Sheet Cavitation Inception Mechanisms on a NACA 0015 Hydrofoil

Experimentally measured trajectories of a series of free-stream nuclei and their growth while passing an isolated roughness element on a hydrofoil are compared with Lagrangian tracking computations on bubbles with the same initial size and position. The comparison shows a good agreement between the measurements and simulations, indicating that the proposed numerical tools may be used in order to determine cavitation inception criteria on hydrofoils and complement the interpretation of experiments. It is shown that the nuclei present in the experiments are not likely to have been fully gaseous; the combination of their trajectory and radius history indicate that they are made from an oil-gas mixture or a solid particle with a gas layer around it.
Two series of experiments also investigated the similarities and differences between cavitation inception induced by free-stream nuclei passing over a roughness element and pits placed around the minimum pressure point. The protruding roughness element causes bubble expansion due to hydrodynamic pressure reduction and traps gas and water vapour in its wake. The (hydrophilic) pits trap gas and water vapour from a passing bubble. The cavity retracts in the pit with increasing ambient pressure until it becomes invisible. The bubble in the pit expands again with decreasing ambient pressure to a vaporous cavity.