By H. J. Richter and K. C. Horrigan, Thayer School of Engineering, Dartmouth College, Hanover, NH; J.B. Braun, North Sails Performance Resource Group, Marblehead, MA; and K. H. Kuehlert, Fluent Inc.

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Two Stars and Stripes IACC yachts under sail

Yacht racing, in particular the America's Cup, has always been at the forefront of sailing research and development, with the single goal of making boats go faster. The research, design, and building of an International America's Cup Class (IACC) yacht is extremely expensive, so participants and suppliers are continually looking for inexpensive yet accurate ways to reduce R & D costs and expedite results. In a collaboration between North Sails' Performance Resource Group and Dartmouth College's Thayer School of Engineering, a "Virtual Wind Tunnel" (VWT) has been developed to meet this need. The VWT enables North Sails engineers to do performance evaluations of full-scale sails and sail plans on the computer and study the fluid-structure interaction between the wind and sails. Initial emphasis has been put on downwind sails that use relatively lightweight, stretchy materials compared to modern, relatively rigid upwind sails.

The VWT is composed of three codes: MemBrain, North Sails' proprietary software for the structural analysis of sails, masts, and rigging, GAMBIT, and FLUENT. These three components are linked together in an iterative process, automated through the use of GAMBIT templates. A VWT analysis begins with an assumed sail shape and position (trim) and surface pressure distribution for a given set of wind and boat velocities. MemBrain uses these initial conditions to compute deformations in the sail geometry by balancing external aerodynamic pressure loads with internal stresses, which are governed by the characteristics of the sail material. Once a new sail shape has been determined, the new sail geometry is transferred from MemBrain into an IGES file. GAMBIT automatically reads the IGES file and generates a mesh. FLUENT is then started by template commands, and a journal file instructs the code to read the mesh, set boundary conditions, and launch the calculation. The flow field and pressure distribution are computed for the deformed geometry, and upon convergence, the new pressure distribution is exported into a file and used to update the sail shape in MemBrain once again. An iterative process coupling GAMBIT, FLUENT, and MemBrain ensues until the sail shape reaches static equilibrium, i.e. when the maximum displacement between pressure updates is less than a preset value. GAMBIT templates allow the entire process to be run with no intervention by the user.

The initial and final sail shape and trim before and after the optimization process

After static equilibrium has been reached, the sail forces and moments are evaluated to see if re-trimming, or repositioning the sails on the boat is needed in order to optimize the sail's performance, i.e. obtain the maximum driving force or the maximum drive over heeling force of the boat under the given wind conditions.

The VWT has many advantages over traditional wind-tunnel testing methods. First, since VWT tests are performed on the computer, they are done at full scale, so the problems encountered when using scaling laws in real wind tunnels are avoided. Second, the computational flow domain around the sails can be very large, and since no wind tunnel walls are present, edge effects are non-existent. Finally, since the boundary layer above the water is computed in the upstream computational domain, a more accurate description of the angle of attack at the site of the vessel as a function of rig height is incorporated into the simulation.

In addition to optimizing sail performance, this promising technology will be used in the future for instrument calibration and predictions of bad air zones, where sailboats in the wake of nearby sailboats experience greatly diminished and changeable winds.

Picture of an optimized sail showing pathlines
and the pressure distribution on the sails (left) and The fiber layout in an asymmetrical spinnaker; changes to the fiber layout cause changes to the stretching characteristics, and thus the sail's response to external forces (right)

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