On the hysteresis of cavitation incipience and desinence in hydraulic machines

Abstract

The omnipresent vortical structures in hydraulic machines are extremely prone to the occurrence of cavitation. It is well known that besides the flow parameters, the incipience, development, and disappearance of cavitation within a vortex is very sensitive to the gas content. It is also known that the pressure threshold for vortex cavitation desinence may be significantly higher than that of its incipience. This hysteresis, which is not yet well understood, is the scope of the current work. The case study is made of an elliptical NACA 16020 hydrofoil, placed in the test section of EPFL high-speed cavitation tunnel. We have observed the inception and the desinence of tip vortex cavitation (TVC) for different flow conditions and gas contents. We found that the pressure threshold for the TVC desinence increases with the dissolved gas content. We have also found that this pressure threshold strongly depends on the flow parameters and may reach atmospheric pressure for specific conditions. We argue that the persistence of a cavity at pressure levels higher than the vapor pressure is due to an outgassing process that sucks air from of the surrounding supersaturated liquid to feed the cavity. The gas diffusion is likely enhanced when a laminar separation of the boundary layer is formed at the tip of the hydrofoil on the suction side.