Numerical Simulations of Growth and Collapse of a Bubble Between Two Parallel Walls

In liquid mercury target systems for high-intense pulsed-spallation neutron sources, the material damage of the vessel wall caused by the cavitation bubble collapse is an important issue to be overcome. One remedy is to install a narrow channel for mercury flows. In such a situation, it is important to clarify the dynamics of the growth and collapse of bubbles in a narrow channel and to predict the damage of the channel walls by the bubble collapse. In the present study, therefore, the growth and collapse of bubbles between two parallel walls are investigated by using two kinds of numerical methods: one is the boundary element method (BEM) and the other is the ghost fluid method (GFM). Main concern in the present study is the bubble dynamics when the equilibrium bubble diameter at its maximum volume, d_eq=2R_eq, is larger than the channel height between two walls, h; in this case, the bubble shape is not spherical when it grows between two walls. The present results show that the bubble shape at the maximum volume is much dependent on η=h/R_eq). In the case of η=1.57, the bubble becomes an ellipsoidal shape at its maximum volume, and when η=0.94, the bubble becomes a flattened hourglass shape. When the ellipsoidal bubble collapses, the bubble splits into two parts leading to the liquid jet impact on nearer walls. On the other hand, the bubble with the flattened hourglass shape collapses neutrally at the middle of the walls without translational motion of its main body. The impulsive pressure on the wall observed in the neutral collapse is lower than that in the case of the splitting bubble.