Vortex rings generated by drops just coalescing with a pool

Abstract

A detailed experimental study has been made of the formation and motion of vortex rings generated when drops of liquid are allowed to come into contact at zero velocity with a quiescent flat surface of the same liquid. In this case the vortex motion is driven principally by the drop surface energy. It is shown that in the case considered, the phenomenon depends on two dimensionless parameters alone, the reciprocal of a Bond number B^-1 and a global Reynolds number Re where the velocity scale is based on the surface energy. Using a video camera, measurements were made, over the whole trajectory, of the ring velocity and diameter for a number of drop sizes and liquids. Vortex rings generated by small drops reach higher peak velocities but decelerate more rapidly than ones generated by large drops. The latter, however, may become turbulent before relaminarizing during deceleration. Although the most interesting finding of the investigation was the discovery of oscillations in the ring translational velocity and diameter during deceleration, the identification of the appropriate scaling laws and a suggestion regarding the early stages of vorticity generation should also be of interest.