Visualization of the Flow Pattern of the Impact of a Freely Falling Drop during the Generation of Sound Packets

The merging of a falling drop with a water surface gives rise to a series of hydrodynamic phenomena that differ in time and space scales. Among them, fast unsteady flows are distinguished, in the process of which deformed underwater caverns and gas cavities separated from them are formed. When the surface of the separating gas cavity closes, the formed bubble undergoes volumetric oscillations, which, in turn, generate short acoustic packets propagating under water and in the air. Experimental data are presented in which the sequence of the formation of caverns is traced, and the processes of the formation and detachment of bubbles and accompanying acoustic signals are identified and detailed. In the composition of the sound packets recorded in the water and air environments, there are differences associated with the characteristics of propagation (transient damping at the water-air boundary) and the influence of the transmission functions of the hydrophone and microphone. In the case of detachment, reattachment, and reseparation of an air bubble from successive caverns, the frequency of the emitted signal increases. Despite the highly irregular shape of the emitting bubbles, the emission frequency remains constant, which indicates the volume of the bubble as the governing parameter of acoustic emission.