We present a study of wetted facemasks to evaluate their capability in blocking respiratory droplets. We show that the increase in wetness progressively weakens the penetration capability of the impacted droplets. Such behavior is observed for hydrophobic and hydrophilic masks, although the underlying mechanism is different.

In addition to their ability 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 a mask by tracking the air exhaled by a person through a mask. We show how a mask confines the exhaled flows within tens of centimeters in front of a person breathing or speaking.

A set of equations for supersaturation (SS) and liquid water content (LWC) fluctuations in cloud turbulence are derived and their spectra are analyzed by using the Lagrangian renormalized approximation. The SS spectrum has three power law ranges, while the LWC spectrum has two power law ranges before the exponential roll off as the wavenumber increases.

Multiscale problems such as hypersonic flows with strong nonequilibrium due to strong shocks and expansions result in flow physics which is no longer accurately described by the Navier-Stokes equations (NSE). Similarly, the NSE break down in rarefied (low density) gas flows. Therefore, hybrid methods, which can combine the continuum description using NSE and the kinetic description (KD), are necessary for efficient high-fidelity numerical simulations. A key input to hybrid methods is a metric to identify regions in the flow-field where the NSE breaks down and the KD should be used. In this Letter, starting from kinetic theory, we develop a rigorous metric to assess where the NSE breaks down.

A phenomenological study is presented of the impact, spreading, and arrest of Galinstan droplets on a rigid substrate. The maximum diameter at which contact line arrest occurs is found to be a power law of the speed and a model is presented to rationalize the observations.

Oil drops placed on surfactant laden water surface exhibit a transition from surface tension driven to surface tension gradient driven spreading as a function of surfactant concentration. Interestingly, this transition occurs at the critical micellar concentration irrespective of surfactant chemistry demonstrating a connection between fluid flows and shuttling of surfactant between the bulk and the interface.

When stirred, is the composition field of the product of a reaction more, or less homogenous than the one of a passive scalar in the same conditions? The composition fluctuations are weaker, and we explain why.

Three-dimensional (3D) turbulence is often studied experimentally in closed containers with energy injected at a large scale from a container boundary. We conduct an experiment with remotely driven small magnetic particles to force fluid in volume, randomly in space and time. Such a forcing, like many direct numerical simulations, generates stationary, homogeneous and isotropic turbulence with almost no mean flow, as characterized by fluid velocity measurements. Different methods are used to estimate the energy dissipation rate consistently. We experimentally confirm Tennekes prediction for 3D turbulence without mean flow, and resolve conflicts among previously suggested values of Tennekes’ constant.

A simple analytical model is introduced for describing the combined effect of thixotropic and nonlocal flow in a homogeneous stress field. The model adequately describes experimental rheo-MRI velocimetry profiles, measured for a milk microgel suspension in a cone-and-plate geometry.

We study active nematic fluids confined in narrow channels with a conflicting anchoring at the walls. In this configuration flows are sustained even in the zero activity limit. We use a numerical approach to span a wide portion of parameter space and provide a unified picture of this system beyond the limiting regime explored previously, which stresses the key role played by the activity and the flow aligning parameter. We also show that solutions depend on two dimensionless parameters and present examples of biaxial solutions and the parameter values for which they are observed.

Mass transfer of gases in liquid solvents is a fundamental process during bubble generation for specific purposes or, vice versa, removal of entrapped bubbles. In our work, we address the growth dynamics of a trapped slug bubble in a vertical glass cylinder under a water barrier after replacing the ambient air atmosphere by a CO${}_2$ atmosphere at the same or higher pressure. The asymmetric exchange of the gaseous solutes between the CO${}_2$-rich water barrier and the air-rich bubble always results in net bubble growth, which we call solute exchange. We compare and explain the experimental results with a simple numerical model, with which the underlying mass transport processes are quantified.

A shock wave impacting an n-dodecane droplet in a nitrogen environment at near-critical conditions results in a transcritical shock-droplet interaction (TSDI). We illustrate the unique behavior of TSDI compared to the classical cases occurring away from the critical point; i.e., shock-bubble interactions (e.g., helium and R22 bubbles) and shock-droplet interactions (e.g., water droplet).

