THE USE OF COMPUTATIONAL MODELLING IN A STUDY OF DOWNWIND SAILS Peter Richards and Gordon Mallinson University of Auckland New Zealand
Auckland - The City of Sails
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Images created by Gordon Mallinsons package SeeFD (Son of GRAFFIC)
Wind direction Americas Cup Course
VMG maximum with a wind speed of 5 m/s True wind direction
Wind direction
Spinnaker Mainsail Spinnaker pole Boom Spreaders Masthead
Velocity Triangles A VTVT VAVA VBVB VTVT VAVA VBVB A (a) Boom Height(b) Masthead
Apparent Wind Twist
Test Section of Tamaki Twisted Flow Wind Tunnel
Schematic of Tamaki Twisted Flow Wind Tunnel Test section Honeycomb and mesh 25m 3m Wind Flow Fans Wooden blocks Pitot tube Flow twisting vanes (Not to scale)
Twisted flow?
Interaction Test Scenarios Mainsail onlySpinnaker onlyCombined sails
Lift and Drag Coefficients - Wind Tunnel
Lift and Drag on the Spinnaker - CFD
Lift and Drag on the Mainsail - CFD
Thrust and Side Forces: Spinnaker, Main and Both
Thrust and Side Force: Summed and Both
Twisted Onset Flow
Thrust and Side Force: Summed, Both and Twisted
CONCLUSIONS Both straight and twisted flow around a pair of downwind sails have been modelled. Under some sailing conditions there is significant interactions between the sails, however under normal sailing situations this interaction is quite small. With twisted flow the thrust and side force coefficients are similar to those for straight flow if referenced to the masthead apparent wind speed and direction.