Abstract:
This work presents an investigation into the aerodynamics of downwind sailing using different methods for modelling turbulence, comparing Large Eddy Simulation (LES) and Reynolds-Averaged NavierStokes (RANS) methods. Pubblished experiments on a downwind sail plan [2] [8] showed areas of flow separation and vortex structures at the leading and trailing edge of the gennaker. The ability of LES to model the transition to turbulence within the shear layer leads to an accurate prediction of the leading edge separation bubble, which significantly influences the flow field around the top half of the sail. The transient nature of the LES solution allows the computation of the creation and shedding of unsteady vortices at the leading edge and downstream of the sail draft. The effect of the vortex rolls being convected towards the trailing edge is to generate a boundary layer which is more resistent to separation. Comparison with the experimental pressure distribution shows the correct prediction of the separation by LES, while the RANS result shows a large area of stalled flow which limits the suction on the sail. As a result, the overall drive and side forces computed by the LES are in good agreement with the experiments with less than 3.5% error, while RANS underestimates their magnitude by more than 14%.