Tank Testing of a Windfoil Hydrofoil with Free Surface Effects

Hydrofoils used on many water crafts and boats, operate in close proximity to the free surface. While the behavior of wings in a single-fluid medium is well known, the effects of the free surface on hydrofoils are not precisely known. The high computational cost of CFD simulations with the air-water interface makes them difficult to be used to build a full hydrodynamic surrogate model with many input parameters, as needed for a Velocity Prediction Program (VPP). For a small hydrofoil,it is possible to test the full-scale foil in a towing tank and measure the hydrodynamic forces. This work presents a test campaign of the main lifting wing of the Olympic Class windfoil (IQFoil) hydrofoil in the towing tank of Centrale Nantes. The measurements carried out in different configurations of immersion, heel and trim, allow for exploring the operation of the hydrofoil and the impact of theproximity of the free surface on its performances. It is shown that the lift force and slope with angle of attack are reduced when approaching the free surface and that the variations of drag are dominated by the induced drag. When the wing pierces the free surface, ventilation may occur resulting in a sharp drop in lift. These experimental results can be used as a validation benchmark for numerical simulations and are useful to build the hydrodynamic force model intended to supply a VPP.