A Multiphase RANSE-based Computational Tool for the Analysis of Super-Cavitating Hydrofoils

Hydrofoils have been traditionally used in marine systems for propulsion and stabilization purposes. During the twentieth century, planning craft started to be partially sustained by lift forces generated by immersed hydrofoils with the aim to decrease the hull wetted area, and hence the resistance. It is clear that hydrofoil design becomes a very important aspect for very high-speed craft. For this reason, flow details have to be accurately solved to capture the complex hydrodynamic phenomena. A complete simulation framework consisting of an automatic grid generation module, a high fidelity CFD solver, and a post-processing tool has been developed with the final goal to be included in a shape optimization process, specifically addressed to the design of cavitating or super-cavitating hydrofoils. The simulation framework has been conceived to deal with any foil geometry with the minimum required input data. The very complex fluid-dynamic aspects involved in hydrofoils design, such as cavitation, laminar-turbulent transition, flow separation, and vortex shedding are solved using a non-linear, fully viscous method based on URANS equations that has been carefully tuned for the solution of the flow around 2D foil geometries. Results post-processing is performed using algorithms specifically designed for the analysis of cavitating flow around hydrofoils, the output consists in time-averaged lift and drag coefficients, as well as pressure and friction coefficients over the hydrofoil surface. Flow velocity, pressure and cavity thickness are probed in specific locations defined in the input file. The grid strategy and the CFD solver setting have been specifically studied with the goal to obtain a relatively fast computational method while still maintaining a high level of accuracy. The simulation framework has been validated with two different geometries tested in Caltech's high-speed water tunnel over a wide range of cavitation indexes and different angles of attack. Interesting results are critically discussed describing the fully cavitating flow enveloping the whole hydrofoil pressure side (super-cavitating) and unsteady behavior of the hydrofoil working at partially cavitating conditions. The multiphase flow is numerically solved considering water and vapor as a single fluid whose characteristics depend on an indicator scalar function, as in the volume of fluid approach. Results have been verified on a successively refined grid to understand the influence of mesh resolution on capturing the dynamic of the cavity. The main advantage of these methods is that there is no boundary condition on the cavity surface and the vapor flow is fully resolved, allowing for a better solution of the pressure recovery at the cavity closure