Computer modelling of downwind sails

Abstract

Abstract An investigation into the use of Computational Fluid Dynamics to model downwind sails has been carried out to determine its place in the design process. The finite volume software package CFDS-FLOW3D was used to model the flow around a spinnaker/mainsail combination. The sail shapes used were those designed by North Sails NZ Ltd for the Whitbread 60 yacht Winston. Both uniform flow and logarithmic flow with wind twist can be modelled using this method. The sails were modelled in a uniform flow at apparent wind angles from 70° to 180 at intervals of 10. Force coefficients were calculated at each interval to compare with wind tunnel data at 90 apparent. Sail trim was carried out by altering the spinnaker pole and boom angles. The differences between the computed and wind tunnel results were 15% for lift and 3% for drag. Comparisons between uniform and logarithmic flow show that the logarithmic flow resulted in a decrease in the force coefficients of approximately 30%. This is comparable with wind tunnel tests. Flow visualisation, using the software package SeeFD, was also carried out to gain an understanding of the resulting flow. This investigation has shown that CFD is a useful tool for the design of downwind sails.