Study on Unsteady Tip Leakage Vortex Cloud Cavitation in an Axial Flow Pump Using an Improved Numerical Method

The aim of the present investigation is to simulate and analyze the formation of three-dimensional tip leakage vortex (TLV) cloud cavitation and the periodic collapse of TLV-induced vortices cavitation. The improved SST k-ω turbulence model and the homogeneous cavitation model were validated by the simulation of unsteady cavitation shedding flow around the NACA66-mod hydrofoil, and then the unsteady TLV cloud cavitation and unstable vortices cavitation in an axial flow pump were predicted by using the improved numerical method. The predicted three-dimensional cavitation structure of TLV and vortices as well as the collapse features show a qualitative agreement with the high speed photography results. Numerical results show that the TLV cavitation cloud in the axial flow pump mainly includes tip clearance cavitation, shear layer cavitation and TLV cavitation, and TLV-induced vortices cavitation occurs in the downstream of blade trailing edge (TE). TLV cavitation cloud is relatively steady before about 80% blade chord with the high vapor volume fraction inside the TLV core. The unsteady TLV cavitation cloud occurs near the TE of blade where the transient cavity shapes of sheet cavitation and TLV cavitation all fluctuate, which results in the decrease of the axial velocity in the tip region. It is found that the unstable vortices cavitation in shear layer in the downstream of TE collapses periodically. The correlation analysis shows that TLV cavitation cloud and vortices cavitation collapse are significantly associated with the interaction between TLV breakdown and boundary layer in the downstream of blade TE.