Packets of Capillary and Acoustic Waves of Drop Impact

Abstract

Flows, capillary waves, and acoustic signals generated by a drop of water falling into a pool of degassed liquid were recorded by a high-speed video camera, hydro-phone, and microphone. A large-scale analysis of the system of equations was performed. The fast conversion of available surface potential energy is traced. The converted energy is stored in a thin layer in the vicinity of the merged free surfaces and creates large perturbations of temperature, pressure and flow velocity. Capillary waves start to radiate simultaneously with the formation of a cavity and the rise of the crown. New groups of capillary waves arise with all changes in the flow structure --- the formation and immersion of a splash, come back of secondary drops, the formation of cavities, the immersion of a streamer and droplets. Simultaneously with the waves, ligaments --- thin near-surface flows are formed that affect the transport and rupture of gas cavities. Thin flows quickly decay and form again when a new group of capillary waves is generated. A comparison of flow patterns and acoustic signals indicates that the generation of resonant sound packets is synchronized with the pinch-off gas fragments from the cavity or their breaking. The duration of the sound depends on the initial heterogeneity of the geometry of the sounding cavity, gradually transforming into a smooth spheroidal form