Hydrodynamics, Energetics and Acoustics of Free Falling Drop Impact at a Motionless Fluid

Abstract

The results of coordinated studies of the flow structure and acoustic signals generated in a fluid at rest by a freely falling drop of clean or dye colored water are presented. Highresolution video recording of the flow revealed the structure of plane ejecta, spikes, droplets sprays cavity, splash, secondary cavity, streamer, secondary droplets and sequence of capillary waves. Recurrence of the fine structure as the appearance of fast fine spikes after decay of initial perturbations on the free surface was observed in the center of the cavity and at the head of a growing splash. Groups of capillary waves accompanied the formation and elimination of every structure componentsplash, separating drop and coming back. A measuring hydrophone and a sensitive microphone measured acoustic signals consisting of a high-frequency pulse of the primary contact and a delayed lower-frequency packet(s). The relative poverty of the acoustic signal indicates that from a large number of observed gas bubbles only one, the formation of which included the break of a thin air bridge, was actually sounding. The shock of a fast-moving remnant of the bridge generates volumetric oscillations of the bubble. The sound stops as soon as the bubble adopts a smooth elliptical or spherical shape. The formation of fine flow components is governed by the release of available potential energy in the process of the elimination of the free surface during the merging of liquids. Perturbations that arose are saved in a thin "double layer" around the former boundary surface inside fluid which is deformed and transported by the main flow.