Improving the Design of Sails Using Cfd and Optimization Algorithms

Two current Stanford Yacht Research projects are presented, both related to the analysis and design of upwind sails, and part of a larger research effort to develop efficient and robust sail and hull shape optimisation methods. First, we discuss the in-house development of a methodology to predict and improve the flying shape and forces on America's Cup sails. We adapted and parallelized a 3D unstructured incompressible Euler solver developed by Jameson and colleagues. The computed pressure loading is transferred to the structural package NASTRAN, which computes the deflected shape of the sail. The mesh is displaced accordingly, and a new pressure loading is computed. This process is repeated until convergence. The computed lift and induced drag for a main sail with elliptic planform compares well with results from lifting surface theories. The flow solver is more efficient than commercial solvers and highly suitable for the computationally intense viscous sail flow calculations. Second, we present a sail shape optimisation method, which combines the commercial CFD package FLUENT with gradient-based cost function minimisation. Results are presented for the optimisation of sheeting angles for the rig of a three masted clipper yacht. We investigate two cost functions, both characteristic of a sail’s aerodynamic performance: the reciprocals of the driving force coefficient and the ratio of driving to heeling force coefficients. Comparing results for upwind and close reaching apparent wind angles shows that the latter leads to well trimmed sails, whereas the former causes the sails to be over trimmed, which is as expected. 1 Doctoral Candidate, Department of Aeronautics and Astronautics, Stanford University 2 Doctoral Candidate, Department of Mechanical Engineering, Stanford University 3 Assistant Professor, Stanford Yacht Research (http://syr.stanford.edu ), Stanford University 4 Research Associate, Center for Turbulence Research, Stanford University 5 Thomas V. Jones Professor of Engineering, Department of Aeronautics and Astronautics, Stanford University