This paper presents a numerical method to study the pulsing behavior of soft corals. Evidence indicates that the pulsing behavior of soft corals in the family Xeniidae facilitates photosynthesis of their symbiotic algae. One way to investigate this complex behavior is through mathematical modeling and numerical simulations. The immersed boundary method is used to model the interaction of the tentacles with the surrounding fluid. The flow is then coupled with a photosynthesis model. The photosynthesis is modeled by advecting and diffusing oxygen, the byproduct of photosynthesis, where the coral tentacles act as a moving source of the oxygen. This study develops a methodology for solving a partial differential equation with boundary conditions on a moving immersed elastic boundary. In this study, the Reynolds and Péclet numbers are varied in the simulations to gain an understanding of how these parameters affect the mixing and photosynthesis. The mixing is quantified using both the fluid flow and oxygen concentration dynamics. The results show that for the biologically relevant Péclet number, the fluid dynamics significantly affect the photosynthesis and that the biologically relevant Reynolds number is advantageous for mixing and photosynthesis.

The Levitation of a droplet on a hot surface due to the rapid vaporization of the liquid at the proximity of the heat source is known as the Leidenfrost effect. This effect has been demonstrated when the surface is a solid or a liquid. Here, we explore the interaction between a liquid droplet and a hot granular material. The granular material is, by essence, a solid but adopts some behaviors similar to fluids. Surprisingly, we found that the droplet deeply penetrated the hot granular material. During the penetration process, the vapor production is so intense that the grains underneath the droplet are ejected. Consequently, the droplet moved downward under the action of gravity. A mechanism based on the Leidenfrost effect is proposed considering that the granular material can be modeled as a rough surface that can be eroded when the vapor speed is sufficient.

We present a deformation-dependent propulsion phenomenon for soft particles such as cells in microchannels. It is based on a broken time-reversal symmetry generated by a fast forward and a slow backward motion of a fluid which does not progress on average. In both sections, soft particles deform differently and thus progress relatively to the liquid. We demonstrate this by using Lattice-Boltzmann simulations of ubiquitous red blood cells in microchannels, as well as simulations for capsules and minimal-soft-tissue models in unbounded Poiseuille flows. The propulsion of the soft particles depends besides the oscillation asymmetry on their size, deformation type, and elasticity. This is also demonstrated by analytical calculations for a minimal model. Our findings may stimulate a rethinking of particle sorting methods. For example, healthy and malignant cells often differ in their elasticity. With the proposed method, several cell types with different deformabilities can be separated simultaneously without labeling or obstacles in a microfluidic device.

The role of streamwise length scales (${\lambda }_x$) in turbulent skin friction generation is investigated using a direct numerical simulation data set of an incompressible zero pressure gradient turbulent boundary layer and the spectral analysis based on the Fukagata-Iwamoto-Kasagi (FIK) identity by Fukagata et al. [K. Fukagata et al., Phys. Fluids 14, L73 (2002)]. The total skin friction generation associated with motions scaled with local boundary layer thickness $\delta$ of ${\lambda }_x<3\delta$ and ${\lambda }_x>3\delta$ is assessed. The FIK-identity-based spectral analysis is further extended to include the quadrant analysis of Reynolds shear stress. This allows one to relate the turbulent skin friction generation to the quadrant events of Reynolds shear stress, which plays a central role in the momentum transport in turbulent wall-bounded flows. The small-scale ejection and sweep events (${\lambda }_x<3\delta$) contribute to a significant portion of turbulent skin friction. However, it is found that the large-scale ejection and sweep events with streamwise length scales at ${\lambda }_x>3\delta$ are equally important. The turbulent skin friction reduction associated with the modification of large- and small-scale quadrant events is studied, using well-resolved simulation data sets of a large-eddy break-up (LEBU) device in a turbulent boundary layer. The results reveal that LEBUs modify both the large- and small-scale ejection and sweep events, yielding a substantial turbulent skin friction reduction.

