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| Fast, Effective Sail and Mast Design |
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Posted Tue July 20, 2004 @12:24PM
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by Kai Graf and Eric Wolf,
University of Applied Science Kiel, Yacht Research Unit,
Germany
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.
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An upwind sailset of an IACC yacht was modeled using two hexahedral grids in close vicinity of the mast with mainsail and jib with headstay. A second test case used hexahedral grid cells only around the mast. The farfield was modeled using tetrahedral grid cells taking advantage of simple generation methods. This worked surprisingly well with only minor influence of non-matching interfaces on convergence of the solver. CFX-5 results showed good agreement with wind tunnel measurements.
This approach simplified the investigation of mast design alternatives significantly. To study mast profile variants, only the hexahedral grid around the mast has to be regenerated. Results showed strong influence of the separation bubble on the lift-to-drag ratio of the mainsail. A systematic variation of mast profile, profile length and mast twist allowed us to determine an optimized mast profile.
Yacht geometry showing sails.
To model the sails, two separate hex grids were created for the sails along with a tet grid for the field.
Using CFX's generalised grid interface, hybrid grids could be used to model the mast.
IACC upwind sailset.
CFX analysis of flow around mast top.
CFX analysis shows downwind flow separation around gennaker of IACC yacht.
Results showed good agreement with wind tunnel measurements.
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