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Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec 7. CFD GRID SENSITIVITY


A grid sensitivity study for the CFD part of the simulation was carried out on a single fixed spinnaker geometry. For this study boundary


general grid resolution, layer resolution and timestep length,


respectively Courant number were varied. The grids consisted of tetrahedrons for the majority of the volume and prism elements for boundary layer resolution. The SST turbulence model with a turbulence level of 5% was used for all computations.


Very little dependency of forces on timestep length was observed as soon as the timestep was small enough not to induce artificial instationarities.


General grid resolution and boundary layer resolution have a much more significant impact on resultant forces. Figure 9 shows driving and side forces for the selected testcase depending on total grid size and y+ value in the state of timestep insensitivity.


The tested spinnaker had the following dimensions: SL [m] = 1.43


SMG [m] = 0.766 SF [m] = 0.808 Area [m²] = 0.923


8.1 WIND TUNNEL TESTING AND FLYING SHAPE CAPTURING


For validation purposes, the spinnaker was tested over an AWA-range of 90° to 180° at an AWS of 5 m s-1. Trim settings were


recorded during the tests for use in


simulations. The resulting force areas are given in Figure 10, reproducibility of the results were confirmed during tests measurements using the recorded trim settings. For scaling and comparison purposes the forces are normalized by the dynamic pressure of the apparent wind


( PDyn   Air AWS ), resulting in force areas. The 1/ 2 2


trends of the measured forces appear to be quite peculiar with significant jumps between AWA = 120° and 127.5°. This appears to be caused by the flow being at least partially attached to the spinnaker at AWA ≤ 120° and fully separated at AWA ≥ 127.5°. At AWA = 120° the spinnaker trim for maximum driving force was quite unusual, especially compared to AWA = 112.5° & 127.5°. At this AWA trimming for maximum driving force lead to an exceptionally high spinnaker


pole


Figure 9: Driving (continuous) and side (dashed) forces depending on general grid and boundary layer resolution


While grid insensitivity has obviously not been reached, a definitive trend can be observed. Typically further refinement of the grid would be deemed necessary. Unfortunately finer grids were found to be impractical for the FSI case.


Results of grid sensitivity analyses for the FE and FSI cases are not available for publication to date.


8. VALIDATION TESTCASES


For the validation of FlexSail a symmetrical spinnaker was tested in the Yacht Research Unit Kiel’s Twisted Flow Wind Tunnel (TFWT) [3]. This spinnaker’s design was the result of a development for sailmakers Holm Segel Schleswig/Germany. The spinnaker design was developed as a generic mould based on a 40’ cruiser/racer. During the tests it emerged to be beneficial to trade area for a more stable and controllable shape, maintaining more attached flow over a wider range of AWAs. Therefore the spinnaker has less than maximum surface area within the given design envelope. During


position for keeping the upper part of the sail flat. The gap in the curves is to account for this transitional behaviour. During the tests the spinnaker sheet lead was adjusted to prevent the sheet from being deflected by the main boom.


]


m ²


y [


A,]


A x [


m ²


-0.5 0.0 0.5 1.0 1.5 2.0


wind tunnel tests the final design has proven to be quite stable and forgiving while having


driving forces


comparable to maximum sized spinnakers at significantly reduces sideforces. This was corroborated during testing this spinnaker at full scale.


90


120 Driving Force Area (Ax)


150


AWA [deg] Side Force Area (Ay)


Figure 10: Driving and side force areas for spinnaker and main sail from TFWT-measurements


The general arrangement of the TFWT is shown in Figure 11.


B-76 ©2011: The Royal Institution of Naval Architects


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