Fast Superfine Components and Sound Packets in Phenomena Induced by the Impact of a Drop on a Target Fluid in Quiescent Conditions
Fluid Dynamics & Materials Processing
The structure of the flow and the acoustic signals generated by the impact of a freely falling drop of water with an underlying quiescent fluid have been investigated for droplets having diameter 0.5 cm and a contact velocity in the range 1.5 < U < 5 m/s. The experimental study has been supported by high-resolution videos of the flow (as seen from above and from the side). The evolution of ejecta, spikes, droplets spray, cavity, splash, secondary cavity, streamer, secondary droplets and
... of capillary waves is reported accordingly. In particular, perturbations of the smoothed free surface with transverse dimensions from 0.03 cm to 0.15 cm have been observed in the center of the cavity and at the head of a growing splash. The total duration of the fine structure's existence has been found to range from 30 to 50 ms. Related acoustic packets have been recorded by means of a measuring hydrophone and a sensitive microphone. Registered acoustic signals have revealed a high-frequency pulse associated with the primary contact and a delayed main lower-frequency packet (with a time shift ranging between 50 ms and 210 ms). The content of the acoustic signals indicates that from a large number of observed gas bubbles, only one was actually sounding. For such a case (the formation of the bubble resulting from the break of a thin air bridge), the shock generated by the fast-moving remnant of the bridge produces volumetric oscillations of the bubble. The sound stops as soon as the bubble takes a smooth elliptical or spherical shape. Several details are reported about the relationship between the generation of capillary and acoustic waves and the features of hydrodynamic flows relating to the cavity, or the formation of a splash and the flows caused by the interaction of secondary drops with the perturbed surface of the target fluid. The formation of fine flows components is associated with the release of available potential energy and the conservation of perturbations in a thin "double layer" located in proximity to the original contact surfaces.