A Velocity Prediction Procedure for Sailing Yachts with a hydrodynamic Model based on integrated fully coupled RANSE-Free-Surface Simulations

Abstract

One of the most important tools in today's sailing yacht design is the Velocity Prediction Program (VPP). VPPs calculate boat speed from the equilibrium of aero- and hydrodynamic flow forces. Consequently their accuracy is linked to the accuracy of the aero- and hydrodynamic data used to represent a yacht. These data are usually derived from Experimental Fluid Dynamics (EFD) or Computational Fluid Dynamics (CFD) results and processed by means of linearisation and interpolation to represent the actual sailing state of the yacht. This interpolation is a potential source of inaccuracy. Furthermore, viscosity related effects are often estimated by simplified theoretical or empirical models potentially neglecting complex physical phenomena. As an alternative, a new method is proposed and implemented, which links the hydrodynamic model directly to a RANSE flow simulation in order to avoid linearisation and interpolation errors associated with traditional VPPs. Within this method, aerodynamic sail forces are taken into account as functions of actual boat state. They are modeled using a sail force model and coupled to the hydrodynamic model via a rigid body motion solver. This way one is able to directly evaluate the hydrodynamic forces at the actual boat state of the yacht for a given wind condition. Within this work, the inherent theory and algorithms of the method are described, and the hydrodynamic model is verified and validated against experimental data from towing tank test. Based on this verification and validation of the simulation model, the functionality of the new method is illustrated by calculating VPP polars. Finally, these VPP polars are compared with results from conventional VPPs and an assessment of the method's accuracy is provided.