Aerodynamics of Sailing Yachts : Force, Pressure and sail shape measurements as bridge between experiments and numerical simulations

Sammanfattning: The present work reports the first systematic results obtained in wind tunnel and at full scale with theexperimental apparatus developed within the joint project among Politecnico di Milano, North Sails andCSEM. The steady state upwind aerodynamics of sailing yachts are investigated through the contemporarymeasurement of global forces, distributed pressures and sail flying shapes, and with numerical simulationsbased on Potential Theory and Navier-Stokes equations.The wind tunnel of Politecnico di Milano and the Sailing Yacht Lab (SYL), used for full scale investigations,are described together with the measurement systems adopted for recording forces, pressures and sailshapes. The aerodynamic loads are obtained through dedicated arrangements of load cells; the pressuredistributions on sail sections are evaluated with integrated systems of customized local measurementsolutions and MEMS sensors; the sail flying shapes are detected thanks to two laser scanners based of theTime of Flight technology. The experimental procedures adopted during tests are presented and discussedin relation with the aim of the work.Numerical simulations of selected wind tunnel cases are performed in order to assess the capabilities of theempirical techniques to validate Computational Fluid Dynamics (CFD) codes and to propose solid numericalset-ups for investigating sailing yacht aerodynamics. A Vortex Lattice Method (VLM) code, written inMatlab, is used for quick preliminary analyses of the global forces developed by the sail plan, whereas theopen source environment OpenFOAM is adopted to perform 3D simulations for investigating in details thelocal flow patterns.The experimental apparatus, both at model and full scale, proved to be extremely well suited for thepurposes of the study, giving remarkable results regarding, in particular, pressures and sail shapes. Theexpected distributions are obtained during wind tunnel tests and interesting considerations arise from thecomparison with the full scale outcomes. Numerically, the results of simulations meet the preliminaryintuitions with the VLM code capable of accurately predicting global forces up to certain wind angles andthe RANS-based computations providing notable agreement with the measured local pressures.