Numerical study of the cavitating flow over backward facing step with a polydisperse two-phase flow model

We study the cavitating flow over a backward facing step with an incompressible polydisperse cavitation model. The model can predict experimental observations for this flow reasonably well, including the shedding cloud characterized by the condensation front, cavity length, void fraction, and shedding frequency. All model variations produced shedding cavities, but the turbulence model and grid resolution are essential for better predictions, with delayed detached eddy simulation (DDES) performing better than Reynolds-averaged Navier-Stokes. Quantities, such as pressures at key points, maximum void fraction location, and shedding frequency, are mildly sensitive to those factors. Finer DDES grid resolution, crucial to resolve small vortices where cavitation occurs in their low pressure cores, improves predictions. Since a fully incompressible model produces a condensation front that follows well the experimental trends, it is concluded that compressibility is not a necessary condition for the formation of a condensation front. Consequently, the speed of sound in the mixture does not appear to play an important role in the front formation and evolution. The polydisperse nature of the model allows prediction of the bubble size distribution. Small bubbles concentrate on the downstream section of the cavity, where cavity collapse is strongest and bubble fission is most intense, while larger bubbles reside near the step where the flow is milder. The condensation front is a moving source of vorticity for the liquid phase where the "compressibility," in the sense of mixture density changes due to void fraction changes, and baroclinic effects are significant, but the buoyancy effect is negligible.