Experimental Study of Free Surface Mixing in Vortical and Chaotic Flows

Abstract

The free surface mixing properties of a scalar advected by a quasi-steady or unsteady electromagnetically forced flow are investigated. The scalar statistics are related with the topology of the velocity fields stirring them. The benefits and consequences of topologically folding a scalar to enhance homogenization are discussed, identifying how this process may lead to the attenuation of diffusion in vortical and chaotic flows. A pair of magnets, whose attitude is controlled during the experiment, is employed to generate a wide range of velocity fields in a shallow layer of conductive stratified brine. The simplicity of the system makes it possible to analyze the basic properties of the flows generated, relating them with more complex geometries found in literature. The concentration measurements characterizing the scalar field are based on LIF, for which a novel experimental procedure (including calibration, error management and statistical estimators) is presented. Special attention is paid to the relation between the variance decay rate and the mean gradient square, identifying several mechanisms that reduce the fidelity of Q2D experiments in reproducing some features of the transport equation. Evidence of the scalar spiral range is presented in the wavenumber and physical spaces for particular quasi-steady samples. When required, the system unsteadiness is generated by modifying the body forcing geometry throughout the experiment, producing chaotic advection regardless of the flow Re. The periodic nature of the forcing oscillations leads to an exponential variance decay dominated by a strange eigenmode. It is shown that such a system contains recurring temporal patterns and becomes independent of the scalar initial condition.