A phenomenological study focusing on investigation of ionic transport within one- and two-dimensional electrolytic cells operating at high voltage and confined by two open electrodes is presented. The cell is filled with a nonsymmetric electrolyte consisting of one positively charged and two negatively charged species. The study offers a physical interpretation and detailed discussion of the origin and development of electroconvective instability typical of configurations relevant to digital printing technologies.

The long-term thermal evolution of rocky planets like Earth, Venus, Mercury, and Mars is governed by mantle convection: the buoyancy-driven creeping flow of silicate rocks. Yet, some key parameters and initial conditions are poorly constrained. Using deep learning, we model two-dimensional surrogates of mantle convection over a wide range of parameters. This paves the way for a rapid evaluation of the parameter space.

The nonlinear phase of Richtmyer-Meshkov Instability (RMI) is characterized by the development of asymmetric interfacial perturbation structures known as ‘bubbles’ and ‘spikes’, which could potentially be curbed by surface tension. Our work systematically examines such curbing effects of surface tension within a wide range of Weber numbers. Based on numerical results of two-phase compressible simulations, we confirm two existing analytic theories predicting RMI perturbation development respectively at small and asymptotically large Weber numbers, and propose a heuristic criterion identifying the transitional development of RMI at intermediate Weber numbers.

We investigate the effects of wall suction and injection on Chemical Vapour Deposition (CVD), a microfabrication process in which a gas mixture is pumped into a reactor and flows over a heated reactant porous surface to chemically deposit a thin film. Regular and cohesive film growth requires that laminar flow be maintained. The energy balance with wall suction for temperature dependence parameter $ϵ<0$ leads to increasing total mechanical energy due to reducing energy production and dissipation with increasing flow suction; for $ϵ>0$ there is a stabilization effect on the Tollmien-Schlichting waves mainly due to a large reduction in the energy contribution term.

In a wide gap spherical Couette flow with $\eta =1/2$, the nonlinear branches of the 4,3,2-fold spiral states and the critical Reynolds numbers at which they sequentially bifurcate from the axisymmetric state are solved. The evidence that these states are established as stable states even over the successive critical Reynolds numbers is discussed based on the formation of their unstable manifolds and basins of attraction in the state space. This result substantiates that both the 4-fold and 3-fold spiral state may coexist stably in experiment.

We study different flow control methods on turbulent flow around a NACA4412 airfoil, using resolved large-eddy simulation (LES). We find that changes in total skin friction due to blowing and suction are not very sensitive to different pressure-gradient conditions nor the Reynolds number. However, the boundary-layer thickness, the intensity of the wall-normal convection, and turbulent fluctuations are much more affected, mostly due to the adverse-pressure-gradient conditions, as skin-friction decompositions show. Overall, we conclude that it is not possible to simply separate pressure-gradient and control effects, which is important for control design in practical applications.

Flow focusing of colloidal dispersions can be used to assemble multiscale structured macroscopic materials. In this combined experimental and numerical study, we have investigated the effect of (partly) co- and counter- flowing sheath flows, as compared to the reference case with sheath flows normal to the core flow. The effect of non-normal sheath flows is substantial and, perhaps surprisingly, independent of direction for variations of +/- 30° from normal flow.

The interaction between an evaporating liquid film and a shearing gas is a complicated problem involving rich and interesting dynamics. We tackle this problem by deriving a Benney-like equation governing the flow in the long-wave limit. Subsequently, the temporal and spatiotemporal characteristics of the flow are investigated both (i) in the linear regime using the analytically derived dispersion relationship, and (ii) in the nonlinear regime by solving the governing equation numerically. Finally, we use self-similarity analysis to study the role of the shearing gas on the rupture mechanics of the film.

Bendotaxis is a mechanism in which a liquid droplet propels itself along an elastic walled channel: the deformation induced by the droplet’s capillary pressure leads to an imbalance in that pressure that generates motion. However, droplets on real surfaces often suffer contact angle hysteresis. We study the conditions under which contact angle hysteresis is strong enough to trap a droplet, preventing bendotaxis.

