The three-dimensional vortex structure of an impacting water drop

Abstract

Observations are reported for the three-dimensional vortex structure created by a dyed water drop impacting a pool of water. The structure and evolution of the vorticity is studied for a Weber number of 22–25 and a Froude number of 25–28. The drop and pool do not make first contact at the bottom of the drop but at latitudes away from the bottom pole of the drop. This traps a thin, curved, pancake-shaped air bubble beneath the drop which rapidly contracts into a sphere. As the drop impacts the pool its impulse produces vorticity which rolls up into a primary vortex ring. As the vortex ring travels down through the pool, vortex filaments extend from the central axis of the vortex ring to form a ‘‘stalk.’’ This reaches from the primary ring to another ring of vorticity which has formed in the now reversing free surface impact crater. As the primary ring convects downward some vortex filaments undergo an azimuthal instability which grows until the filaments escape the trapped orbits of the primary vortex ring and are ‘‘shed.’’ This results 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 similar to that of the separation bubble on a blunt flat plate. The structure’s appearance is also strikingly similar to the appearance of an aboveground atomic blast.