Identification of zones in a free evolving vortex ring

A direct numerical simulation of a three-dimensional free evolving vortex ring has been performed to understand some features of the dynamics of vortex rings. In previous experiments, a persistent wake has been observed and the direct simulation has clarified the causes of this wake formation. A kinematic alignment analysis of the vorticity vector with the eigenvectors of the strain-rate tensor, together with vorticity visualisations, have been employed to explain the growth of the Widnall instabilities. The analysis has shown the presence of two different zones, the first characterised by intense vorticity and low strain rate and the second with opposite features. In the former, there is a ring core which maintains its coherence even in the long term, in the latter there are elongated structures contributing to the deformation of the ring.