Hydrodynamic mechanisms of aggressive collapse events in leading edge cavitation

Transient cavities generated from unsteady leading-edge cavitation may undergo aggressive collapses which are responsible for cavitation erosion. In this paper, we studied the hydrodynamic mechanisms of these events in the leading edge cavitation formed over a modified NACA0009 hydrofoil using experimental and numerical methods. In the experimental investigation, high-speed visualization (HSV) and paint test are employed to study the behavior of the cavitating flow at sigma = 1.25, beta = 5 degrees, U-infinity = 20 m/s. In the numerical part, the same cavitating flow is simulated using an inviscid density-based compressible solver with a barotropic cavitation model. The numerical results are first compared with the experimental HSV to show that the simulation is able to reproduce the main features of the cavitating flow. Then, as the compressible solver is capable of capturing the shock wave upon the collapse of cavities, the location of collapse events with high erosion potential are determined. The location of these collapse events are compared with the paint test results with a qualitatively good agreement. It is clearly observed, in both the experiments and the numerical simulation, that there exists four distinct regions along the hydrofoil with higher risks of erosion: (1) A very narrow strip at the leading edge, (2) an area of accumulated collapses at around 60 percent of the sheet cavity maximum length, (3) an area around the closure line of the sheet cavity with the highest erosion damage, and (4) a wide area close to the trailing edge with dispersed collapse events. A combined analysis of the experimental and numerical results reveals that the small-scale structures generated by secondary shedding are more aggressive than the large-scale cloud cavities (primary shedding). It is also observed that the high risk of cavitation erosion in regions 2 and 3 is mainly due to the collapses of the small cavity structures that are formed around the sheet cavity closure line or the rolling cloud cavity.