On the airflow in a cavity during water entry

The airflow physics in the cavity during the entry of a symmetric wedge into water are investigated experimentally using flow visualization techniques and particle image velocimetry (PIV) measurements. The spatial and temporal evolutions of the airflow velocity in the cavity are presented. Smoke visualization indicates some small-scale flow phenomena like secondary vortexes, free shear layers, and Kelvin-Helmholtz (K-H) instabilities within the water entry cavity. The information of airflow velocity in the cavity is obtained via PIV measurements. The results show that ambient gas is entrained into the transient air bubble at the wake of the wedge and the air flow rate is nearly equal to the cavity volume change rate. Thus, it is feasible to use the airflow rate to predict the cavity volume change rate. When the wedge descends, the air-water interface has a significant effect on the wake vortex behaviors. The vortex areas shrink gradually and wake vortex circulation decreases over time. Thus, the air-water interface not only suppresses vortex growth but also leads to vortex dissipation.