Experimental study of the impact of splash closure on the cavity evolution behind a sphere entering water

Abstract

In this paper, we conduct an experimental study on the cavity evolution behind a hydrophobic sphere entering the water. It is found that there is a correlation between splash and cavity evolution. Two dominant regimes for cavity pinch-off, including hydrostatic pressure and pressure pulsation inside cavity, are confirmed. When the splash is removed by a guard, the cavity pinch-off is dominated by hydrostatic pressure. The theoretical model is well consistent with the experimental results, including the cavity profile and the dimensionless pinch-off depth. However, when the splash domes, the theoretical solution fails with the increase in the effect of pressure pulsation inside the closed cavity. With the increase in the Froude number Fr, the splash closure most significantly impacts the necking position. The impact of the splash closure is dependent of Fr, and there is a critical value of Frcri separating the deep pinch-off into two regimes. The critical value of Frcri = 134 is theoretically proposed, which is a reasonable predication for the present experimental observations. In addition, the expansion rate of cavity volume is measured to confirm the transition of distinct regimes. Based on our theoretical analysis, the critical expansion rate is determined as 4.9, which agrees well with present experimental result 4.6. Distinct pinch-off regimes significantly impact the subsequent cavity evolution and the forces on sphere. A deep seal controlled by hydrostatic pressure is constantly accompanied by severe cavity resonance and a dramatic decrease in the cavity length, whereas pinch-off controlled by pressure pulsation leads to weak ripples.