CFD Analysis of Windsurfing Aerodynamics | Posture Study
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With significant advancements in composite materials, manufacturing technology and better understanding of sailing mechanics, consistent leaps forward are made in creating lighter and more efficient sail craft every passing year. One challenge that designers and sailors alike have taken on themselves is pushing the speed barriers for sail powered water craft. |
Radical Hidropter Design breaking the 50knot Barrier |
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Antoine Albeau breaking the World Speed Sailing Record |
In the past few decades, very innovative but also costly multi-hull vessels have been successful in breaking the world sailing speed records. However, much simpler and lower cost designs such as windsurfers and kiteboarders have been very consistent in leading the way. The last six records were broken by windsurfers and kiteboarders - the latest at 50.57knots. One common ground to these watercraft is the large impact of the sailor himself on the aerodynamic performance. |
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At the time the CAEbridge study was initiated, the world speed sailing record was held in the hands of windsurfers at 49.09 knots. With intentions of helping to challenge the 50 knot barrier, we decided to build a CFD model for a windsurfer assembly with detail level down to the fingers of the sailor. In order to keep tack of their individual impacts, we broke the complete assembly into meaningfully chosen parts. With a specialized human form morphing tool, we tested different sailor postures and inherent sail angles to increase the sail efficiency and to reduce the aerodynamic impact of the sailor on the sail. |
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CAEbridge Windsurfer Model with Alternative Sailor Postures |
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The wind direction was set to 120 degrees relative to the board heading angle, and the board speed was set at about 1.2 times that of the wind. The wind was imposed with a parabolic profile, increasing from 0 on the sea surface to its imposed value at 2.5m height, which is typical for a wind meter location. The changing angle of the apparent wind with height allowed us to assess the optimality of the sail head twist for the given scenario. Usually the head twist is not optimized for such extreme sailing angles for versatility. |
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Observing the mild separation of the flow near the bottom of the sail, we observed the sail was over sheeted. In addition, the relatively more severe separation at the top showed us the need for allowing for additional head twist to open it up. With these changes, optimal angle of attack through the whole height of the sail was feasible. Markers for the optimal sheeting angle may be included for the sailor's reference. |
Flow Separation at Sail Foot & Head due to Oversheeting & Inadequate Head Twist |
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Pressure Difference between Sailor Front and Rear |
Raking streamlines originating from the sailor, we observed that he does not interfere with the flow to the sail. However, observing that significant drag is induced due to the pressure difference between fore and aft projections of the sailor's body, we decided to evaluate the local flow patterns around his body. Of all the regions evaluated, the head, the upper back and the leading (right) leg turned out to create significant vortices creating suction behind. |
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One way to remedy the problem might be to fit a wet suit with turbulators on the front portions of these body parts to trip turbulence and help the flow to remain attached longer. Another way is to fill the voids where the vortices exist. For this, a stretchy skirt or a web can be fitted to fill the gap between the legs. A similar idea can be utilized for the arm pits. For the section above the shoulder a tear drop shaped helmet aligned with the flow and conforming to the shoulder can be utilized. |
Regions of Severe Vortex Formation |
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Once all the aerodynamic improvement ideas are tested and verified, we will focus on the hydrodynamic aspects, utilizing more advanced simulation techniques that model the interaction of water with air and the board/fin surfaces. |
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