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Abstract Cloud cavitation is known as a typical phenomenon of cavitating flow, in which aggregation of bubbles repeats collective growth and collapse behavior and induces shock waves. To understand the mechanism of the cloud cavitation phenomenon, it is crucial to clarify the relation between flow fields and the unsteady behavior of the cloud. In this paper, we experimentally investigated how velocity and vorticity fields appear in association with the unsteady behavior of the cloud cavitation (i.e., from its inception, growth, collapse, and finally rebound). To do this, we made the cloud by injecting a pulsed water jet into still water, and the fluorescent particle image velocimetry (PIV) method was employed to obtain two-dimensional velocity fields surrounding the cloud. The particle images were recorded by a high-speed camera with 300,000 fps, and then the velocity and vorticity fields were computed by PIV analysis. Thus, we illustrated that flows of twin vortices move along the boundary of the cloud associated with its growth and shrink behavior and collide with each other as well as disappear before the collapse. Furthermore, we showed that high-vorticity regions appear together with the twin vortices induced by the collapse. These experimental observations suggested that the growth and shrink behavior of the cloud induce the motion of the twin vortices and the collapse of the cloud creates the twin vortices with high vorticity. Finally, we made a comparison between the experimental results and the numerical simulation performed in our previous work and demonstrated the consistency of the flow structures.
USHIOKU et al. (Mon,) studied this question.