Effect of Gas Content on Tip Vortex Cavitation

Occurrence of tip vortex cavitation (TVC) is a common problem in axial hydraulic machines. The behavior of this type of cavitation is so sensitive to variations in the flow parameters and the gas content level of water. Regarding this, we have investigated TVC generated by an elliptical hydrofoil with NACA-16020 cross-section in the high-speed cavitation tunnel of EPFL. We measured the cavitation incipience and desinence thresholds for different flow conditions as well as various gas content levels of between 50 and 100 % oxygen saturation at 18 °C. We observed that the disappearance threshold of TVC increases with the gas content level, which is an indication of the fact that once the cavitation develops in the tip vortex, the dissolved gas in the surrounding supersaturated liquid diffuses into the cavity. Moreover, we found that the extent to which the desinence is delayed depends on the incidence angle and the upstream flow velocity; the parameters that determine the characteristics of the boundary layer on the hydrofoil. According to the flow visualization results, we argue that this gaseous cavitation is enhanced when a laminar separation bubble is formed on the suction side of the hydrofoil.