**4. Conclusions**

This study utilized a U-shape platform device to generate a single cavitation bubble and sequential images of the bubble collapse flow are recorded by a high speed camera for the detail analysis of the flow field characteristics and the cause of the counter jet during the process of bubble collapse induced by pressure wave. A series of bubble collapse flows induced by pressure waves of different strengths are investigated by positioning the cavitation bubble at different stand-off distances to the solid boundary. It is found that the Kelvin-Helmholtz vortices are formed when the liquid jet induced by the pressure wave penetrates the bubble surface. If the bubble center to the solid boundary is within one to three times the bubble's radius, a stagnation ring will form on the boundary when impacted by the penetrated jet. The liquid inside the stagnation ring is squeezed toward the center of the ring to form a counter jet after the bubble collapses. At the critical position, where the bubble center from the solid boundary is about three times the bubble's radius, the bubble collapse flows will vary. Depending on the strengths of the pressure waves applied, either just the Kelvin-Helmholtz vortices form around the penetrated jet or the penetrated jet impacts the boundary directly to generate the stagnation ring and the counter jet flow. This phenomenon used the particle image velocimetry method can be clearly revealed the flow field variation of the counter jet. If the bubble surface is in contact with the solid boundary, the liquid jet can only splash radially without producing the stagnation ring and the counter jet.

For all the experiments performed in this study, the strength of the pressure wave adopted to induce the bubble collapse flow was kept as low as possible so that the bubble collapsed in a longer period of time. The characteristics of the bubble collapse flows at different standoff distances can thus be clearly manifested. However, different strengths of the pressure waves are needed to induce the bubble collapse flow at different γ locations. A lower strength of the pressure wave is needed for an increasing γ value and vice versa.
