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High framing rate PIV studies of an impinging water drop

The flow field formed by the impact of a water drop in the vortex ring region was studied using high framing rate particle image velocimetry (HFRPIV). A survey of the current hardware available for HFRPIV is given followed by the results for a HFRPIV study at 1 kHz (double frames). The HFRPIV results are matched against observations from a dyed drop experiments with similar impact conditions. The results show that the vortex ring is formed by the downward flow of the fluid during the collapse of the cavity. The vortex ring that propagates downwards has a rotation opposite in sign to the vortex ring that propagates with the capillary waves that radiates outwards along the free surface from the impact site. The vortex ring that propagates downwards originates in the bulk fluid and entrains the dye in the drop fluid during the cavity collapse stage. The results show that the vortex ring is not shed from the cavity interface with the drop fluid as is often interpreted from observation with dyed drop results.