Transition from Rotating Cavitation to Cavitation Surge in a Two-Dimensional Cascade

Cavitation instability in the turbo-pump can be divided into two different stages: the local instability such as the rotating cavitation and the system instability such as the cavitation surge. In our study, a numerical analysis of cavitating flow in a two-dimensional cascade, which represents the tip region in an inducer of liquid-fuel rocket engine, was conducted to find an indicator of the onset of cavitation surge. The response characteristic of cavitating flow field to three types of inflow boundary conditions was examined: steady inflow, inflow with sinusoidal fluctuation and inflow considering the characteristic of piping system. The last one is modeled by the unsteady Bernoulli's equation. Various cavitation modes that cause local instability and system instability were reproduced by considering flow rate variation in piping system. It is found that the specific mode, which is hidden behind the fluctuation of rotating cavities, is to be developed to the cavitation surge. The frequency of the surge was confirmed to be in the experimentally observed range and the ratio of the surge frequency to the propagating frequency, which is generated by rotating cavitaion, is almost constant while inflow changes.