Trans RINA, Vol 152, Part B2, Intl J Small Craft Tech, 2010 Jul-Dec


TECHNICAL NOTE A THREE-DIMENSIONAL INVERSE SAIL DESIGN METHOD


J P Pilate, F C Gerhardt and R G J Flay, Yacht Research Unit, University of Auckland, New Zealand (DOI No: 10.3940/rina.ijsct.2010.b2.104tn) SUMMARY


Today sail shapes are usually designed using analysis methods i.e. based on experience the designer specifies a certain sail shape and then proceeds to determine the aerodynamic characteristics of this sail. Finding optimum sail shapes using such a method can involve a lot of trial and error. A new approach in sail design is proposed in this paper, where an inverse method is considered. The inverse method involves specifying the aerodynamic characteristics, and working backwards to obtain the corresponding sail shape to produce those characteristics. The paper investigates a single sail in an upwind condition. Because the solution of the inverse process is not unique, some variables have to be fixed. The sail shape is defined by three parameters: the planform, the camber, and the twist. In the present work, the planform is assumed to be defined by the class-rules of the yacht and is thus known. The sail designer has to specify one of the two possible trims: the twist or the camber. Then the theory, described in the paper, shows that there is a unique solution of the inverse process. Thus two cases are considered. The first involves a fixed twist and planform. There, the code generates the camber of the sail which will produce a given pressure distribution. The second case considers a fixed camber and planform. Here the code trims the sail twist to match the desired pressure distribution. Validation tests have been performed and results are presented. To validate the current approach, the pressure map was first computed from a specified shape. Then the resulting pressure distribution was employed as an input to the inverse method. The shape of the sail obtained with the inverse method is compared to the shape initially used in the analysis. The agreement is good in both inverse computations.


NOMENCLATURE i


a c


C i


U∞ x z


α


Δ Cp γ


η θ


1.


and the deformation of the cloth has to be found. The resulting solution corresponds to the flying shape.


Fourier coefficients (-) Chord length (m)


Polynomial coefficients (m 1-i) Free-stream velocity (m s-1) Chordwise coordinate (m)


Camber coordinate (m) Angle of attack (rad)


Differential pressure coefficient (-) Bound vorticity (m s-1)


Auxiliary coordinate, x-direction (m) Angular coordinate (rad)


INTRODUCTION


Over the last decade, a scientific approach has become more and more important in the sailing world. During the 2007 America’s Cup, big improvements were made to the sail aerodynamics of the IACC boats. Thanks to new powerful


computational tools, engineers simulated-


optimised-improved the design of their boats. Time and money were saved on experimental and full-scale testing. This paper focuses on the aerodynamics of sails and proposes a new approach for the sail design process. In the current sail design process, the designer uses CFD codes coupled to finite element analysis to predict the flying shape under a given wind condition [1,2,3,4]. The elasticity of the sailcloth causes deformation of the shape and thus equilibrium between the aerodynamic forces


©2010: The Royal Institution of Naval Architects


The aerodynamics of sails can be divided into two areas: upwind and downwind. The aerodynamics in upwind sailing appears a little less complicated. The flow over the sails stays attached over major portions of the sails and potential flow theory can be applied. The downwind case is much more complicated because it involves flow separation and high flow curvature. The full Navier- Stokes Equations have to be solved for this situation. Results from these codes give vital information to the designer, such as the aerodynamic coefficients of the sails, the “look” of the flying shape, the non-presence of wrinkles, the loads in the cloth, etc. An important check is to ensure that the sails generate


the required


aerodynamic characteristics as specified by the architect of the boat (e.g. rolling moment for a given thrust).


In the sail-making industry, the use of a Vortex Lattice Method Code is common practice in order to obtain the forces generated by a sail in upwind conditions. More sophisticated CFD codes (RANS, LES) are employed for downwind conditions.


The latter codes are time


consuming and expensive, and they are mainly used for campaigns like the America’s Cup, the Volvo Ocean Race, the Vendée Globe, etc, which have the required large budgets.


At the present time, sails are designed using a so-called “analysis” approach. Here the shapes of n different sails are specified and then the flow around them is simulated to yield the pressure distributions over the n sails and the


B-107


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  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66