The organization of scalar mixing by coherent structures

Abstract

Turbulent scalar mixing is prominent in all sorts of situations, being it in air pollution or making tea. However, this does not often lead to a homogeneously mixed fluid, but rather leads to regions of higher concentrations next to regions with significantly lower concentrations. This can be characterized by dividing the concentration field in a turbulent flow into a few regions that have almost the same concentration: Uniform Concentration Zones. How these Uniform Concentration Zones are organized is yet unclear, but a possible candidate in the velocity field is the use of Lagrangian Coherent Structures. These structures are, in principle, transfer barriers in the flow and defined as local maxima of the Finite Time Lyapunov Exponent (FTLE), which is a measure of exponential fluid-separation over time. In order to successfully apply the concept of Lagrangian Coherent Structures in an experiment, turbulent structures should remain in focus long enough to measure the exponential separation of fluid parcels. As almost all fully developed turbulence comes with a mean flow, the observation time in an Eulerian field-of-view is too short. Therefore an experiment was designed to move the observation equipment with the mean flow. With this experimental setup, the concentration field and the velocity field are measured simultaneously via respectively LIF and PIV. Comparing the Uniform Concentration Zones with both the forward and backward time FTLE-field, shows that there is not much correlation between the concentration field and the forward time FTLE. However, local maxima of the backward time FTLE seem to overlap often with boundaries between Uniform Concentration Zones.