The response of an aquatic plant and an elastic fiber in an oscillatory flow field induced by gravity waves is studied. The flexible structure behaves like a driven harmonic oscillator. In the limit of drag-dominated oscillations, the frequency response of the structures exhibits no resonance. Our results show that in natural settings, the oscillations of such slender biological structures are generally in the drag-dominated regime and hence sharp resonance is unlikely.

The thinning of the liquid films separating bubbles in a foam or in a bubbly liquid controls the coalescence process and the foam stability, and is highly relevant in many industrial processes. This thinning process leads to films of heterogeneous thickness, in which spontaneous symmetry breaking occurs, shown to be driven by surface minimization only.

Glassy thin films have been abundantly studied in the past decades and shown to exhibit a liquid-like surface mobile layer. When immersed in water, nanobubbles spontaneously form at their free surface. Here, from a combination of glassy lubrication theory and finite-element numerical integration, we demonstrate that, and study how, the surface is dynamically deformed through a surface flow driven by the inner pressure of the bubble. As a consequence, the film eventually undergoes a dewetting event, for which we predict a typical time scale from the model.

Imbibition, the displacement of a nonwetting fluid by a wetting fluid, plays a central role in diverse energy, environmental, and industrial processes. While this process is typically studied in homogeneous porous media with uniform permeabilities, in many cases, the media have multiple parallel strata of different permeabilities. Here, we use numerical simulations to examine the fluid dynamics of imbibition in stratified media. Our results highlight how stratification can fundamentally change the dynamics of imbibition, and provide quantitative guidelines for predicting and controlling this process.

The membrane permeation of a pure fluid driven by lubrication pressure is studied in the range of wall-membrane gap widths that cannot be treated by the Reynolds lubrication equation. This lubrication effect is modeled by including a higher-order pressure component described by the wall-tangential derivative of the local Couette-Poiseuille velocity. A permeate flux model is developed by renormalizing the Couette component, which shows divergent behavior, into the lowest order pressure component, and successfully predicts the permeate flux from infinitesimal to finite permeability.

The relaxation dynamics of a bent rod immersed in a fluid is strongly affected by its bending stiffness and the fluid viscosity. For large bending stiffness or low viscosity, the dynamics is underdamped and the rod oscillates around its equilibrium position with a well-defined frequency and a damped amplitude. For low bending stiffness or large viscosity, the dynamics is overdamped and the rod relaxes to its equilibrium position without oscillating. We show the existence of two overdamped regimes where the relaxation dynamics is characterized by two different polynomial decays showing that the viscous force is not proportional to the rod velocity in our experiments.

In experiments, chemically active drops most often swim along a rigid wall, the impact of which on self-propulsion is in general completely overlooked in models. Using linear stability analysis, we demonstrate here that the proximity to a rigid surface promotes droplet propulsion as a result of the localization of the strongest interfacial flows within the thin lubrication gap separating the droplet from the wall.

Motivated by the current pandemic we analyze the use of cloth masks as effective alternatives to medical masks for the general public. In this paper, we establish a quantitative framework for estimating the filtration efficacy of cloth masks by deriving analytical estimates for the pressure drop across woven heterogeneous fabrics. We then introduce a filtration quality factor to compare the intrinsic filtration capabilities of diverse materials for submicron aerosols. Finally, we present a decision map to illustrate the trade-offs between filtration efficiency and breathability and to provide practical guidance on the selection of cloth masks.

We present an experimental study of the time-dependent velocity and pressure fields during high-speed microbubble production in a flow-focusing device. Particle tracking velocimetry is used to resolve velocities up to 30 m/s at timescales down to 100 ns, while numerical simulations give access to both the gas and liquid velocity and pressure oscillations.

This paper reports the separation dynamics of a binary solution placed in a temperature gradient. We show the existence of three separation regimes, depending on the volume fractions of the components. One of the regimes is associated with a capillary instability of which a typical pattern is shown in this picture. All the observed regimes are rationalized.

