Coastal erosion caused by cavitation bubble implosion: Micro-view on the bubble dynamics

Recently, coastal erosion in the form of inertial cavitation has been generally accepted. Air pockets in an incoming wave are forced into the surface of the coastal cliff, and then the force of the wave compresses the air pockets until the bubble implodes, giving off various forms of energy that erode the rock. This paper studies the behaviour of a cavitation bubble in a compressible liquid near a rigid boundary, when it is subjected to a harmonic wave. Because of the extremely short life time of the bubble, we assume the wave pressure acting on the bubble surface is constant. An approximate perturbation theory by using the method of matched asymptotic expansions is used to incorporate the influence of the compressibility of the liquid. The rigid boundary creates an asymmetry in the flow field that causes bubble to collapse non-spherically. A numerical model based on the mixed Eulerian-Lagrangian (MEL) method and the boundary integral method (BIM) for bubble dynamics in a weakly compressible liquid is developed. The shock wave pressure caused by the high speed jet hitting on the rigid boundary is found to be of sufficient magnitude to initiate erosion of the loose boundary.