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When a drop falls from a moderate height into a shallow fluid pool whosedepth is of the order of the drop-radius two pairs of vortex rings aregenerated. The inner pair forms at the edge of the crater created by theimpacting drop while the outer pair is laid off from the spreading wave. Onering of each of these pairs is short-lived while the other persists. Each ofthe rings has a measureable non-zero circulation (whose sign, however, isopposite to that of the well-known deep-water drop vortex rings) which persistslong after the wave has receded. Furthermore under certain conditions theydevelop instabilities typical to ring vortices. Although the rings are wellreproducible, aspects of their later development are very sensitive to changesin some of the experimental parameters. This paper reports on experimentsdistinguishing reproducible aspects of these drop- generated rings anddocumenting their dependence on material and geometrical parameters.

Vortex ring generation during drop impact into a shallow pool