  When a drop falls from a moderate height into a shallow fluid pool whose
depth is of the order of the drop-radius two pairs of vortex rings are
generated. The inner pair forms at the edge of the crater created by the
impacting drop while the outer pair is laid off from the spreading wave. One
ring of each of these pairs is short-lived while the other persists. Each of
the rings has a measureable non-zero circulation (whose sign, however, is
opposite to that of the well-known deep-water drop vortex rings) which persists
long after the wave has receded. Furthermore under certain conditions they
develop instabilities typical to ring vortices. Although the rings are well
reproducible, aspects of their later development are very sensitive to changes
in some of the experimental parameters. This paper reports on experiments
distinguishing reproducible aspects of these drop- generated rings and
documenting their dependence on material and geometrical parameters.
