Physics

In electrochemical systems, an understanding of the underlying transport processes is required to aid in their better design. This includes knowledge of possible near-electrode convective mixing that can enhance measured currents. Here, for a binary acidic electrolyte in contact with a platinum electrode, we provide evidence of electroconvective instability during electrocatalytic proton reduction. The current-voltage characteristics indicate that electroconvection, visualized with a fluorescent dye, drives current densities larger than the diffusion transport limit. The onset and transition times of the instability do not follow the expected inverse-square dependence on the current density, but, above a bulk-reaction-limited current density are delayed by the water dissociation reaction. The dominant size of the electroconvective patterns is also measured and found to vary as the diffusion length scale, confirming previous predictions on the size of electroconvective vortices.

We study the electrokinetic flow of viscoelastic fluids subjected to an oscillatory pressure gradient, and particularly focus on the resonance behaviors in the flow. The governing equations are restricted to linear regime so that the velocity and streaming potential fields can be solved analytically. Based on the interaction of viscoelastic shear waves, we explain the mechanism of resonance, and derive a critical Deborah number Dec = 1/4 which dictates the occurrence of resonance. Using the Maxwell fluid model, we show that the resonance enhances electrokinetic effects and results in a dramatic increase of electrokinetic energy conversion efficiency. However, by applying the Oldroyd-B fluid model it reveals that the amplification of efficiency is suppressed even for a very small Newtonian solvent contribution. This may be one of the reasons that experimental verification regarding the high efficiency predicted by Bandopadhyay & Chakraborty (Appl. Phys. Lett., vol. 101, 2012, 043905) is unavailable in the literature. Furthermore, the damping effect of solvent viscosity is more significant for higher-order resonances. Introducing the factor of multiple relaxation times, we show that the occurrence of resonances for the streaming potential field and the flow rate are still dominated by Dec. For the efficiency in the multi-mode case, the occurrence of resonance is dominated by the Deborah number De and the mode number N, and the resonance disappears for small De or large N. In addition, a new type of scaling relation between the streaming potential field and EDL thickness can be identified at large De.

We examine a passive scalar diffusing in time-varying flows which are induced by a periodically oscillating wall in a Newtonian fluid between two infinite parallel plates as well as in an infinitely long duct. These shear flows yield the generalized Ferry waves which are exact solutions of the Navier-Stokes equations. First, we calculate the second Aris moment for all time, and its long time limiting effective diffusivity as a function of the geometrical parameters, frequency, viscosity, and diffusivity. We show that the viscous dominated limit results in a linear shear layer for which the effective diffusivity is bounded with upper bound κ(1+ A 2 /(2 L 2 )) , where κ is the tracer diffusivity, A is the amplitude of oscillation, and L is the gap thickness. Alternatively, we show that for finite viscosities the enhanced diffusion is unbounded, diverging in the high frequency limit. Physical arguments are given to explain these striking differences. Asymptotics for the high frequency behavior as well as the low viscosity limit are computed. Study of the exact formula shows that a maximum exists as a function of the viscosity, suggesting a possible optimal temperature for mixing in this geometry. Physical experiments are performed in water using Particle Tracking Velocimetry to quantitatively measure the fluid flow. Using fluorescein dye as the passive tracer, we document that the theory is quantitatively accurate. Further, we show that the scalar skewness is zero for linear shear at all times, whereas for the nonlinear Ferry wave, using Monte-Carlo simulations, we show the skewness sign (as well as front versus back loaded distributions) can be controlled through the phase of the oscillating wall. Lastly, short time skewness asymptotics are computed for the Ferry wave and compared to the Monte-Carlo simulations.