Cablelike bodies play a key role in many interdisciplinary systems but are hard to simulate. Asymptotic theories, called slender-body theories, are effective but apply in specific regimes and can be hard to extend beyond leading order. In this paper we develop an exact slender-body-like theory for the surface traction of cablelike bodies in viscous flow. This theory expresses the traction as a series of solutions to a well-behaved one-dimensional Fredholm integral equation of the second kind. This process can be simply generalized to other systems.

The wake behind a bluff body in the presence of a density gradient is characterized by competing effects of buoyancy, momentum, and ultimately viscosity as the flow decays. Consequently, there are a number of distinct flow regimes into which the wakes can be placed, and there is some interest in whether any given wake can be traced back to its source through interrogation of selected fluid mechanical properties in the wake. Here we use dynamic mode decomposition (DMD) to find modes that can be used to characterize and automate such a process. A custom-designed algorithm is proposed that can sort and classify stratified wakes based on a selection of the most energetic DMD modes. The approach is very, or partially successful, depending on the quality and dimension of the input data. The success of these first steps may be used to develop similar methods for more challenging and fully turbulent wakes, and also can serve as a guide for data-driven methods that require no prior knowledge of the flow structure.

Ciliates are ubiquitous in the marine environment and important consumers of phytoplankton and flagellates. The feeding on suspended food particles in ciliates is complex and relies typically on coordinated motion in bands of transversal rows of cilia known as membranelles. Many ciliates are upstream collectors that use a single membranelle band to generate feeding flow, retain food particles, and transport them to the mouth region. To explore a representative upstream collector, we consider the ciliate Euplotes vannus. We determine the clearance rate using particle tracking and estimate the flow-generating force by fitting a point force model to the observed flow. To obtain a mechanistic understanding of feeding flow and particle retention, we use high-speed video microscopy. The cilia move parallel to the membranelle band towards the mouth region in the power strokes, and a metachronal wave propagates away from the mouth region parallel to the band and outwards along the membranelles. A gap, therefore, opens between neighboring membranelles from the inner side of the band where the mouth region is located, and while food particles are retained, water is drawn in and pushed outwards across the membranelle band as the gap closes from the inner side. We suggest a model that rationalizes our observations and quantifies the mechanism by which the membranelle band can both generate feeding flow and retain food particles. The model compares favorably with our observations and suggests a trade-off between the clearance rate and the membranelle gap that determines the lower cutoff in the prey size spectrum. We quantify the trade-off and discuss the ecological significance of our findings.

Recently the dynamics of flows beneath ice cover has garnered much interest in the environmental fluid dynamics community. Of particular importance is understanding how heat introduced via solar radiation is transported within a body of cold fresh water (characteristic temperatures are below the temperature of maximum density), and how a subsurface current affects this process. In this study we present high-resolution numerical simulations of the interaction between cold water convection and a parallel shear flow. The results of this paper show that for a sufficiently strong shear current, the growth phase of instabilities generated by volumetric thermal forcing (which represents solar radiation) is nearly two-dimensional and that the transition to more vigorous three-dimensional motion is initialized by baroclinic production of vorticity by convective instabilities followed by a rapid increase in streamwise vorticity generated by vortex tilting and stretching. We describe how this process is modified by differences in shear strength and thermal forcing attenuation length.

Although liquid behaviors in micrometric scales can be observed optically and those below nanometric scales can be probed with atomic force microscopy, those in the ranges from tens to hundreds of nanometers have seldom been visualized due to limits in optical and electron microscopy. Here we report direct visualization of condensation and wicking of water on arrays of solid walls with gaps ranging from tens to hundreds of nanometers, by reducing electron-beam-induced heating in the environmental scanning electron microscopy. We find that the water condensing and wicking on hydrophilic nanopatterns exhibits little difference from what has been observed on micropatterns. However, the condensation behavior of water on hydrophobic nanopatterns is found not to follow the conventional capillary condensation theory. The hydrophobic grooves of 40–80 nm widths are observed to be completely wetted by condensing water vapor, but do not allow externally approaching liquid water to invade themselves. We expect our experiments to trigger further experimental attempts to observe nanoscale liquid menisci in various situations. In addition, those peculiar nanoscale behaviors of water reported here will broaden our knowledge of nanoscale wetting and feed empirical inputs for theoretical models.

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.