We conducted a two-dimensional numerical analysis on the unsteady behavior of cloud cavitation and associated induced shock waves using the smoothed particle hydrodynamics method in conjunction with a pulsed submerged water jet injection into still water through a nozzle. We modeled a bubbly water jet as a mixture of liquids and gases to develop Navier–Stokes (NS) equations for multiphase flows in the Lagrangian description, which can naturally describe the inception of cavities without phase change models or initial bubble conditions. Subsequently, we numerically investigated the inception, growth, collapse, and rebound of clouds generated in multiphase flows. Our investigation revealed the appearance of twin vortices in association with the water jet injection, which moves along the boundary of the cloud synchronized with its growth and collapse behavior. Furthermore, we investigated the pressure fields to showcase the emission of the shock wave associated with the cloud collapse, noting the release of weak pressure waves prior to the main shock wave generation. Finally, a comparative study between the numerical analysis and the experimental results outlined in Part 1 of this study demonstrated consistency in the numerically observed unsteady behavior of the cloud and main shock wave phenomenon.

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.