COMPUTER MODELING OF DOWNWIND SAILS

An investigation into the use of Computational Fluid Dynamics to model downwind sails has been carried out to determine its place in the des ign process. The finite volume software package CFDS-FLOW3D was used t o model the flow around a spinnaker/mainsail combination. The sail sha pes used were those designed by North Sails NZ Ltd for the Whitbread 6 0 yacht Winston. Both uniform flow and logarithmic flow with wind twis t can be modelled using this method. The sails were modelled in a unif orm flow at apparent wind angles from 70 degrees to 180 degrees at int ervals of 10 degrees. Force coefficients were calculated at each inter val to compare with wind tunnel data at 90 degrees apparent. Sail trim was carried out by altering the spinnaker pole and boom angles. The d ifferences between the computed and wind tunnel results were 15% for l ift and 3% for drag. Comparisons between uniform and logarithmic flow show that the logarithmic flow resulted in a decrease in the force coe fficients of approximately 30%. This is comparable with wind tunnel te sts. Flow visualisation, using the software package SeeFD, was also ca rried out to gain an understanding of the resulting how. This investig ation has shown that CFD is a useful tool for the design of downwind s ails.