When placed in a canonical flow (straining, rotational, and vortex stretching corresponding to the top, middle, and bottom row), finite-density particles can follow various paths. The present article analytically predicts their long-term behavior, namely, whether they will cluster or disperse or whether their trajectories cross.

We perform experiments and simulations of underwater granular column collapses and report an enhanced granular flow mobility as the column size increases. The size effect is attributed to several mechanisms due to the complex fluid-particle interactions. Strong fluid eddies in large cases penetrate through the flowing layer of the granular phase and tend to mobilize more particles disproportionally. The interstitial water lubricates interparticle contacts, causing enhanced basal slip of the fast-moving granular front. This contact lubrication or resistance reduction becomes more prominent in large underwater granular column collapses.

The relevance of the generalized/nonlinear Fourier’s law in a driven granular gas such as in dust storms is underscored by demonstrating that its linear, isotropic version does not hold even in the continuum limit of zero Knudsen number.

We present a numerical study of settling and clustering of small inertial particles in homogeneous and isotropic turbulence. Particles are denser than the fluid, but not in the limit of being much heavier than the displaced fluid. At fixed Reynolds and Stokes numbers we vary the fluid-to-particle mass ratio $\gamma$ and the gravitational acceleration. We report nonmonotonic behavior of the particles’ velocity skewness and kurtosis with the second parameter, and an associated anomalous behavior of the settling velocity compared to the free-fall Stokes velocity, including some cases of loitering. Clustering increases for increasing gravitational acceleration, and for decreasing $\gamma$.

Direct numerical simulations are performed to study the influence of the flow Reynolds number (Re) on the dynamics of inertial, ellipsoidal particles in a turbulent wall-bounded flow. In wall units, the time required for the particle distribution to reach a statistically steady state increases with Re. Once the distribution reached a statistically steady-state, a more uniform concentration profile is observed for higher values of Re, regardless of particle shape, but with a noticeable influence of particle inertia. Finally, in the near-wall region, preferential orientation of all ellipsoids is modified when Re increases, with a strong dependence on the ellipsoids characteristics.

Spatiotemporally chaotic dynamics of transitional plane Couette flow may give rise to regular turbulent-laminar stripe patterns with a large-scale pattern wavelength and an oblique orientation to the laminar flow direction. How the stripe pattern forms remains an open problem. To understand stripe pattern spatiotemporal dynamics we identify unstable periodic orbits of the 3D Naiver-Stokes equations that show oblique large-scale spatial amplitude modulation and a temporal evolution of standing waves. The unstable periodic orbits are embedded in the edge of chaos in a symmetry subspace of plane Couette flow and thereby may mediate transition to and from turbulent flows with oblique patterns.

Machine learning is becoming an increasingly popular method for building mathematical models from observations of natural processes. Here, the central challenge is to impart structure into the model parameterization to enforce physical relevance, yet retain a degree of generality so that a large variety of dynamics can be learned. We introduce a novel methodology, based on a data-driven extension of the classical Onsager principle, that strikes a balance between these competing aspects. We demonstrate its efficacy by learning quantitatively accurate and qualitatively faithful reduced order models of the Rayleigh-Bénard convection equations.

We introduced a new chemical rection for mixing studies, forming a fluorescent product from transparent reactants, with tunable kinetics.

In a finite size wind farm turbulence is streamwise inhomogeneous. This paper examines the evolution of large scale motions on the order of 10 turbine rotor diameters influenced by wake-mixing from turbine rows. Using wavelet-based methods, the analysis illustrates that streamwise and vertically coherent length scales grow significantly past the first row of turbines. The first observation is primarily due to turbulence from the superposition of turbine wakes while the latter one is associated with the development of a global energy transfer mechanism (“sweeps”) between the outer layer and the wind turbine region.

The compressibility effect on opposition drag control is investigated using direct numerical simulation of turbulent channel flows at a bulk Reynolds number Re${}_b$ = 3000 for three different bulk Mach numbers: Mb = 0.3, 0.8, and 1.5. With increasing Mb, drag reduction slightly decreases at small sensing plane location $y_d^{+}$ but increases at large $y_d^{+}$. Interestingly, for large $y_d^{+}$ cases, a resonance buffer layer characterized by a streamwise periodic array of spanwise-coherent rollers is established, one of the primary reasons for the degradation of drag reduction performance.