In this paper, we study wave transmission in a rotating fluid with multiple alternating convectively stable and unstable layers. We have discussed wave transmissions in two different circumstances: cases where the wave is propagative in each layer and cases where wave tunneling occurs. We find that efficient wave transmission can be achieved by `resonant propagation' or `resonant tunneling', even when stable layers are strongly stratified, and we call this phenomenon `enhanced wave transmission'. Enhanced wave transmission only occurs when the total number of layers is odd (embedding stable layers are alternatingly embedded within clamping convective layers, or vise versa). For wave propagation, the occurrence of enhanced wave transmission requires that clamping layers have similar properties, the thickness of each clamping layer is close to a multiple of the half wavelength of the corresponding propagative wave, and the total thickness of embedded layers is close to a multiple of the half wavelength of the corresponding propagating wave (resonant propagation). For wave tunneling, we have considered two cases: tunneling of gravity waves and tunneling of inertial waves. In both cases, efficient tunneling requires that clamping layers have similar properties, the thickness of each embedded layer is much smaller than the corresponding e-folding decay distance, and the thickness of each clamping layer is close to a multiple-and-a-half of half wavelength (resonant tunneling).

The task of dynamic flow estimation is to construct an approximation of an evolving flow---and particularly, its response to disturbances---using measurements from available sensors. Building from previous work by Darakananda et al.~(Phys Rev Fluids 2018), we further develop an ensemble Kalman filter (EnKF) framework for aerodynamic flows based on an ensemble of randomly-perturbed inviscid vortex models of flow about an infinitely-thin plate. In the forecast step, vortex elements in each ensemble member are advected by the flow and new elements are released from each edge of the plate; the elements are aggregated to maintain an efficient representation. The vortex elements and leading edge constraint are corrected in the analysis step by assimilating the surface pressure differences across the plate measured from the truth system. We show that the overall framework can be physically interpreted as a series of adjustments to the position and shape of an elliptical region of uncertainty associated with each vortex element. In this work, we compare the previously-used stochastic EnKF with the ensemble transform Kalman filter (ETKF), which uses a deterministic analysis step. We examine the response of the flat plate at 20 ∘ in two perturbed flows, with truth data obtained from high-fidelity simulation at Reynolds number 500. In the first case, we apply a sequence of large-amplitude pulses near the leading edge of the plate to mimic flow actuation. In the second, we place the plate in a vortex street wake behind a cylinder. In both cases, we show that the vortex-based framework accurately estimates the pressure distribution and normal force, with no {\em a priori} knowledge of the perturbations. We show that the ETKF is consistently more robust than the stochastic EnKF. Finally, we examine the mapping from measurements to state update in the analysis step through SVD of the Kalman gain.

A small drop of a heavier fluid may float on the surface of a lighter fluid supported by surface tension forces. In equilibrium, the drop assumes a radially symmetric shape with a circular triple-phase contact line. We show theoretically and experimentally that such a floating liquid drop with a sufficiently small volume has two distinct stable equilibrium shapes: one with a larger and one with a smaller radius of the triple-phase contact line. Next, we experimentally study the floatability of a less viscous water drop on the surface of a more viscous and less dense oil, subjected to a low frequency (Hz-order) vertical vibration. We find that in a certain range of amplitudes, vibration helps heavy liquid drops to stay afloat. The physical mechanism of the increased floatability is explained by the horizontal elongation of the drop driven by subharmonic Faraday waves. The average length of the triple-phase contact line increases as the drop elongates that leads to a larger average lifting force produced by the surface tension.

This study deals with the estimation of the trajectory of coronavirus COVID-19 adhering to respiratory droplets projected horizontally, considering the geographical altitude. The size of viruses and respiratory droplets is the factor that determines the trajectory of the microparticles in a viscous medium such as air; For this purpose, a graphical comparison of the diameters and masses of the microparticles produced in respiratory activity has been made. The estimation of the vertical movement of the microparticles through the air is based on Stokes' Law, it was determined that respiratory droplets smaller than 10 {\mu}m in diameter have very small speeds, in practice they are floating for a few seconds before evaporating in the air; Regarding the horizontal displacement of respiratory droplets, frames from Scharfman et al. to determine its scope. In the case of a sneeze, the respiratory droplets can reach a distance of 1.65 m in 1 s, stopping rapidly until reaching 1.71 m in 2 seconds, then an analysis of the effect of geographical altitude on the movement of the micro-droplets was made, determining minimal change in kinematic variables.

