CD-adapco propels Luna Rossa into the lead

Anthony Massobrio talks to Mario Caponnetto of Team Luna Rossa

When Mario Caponnetto compares Americas Cup Racing to another CFD intensive sport, he does so with certain a glint in his eye. “In many ways, it’s very like Formula 1”, says Luna Rossa’s CFD Team Leader, “We have to contend with the same ontradiction. The sponsors and spectators want close races, but as designers, we want to win by any means – and if possible by a large margin”.

While Caponnetto’s comparison is certainly valid – Formula 1 and Americas Cup yacht racing both rely heavily on numerical flow simulation – it betrays a degree of modesty. Whereas Formula 1 CFD engineers need only concern themselves with a single medium (air), Caponnetto’s team (Francis Huber and CD-adapco Thesis prize winner Evan Spong) have to account simultaneously for the influence of both air and water as well as the interaction between them.

From a designer’s point of view, the 32nd edition of the Americas Cup is the toughest ever. In an attempt to reduce winning margins to just a few seconds, organizers have fixed the length, weight and sail area of competing boats. By fixing the critical dimensions of the boat, designers are essentially limited to adding extra appendages and modifying the shape of the hull.

This means that each design team is working to a very similar specification. Although original “ground-breaking” solutions are still possible, yacht design (like Formula 1) now tends to proceed with small and continuous improvements in every aspect of the boat (dynamics, structures, hydrodynamics and materials). It is within these constraints that CFD demonstrates its true value.

Caponnetto explains: “Without the help of CAE tools such as CFD, such incremental changes would be very difficult to analyse. Using CD-adapco’s CFD tools we can quickly assess the influence of design changes without having to worry about the influence of scaling.”

Scaling is important. The standard tools of Naval Architecture – towing tank and wind tunnel analysis – operate on a scaled model of the actual boat (competition rules restrict models to a maximum of one-third scale). Since different scaling factors are required for similarity of both Reynolds number (relating to aerodynamics) and Froude number (relating to wave dynamics), comparison with the full-scale design is always subject to a degree of uncertainty.

“Testing of sails in a wind tunnel with the correct Reynolds number is particularly difficult”, says Caponnetto. “This makes it very difficult to test the complete vessel in a situation that simulates the true dynamics of the sea. In order to do this, the test would need to take account of the varying intensity of the waves, the direction and force of the wind and of course the route the boat is taking.” Until recently CFD was typically used in as a numerical alternative to model testing in naval design. Hydrodynamic and aerodynamic calculations were performed independently. The hydrodynamic calculations accounted for the flow around hull and appendages aimed to assess the impact of the design on resistance and stability. Aerodynamic simulations concentrated on the sails and mast and were concerned with propelling force and tilting momentum. Although interesting in their own right, these uncoupled calculations provide little direct information on the overall stability of a boat, which relies on a combination of aerodynamic and hydrodynamic influences. Results from the uncoupled calculations were typically combined manually using a VPP (velocity prediction program) that combined the results from testing or analysis and predicted the equilibrium position of the boat. Although this technique has been used to great effect by Luna Rossa and others, it is time consuming to perform and requires a degree of interpolation between conditions.

The Luna Rossa CFD team are busy pioneering a new type of coupled CFD calculations that combine hydrodynamic and aerodynamic calculations in a single model. Using CD-adapco’s unique 6 degreesof- freedom solver, it is possible to accurately calculate the behavior of the boat design in a number of wave conditions and through a full range of maneuvres. The interaction between air and water is accounted for using a free-surface technique that accurately predicts the shape of waves generated at the waterline.

As well as providing the type of repeatable condition that allows the influence of small design changes to be accurately assessed, coupled CFD also allows detailed flow visualization that can help discover hitherto unnoticed flow phenomena that have a strong influence on the boat.

“ It is the future”, says Caponnetto. “CD-adapco’s CFD tools are already making a huge contribution to the excellent performance of the Luna Rossa team.”

The signs are good. After 5 out of 13 acts, Luna Rossa lead the Louis Vuitton Cup that will determine the challenger for the 2007 Americas Cup.

Details correct at time of going to press.