By DE Editors
Anthony Massobrio ]AM]. Why this choice of a rigid wing on your trimaran, instead of a conventional sail? Isn’t this a radically new and risky choice?
Mario Caponnetto ]MC]. Rigid wings are not really radically new in yacht racing. They have been used for many years in high-performance catamaran races and by others racing boats. By the way, a rigid wing first appeared in America’s Cup in 1987. What is radically new is its size. The wing, with its 57 m above deck, is the largest wing ever. It is 80% larger than a 747 aircraft wing. No one on our team had designed anything like this before, and this scared us a little bit at the beginning. Starting from the white paper and evaluating pros and cons, we decided to quickly move forward in the project. This project came true thanks to the enthusiasm of our chief designer, Mike Drummond.
]AM] What are the benefits and the shortcomings (if any) of a rigid wing, with respect to a conventional sail?
]MC] The main advantage of a rigid wing is shape control. In other words, depending on the angle and the velocity of the wind, there is an optimal sail geometry, optimizing the aerodynamic pressure field. This makes it possible to extract maximum propelling power from the wind, or in other terms to maximize efficiency. On a conventional sail, material works from the structural point of view like a membrane, and shape control is difficult. Some specific shapes are impossible to obtain, and the final shape is a compromise. With a rigid sail, shape is much easier to control without compromises. Furthermore, during navigation, there is always feedback between imposed shape and achieved shape, whereas with traditional sails, it is already an issue to identify the sail shape during navigation.
]AM] I guess the rigid wing benefits have its downside in terms of weight?
]MC] Not quite so. A conventional sail supports only traction loads and not bending loads. The wing having a thickness makes it possible to distribute loads on the two sides of the structure, which results in its being very light.
To sum up, the rigid wing weight is comparable to a conventional mast/sail system. With a one-dimensional analogy, we should think of a sail as a rope supporting a weight (the wind pressure) at its center. If one wishes to reduce its sag, tension will increase. Therefore, its thickness and weight should be increased to avoid a breakdown. If we replace the rope with a cantilever, the weight of the structure will be smaller, given the same displacement. Understand that huge forces are required to put tension on a conventional sail to the point of stressing the boat structure itself. In comparison, a finger is enough to control the rigid wing.
]AM] What are the aerodynamic benefits of the rigid wing?
Once again, one of the main benefits is shape control, aiming to control lift forces and to reduce drag forces. To do so, the wing is made of a front rotating element and eight independently rotating flaps. This makes is possible to change the vertical aerodynamic load. Between every flap and the frontal element lies a slot that favors air flow between the two sides of the wing. This makes it possible to delay the stall and to dramatically increase the maximum lift. In practice, the wing is able—even with light wind—to lift the central hull of the trimaran out of the water and reduce its resistance, even though the wing lateral surface is less than half that of a conventional sail.
The wing’s horizontal sections are more aerodynamically shaped than a thin sail. A sail profile is efficient at a certain angle of attack, more or less, when the flow is tangential to the frontal edge of the sail. At smaller or larger angles, a flow tends to separate from the sail, thus reducing its efficiency. The rigid wing, with its rounded front edge, is much more tolerant to variations in the angle of attack. Even at a small angle of attack, the wing will still create lift and push the boat, whereas the sail will beat like a flag and restrain the boat. This is a noticeable advantage during maneuvering—in particular when tacking—and is one of the benefits that is most valued by our team’s sailors.
]AM] How did you develop the wing project?
]MC] It was developed during a very few months, in house. The project was headed by Joseph Ozanne, who linked aerodynamic, structural, electronic, and shipyard engineers. The entire aerodynamic project has been based on numerical simulations without ]the use of] a wind tunnel.
CFD work has been carried on by Francis Hueber and me. In a very short time, the optimization work on the wing profile has been carried out with the STAR-CCM+ CFD code by our partner CD-adapco, exploiting a remote supercomputing cluster.
For us, it was very important that the CFD code was able to give indications of the wing’s behavior as far as stall is concerned. That behavior was later validated during sea trials. Furthermore, we created a database of optimal wing shapes, based on all the possible wind situations. The database is installed onboard and allows us to optimize wing efficiency at any moment.
