Nonlinear behavior of the collapse of a spherical bubble cloud

Cloud cavitation is known well as one of the most destructive cavitation. High pressure generated by the violent collapse of a bubble cloud causes severe erosion, noise and vibration in hydraulic machines. However, this high concentration of the energy has been utilized recently for biomedical, environmental and other industrial applications. In the medical field, the behavior of the bubble cloud has much influence on HIFU (High Intensity Focused Ultrasound) applications, where acoustic cavitation is generated due to the high pressure amplitude in the focal region. 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. We consider the compressibility of the liquid, the evaporation and condensation of the liquid at the bubble wall, heat transfer through the bubble wall 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.