Study on the mechanism and load characteristics of secondary cavitation near free surface in underwater explosion

Although the cavitation phenomenon in underwater explosion has been researched for more than 100 years, the phase transition models based on mass and heat exchange between liquid and its vapor phases have only been established in the past decade. In this study, the secondary cavitation phenomenon was first captured by phase transition based on a four-equation system. The bulk cavitation near the free surface induced by underwater explosion was numerically investigated, and three typical bulk cavitation cases were investigated to explore their motion mechanisms and load characteristics on hydrodynamics and phase transition generation. It was found that secondary bulk cavitation will occur only under the condition that both the initial shock wave intensity and the distance between the water surface and the explosion bubble are satisfied in a specific relationship. Producing bulk cavitation was difficult at a relatively deep detonation depth because of the weak rarefaction wave reflected from the water surface by smaller charges. The statistical data under the condition of small charge indicated that the duration of cavitation increases with the increase in charge weight but the growth trend slows down gradually. However, the maximum volume of cavitation increased linearly with an increase in the charge weight. The present results can expand the currently limited database of underwater explosion multiphase fluids and provide insight into the interactions between the shock wave, bulk cavitation, explosion bubble, and water surface.