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.