Numerical analysis of unsteady cavitating flow by using a modification based on an assumption of apparent phase equilibrium

The prediction accuracy of cavitation by CFD is still not so high even in a simplest flow field around a single hydrofoil especially in transient condition at higher angle of attack, which is common problem in both commercial software and in-house solvers. In the transient condition, unsteady cavitation occurs, in which sheet cavity breaks off and cloud cavity sheds downstream periodically. At that time, the sheet cavity length tends to be underestimated in usual CFD. In the present study, modification for the phase change model is suggested, which is based on an idea of apparent phase equilibrium on gas-liquid interface with unsteady and disturbed flow. At first, a preliminary experiment has been done for evaporation on two gas- liquid interfaces with and without flow, the result contributes the evidence of the idea of apparent phase equilibrium with flow. In the result, the pressure around gas-liquid interface with flow was higher than that without flow on the occasion of evaporation, it means flow accelerates evaporation. I treat the gap of the pressure as a gap of phase equilibrium pressure macroscopically. Then, numerical simulation of cavitating flow around a hydrofoil is performed with a modification of phase change model in the transient condition at higher angle of attack which is most difficult to predict by the present solvers. In the modification, the gap of the pressure with and without flow is taken into account according to a value of a local variation of velocity in the cavitating flow filed. The formulation is similar to the PDF model for phase change model in cavitation by Singhal. The numerical results by the present modification are compared among few pressure variation components which are assumed to accelerate the evaporation in transient cavitation.