What really impressed us during the very first trials was better wing performance with respect to conventional sails. Therefore, at the end of the testing phase at our San Diego base, it was decided to use the wing for the next America’s Cup matches. This shows the value of the project we carried out.
]AM] Could you please give us more details on the aerodynamics simulation aspects?
Mario Caponnetto is currently responsible for CFD at BMW ORACLE Racing. He graduated from University of Genova in Naval Architecture and Marine Engineering, and he has worked with industrial CFD since 1989. He is the co-founder of the CFD consultancies company Caponnetto-Hueber.
]MC] STAR-CCM+ is a finite-volume approach to CFD. This is really nothing new at all. Its theory can be found in textbooks. What interested us was the practical implementation.
First of all, we exploited the “client-server” architecture of the CD-adapco software. We could use a remote supercomputing cluster facility located in Italy. While sitting in our offices in Valencia or San Diego, we could check in real time the progress of the simulations running on the cluster. This happened thanks to a lightweight client—or if you like, the final user—based on a Java interface and a C++ server—or if you like, the supercomputing cluster.
Second, of course, use of the supercomputing cluster leveraged the STAR-CCM+ capability to scale well (i.e., to exploit the capability to divide the processing tasks between several processors in parallel). This was necessary because computational meshes for aerodynamics can reach several million elements.
The third success factor was process automation. STAR-CCM+ includes a CFD simulation engine (the solver), but also all the preprocessing (including construction of the computational mesh) and post-processing phases. This means we could build one complete workflow, or pipeline, and implement it over and over again during our optimization studies.
]AM] So CFD is a tool for the happy few?
]MC] Situations like America’s Cup or Formula 1 require tremendous accuracy and detail because the engineering situation is pushed to the limit and the optimization requirements for quantities like aerodynamic drag can be orders of magnitude more sensitive than in mass production boats or cars.
I think that A.C. will continue to be one of the best benchmarks for CFD tools that can, in industrial situations, be applied in standard design offices, based on small clusters or even PCs.
Nowadays, all CFD processes should be automated in industrial situations, whereas A.C. pushes the application of the code to its limits, in terms of physics, computational mesh, and hardware resources. This creates a feedback process between the STAR-CCM+ developer, CD-adapco, and CFD teams in America’s Cup or Formula 1, and the feedback has a positive fall on other sectors.
For instance, we evaluated several models representing turbulence, from the standard k-e to k-w SST to almost direct simulation via LES, whereas in repetitive industrial automotive or marine simulations just k-e or k-w will be adopted as daily model.
]AM] Could you disclose to the public some tips and tricks you implemented in your CFD activity?
]MC] What I can disclose is that we used the STAR-CCM+ technology for automatic meshing. Both arbitrary isotropic polyhedral and Cartesian ]oriented] trimmed cells are usable. There is no absolute rule on using the former or the latter. Polyhedra may be preferable to capture vortex phenomena, whereas the Cartesian grid underlying trimmed cells may be preferable when a preferred flow direction is present. In both cases, a special treatment is used for boundary layer phenomena.
]AM] Coming back to sea trials—what were the changes for your sailors?
]MC] Several changes. It goes without saying that America’s Cup sailors are among the best, especially when talking about trimmers. We talk about people who developed in a lifetime the sensitivity, based on talent and experience, on how to make sails “breath.” Then, engineers (all of them yachtsmen but amateurs) asking yachtsmen to follow our graphs and tables, so contrary to intuition… it was not easy at the beginning, but sailors, after testing out our idea in practice, became its strongest supporters. Because they were asking designers why one wing shape was better than another, CFD visualization capabilities were really useful to support the engineers’ explanations to sailors. I think that in a high-tech sports activity, it is important to find a common language between engineers and “pilots,” and in that sense, CFD has been a very good communication tool.
]AM] What is your America’s Cup forecast?
]MC] It is difficult to say. Anything could happen due to meteorological conditions. Also, boats are quite different from each other.
Our competitors did a good job with the advantage of designing their boat around rules they made themselves after seeing our boat. For instance, they decided an engine could replace arms’ force and allowed movable ballast.
We tracked the new rules and adapted our boat accordingly. Fortunately, there is still not a lot of time to wait. The America’s Cup match will take place in Valencia on February 2010.
]AM] Mr. Caponnetto, we wish you and your team good luck. Thank you for the interview.
BMW ORACLE Racing