In this paper a novel methodology is proposed for wall-modeled large eddy simulation (WMLES) on non-body-conforming Cartesian grids. The proposed WMLES employs a partial-slip velocity boundary condition to reduce conservation errors at the wall. In addition, since the slip velocity reduces the shear stress in the near-wall region, a modeled turbulence shear stress is introduced to maintain the shear-stress balance in the near-wall region. The proposed WMLES robustly predicts turbulence statistics in turbulent boundary layers developed on an inclined flat plate without showing log-layer mismatch.

This work concerns the simulation of a homogenized streamwise periodic boundary layer which reduces the computational cost of direct numerical simulations of incompressible flat plate turbulent boundary layers. It expands upon the understanding of how the transpiration velocity changes dependent on whether a blending function is used to account for inner and outer self-similar scaling effects. With this rescaling correction, lower Reynolds number effects on the transpiration velocity are captured by the homogenized streamwise periodic boundary layer simulation.

To better understand turbulent reattachment downstream of canopy edges we conducted analysis of velocity and visual data from flume experiments. We find that the mean flow and turbulence statistics suggests the presence of both canopy shear and backward facing step (BFS) dynamics. Depending on the canopy characteristics, the dominance of either dynamics varies spatially within the wake. In regions dominated by canopy-shear turbulence, the separation induced by the canopy edge (BFS dynamics) modifies the canopy signal by introducing both larger and smaller scales to the flow. We also find that turbulence development over the upstream canopy influences the reattachment length in the wake.

General solutions for closely spaced corotating helical vortices are obtained using a filament approach. For these vortex structures, helical symmetry is broken but solutions maintain a form of spatial periodicity. We show there exists a moving frame where vortex elements move along the structure without distorting it. Locally, the structure behaves much like a helical pair aligned with the locally tangent flow. However, away from the cores the induced flow is reminiscent of a helical vortex.

Vortex-Induced Vibration (VIV) is studied extensively for cases where the oscillations are possible in the direction of flow (IL VIV) or perpendicular to it (CF VIV). In this work, we study VIV for the angles in between and show how the cylinder’s response transitions from a purely IL VIV to a purely CF VIV.

A numerical study of damping effects on aircraft wings caused by liquid sloshing inside the fuel tanks has been performed. To model this phenomenon, a tank-fluid system is vertically excited with a periodic law of motion. The work has two parts. In the present Part I, a mathematical formalism of the global energy balance in different frames of reference is provided. Then, an enhanced Smoothed Particle Hydrodynamics model, (delta-LES-SPH), has been applied for the simulation of this violent flow. Multiple energy dissipation mechanisms such as liquid impacts and turbulence are discussed. Comparison with experimental results and the fluid-structure interaction case is carried out in Part II.

As a continuation of Part I, where the energy balance of a periodically excited confined flow was studied, in this second part the numerical model is validated against experiments on a vertically excited tank-fluid system in decaying motion. In one of two numerical models, the decaying tank movement is prescribed according to the experimental measurements; in the second the tank is coupled to a mass-spring-damper equation resembling the experimental one. Both problems were studied for two different fluids with a difference in two orders of magnitude in Reynolds number. The numerical force and energy balance inside the fluid tank are compared to the experimental data showing good agreement.

Turbulent/nonturbulent interfaces typically have a thickness of the order of the Kolmogorov scale. However, when we add a density or temperature stratification, we observe that the interface thickness scales differently. We analyze two different turbulent flows at moderate Reynolds numbers using direct numerical simulation and demonstrate that the results collapse on a single curve using a lengthscale based on the potential enstrophy.

Experiments with an oscillating torus in a stratified and/or rotating fluid on the 13 m-diameter Coriolis platform show that, at such large Stokes number, geometrically focusing internal waves compare to some extent with the presented linear theory, but exhibit surprising results in the nonlinear regime: though the waves are very nonlinear with wave triads and increasing energy in the higher harmonics, they do not overturn, and wave breaking is not observed.