Experimental and numerical investigations of the collapse of a laser-induced cavitation bubble near a solid wall

With the collapse of cavitation bubbles near the wall, micro-jets and shock waves will be formed, to generate a high-pressure load and to cause the cavitation damage on the surface of the hydraulic machinery. Due to the rapid development of the cavitation bubble collapse process (in the time scale of hundred nanoseconds), the time resolution of the conventional high-speed cameras should reach more than one million frames per second, which will limit the spatial resolution, and obscure the details of the cavitation bubble shape near the cavitation bubble collapse moment. In this paper, with the help of the laser cavitation bubble photogrammetry system with nanosecond-micron space-time resolution, the experiment is carried out for the cavitation bubble collapse morphology evolution near the wall. The morphological characteristics of the cavitation bubble collapse at specific times are analyzed. With the help of the OpenFOAM code, the collapse process of the cavitation bubble near the solid wall is calculated. It is shown that the cavitation bubble near the wall collapses in an axial symmetric heart shape and the micro-jet directed to the wall will pull the cavitation bubble towards the wall. The counter-jet generated in the rebound stage will drive the cavitation bubble to move away from the wall. The numerical simulation of the cavitation bubble shape in the collapse period is well consistent with the experimental results, but the ability to capture the shock wavefront needs to be improved. Under the conditions studied in this paper, the cavitation bubble collapse micro-jet velocity can reach up to a hundred meters per second both in the experiment and the numerical simulation.