Multi-cycle dynamics of underwater explosion bubbles: An experimental investigation

An investigation of the multi-cycle dynamics of underwater explosion bubbles near hybrid boundaries is conducted using the experimental method. The experiments are carried out within a square steel tank. The free surface and a freely hanging steel plate comprise the hybrid boundary condition. Bubbles are initially positioned at equal distances from the two boundaries to facilitate a comparison of the relative influence exerted by each boundary. High-speed photography and pressure sensors are utilized to document the dynamics of bubbles and pressures in the flow field. An examination of the bubble behavior is conducted through the analysis of experimental imagery, elucidating the formation of a cone-shaped cavity near the free surface and the subsequent complex distortion behavior of the bubble as it interacts with the wall during the rebound phase. The investigation concentrates on the migratory properties of bubbles across various regions, revealing a spectrum of trajectories due to the varying boundary and buoyancy effects: vertical descent, "L"-shaped trajectory, "U"-shaped trajectory, and diagonal upward trajectory. Unless very close to the free surface, the direction of bubble migration changes at the end of each cycle due to local fragmentation of the bubble surface. Finally, the discussion culminated in an analysis of bubble energy conversion, revealing that the complex bubble behavior near the free surface weakens bubble collapse, resulting in minimal impact on the surrounding fluid dynamics from bubble oscillation. In the region distant from the free surface, when the bubble oscillation is weakly affected by buoyancy and boundaries, the energy emitted by bubble oscillation peaks due to the nearly spherical collapse.