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. In the limit of a shallow angle, the vortex ring produces an interfacial wave in its wake when sufficiently close to the interface. The flow remains largely laminar and the ring acts minimally to mix the stratification. Conversely, at low Ri, a vertically downward propagating vortex ring will cause significant volumes of lower layer fluid to be transported into the upper layer during the turbulent energy cascade, leading to 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 axisymmetric, vertically propagating case impacts the evolution of the density field and the long-term mixing regime.