The vortex ring velocity resulting from an impacting water drop

Experimental observations are reported on the evolution of a vortex ring for the first 70 ms after it is created by the impact of a dyed water drop upon a pool of clear water. The 2.6 mm diameter drops were released from two heights. The drops impacted the pool with Weber numbers of 23.2 and 16.6 and Froude numbers of 25.2 and 18.0. The Reynolds number of the resulting relaminarized vortex rings based on their diameter was in the range of 320 to 390. Precisely controlled multiple exposure photographs were used to measure the position and shape of the vortex ring versus time, and calculate velocity. It is proposed here that the appropriate time scale is the time it takes for the impact crater to reach its maximum depth. Excellent agreement was found when using this scaling to compare both the present data sets and that previously published for a larger drop.This work was supported by the Natural Sciences and Engineering Research Council of Canada Grant No. OGP 00 41747. Mr. B. Faulkner is thanked for help with the strobe-timing