DETAILED OBSERVATIONS ON A STARTING MECHANISM FOR SHEDDING OF CAVITATION CLOUD

Cavitation is observed in a convergent-divergent nozzle using a high speed video camera. For this kind of flow, the cavitation has unsteady character with periodic shedding of cavitation clouds. The periodicity is accompanied with a re-entrant motion, the growth of vortex cavity on the separated shear layer as well as the shedding of a large-scale cavitation cloud. In this process, the formation of attached cavity is repeated with shedding of cavitation clouds. The switching mechanism from the re-entrant motion to the formation of shedding cavitation clouds has not been fully understood. In the present study, it is shown in details by experimental results based on high speed video observations that one of the mechanisms is the leading-edge bubble collapse in the separated region to interrupt continuous supply of vorticity from upstream boundary layer and the formation of micro-vortex cavities on the separated shear layer. In addition, various appearances of cavitation bubbles are also shown from a viewpoint of unsteady cavity behavior