The dynamics and mixing properties of vortex rings obliquely impacting a density interface

Abstract

This thesis presents an experimental investigation into the dynamics and mixing properties of vortex rings obliquely impacting the density interface in a two-layer density stratification. For turbulent two-layer zero-mean-shear flows, the classical grid-mixing experiments of J.S. Turner (J. Fluid. Mech, 33:639–656, 1968) demonstrated the intermittent interaction of strong, coherent eddy-like structures with the density interface to be a dominant mixing mechanism. Previous studies have made an analogy between this mixing mechanism and the mixing induced by a vortex ring vertically impacting the density interface in an otherwise stationary flow, to study the mixing mechanism in isolation. Our research extends this analogy by investigating vortex rings obliquely impacting the density interface, at propagation angles θ0 ≤ 25◦ relative to the vertical.

The dynamics of the ring-interface interaction were explored using a double-pulsed laser system to take two-dimensional planar simultaneous Particle Image Velocimetry (PIV) and Laser-Induced Fluorescence (LIF) measurements. To be able to obtain a high signal to noise ratio in all the observable velocity scales of the flow, we developed a ‘multi-frame’ PIV algorithm that makes use of interrogating pairs of PIV images at several different time intervals apart. This algorithm was used to process our PIV data, and can easily be adapted to process PIV data for other flows with regions of localised turbulence.

Ring–interface interactions are classified as ‘penetrative’ or ‘non-penetrative’, corresponding to whether downward entrainment across the interface occurs or not. For both types of interaction, our PIV/LIF measurements reveal that oblique ring impacts lead to an azimuthally asymmetric production of baroclinic vorticity, triggering instability mechanisms that are not present in the θ0 = 0◦ case. With the aim of investigating the influence of θ0 on the mixing properties of the ring–interface interaction, experiments were conducted in which a periodic sequence of 600 vortex rings were generated to mix an initially two-layered stratification. Insights made after the completion of these experiments revealed that, in the non-penetrative regime, the system converges to a state where a significant fraction of the total mixing is convective, as opposed to being directly associated with the ring interacting with the interface. It is argued that the high mixing efficiency observed is attributable in large part to the convective mixing, rather then directly to the ring–interface interaction as has been previously reported. These results are discussed and contextualised with previous grid-mixing experiments.