Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun


One open question for further investigation is if a twist reduction of the sail would also increase the drive force. In fact, by decreasing the twist of the sail, or by introducing the twisted flow, a smaller angle of attack on the highest sections compared to the lowest is achieved.


It should be noted that the twisted flow can increase the drive force of each sail


slightly this reason when sail crossovers to


investigated, the twisted flow device should be used. 5.


CONCLUSIONS Experimental tests were performed on a 1/15th scale


model in the Yacht Research Unit’s Twisted Flow Wind Tunnel in preparation for Emirates Team New Zealand’s challenge for the 33rd America’s Cup. Three off-wind sails, named A1, A2 and A3 respectively, were tested with the same mainsail at apparent wind angles of 40°, 55° and 70° and at heel angles of 0°, 10° and 20° and their pressure distributions measured. These pressure measurements showed that the measured force trends are clearly correlated with the pressure distribution on the leeward side of the asymmetric spinnaker.


In particular the pressure distribution results enable the following conclusions to be drawn.


1.


The sail’s trim changes significantly when the apparent wind angle increases, which leads to a different sail shape and different angle of attack. When larger AWAs are sailed, the spinnaker sheet is eased, which increases the camber and decreases the


angle of attack. But the


combination of the increase in apparent wind angle and the sheet ease is still an increased angle of attack. The pressure measurements show that, in the first stage, increasing the apparent wind angle causes the suction to increase. Then, when the apparent wind angle is further increased, the suction cannot increase anymore because the higher adverse pressure gradient causes trailing edge separation to occur first.


2.


Some asymmetric spinnakers (e.g. the A2) show the maximum drive force when slightly heeled. The pressure distribution on the leeward side of the A2 shows that the drive force increase at low heel angles is correlated with a larger suction peak, whilst the drive force decrease at large heel angles is correlated with an earlier trailing edge separation.


3.


The 3 sails showed good correlation between their design purpose and the measured pressures. The pressure measurements gave an explanation of the force trends, allowing a deeper understanding of


The authors would like to thank Frederik Gerhardt for his enthusiasm in providing new ideas und suggestions for the research. The support of the YRU, especially David Le Pelley and


Nick


acknowledged. 7. 1.


REFERENCES


VIOLA I.M. AND FLAY R.G.J., Force and Pressure Investigation of Modern Asymmetric Spinnakers, International Journal of Small Craft Technology, Trans. RINA, vol. 151, part B2, pp. 31-40, 2009..


2. MARCHAJ C., Aero-Hydrodynamics of Sailing, Dodd Mead and Company, New York, 1979.


3. GENTRY A., The Aerodynamics of


Interaction, In the proceedings of the 3rd AIAA Symposium on the Aero/Hydronautics of Sailing, Redondo Beach, California, November 20th, 1971.


Sail


4. GENTRY A., The Application of Computational Fluid Dynamics to Sails, In the proceedings of Symposium on Hydrodynamic


Enhancement for Marine Applications, Newport, the sail’s Performance Velychko is gratefully 4.


consequently it affects the crossover between the sails. For


differently, and have


be


characteristics. In particular the A1, which is designed for close apparent wind angles, shows a high suction on the forward region, due to a small camber. This projects the aerodynamic force forward, which is necessary when sailing at close angles. On the contrary, when sailing at deeper angles the A1 shows a larger trailing edge separation, which compromises its performance, while in the same sailing condition the A3 shows the maximum suction due to its larger camber.


The A2 pressure


distribution doesn’t change significantly when the apparent wind angle is increased, and it achieves the maximum suction at 55° apparent wind angle, which confirms the A2’s all- purpose design.


Finally, the effect of the twist on the force and pressure measurements was investigated. The same tests were performed with and without twisted flow onto the yacht model. The twisted flow decreases the angle of attack onto the lower sections and increases it onto the highest sections of the sail. All the tests performed at the 3 apparent wind angles and at the 3 heel angles showed a force increase when the twisted flow was used. The pressure measurements showed that the suction on the leeward side of the spinnaker increased both on the lowest and highest sections, which suggests the existence of dominant and important 3D aerodynamic effects that are incorrectly modelled in uniform flow.


6. ACKNOWLEDGMENTS


©2010: The Royal Institution of Naval Architects


B-49


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