It is well known that a collective collapse of cloud cavitation induces a shock wave emission, whose destructive power has been utilized in both medical and environmental fields. Therefore, it is important to elucidate the mechanism of how the shock wave is generated in association with the cloud collapse. In our previous study, we reported that the main shock wave is emitted by the collapse of the high-void fraction region in the cloud (referred to as the “nucleus of the cloud”) from macroscopic perspective; however, the detailed mechanism on how the main shock wave is formed has not been sufficiently understood, and further experimental investigations are required to clarify it. In this study, we made an experimental observation of the cloud-induced shock wave by using ultra-high-speed photography. In our experiment, the cloud was created by submerged pulsed water-jet injection, and its unsteady motion and the shock wave phenomena were recorded by a high-speed camera operating at 1,700,000 fps. Finally, our observations revealed multiple shock wave emissions during the cloud collapse as well as weak pressure waves prior to it. These findings suggest that the main shock wave from the cloud collapse originates from the accumulation of these weak pressure waves.
USHIOKU et al. (Wed,) studied this question.