Cavitation bubble-induced hydrodynamic loading and interfacial deformation in an air pocket

The presence of trapped air on a solid surface can alter the direction of the liquid jets induced by cavitation bubbles, which prevents or reduces erosion. In this study, we numerically investigate mutual interaction between air trapped in a pocket on a wall and a nearby bubble in water, as well as the resultant hydrodynamic loading. Both the depth and radius of the cylindrical pocket are similar to the maximum bubble radius. The pressure imposed on the inner wall of the air pocket is assessed for various values of the air pocket size and the stand-off parameter. The deformation of the air pocket and the bubble is analysed in each of three sequential stages. During the bubble expansion stage, a shock wave reflects at the water–air interface of the pocket, and the wall inside the compressed pocket is protected from the shock wave. As the bubble jet induced during bubble contraction tends to move away from the air pocket, other liquid jets formed at the water–air interface, namely central and lateral pocket jets, can directly collide with the inner wall of the pocket after the bubble collapses. These collisions exert significant pressure on the wall under certain conditions. The formation of the central pocket jet originates from the strong fluctuation of the water–air interface by the expanding and contracting bubble. The development of the lateral pocket is related to changes in the potential energy of the air under its second contraction.