Designers of high performance yachts regard CFD as a valuable tool in the design stage of yachts, especially when competing in races such as the America’s Cup or the Volvo Ocean Race. At the Institute of Naval Architecture, University of Applied Science Kiel, we have been using CFX-5 to investigate the flow around mast and sails, focusing on flow separation phenomena and it's impact on sail performance. While modeling the flow around a downwind sailset with mainsail and gennaker is relatively straight forward, the simulation of flow around upwind sailsets can be challenging, especially if accounting for local flow separation behind mast.

Typically, a mast has a height 50 to 60 times its diameter. To optimise the mast profile and envelope, we must model the mast geometry accurately. Using tetrahedral/prism grids leads to a large number of surface triangles and a correspondingly large number of tetrahedral grid cells. Hexahedral grids are more efficient here but the model becomes quite complex if a full sailset including mainsail, jib, mast and headstay is to be simulated. As a remedy we made use of CFX-5’s generalized grid interface ability, using compound grids of two or more grids stitched together with non-matching grid interfaces.