THE 3-DIMENSIONAL VORTEX STRUCTURE OF AN IMPACTING WATER DROP

Observations are reported for the three-dimensional vortex structure c reated by a dyed water drop impacting a pool of water. The structure a nd evolution of the vorticity is studied for a Weber number of 22-25 a nd a Froude number of 25-28. The drop and pool do not make first conta ct at the bottom of the drop but at latitudes away from the bottom pol e of the drop. This traps a thin, curved, pancake-shaped air bubble be neath the drop which rapidly contracts into a sphere. As the drop impa cts the pool its impulse produces vorticity which rolls up into a prim ary vortex ring. As the vortex ring travels down through the pool, vor tex filaments extend from the central axis of the vortex ring to form a ''stalk.'' This reaches from the primary ring to another ring of vor ticity which has formed in the now reversing free surface impact crate r. As the primary ring convects downward some vortex filaments undergo an azimuthal instability which grows until the filaments escape the t rapped orbits of the primary vortex ring and are ''shed.'' This result s in three to five loops or ''petals'' left behind the primary ring. A three-dimensional vortex skeleton of this structure is presented. The results confirm the hypothesis that the structure is topologically si milar to that of the separation bubble on a blunt flat plate. The stru cture's appearance is also strikingly similar to the appearance of an aboveground atomic blast.