Investigation of flow patterns and drag reduction effects of a ventilated supercavity regulated by an extra annular hydrofoil

In this paper, experimental and numerical investigations are presented on gas entrainment and drag reduction effect of ventilated cavity flow patterns around a disc-shaped cavitator equipped with an annular hydrofoil. The compressed air is injected from the cavitator and then modulated by the hydrofoil to form a hydrofoil-guided cavity (HC). Compared to regular ventilated super cavity (SC) that has lower cavity pressure than the freestream static pressure, it is found that the HC has significantly increased cavity pressure. The cavity pressure of HC becomes even higher than the freestream static pressure at some internal locations, leading to much different hydrodynamic characteristics of HC. To inspect such special cavity, this study comparatively investigates the flow patterns and drag reduction effect of HC and SC under same Froude number (Fr) and ventilate rate (Q) by both experimental measurements and numerical simulation. It is observed that HC has more intense internal flow, indicating higher velocity of the recirculating gas inside the cavity body. Meanwhile the overall flow resistance of HC is significantly reduced compared to those of SC. Moreover, by examining the internal pressure distributions of HC under different Fr and Q conditions, it can be found that a high-pressure region is presented near the location of the foil trailing edge. Such region is directly induced by the diverging water flow after it comes out from the gap between the inner surface of the hydrofoil and the gas-liquid interface of HC. Consequently, the discrepancy in pressure distribution inside the cavity causes the HC to possess more significant drag reduction compared to the SC.