Vibration characteristic of bubbles inside a spherical cluster

Abstract

The motion of a cavitation bubble can be affected by many factors, such as acoustic frequency, acoustic pressure, bubble sizes and liquid properties. Furthermore, the size distribution of bubbles in the cavitation liquid is influenced by the acoustic condition and liquid properties. It is important to control the size distribution of cavitation bubble because the oscillation of cavitation bubble is related to the cavitation intensity. According to the dynamic model of bubbles in a spherical bubble cluster, the oscillating behavior of bubbles has been studied numerically to evaluate the effect of the driving frequency, pressure amplitude and the liquid properties on the vibration characteristic of cavitation bubbles. The ratio of maximum radius to minimum radius for a oscillating bubble is investigated and the numerical results suggest that, the unstable acoustic response parameter region exists when the size of microbubbles is less than 10 μm and bubbles are driven by acoustic wave whose frequency is lower than 50 kHz. Bubbles of 3 μm radii are more sensitive to the change of the acoustic parameters and vibrate more violently, and it is easy to grow up to a larger one because of the rectified mass diffusion. The nonlinear resonance of forced oscillating bubble can be observed on the response curves of bubbles, and it is found that the resonant frequency of bubbles decreases with the increment of the driving pressure amplitude. Therefore, a smaller resonant size of bubbles could be obtained in high-intensity ultrasound field. In a multibubble system, the nonlinear resonant response is mainly in the region that the acoustic frequency is smaller than the natural frequency of bubble and a microbubble smaller than the resonant one is easy to resonate with the acoustic condition although cavitation pressure threshold decreases with the size increment of cavitation bubbles. Based on the analysis of the effects of gas concentration on the bubble size distribution in the multibubble system, it is found that there are threshold and stable radii of bubbles in the high intensity acoustic field, and the equilibrium radii of cavitation bubbles should vary with acoustic condition, liquid properties and bubble density. Therefore, the cavitation field is a transient system. However, the sizes of cavition bubbles are mainly in the range between threshold radius and stable one, and these bubbles are more active in the bubble liquid.