Experimental and numerical investigation on bubble dynamics near a free surface and a circular opening of plate

Dynamics behavior for a submerged oscillating bubble is closely related to the physical properties of boundaries adjacent to the bubble. For an incomplete boundary, such as a ship structure broken by the shock wave from underwater explosion attack, the bubble load formed after the shock wave may cause secondary damage to it, and the jet characteristics generated during the bubble collapse are worth exploring. Based on the incompressible potential flow method, a boundary integral method with axisymmetric code is employed to investigate the jet features of the bubble under the combined action of a free surface above and a fixed plate with a centric circular opening below. For the numerical divergence caused by the expanding bubble infinitely approaching the rim of the circular opening, the bubble walls above and below the opening are, respectively, fused with the rigid wall, and the initial infinite flow field surrounding the bubble is then divided into two independent basins that are both required to be solved. Several experiments of a spark-generated bubble oscillating near a free surface and a rigid plate with a circular opening are conducted and the numerical results are validated. Following that, two typical cases with a dominant jet, respectively, directing upward and downward are analysed in detail, and some new phenomena are discovered. Additionally, another particular case that the upward and downward jet collides in the vicinity of the opening centroid is also presented. Finally, the initial bubble-free surface distance effects upon the bubble movement and the jet developments are analysed and discussed.