Investigation on energy loss mechanism of hydrofoil cavitating flow based on velocity gradient tensor decomposition

The presence of tip clearance can generate a leakage vortex, which complicates the cavitating flow characteristics and significantly affects the energy loss. This study numerically investigates the cavitating flow around a National Advisory Committee for Aeronautics 0009 hydrofoil with a tip clearance. Based on velocity gradient tensor (VGT) decomposition and entropy production (EP) theory, the fluid motion patterns and the sources of vortices in hydrofoil cavitating flow are investigated; furthermore, the energy losses and their underlying mechanisms are revealed. The results show that pure shear (PS) is the predominant deformation in hydrofoil cavitating flow, while rigid rotation accounts for a relatively small proportion. The vortices can be divided into three categories based on their causes, including cavity growth induced vortex, cavity shedding induced vortex, and cavity collapse induced vortex. The detached cavity moves downstream with internal rotation rather than a simple translation. With increasing tip clearance, the energy loss presents a declining trend. The turbulence dissipation EP serves as the primary contributor, constituting over 90% of the total. The changes in cavity volume and EP exhibit opposite trends during one cycle. The distributions of EP rates exhibit a strong correlation with those of VGT components. The energy loss predominantly arises from the PS and compression/stretching (CS) deformation. Among them, direct dissipation EP mainly comes from CS, and turbulent dissipation EP is mainly caused by PS.