Numerical Study On The Collapse Of Bubbles Near A Wall

Cavitation can be observed in all kinds of hydraulic machinery, which contains a large amount of gas (vapor) bubbles. The sizes of gas nuclei or cavitation bubble are in μm-mm scale and clouds of cavitation is at the millimeter level, however cavitating flow is in meter scale. From the micro perspective, the cavitation is an collection of cavitation bubbles. In order to understand, control and suppress cavitation, we must understand cavitation from micro respect. Based on OpenFOAM platform, Direct Numerical Simulations and the volume of fluid method are used to study the collapse of 1 and 27 bubbles near the wall.In the study of the single bubble, vapor bubble and gas bubble are chosen as the bubble models respectively. The results of models both show that bubble collapses in "heart" type and forms a high-speed micro jet. When the bubble reaches the smallest size, the pressure on the wall reaches the largest, which can erode the wall. With the increasing of the normalized standoff y, the interaction between the bubble and the wall becomes weaker. Meanwhile, the prolongation factor k decreases gradually approaching to 1. The pressure of the wall is independent of the bubble size, but depends on the normalized standoff y. Under certain normalized standoff y, the evolutions of bubble radius are almost same for different bubbles. The pressure acting on the wall has a very strong dependency on the driving pressure.In the study of 27 vapor bubbles, we find that both the interaction of bubbles and the effect of wall have significant influence on the collapse process of bubbles. When the last bubble reaches the smallest size, the pressure on the wall will reach the peak. The impulsive pressure is affected by the void fraction a, and especially the standoff distance γ-The collapse of 27 bubbles will produce larger pressure than 1 bubble. The propagation velocity of pressure wave is approximately equal to the speed of sound in water. The impulsive pressure produced by 27 bubbles is a linear function of the square root of the driving pressure.