Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec
around 30. Following the findings from simulations of the Wilkinson testcase, the SST turbulence model was used with a free stream turbulence of 5%.
Figure 13 shows the design output from the sailmaker’s lofting program ProSail.
Figure 11: General arrangement of YRU-Kiel Twisted Flow Wind Tunnel
During the TFWT measurements the flying shape of main sail and
spinnaker was recorded using
photogrammetric techniques [10]. Figure 12 shows the relation between an exemplary picture used for the measurements and the resulting CAD model. The images were processed using Photomodeller®.
Figure 13: Designed shape from ProSail
The result of the simulation shows a significant change from designed shape to flying shape, Figure 14 compares design and flying shape at AWA = 90°.
Figure 12: Exemplary photo from photogrammetric measurements and corresponding CAD model
8.2 SIMULATION USING FLEXSAIL
The flow around the main and spinnaker and the spinnakers structural behaviour were simulated using FlexSail. The simulations were carried out in model scale. For the simulations the main sail was assumed to be rigid with a flying shape as recorded during the wind tunnel tests, the spinnaker was trimmed according to the settings recorded during these measurements. To ensure comparability the incident flow was modelled based on measurements of the wind tunnel flow conditions. The simulations were carried out at model scale to keep Reynold’s-similarity. The structural properties of the spinnaker were the same as in the wind tunnel.
The discretisation mesh for the spinnaker consisted of 12479 triangular elements with local refinement near the three corners of the sail. Total control volumes in the domain numbered approx. 2.2*106, this includes refinement by prism cells close to sails and hull to resolve the boundary layer. On the spinnaker the same surface mesh is used for the structural computations and the flow simulation. In all flow simulations the typical y+ was between 1 and 4, the average Courant Number was
©2011: The Royal Institution of Naval Architects
Figure 14: Designed (red) and computed flying (green) shape at AWA = 90°
This comparison indicates a significant displacement respectively change of shape from designed to flying shape. Especially the projection of the luff can be clearly seen.
The streamlines in Figure 15 show clean flow behaviour around the body of the spinnaker. Only near head, foot and close to the leech some divergence of the streamlines is visible, indicating separation in these areas. This is supported by the pressure distribution shown in Figure 16. Near the luff an area of low pressure can be observed indicating a suction peak. The interruption of this area close to mid-luff indicates a locally non-optimum luff twist distribution. The towards the leech.
pressure increases gradually B-77
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