Droplet Motion in Purified Systems

Experimental results are presented of the behavior of droplets, falling in water at Reynolds numbers ranging from 138–971. The location of flow separation and the wake configuration were studied with a shadowgraph optical system. The measured drag coefficients were found to be higher than predicted by Chao, who assumed complete internal circulation and negligible flow separation. The agreement was improved when a correction for the pressure drag was introduced. The latter was found to be essentially due to flow separation but not to the pressure differences across the boundary layer as asserted by Moore. In the nonoscillating range the viscous drag remains predominant, and there exists a systematic dependence of the drag coefficient on a property parameter associated with the propensity of the droplet to develop internal circulation. That parameter is also a significant group in correlating droplet deformation in pure systems. The onset of sustained droplet oscillation was observed to coincide with the onset of the periodic shedding of vortices. The transition to oscillation occurs at a critical Weber number of approximately 4, at which point there is a sharp increase of the drag coefficient.