2022 年 36 巻 1 号 p. 95-106
This study deals with numerical simulations of the growth of bubble nuclei and the corresponding bubble cloud formation in a pressure field given by high intensity focused ultrasound (HIFU) backscattered from a laser-induced bubble interface. The imposed pressure on the nuclei is calculated with the Ghost Fluid Method (GFM). The growth of nuclei in a microscopic field that cannot be resolved under Eulerian grids in the GFM is evaluated by using the Keller equation for bubble dynamics. Once the nuclei grow so lager that they can be captured with the level-set function, the GFM with the level-set method takes over the computation in the macroscopic field. The validity of present method is confirmed by the growth of a single bubble nucleus placed on the HIFU propagation axis near the laser-induced bubble. The first growth and the following collapse of the bubble captured with the level-set function in the macroscopic field are in fairly good agreement with those predicted by the Keller equation with keeping acceptable mass conservation, although the accuracy depends on the grid resolution. It is also shown that there exists a bubble configuration where multiple bubbles grow more stably than those when each bubble grows solely: the stable multiple bubbles become a new reflector of HIFU leading to a high negative pressure field where next cavitation inception can occur. Finally, we demonstrate the cone-like bubble cloud formation observed in experiments (Ogasawara et al. (2018); Horiba et al. (2020)) using the present method. The simulated cloud formation process supports the experiments.
