Experimental and numerical investigation of the dynamics of a coalesced oscillating bubble near a free surface

Understanding the dynamics of oscillating bubbles beneath a free surface is crucial to many practical applications including airgun-bubble clusters, underwater explosions, etc. In this paper, an experimental and numerical study of the dynamic behaviors of a coalesced bubble near a free surface is conducted, which shows quite different physical features from single bubble dynamics. Firstly, two similar sized underwater discharge bubbles are generated simultaneously beneath a free surface and their complex interactions are experimentally studied with high-speed photography imaging. A strong interaction between two bubbles and the subsequent coalescence are observed when the initial distance between two bubbles is smaller than the maximum equivalent bubble radius. Secondly, both axisymmetric and three-dimensional (3D) boundary integral models are used to simulate the pre-coalescence and post-coalescence of two bubbles. The results obtained by the two models agree well in axisymmetric conditions. The essential physical phenomena in representative experiments are well reproduced by the present 3D model. The pressure field is calculated by the auxiliary function method, which helps to reveal the underlying mechanisms of bubble collapse patterns and jetting behaviors. A parametric study reveals the dependence of the coalesced bubble dynamics and free surface motion on the governing dimensionless quantities.