The penetration of drop-formed vortex rings into pools of liquid

Abstract

Drops falling certain distances before impact with a pool of the same liquid produce vortex rings that penetrate more deeply into the pool. As a drop falls it oscillates in shape between prolate and oblate through spherical. Earlier researchers have stated that a spherical shape changing to prolate correlates with maximum vortex ring penetration and a spherical shape changing to oblate with minimum penetration. The work reported here in which the drop shape on impact was photographed does not support these views. Analysis of high-speed motion pictures of drop impact reveals that the shape of the crater caused by drop impact exerts a crucial influence on how deeply the drop-formed vortex ring penetrates the pool. A narrow crater formed by the impact of a prolate-shaped drop accompanies maximum vortex ring penetration. The narrow crater appears to aid vortex ring formation with minimum subsequent distortion and helps the vortex ring to detach readily from the crater.