Sail A
Within a remarkably short period most bigger modern performance sails became complex composite structures with intricate fibre patterns laid out for shape retention and durability. Here (left) the fibre patterns of the Imoca 60 PRB’s DFi mainsail and Code 0 are shown (along with the boat’s rigging). Heavy reinforcement at the bottom batten reflects the abuse to which such fast singlehanded boats are inevitably subjected. The Code 0 ‘Sail A’ (right) referred to in the text below – before (green) and after a virtual ease (red)
meant to do: design sails! This ties into the biggest obstacle of CFD-based fluid/ structure interaction which is cost. Unlike panel codes which run in seconds on a laptop, CFD solvers require large comput- ing clusters to obtain a solution in a few hours. Buying a cluster and maintaining it doesn’t come cheaply. Commercial CFD software is also very
expensive, costing tens of thousands of dollars for licences, while open source soft- ware requires even greater expertise to use. Add in the cost of engineers to run it all and it is easy to see why the majority of sailmakers have shied away from integrat- ing CFD into their design process. To make CFD-based FSI more wide-
spread the entry barrier must be made lower. To this end, K-Epsilon teamed up with BSG to develop an automated web- based service to allow SailPack users to run CFD on their designs on an as-needed basis using K-FSI-RANS, the CFD-based FSI code that couples K-Struct to the com- mercial CFD software FINE/Marine. This solution dramatically reduces the
investment required and eliminates the need for the designer to spend time performing CFD themselves. By automating the mesh- ing, running and post-processing, large studies become feasible at a reasonable cost. With an automated FSI process in place,
a designer can obtain the sail’s flying shape for a given sail set-up, but the shape they really want is the flying shape that max- imises performance. The flying shape is obviously dependent on the sheet length and angle and, while the sail designer will probably have an idea of the correct sheet length based on experience, it may not be the exact optimum. To home in on the opti- mum sail trim K-Epsilon also developed an automated trimming routine to ease or trim
62 SEAHORSE
the sheets during the computation to find the optimum flying shape. By automating the trimming we avoid manually finding best trim through iterative trial and error.
Case study In France Incidence Sails are using all of these tools to create sails that employ their proprietary DFi manufacturing solution. These sails consist of a very lightweight pre-preg in place of the fibres and PET film that are traditionally used. The result is a membrane where the ratio of fibres to sail weight is particularly high. However, lightness is not enough and
these fibres must be precisely placed to maximise effectiveness while minimising weight. Seamless integration between the design, analysis and production of the sails is critical to ensure that what is designed is what is analysed and then built. The flying shape of the sail therefore includes two main elements: l The initial aerodynamic shape l The structure of the membrane, ie the orientation and number of the fibres Numerical calculation is now an essen-
tial element in modern sail design. Indeed, a high precision in the prediction of the flying shape of the sail allows us to make more efficient sails, and to better predict the range of use for those sails. In addition to improving the boat’s intrinsic perfor- mance, this avoids the need for several prototypes whose cost is very high given the technology now being used. The numerical calculation performed
here has therefore made it possible to adjust the design and structure of the sails in the range where they are intended to be efficient. With the calculation typically used previously (FSI with panel code or ‘manual’ coupling RANS/structure) we
would of course have improved the initial design of the sails, but with far less preci- sion and so the resulting sails would prob- ably have ultimately been less efficient.
Results The candidate Code 0 (‘Sail A’, above) was provided trimmed to a prescribed baseline. A K-FSI-RANS computation was then launched on the sails and rig and a flying shape obtained without modifying the sheet length. Let us call this the trim 0m condition. The trimming routine was then activated
for the Code 0 sheet and the K-FSI-RANS computation continued with the sheet length being moved. After varying a bit the Code 0 sheet length settles eased out +0.80m. The difference in flying shape is apparent
while driving force has increased dramati- cally by 37%. The eased Code 0 is much fuller along its entire height giving the sail a greater camber and hence increased lift. Furthermore, the surface of the sail is
rotated forward so the resulting new lift pro- duces driving force. Without an automated trimming routine the sail designer would be forced to manually adjust and rerun the computation, which quickly becomes a laborious process.
Conclusions The integration of numerical analysis of the sail’s behaviour into the sail design process is critical to maximising sail performance. The use of an integrated, on-demand web- service for such analysis promises to dramatically increase the prevalence of CFD in sail design and push more designers to make even better sails. David Gross, naval architect and CFD engineer, K-Epsilon. Thanks also to Jules Poncin, Yann Roux, Pierre-Antoine Morvan and Laurent Guillaume
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