The influence of propagation angle on vortex ring-induced mixing

The propagation angle of a vortex ring travelling towards the interface in a two-layer density stratification has a considerable influence on the mixing induced by the interaction that ensues. For example, at bulk Richardson number equal to 1, a vortex ring propagating at 75 degrees to the vertical produces an interfacial wave when sufficiently close to the interface. The ring is unable to penetrate the interface, the flow remains largely laminar and the ring acts minimally to mix the stratification. Conversely, for the same Richardson number, a vertically downward propagating vortex ring will cause significant volumes of lower layer fluid to be ejected into the upper layer as the vortex ring destabilises, leading to a turbulent energy cascade and considerable mixing. Here, we present new experimental results investigating the mixing induced by the periodic generation of vortex rings fired at an angle in a two-layer stratification. We demonstrate how deviating from the vertically propagating case impacts the evolution of the density field and the long-term mixing regime.