velocity ($U_w$). Compared to the same pure wave condition, wave energy dissipation was enhanced by large $U_c/U_w$ but can be decreased for small $U_c/U_w$. Once validated, the wave-damping model was used to explore a wider range of wave, current, and meadow conditions in order to illustrate the influence of reconfiguration on wave forces; the impact of current on wave group velocity; and the modification of in-canopy time-mean and wave orbital velocity associated with canopy drag.

We consider the influence of transverse confinement on the instability properties of velocity and density distributions reminiscent of those pertaining to exchange flows in stratified inclined ducts, such as the recent experiment of Lefauve et al. [J. Fluid Mech. 848, 508 (2018)]. Using a normal mode streamwise and temporal expansion for flows in ducts with various aspect ratios $B$ and nontrivial transverse velocity profiles, we calculate two-dimensional (2D) dispersion relations with associated eigenfunctions varying in the “crosswise” direction, in which the density varies, and the spanwise direction, both normal to the duct walls and to the flow direction. We also compare these 2D dispersion relations to the so-called one-dimensional (1D) dispersion relations obtained for spanwise invariant perturbations, for different aspect ratios $B$ and bulk Richardson numbers ${\text{Ri}}_b$. In this limited parameter space, the presence of lateral walls has a stabilizing effect, in that the 1D growth-rate predictions are almost systematically an upper bound to the 2D growth rates, which in turn decrease monotonically as lateral walls are brought together with increased spanwise confinement ($B\to 0$). Furthermore, accounting for spanwise-varying perturbations results in a plethora of unstable modes, the number of which increases as the aspect ratio is increased. These modes present an odd-even regularity in their spatial structures, which is rationalized by comparison to the so-called one-dimensional oblique dispersion relation obtained for oblique waves, characterized by a continuously varying spanwise wavenumber in addition to the streamwise wavenumber. Finally, we show that in most cases, the most unstable 2D mode is the one that oscillates the least in the spanwise direction, as a consequence of viscous damping. However, in a limited region of the parameter space and in the absence of stratification, we show that a secondary mode with a more complex “twisted” structure dominated by crosswise vorticity becomes more unstable than the least oscillating Kelvin-Helmholtz mode associated with spanwise vorticity.

We report existence of nonequilibrium turbulent dissipations in buoyant axisymmetric plumes at infinite Reynolds number limit. The plume statistical equations remain form invariant under one translation parameterized by $a_0$ and two unequal stretching transformations parameterized by $a_1$ and $a_2$ of flow variables, except the pressure-strain-rate equation. The dissipation coefficients, unlike Kolmogorov theory, in the scaling of dissipations ${\epsilon }_u$ of velocity fluctuation $u^{\prime }$ and ${\epsilon }_{\theta }$ of thermal fluctuation ${\theta }^{\prime }$ evolve linearly with spreading rate $d\delta /dz$. The spreading rate is expressed as inversely proportional to local Reynolds number ${\mathrm{Re}}_{\delta }$ raised to the exponent $m=3(a_1-a_2)/(a_1+a_2), a_1\ne -a_2$ and is varying with ${\mathrm{Re}}_{\delta }$ for $m\ne 0$. The components of Reynolds stress tensor have different streamwise (s) evolutions as ${\left(\overline{w^{\prime 2}}\right)}_s={\left(\overline{v^{\prime 2}}\right)}_s\propto {(d\delta /dz)}^2{\left(\overline{u^{\prime 2}}\right)}_s$ and ${\left(\overline{u^{\prime }{v}^{\prime }}\right)}_s\propto d\delta /dz{\left(\overline{u^{\prime 2}}\right)}_s$ unless $d\delta /dz$ is a constant, which holds only if $m=0$ giving $a_1=a_2$. This implies that Kolmogorov equilibrium theory ($m=0$) is intertwined inextricably with complete self-preservation. There is a direct universal relationship between turbulent dissipation and entrainment coefficient $\alpha$ as $d\delta /dz\propto \alpha$, the proportionality constant depends on integrals of mean axial velocity, temperature difference, and turbulent stresses. The power exponent in power-law streamwise evolutions of flow quantities are specified by the ratio $a_2/a_1$ and differs from the conventional results unless $m=0$. Both the local axial velocity and temperature difference widths are increasing with the increase of vertical distance from the source, as same power-law scaling exponents but differ due to different prefactors. However, the spreading rates and entrainment coefficient decrease in the non-Kolmogorov dissipation region, which occurs when $m\ne 0$ preferably lying in (0,1] ($a_2/a_1\in [1/2,1)$). Similar trends of spreading rate and entrainment were also measured experimentally in planar jet [Cafiero and Vassilicos, Proc. R. Soc. London A 475, 20190038 (2019)] and established theoretically in planar jet and plume [Layek and Sunita, Phys. Rev. Fluids 3, 124605 (2018); Layek and Sunita, Phys. Fluids 30, 115105 (2018)].

Extremely small amounts of surface-active contaminants are known to drastically modify the hydrodynamic response of the water-air interface. Surfactant concentrations as low as a few thousand molecules per square micron are sufficient to eventually induce complete stiffening. In order to probe the shear response of a water-air interface, we design a radial flow experiment that consists in an upward water jet directed to the interface. We observe that the standard no-slip effect is often circumvented by an azimuthal instability with the occurrence of a vortex pair. Supported by numerical simulations, we highlight that the instability occurs in the (inertia-less) Stokes regime and is driven by surfactant advection by the flow. The latter mechanism is suggested as a general feature in a wide variety of reported and yet unexplained observations.

We experimentally examine the formation of viscous fluid threads in hydrodynamic focusing sections using both miscible and immiscible fluid pairs at relatively low flow rates. A systematic comparative study is conducted between diffusive and capillary regimes using a viscous oil and a variety of polar organic solvents in a simple microflow geometry. Silicone oil and various low molecular weight alcohols are used as model fluids to investigate the dynamics of viscous multiphase flows at ultralow interfacial tension and with partially miscible systems. An original methodology based on analysis of thread width and detachment length in the viscous regime is developed to quantify various interfacial destabilization processes over a wide range of injection flow rates. For miscible fluid pairs, we investigate several regimes of thread swelling and, for immiscible fluid pairs, we discuss diverse modes of droplet formation and wetting dynamics. This work provides a comprehensive general classification of immiscible and miscible fluid dynamics with large viscosity contrasts in microchannels together with a unifying phenomenological description of thread behavior based on simple functional relationships to better delineate the role of flow parameters and fluid properties on viscous microflow processes.