NUMERICAL INVESTIGATION OF BUBBLE CLOUD FORMATION BY HIGH-INTENSITY FOCUSED ULTRASOUND USING THE GHOST FLUID METHOD COUPLED WITH BUBBLE DYNAMICS

Abstract

In this study, numerical simulations were conducted to investigate the growth of bubble nuclei and the corresponding bubble cloud formation in a pressure field given by high-intensity focused ultrasound (HIFU) backscattered from the bubble interface. A multiscale numerical method was developed to simulate the corresponding experiment, in which the overall pressure fields obtained by HIFU were solved using the ghost fluid method coupled with the bubble dynamics in the microscopic field. The validity of the present method was confirmed by the growth of a single bubble nucleus placed on the HIFU propagation axis. The formation of a cone-shaped bubble cloud that consisted of multiple layers of bubbles observed in the experiment was simulated successfully under the random distribution of initial bubble nuclei. The influence of the initial distribution of bubble nuclei on the cone-shaped bubble cloud formation was also investigated. The results showed that the shape of each bubble layer affects the final form of the bubble cloud since the negative pressure due to backscattering from the HIFU is dependent on the shape of each bubble layer. A final bubble cloud shape was predicted by averaging the void fractions in all of the bubble nuclei distribution cases. The results showed that the averaged final shape of the bubble clouds matched the results observed in the experiment: the distances between the first cavitation inception point and the laser-induced bubble interface and between the bubble layers in the cloud were in quantitative agreement with the experiments.