Integrating RANS into a design process – America’s Cup sails… part 1

In many situations choosing the best way to integrate CFD into an existing design process can have as much influence on the effectiveness of the results as the technical aspects of the CFD solution itself. As we saw in issue two, with the America’s Cup bulb, CFD can be a great tool in optimising a design at its inception but that might not be the best way to get value out of CFD in your design process. Over the next two editions FLUID is going to look at two different ways we integrated aerodynamic RANS analysis into the design process of America’s Cup sails.

Sails represent an entirely different design problem to a hull or appendage. The sail trimmers have numerous tools at their disposal to change the flying shape of the sail once it is built and once the limit of these is reached it is often an overnight job to have the sail re-cut to change its shape. In this case we utilised the capability of RANS differently. Our sail analysis program took on two distinct paths, conventional analysis of prospective shapes and in depth analysis of current shapes concurrent with testing.

In the conventional analysis we compared design shapes. The advantage of computational methods in this case was we could ascertain which feature of a design change was causing the change in performance. As an example, two mainsails with quite different roach profiles were compared. One, our current setup, the other, a shape favored by a competitor.

In this case initial analysis showed a significant improvement in light air performance with the second profile. Further analysis in the computer, however, showed that the gain was due to a difference in sail depth caused by this change not the profile itself.

With this knowledge we were able to re-cut an existing main to add depth in the upper part of the sail and capture the performance gain without major changes to our roach profile. By isolating the particular change that had caused the improvement in CFD we saved ourselves the cost of a new sail and the downstream issues that can be caused by a change of roach on battens, structure and mast setup.