Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun
increased heel. Conversely, the A3 performs better at higher AWAs but drive force quickly decreases with increased heel. The A2 shows an intermediate behaviour. These trends
are confirmed by the pressure
measurements showing that the main force differences are related to the different pressure distributions on the leeward side of the spinnaker. As an example, the pressure measured at the mid-section of each of the 3 sails is presented in Figures 6, 7 and 8 for AWAs of 40°, 55° and 70°, respectively and 10° heel.
At 40° AWA (Figure 6), the A1 shows a higher leading edge suction peak and a minor pressure recovery, which is correlated to a larger angle of attack due to the lower camber of the horizontal sections of the A1. This allows the A1 to generate the maximum suction peak after a small pressure recovery.
At 55° AWA (Figure 7), the A2 shows the maximum suction peak after the reattachment of the leading edge separation. At the lowest sections (not shown in figure) the difference between the suction of the A2 and suction of the other two sails becomes even larger. The trailing edge separation does not change significantly and is about 60% of the curve length for all the sails.
Finally, at 70° AWA (Figure 8), the A3 shows the maximum suction peak after the reattachment of the leading edge separation. The trailing edge separation point occurs at about 50% of the curve length for both the A3 and A2, and the A3’s highest suction peak is related to the largest camber of the sail. At the lowest sections (not shown in figure) the differences between the Cp of the A3 and Cp of the other two sails becomes larger than 0.5.
In conclusion, the A1 takes advantage of a flatter
entrance profile which allows a shorter separated region at the leading edge and hence a large suction on the first quarter of the curve length. The A3 takes advantage of the increased camber to maximise the suction, and the A2 is a compromise optimised for mid-range AWAs.
4.4 EFFECT OF TWISTED FLOW
The flow was twisted using the vanes shown in Figure 1 to approximately simulate the onset flow that a yacht sailing at a speed of 7 m/s in a true wind speed of 6 m/s at a true wind angle of 140° would experience. The twist profile was kept unchanged for tests discussed in this section. Figure 9 shows the resultant deflection angle of the wind tunnel flow measured at different heights at the model location in the empty test section. The ordinate shows the ratio between the vertical height z and the model height h, the abscissa shows positive values when the flow deflection acts at an increased yaw angle. The twisted flow device changes the angle of attack of the horizontal sections of the sails. Comparing the twisted flow testing condition with the uniform flow testing condition, it is evident that above a height of 0.26h (10m
Figure 7: Cp at 55° AWA on the leeward side of the A1, A2 and A3 mid-sections respectively.
height in full-scale) the higher the section, the more the angle of attack increases. Conversely below 0.26h the lower the section, the more the angle of attack decreases. Note that the deflection angle is defined in the horizontal plane. At the lowest measured point the local AWA is reduced by about 15°. Conversely, at the top of the mast the AWA is increased by about 10°. The deflection of the flow leads also to a deviation angle in the vertical plane. However, the deviation angle has opposite sign on the two sides of the test section and it is negligible in a wide region in the centre of the test section, where the model is located.
Figure 6: Cp at 40° AWA on the leeward side of the A1, A2 and A3 mid-sections respectively.
Figure 8: Cp at 70° AWA on the leeward side of the A1, A2 and A3 mid-sections respectively.
Figure 10 shows drive force coefficient Cx achieved by the A3 at 10° heel at the various AWAs when the spinnaker is trimmed to maximise the drive force and when it is trimmed by tightening the sheet just enough to avoid the luff flapping. The two trims were measured both in uniform flow conditions and in twisted flow conditions. Easing the sheet of the A3 enough to let the luff flap leads to the force increasing. Moreover, both of
©2010: The Royal Institution of Naval Architects
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