Comparison of two commonly used hydrofoils and their propensity to cavitation and ventilation

Cavitation occurs when vapour pockets form in a liquid flow because of local pressure reductions. This kind of phenomena can occur on hydrofoils operating in a liquid domain near a free surface (seawater and air) this boundary induces persistent ventilated cavities to form and attach to the hydrofoil surface. As a result, ventilation and cavitation can coexist to reduce the efficiency of a hydrofoil operating in a liquid domain near a boundary (free surface) with air. The objective of the research project is to determine if the same trends can be seen in both cavitation and ventilation propensity for two commonly used hydrofoil sections (NACA-0012 & Mach 2) whilst using realisable design methodologies. The hydrofoils were configured around a typical main foil fitted to a moth sailing dinghy. The cavitation propensity of the hydrofoil sections was analysed numerically by using Computational Fluid Dynamics (CFD) to determine design conditions where cavitation is likely to occur when operating near a free surface. The two-dimensional CFD analysis was performed purely to determine the effect of cross-sectional shape to cause cavitation propensity at certain design conditions. The CFD analysis showed that both hydrofoils had a high reduction in lift and increase in drag when operating at Depth to Chord Ratio (D/C) of one. The proximity relationship with the free surface also showed that cavity length increased as submergence was decreased. The NACA0012 had a greater propensity to cavitation over the design conditions analysed. The ventilation propensity of the hydrofoils was analysed by conducting a full-scale experiment where the number of ventilation events were analysed over a set period. The full-scale experiment showed the existence of ventilation caused by the hydrofoil tip breaking the free surface which allowed a tip vortex to propagate from behind the trailing edge and attached to regions on the upper surface of the hydrofoil. The NACA-0012 experienced this phenomenon on more occasions than the Mach 2 and more importantly had greater period of persistent ventilated cavity attachment on the leading edge. This location of persistent ventilated cavity attachment also coincided to be the same region of maximum cavitation propagation as verified in the CFD results, this is where the NACA-0012 hydrofoil had a higher propensity to operate with cavitation where pressure regions below free surface reached cavitation critical pressure due to cross-sectional shape. The research project has shown that there is a correlation between ventilation and cavitation on a hydrofoil operating near a free surface. The primary focus was to determine common trends and explain the commonality of both phenomena. With such information a designer can use a relatively simple measure of two-dimensional cavitation propensity to help reduce the probability of complex three-dimensional persistent ventilated cavities.