Computational Fluid Dynamics for Downwind Sails

In recent years computational fluid dynamics (CFD) has demonstrated the ability to predict sail and appendage forces under upwind conditions or at angles of attack conducive to attached flow. Few sail or yacht designers would be without this tool, at least to check or confirm performance estimates made with other methods. More advanced codes (RANS) solve the full Navier-Stokes equations, thus including viscous effects and placing relatively less importance to fully attached flow. Due to the large proportion of downwind sailing, where the sails might operate in separated airflow, it is useful to evaluate the performance of sails as used off wind despite the added uncertainty resulting from the elasticity of the light material that must be used to allow the sails to fill properly at the low relative wind speeds. While downwind sail forces have been often tested in wind tunnels, CFD codes are now sufficiently advanced to predict such forces with confidence similar to that achieved in prediction of upwind forces. This paper presents a new method of linking a CFD code with a Finite Element Analysis (FEA) computer program, for evaluating the sail shapes and proper trim for known sail materials and fiber orientation. A VPP (Velocity Prediction Program) is used to predict leeway, heel, and boat speed for a given true wind angle and wind speed. Then the CFD code computes the airflow around the sails for the given onset flow conditions and provides the pressure distribution on the sails as needed for the FEA program. This is done in full scale considering the boundary layer above the water. This process of updating the pressure for the FEA program from the CFD code is repeated several times until optimal trim and sail shapes can be obtained for best sailing performance, e.g., the maximum driving force. Thus, this method can be considered a "Virtual Wind Tunnel" (VWT).