Numerical Study Of Gas Bubble Collapse

The cavitation bubble flow is a kind of complex two-stage flows commonly found in liquid dynamics. Owing to the complex characteristics associated with cavitating flows, such as the high unsteadiness and strong pulse, there is still a lack of the comprehensive understanding of the mechanism of cavitation bubble flows, especially in the collapse stage of cavitation bubble. In this paper, taking into account the effects of surface tension, viscosity and compressibility, the collapse of three dimensional bubble is simulated by the direct numerical method (DNS) based on volume of fluid (VOF). All analyses of the single gas bubble, two gas bubbles and multi gas bubbles, from the perspective of micro-dynamics, focus on the evolution of bubble’s shape and nearby flow field.Driven by the pressure difference inside and outside the bubble, the single gas bubble begins to collapse and then rebound periodically. During the first-stage collapse, the numerical result of bubble’s volume evolution is in good agreement with the Rayleigh analytical solution. The single gas bubble maintains spherical deformation, the velocity vectors distribute along the direction of the bubble’s radius, and the pressure inside the bubble is uniform. When the gas bubble reaches the minimum volume, it starts rebounding and dramatic changes occur in the vicinity of the turning point. The single gas bubble can no longer maintain its spherical shape, the significant gas vortices inside the bubble are observed, and the pressure inside the bubble is non-uniform.Both in the stages of collapse and rebound, the two gas bubbles with the same radius evolve symmetrically and their centroids continue to get close to each other. The liquid jet forms in the stage of collapse and the concave shape of the bubbles toward each other occur in the stage of rebound. The results show that the smaller the distance between the two bubbles, the greater the jet intensity and the degree of deformation. For two gas bubbles with different radius, the smaller bubble changes greater and faster than the larger bubble. Meanwhile, as the radius ratio of the two bubbles increases, the degree of deformation of the smaller bubble and the pressure gradient between the two bubbles increase. The collapse process of multi gas bubbles is closely related to the bubble-bubble interaction. The farther from the center, the sooner bubbles begin to collapse. With increasing of the distance from the center, the degree of the bubble deformation increases. Due to the inhibition of surrounding gas bubbles, volume of the center bubble varies slowly, but then fast. Besides, when the number of surrounding bubbles increases, that is to say, when the bubble-bubble interaction increases, the collapse time of center bubble becomes longer and its generated maximum pressure increases obviously.