Transition and gas leakage mechanisms of ventilated cavities around a conical axisymmetric body

In this paper, we present an experimental investigation on gas entrainment and gas leakage mechanisms of different ventilated cavity flow patterns around a conical axisymmetric body, by high-speed video and theoretical analysis. The ventilated cavity is generated around a conical axisymmetric body and studied in the closed-loop water tunnel at Lab of Fluid Machinery and Engineering, Beijing Institute of Technology. The experimental studies revealed three distinct cavity patterns, namely, the foamy cavity (FC), the intermittent and transitional cavity (ITC), and the continuous and transparent cavity (CTC) under different Froude and CQ values. All patterns contain the transparent region at the cavity rear (except FC) and the recirculating vortex that generates foam shedding at the cavity rear. The qualitative gas entrainment and leakage process are then explained based on the schematics of internal gas flow and surrounding water flow. As the flow pattern varies from FC to CTC with increasing ventilation rate, it is observed that the size of shedding foam grows larger while the shedding frequency gradually reduces, leading to balanced gas injection and leakage. Moreover, the mechanism controlling the hysteresis of the ITC-to-CTC transition is quantitatively explained using a gas leakage model of ITC cavities, which assumes the foam is periodically shed in the form of a vortex ring. After extensive validation by both current measurements and previous research, the model can accurately predict the ventilation rate required for the ITC-to-CTC transition. Finally, the mechanism causing closure discrepancy between CTC cavities in this paper and free-standing supercavities (without cavitator body) in past studies are investigated. Due to the impact of stagnation points on the cavitator body, the adverse pressure gradient (APG) in the streamwise direction of CTC cavities is estimated to be 6.8 times that of free-standing supercavities under the same flow conditions, leading to water flow separation at the cavity rear. As a result, after the ventilation hysteresis, the recirculating vortex closure is still observed in CTC cavities under the impact of large APG, different from free-standing supercavities that form vortex tube closure.