The bubble formation process upon an underwater collision of an object with a flat surface

Bubbles may form when the water is subjected to low pressure. Pressure drop can occur due to various causes, including a fast relative motion, acoustic interaction, and significant tension, among others. In previous studies, formation of a cavitation bubble upon the underwater collision of a rigid sphere and a flat surface (e.g., Seddon, et al. 2012, de Graaf, et al., 2015) has been discussed. The bubble size is reported to be related to the cavitation number, which characterizes the rate of pressure reduction in dimensionless form. In this study, we employed a similar set-up, while the impactor has a flattened disc-like shape. We have recently presented the bubble formation process briefly (Kiyama, et al., APS-DFD 2024). We here discuss the detailed visualization with simultaneous measurement for a more comprehensive understanding of the process. We also discuss the relationship between the bubble onset and formation, and the dimensionless number, including cavitation number, with the support of systematic experiments.