Trans RINA, Vol 1521, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun
enable testing in uniform flow. All the tests reported in this paper were performed in uniform flow except for the additional configurations measured with the A3 which were also re-tested with twisted flow to investigate how the twisted flow changes the pressure distribution on the spinnaker.
In uniform flow, the turbulence intensity was less than 3% (while in twisted flow the average turbulence intensity cannot be defined precisely because it varies horizontally due to the presence of the wakes from the vanes). Both in uniform and twisted flow conditions the boundary layer was confined to the lowest 10% of the height of the model.
4. RESULTS & DISCUSSION
The pressure distribution on the asymmetric spinnakers changes in each tested condition but a general trend can be described. Figure 2 shows the pressure coefficient trend on the 5 sections measured.
The pressure
coefficient on the windward side remains roughly constant, equal to one, for most of the curve length and then decreases to match the negative windward pressure coefficient at the trailing edge. The pressure coefficient on the leeward side shows a first suction peak followed by a pressure recovery, which is correlated to the leading edge separation and reattachment. Then a second suction peak occurs due to the sail curvature. The adverse pressure gradient of the following pressure recovery leads to trailing edge separation, which is evident from the constant negative value of the pressure coefficient. The flow is fully separated on the highest sections and, after a high suction peak at the leading edge, the pressure recovery is almost linear. On the lowest sections the trailing edge separation occurs generally before that on the mid-section.
In the following, section 4.1 shows the effect of increasing AWA on the general pressure trends described above. At each AWA the sail was re-trimmed to achieve the maximum drive force leading to a different geometry and a different angle between the wind and the boat model. Section 4.2 shows the effect of increasing the heel angle. When the heel angle is increased, the geometry of the sail remains roughly unchanged and only rotates around the longitudinal boat axis, which leads to a geometrical reduction of the angle between the wind and the sail geometry. Section 4.3 shows the differences between the pressure
distributions on the three 4.1 EFFECT OF AWA
When the AWA was increased, the sail’s sheet was eased to maximise the drive force and, consequently, the geometry of the sail changed significantly. However, despite the sheet easing, the angle of attack between the reference wind direction and the sail (measured, for
sail
shapes. Finally, section 4.4 shows the effect of testing in twisted flow on the pressure distribution.
instance, between the wind direction and the chord between the tack and the clew) also increased. Figure 3 shows 3 photographs of the A1 at 10° heel from a camera on the roof of the wind tunnel. It can be clearly seen that even if the spinnaker sheet is eased, the angle of attack increases and the camber of the foot becomes larger. The increase in angle of attack causes the separated region to enlarge. Moreover, by easing the sheet the camber of the sail increases, which leads to a larger pressure suction. The adverse pressure gradient after a larger suction peak can eventually lead to a larger trailing edge separation. In brief, increasing the AWA leads to a larger suction peak until an excessive separation occurs at the trailing edge.
Easing the sheet results in the direction of the resultant aerodynamic force turning forward in the direction of the drive force component. This effect is stronger than any reduction in the pressure forces due to an earlier separation. Hence the drive force always increases with the AWA in the range being investigated.
Figure 2: Schematic diagram of the pressure distribution over the asymmetric spinnakers at 5 horizontal heights and the corresponding flow field.
B-44 ©2010: The Royal Institution of Naval Architects
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