Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun
the 5 horizontal sections of the A2 sailing at 55° AWA. Force measurements show that the drive force increases when the heel is increased from 0° to 10°, and then significantly decreases when heeled further.
Figure 5
shows that heeling the model from 0° to 10° causes the suction pressure to increase and then heeling the model further to 20°, causes the suction to decrease due to the trailing edge separation point moving forward.
Some possible interpretations that should be investigated in future research are now discussed. It can be assumed that:
i.
The pressure on the leeward side of the profile, measured
decreases with increase in the local angle of attack until incipient flow separation;
ii. When stall occurs, the leading edge leeward pressure increases;
iii. The amplitude of the suction peak after
reattachment increases with increasing angle of attack but decreases when the trailing edge separation point moves a long way forward.
Figure 3: The A1 is photographed from above at 40° (top), 55° (middle), 70° (bottom) AWA. The arrow shows the wind direction and the line shows the chord measured at the sail foot.
Figure 4 shows the pressure coefficients of the 5 measured horizontal sections of the A2 at 10° heel, at ⅞, ¾, ½, ¼ and ⅛ of the mitre, respectively. At each section, the Cp’s measured at 40°, 55° and 70° AWA are compared.
On the top section, at ⅞ of the mitre, the
pressure trend shows a fully separated flow at each AWA. At the ½ mid-section and at the lower sections, the pressure trends show a reattachment in the first 10% of the curve
length and a successive suction peak,
followed by a trailing edge separation at 60%-70% of the curve length. The second suction peak increases when the AWA increases from 40° to 55° because of the increased camber. Then it decreases because the trailing edge separation points moves forward to 30%-50% of the curve length. Increasing the AWA and thus the angle of attack, causes the leading-edge suction peak to increase but it does not significantly change the location of the reattachment point.
4.2 EFFECT OF HEEL
In downwind sailing, inducing the boat to heel can cause the aerodynamic forces to increase. The reason why this happens is still not completely understood and future investigation will be required. This work shows that the force increase is mainly due to the increased suction on the leeward side of the spinnaker. Figure 5 shows the pressure distribution along the curve length measured on
©2010: The Royal Institution of Naval Architects
It should be remarked that the pathlines have not been measured but are not necessary horizontal. Hence, the angle of attack at the leading edge is not in the horizontal plane. The pressure is measured in a body fixed sense and measured sections are roughly horizontal when the model is upright but are inclined when the model is heeled.
The following observations can be drawn from the pressure trends shown in Figure 5. On the lowest sections (first and second plot from the bottom) the leading edge suction decreases with increasing heel, and the trailing edge separation occurs later at 10° heel than at 0° heel. Both these observations may be due to a smaller angle of attack. The sail shape does not change with the heel angle but, considering horizontal sections parallel to the floor, the geometry rotation of the sail causes the camber of the lowest horizontal sections to increase and the camber of the highest horizontal sections to decrease. The flying shape of the A2 sailing at 55° AWA has been detected with a photogrammetric technique. The camber measured on the horizontal section at the clew height increases by roughly 10% when heeled to 10°, and by 20% when heeled to 20°. Increasing the camber causes the suction peak after reattachment to increase. This can explain the earlier separation of the lowest section at 20° heel. On the highest sections (uppermost plot), when the heel angle is increased the camber decreases but the flow is already fully separated and hence is insensitive to this.
Different interpretations of the pressure trends shown in Figure 5 are possible and further investigations are necessary to understand completely what is happening. Interestingly, the present results show that the heel effect is far from being a simple reduction of the effective apparent wind angle and effective apparent wind speed,
very close to the leading edge,
B-45
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