Developing methods of decomposing the physical resistance components acting on a hydrofoil

Abstract

The America’s Cup is a sailing competition contested by teams fielding bespoke yachts designed and built to the AC75 class regulations. These yachts make use of hydrofoils, which permit them to lift their hulls above the water surface. When in this “foiling” regime, the drag forces on the hull are drastically lower, permitting greater sailing speed. Predicting hydrodynamic forces produced from hydrofoils on race yachts remains challenging, due to a wide operating regime and complex hydrodynamical phenomena. In this study, existing wake survey methods of decomposing the total drag force of a hydrofoil into physical subcomponents were developed through examining their sensitivities and accuracy. Computational fluid dynamics (CFD) simulations of a rectangular NACA4412 hydrofoil were performed to generate data for the wake surveys. The hydrofoil was simulated at a submergence depth of one chord. Viscous flow was simulated with the k–ω SST turbulence model in steady state; and the volume-of-fluid (VOF) model was used to capture free surface effects. Chord Froude numbers of 0.5, 1, 2 and 4 were investigated, corresponding to Reynolds numbers of 2.57 × 105, 5.15 × 105, 1.03 × 106 and 2.06 × 106. Force decomposition was performed using two different wake survey methods – one each for the wave and viscous drag – as well as Trefftz plane analysis for the induced drag. Wave drag forces could be reliably isolated at all tested Froude numbers, while reliable predictions of the viscous drag were dependent on the Froude number and method of isolating the hydrofoil’s viscous wake region. Accurately isolating the induced drag component proved challenging due to free surface interactions. Careful selection of the wake survey location was also necessary for accurate predictions of any of the drag components.