Numerical Simulation of Unsteady Cavitation Flows

A previously developed simple numerical code, based on the Navier-Stokes equations of compressible fluid and a virtual single phase equation of state, has been applied to further demonstrate its capability to capture highly dynamic nature of cavitating flows about a two-dimensional NACA0015 hydrofoil. Computational time steps in the order of 10 microseconds were used to capture the detailed unsteady characteristics of bubble cavitation, bubble/cloud cavitation, sheet/cloud cavitation and super cavitation. The formation and collapse of cloud cavity, and the related generation and radiation of shock waves were visualized. The collapsing of cloud cavity is observed to be a highly unpredictable turbulent phenomenon, which most frequently breaks apart into a number of smaller pieces before collapsing. Only rarely it is found to collapse spherically as a whole. If the sheet cavity is long enough so that the cloud cavity can arrive at the tail of the foil before collapsing, refraction of negative pressure wave around the tail may cause cavitation to occur on the pressure side of the foil. The unsteadiness of natural super cavity is strongly influenced by the instability of the positive vortex sheet on the pressure side of the foil