Sailing yacht performance prediction based on coupled CFD and rigid body dynamics in 6 degrees of freedom

Artikel i vetenskaplig tidskrift, 2017

Traditional Velocity Prediction Programs (VPPs) for sailing yachts rely on empirical relations, while modern techniques include CFD. Normally a multi-dimensional matrix of CFD results is pre-computed and forces and moments obtained by interpolation during a VPP execution. A more exact technique is to use the CFD code itself as a VPP, i.e. to simultaneously integrate the flow and rigid body equations in time. In the present paper a technique of this coupled type is introduced. An initial guess is required for speed and attitude, and by time integration in six degrees of freedom this guess is corrected to a steady state solution. Three phases are identified. First the attitude is kept constant over a period of time to stabilize the flow. Thereafter the different degrees of freedom are released successively and the rudder activated to achieve yaw balance. Finally, in the converged phase, the net forces and moments are zero in all degrees of freedom and the results reported. The performance prediction is carried out for a dinghy and a large high-performance racing yacht. Convergence of all variables is demonstrated and the results used for finding the optimum speed to windward and downwind. Details of the numerical setup are reported.