Unsteady behavior of cloud cavitation and its induced shock waves: Part 1: Experimental observations and analysis

Cloud cavitation is associated with a collective unsteady motion, characterized by repeated growth and collapse of the cloud, with the collapse process emitting a shock wave. The mechanism whereby the cloud exhibits the unsteady behavior (i.e., from its inception, growth, collapse, and finally rebound) and generates the shock wave has not been well revealed. In particular, we consider that how the impact pressure which is the source of the outward-propagating shock wave is formed during the cloud collapse remains an open question. To elucidate the unsteady behavior of the cloud and associated shock wave phenomena, detailed experimental and numerical analyses are required, each of which is explored in Parts 1 and 2 of this paper. The main objective of Part 1 is to investigate the structure of the unsteady behavior of clouds induced by pulsed submerged water-jet injection and shock wave phenomena through experimental observations. To this end, we developed an observation system with two high-speed cameras operating at 450 kfps arranged orthogonally, enabling the simultaneous observation of unsteady phenomena as well as shock wave propagation using the shadowgraph and Schlieren methods. Thus, we illustrated how the cloud shows elementary processes and how shock waves are generated and propagated. In particular, we introduced the concept of “nucleus of the cloud” to understand the structure of its collective unsteady motion and the shock waves associated with the cloud collapse.