This February, in the picturesque Spanish port of Valencia, BMW ORACLE Racing’s skipper Russell Coutts will take the helm of one of the most technologically advanced – and hopefully fastest – boats ever built, in a bid to capture the 33rd America’s Cup. The most remarkable feature of the trimaran – named “USA” – is that it will be powered by an enormous wing, rather than a conventional sail.
As Mike Drummond, BMW Oracle’s Racing Design Director explains: “A wing of this scale has never been built for a boat. In terms of size, it dwarfs those on modern aircraft. Towering nearly 190 ft (57 m) above the deck, it is 80% bigger than a wing on a 747 airplane.” It was analyzed using a process very similar to CFD, using CD-Adapco’s Star-CCM analysis tools.
BMW ORACLE Racing USA Trimaran.
In an interview conducted by CD-adapco, Oracle’s CFD Manager Mario Caponnetto explains how STAR-CCM+ was used to optimize the aerodynamic design of the wing, at the expense of traditional wind tunnel testing.
Why this choice of a rigid wing on your trimaran, instead of a conventional sail?
[MC] Rigid wings are not really radically new in yacht racing. They have been used in high performance catamaran races and other racing boats for many years. 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 meters above deck, is the largest wing ever, 80% larger than a 747 aircraft wing. No one in our team had designed anything like this before, and this scared us a little bit at the beginning. Starting from white paper and evaluating pros and cons, we decided to move forward and quickly in the project. This project came true thanks to the enthusiasm of our chief designer, Mike Drummond.
What are the benefits and the shortcomings (if any) of a rigid wing?
[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 that in turn optimizes the aerodynamic pressure field. This makes it possible to extract a maximum propelling power from the wind – 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 rigid sails, shape is much easier to control without compromises. Furthermore, during navigation there is always a feedback between imposed shape and achieved shape, whereas with traditional sails it is already an issue to identify the sail shape during navigation.
What are the aerodynamic benefits of the rigid wing?
[MC] 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 of a conventional sail. The wing 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 benefits that are most valued by our team’s sailors.
Could you share more details on the aerodynamics simulation aspects?
[MC] STAR-CCM+ is a finite-volume approach to CFD. (It allowed us to) exploit 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, usage 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 since 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 phase (including construction of the computational mesh) and post-processing. This means we could build one complete workflow, or pipeline, and implement it over and over again during our optimization studies.
So, CFD is a tool for the happy few?
[MC] Situations like America’s Cup (AC) or Formula 1 require a tremendous accuracy and detail since the engineering situation is pushed to the limit, and the optimization requirements for factors like aerodynamic drag can be orders of magnitude more sensitive than in mass production boats or cars. I think that AC 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 AC pushes the application of the code to its limits in terms of physics, computational mesh or 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 (benefit) on other sectors.
Filed Under: Simulation, Software