Head-on collision of a large vortex ring with a free surface

Abstract

The generation of free surface waves due to the head-on collision of a vortex ring with the free surface is studied experimentally and numerically. The interaction process is investigated by looking at the free surface evolution with the aid of a shadowgraph image, by measurement of the free surface elevation, and by visualizing the vortex ring with hydrogen bubbles. Three different free surface patterns are observed, depending on the strength of the vortex ring: (1) A single circular depression of the free surface for weak vortex rings and at very early times for strong vortex rings, (2) axisymmetric radially propagating waves in the early stage of the interaction of strong vortex rings with the surface, and (3) fully three-dimensional waves that depend on the local structure of the vortex core. Numerical simulations are used to address the observed axisymmetric phenomena.