Structures of supercavitating multiphase flows

Supercavitation around a hydrofoil is studied based on flow visualization and detailed velocity measurement. The main purpose of this study is to offer information for validating computational models, and to shed light on the multiphase transport processes. A high-speed video camera is used to visualize the flow structures under different cavitation numbers, and a particle image velocimetry (PIV) technique is used to measure the instantaneous velocity and vorticity fields. It is shown that the cavitation structure depends on the interaction of the water-vapor mixture and the vapor among the whole supercavitation stage. As the cavitation number is progressively lowered, three supercavitating flow regimes are observed: first, fluctuating cavity with periodic vortex shedding, then, vapor and water-vapor mixture coexist inside the cavity with a turbulent wake, and finally, a cavity largely filled with vapor and with a two-phase tail and distinct phase boundaries in the wake region. Even though the overall cavity boundary seems to be quite steady, the unsteadiness of the pressure fluctuation and mass transfer process between the vapor and the two-phase regions is substantial. Furthermore, in the cavitating region, strong momentum transfer between the higher and lower flow layers takes place, resulting in a highly even velocity distribution in the core part of the cavitating region, and the lower velocity area becomes smaller and, as the cavitation number lowers, moves toward the downstream.