Sheltering the perturbed vortical layer of electroconvection under shear flow

Abstract

Sheltering of a perturbed vortical layer by a shear flow is a common method to control turbulence and transport in plasma physics. Despite the desire to exploit this phenomenon in wider engineering applications, shear sheltering has rarely been observed in general non-ionized fluids. In this study, we visualize this shear sheltering in a generic neutral-fluid situation in electromembrane desalination: electroconvection (EC) under the Hagen Poiseuille flow initiated by ion concentration polarization. Our work is the first demonstration of shear sheltering in electrochemical systems. Experiment, numerical simulation and scaling analysis accurately capture the effect by pinpointing the threshold for shear suppression. Determined by balancing the velocity fluctuation (with EC vortices) and the flow shear (with no-slip walls), the threshold for shear suppression is scaled as the EC height. Stable EC with coherent unidirectional vortices occurs under the threshold height, whereas chaotic EC occurs beyond this height as the EC-induced vortical perturbation overwhelms the flow shear. Attractors in a time-delay phase space illustrate this sequence of steady periodic (stable EC) chaotic transitions precisely. Going one step further, the shear sheltering effect is decoupled from the shear-independent mechanism of vortex suppression, i.e. vortex sweeping by the mean flow. In the frequency domain, this shear-independent effect is negligible for stable EC (when shear sheltering dominates), whereas it can reduce the level of chaotic fluctuations of chaotic EC (when shear sheltering weakens). Lastly, taken together, we describe the EC-induced convective ion transport by the new scaling law for the electric Nusselt number as a function of the electric Rayleigh number and the Reynolds number, This work not only expands the scientific understanding of EC and the shear sheltering effect, but also affects a broad range of electrochemical applications, including desalination, energy harvesting and sensors.