Strongly Coupled Performance Prediction for Sailing Yachts based on CFD

Licentiatavhandling, 2016

Quantifying performance of sailing yachts can be done in many different ways. Historically the most common way is a static approach where a steady state solution is obtained by solving equilibrium equations in 3 or 4 degrees of freedom. More recently, dynamic methods have appeared. In this thesis a method of the latter kind in proposed. The possibility of accurately quantifying performance makes a tool suitable for evaluating changes in design or sailing technique. Motivation for developing such a tool would be the ability to produce a good yacht directly from the drawing board or finding optimum placement of center of gravity. The most challenging aspect of the yacht performance problem is the strong coupling of resultant forces to attitude and speed. As these eects are highly non-linear an accurate method to compute the acting forces is required. In the present project the instantaneous hydrodynamic forces are obtained using Computational Fluid Dynamics, CFD, and the  flow solution is integrated in time together with the rigid body motions. The aerodynamic forces are still modelled, which makes the computation faster and is an advantage if the interest is in optimizing the hull. The emphasis of this study is to report the procedures and setups used in order to make the method as accessible as possible. The outcome is the predicted velocity and attitude of a dinghy and a keel-boat for a different wind speeds and directions. The emphasis in terms of results is on finding maximum VMG upwind and downwind. Predicting performance in this way inherently gives the attitude, linear and rotational speeds and complete  flow field around the yacht. Using these secondary results an investigation of assumptions within classical sailing mechanics is performed. The investigated topics are: center of lateral resistance, sideforce distribution and keel downwash.