Synergistic effects of the free surface and flat wall on the bubble dynamics within confined spaces

The present paper systematically investigates the evolution of the cavitation bubbles under the synergistic effects of the free surface and flat wall within confined spaces. The influences of free surface and flat wall on the flow field and the Kelvin impulse are investigated, and the jet enhancement phenomenon induced by the combination boundary is analyzed and is also compared with the case of the single boundary. Quantitative investigations are conducted to elucidate the impacts of the free surface-flat wall distance and the bubble-free surface distance on the collapse jet formation, bubble interface motions, and liquid velocity. The main conclusions are summarized as follows: (1) The free surface exhibits a more dominant contribution to the high-velocity region, while the flat wall contributes more markedly to the low-velocity region. (2) The opening angle of the bubble jet near the flat wall is smaller than that near the free surface, but the jet velocity is similar. (3) The jet morphology transitions from an arcuate jet to an angular jet as the bubble-free surface distance increases.