Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun


lower compared to the one obtained with the full scale rig in 7.72m.s-1, consequence of a Reynolds effect, namely a dependence of the aerodynamic coefficients with the Reynolds number.


CL Model (1:15) in 4.15m.s-1 Full scale rig in 15kts,(7.72 m.s-1) CD


1.133 0.155 1.242 0.162


Table 2 Numerical prediction of lift and drag coefficients for the Small Main rig with hull, at model scale and at true scale


Discrepancies were also noticed for the axial velocity fields at mid-span. In fact, on the leeward side of the jib, the peak of velocity is 2% lower at model scale in 4.15m.s-1 than at full scale in 7.7m.s-1.


The tip vortex generated by the head of the mainsail has also shown sensitivity to scaling: with the scaled model, the magnitude of the peak of suction in the core of the tip vortex was reduced by nearly 11% compared to the full- scale rig in 7.72m.s-1. These results verify those found in the papers of the recent conference, ([13], [14], [15], [16], [17] and [18]).


5. CONCLUSIONS


The present work aimed at developing a methodology for studying modern square head rigs in upwind conditions by combining wind-tunnel measurements with 3D RANS simulations. To do so, the flying shapes of the models were acquired from pictures taken during the wind-tunnel testing.


The CFD simulations have first highlighted the need for an extreme accuracy when acquiring the flying shapes from pictures. In particular, it was thought that a slight misalignment of the camera with respect to the model’s centreline may have caused errors of the order of 1° in the actual sheeting angles,


resulting in significant


discrepancies between the measured and forces and the predictions.


The second point to be made is the importance of modelling the hull when simulating sail flow in upwind conditions. It has been shown that the hull does not only influence the flow in its vicinity, but has also an impact on the flow speed and direction at mid-span of the mast. Moreover, the hull has a strong influence on the tip vortices generated at the sails’ foot: the presence of a hull tends to tangle these two vortices, which would be clearly separated otherwise. These tie in with wind tunnel practise. It has also been noted that this tangle-up of vortices reduces significantly the magnitude of suction in the vortex core, hence their vorticity. Modelling the boom and spreaders could as well increase the accuracy of the simulation, but at complex mesh.


the cost of an even more


The last part of the work has highlighted the importance of scaling effects. In fact, simulating a 1:15 model in a wind-tunnel or a full scale rig in a realistic breeze can lead to differences of up to 10% for the lift and drag coefficients and 11% for the suction in the upper tip vortex core. These differences are consequences


of


significant Reynolds effects. It would thus be preferable to test the models in stronger wind speeds. However, at this scale, exact similitude would only be achieved with wind speeds of 115m.s-1, which will cause structural issues with the models.


RANS solvers have now reached a mature stage and can be used as high-end design tools to study sail flow and to perform optimization of modern rigs. Not only full scale force predictions can be achieved, but the whole flow field around the sails


can be studied for a better


understanding of the main flow features. However, it is still preferable to couple CFD simulations with some wind-tunnel experiments to validate the numerical model in general and the mesh in particular. Flying-shape acquisition is thus necessary, but requires very high accuracy.


6. ACKNOWLEDGEMENTS


The authors are particularly grateful to Mr. L. Gilbert and Incidences sailmakers for providing the One Metre hull, rig and sails for the model.


7. 1.


2. 3.


4. 5.


REFERENCES MILGRAM,


Proc. 7th Symposium. Naval Hydrodynamics, pp.1397-1434, 1968


J.H., The aerodynamic of sails,


MILGRAM, J.H., The analytical design of yacht sails, Trans SNAME, pp.118-160, 1968


GREELEY, D.S., KIRKMAN, K.L., DREW, A.L., CROSS-WIHITER, J.H., Scientific sail shape design,


Yacht Symposium., pp. 33-80, 1989


RAMSE, W., Numerical methods for flow around lifting bodies with vortex wake rollup, PhD thesis, MIT, 1996


DOYLE, T., IACCARINO, G. GERRITSEN, M., Sail optimization for


6.


and the


Maltese Falcon Clipper Yacht, Proc. HISWA 2002 conference, Amsterdam, The Netherlands, 2002


CHAPIN, V.G., NEYHOUSSER, R., JAMME, S., DULLIAND, G. and CHASSAING, P., Sailing yacht rig improvements through viscous computational


fluid 7. 8. dynamics, Proc. 17th Chesapeake Sailing Yacht Symposium, 2005


COUSER, P. C., DEANE, N,, Use of CFD techniques in the preliminary design of upwind sails, Proc. 14th Chesapeake Sailing Yacht Symposium, 1999


MENTER, F.R., Two-equation eddy-viscosity turbulence models for engineering applications, AIAA-Journal., Vol. 32, No 8, 1994


B-6 ©2010: The Royal Institution of Naval Architects Proc. 9th Chesapeake Sailing


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60