A hybrid Eulerian-Lagrangian solver RYrhoCentralFoam is developed based on OpenFOAM to simulate detonative combustion in two-phase gas-liquid mixtures. For Eulerian gas phase, RYrhoCentralFoam enjoys second order of accuracy in time and space discretizations and is based on finite volume method on polyhedral cells. The following developments are made based on the standard compressible flow solver rhoCentralFoam in OpenFOAM: (1) multi-component species transport, (2) detailed fuel chemistry for gas phase combustion, and (3) Lagrangian solver for gas-droplet two-phase flows and sub-models for liquid droplets. To extensively verify and validate the developments and implementations of the solver and models, a series of benchmark cases are studied, including non-reacting multi-component gaseous flows, purely gaseous detonations, and two-phase gas-droplet mixtures. The results show that the RYrhoCentralFoam solver can accurately predict the flow discontinuities (e.g. shock wave and expansion wave), molecular diffusion, auto-ignition and shock-induced ignition. Also, the RYrhoCentralFoam solver can accurately simulate gaseous detonation propagation for different fuels (e.g. hydrogen and methane), about propagation speed, detonation frontal structures and cell size. Sub-models related to the droplet phase are verified and/or validated against analytical and experimental data. It is also found that the RYrhoCentralFoam solver is able to capture the main quantities and features of the gas-droplet two-phase detonations, including detonation propagation speed, interphase interactions and detonation frontal structures. As our future work, RYrhoCentralFoam solver can also be extended for simulating two-phase detonations in dense droplet sprays.

Transported probability density function (PDF) methods are widely used to model turbulent flames characterized by strong turbulence-chemistry interactions. Numerical methods directly resolving the PDF are commonly used, such as the Lagrangian particle or the stochastic fields (SF) approach. However, especially for premixed combustion configurations, characterized by high reaction rates and thin reaction zones, a fine PDF resolution is required, both in physical and in composition space, leading to high numerical costs. An alternative approach to solve a PDF is the method of moments, which has shown to be numerically efficient in a wide range of applications. In this work, two Quadrature-based Moment closures are evaluated in the context of turbulent premixed combustion. The Quadrature-based Moment Methods (QMOM) and the recently developed Extended QMOM (EQMOM) are used in combination with a tabulated chemistry approach to approximate the composition PDF. Both closures are first applied to an established benchmark case for PDF methods, a plug-flow reactor with imperfect mixing, and compared to reference results obtained from Lagrangian particle and SF approaches. Second, a set of turbulent premixed methane-air flames are simulated, varying the Karlovitz number and the turbulent length scale. The turbulent flame speeds obtained are compared with SF reference solutions. Further, spatial resolution requirements for simulating these premixed flames using QMOM are investigated and compared with the requirements of SF. The results demonstrate that both QMOM and EQMOM approaches are well suited to reproduce the turbulent flame properties. Additionally, it is shown that moment methods require lower spatial resolution compared to SF method.

Pore structures and gas transport properties in porous separators for polymer electrolyte fuel cells are evaluated both experimentally and through simulations. In the experiments, the gas permeabilities of two porous samples, a conventional sample and one with low electrical resistivity, are measured by a capillary flow porometer, and the pore size distributions are evaluated with mercury porosimetry. Local pore structures are directly observed with micro X-ray computed tomography (CT). In the simulations, the effective diffusion coefficients of oxygen and the air permeability in porous samples are calculated using random walk Monte Carlo simulations and computational fluid dynamics (CFD) simulations, respectively, based on the X-ray CT images. The calculated porosities and air permeabilities of the porous samples are in good agreement with the experimental values. The simulation results also show that the in-plane permeability is twice the through-plane permeability in the conventional sample, whereas it is slightly higher in the low-resistivity sample. The results of this study show that CFD simulation based on micro X-ray CT images makes it possible to evaluate anisotropic gas permeabilities in anisotropic porous media.