Numerical study on nonlinear behavior of a collapsing bubble cloud in an ultrasound field

Abstract

Cloud cavitation is known well as one of the most destructive cavitations. High pressure generated by the violent collapse of a bubble cloud causes severe erosion, noise, and vibration in hydraulic machines. In the medical field, many researchers have investigated the behavior of the collapsing bubble cloud to effectively use the highly concentrated energy for HIFU (high-intensity focused ultrasound) applications. In this study, it is assumed that the acoustic cavitation forms a spherical bubble cloud which consists of many microbubbles, and ultrasound focusing in the spherically symmetric cloud is numerically investigated. The compressibility of the liquid, the evaporation and condensation of the liquid at the bubble wall, and heat transfer through the bubble wall are considered in the simulation. The pressure wave focuses to the center of the cloud and the pressure inside bubbles extremely increases when the frequency is near the first mode natural frequency of the cloud. Especially in the cases of relatively high-pressure amplitudes, a shock wave is formed in the cloud and it generates high pressure fluctuation near the center of the cloud even when the frequency is much lower than the first mode frequency.