Large Eddy Simulation of the transition from sheet to cloud cavitation over a wedge

LES of sheet to cloud cavitation over a wedge is performed at Reynolds number Re=. 200, 000 (based on the wedge height and free stream velocity) and cavitation number σ. =. 2.1. The attached sheet cavity grows upto a critical length, after which it breaks into a cloud cavity which is highly three-dimensional and vortical in nature. The mean and RMS void fraction profiles obtained inside the cavity are compared to experiment and good agreement is observed. The frequency of the shedding process is obtained from point spectra at several locations and the obtained frequency is found to agree with the experiment. It is observed that the mean pressure at the wedge apex does not fall below vapor pressure; however cavitation occurs there due to the unsteady pressure falling below vapor pressure. The maximum mean void fraction occurs in the sheet cavity and is about 0.5, while the cloud region has even lesser amount of void fraction. The velocity fluctuations immediately downstream of the cavity show comparable streamwise and spanwise values, while the spanwise values are smaller in comparison inside the cavity region. The probability density function of void fraction examined at several locations inside the cavity show that the mean value obtained from time averaged data is very different from the most probable value of void fraction, indicating the considerable unsteadiness of the flow. The pressure waves produced upon cloud collapse are found to display both cyclic behavior and small scale transient behavior downstream of the wedge. The LES results agree better with experiment than unsteady RANS in predicting this highly unsteady flow.