Enhanced Wind Tunnel and Full-Scale Sail Force Comparison

Abstract

This paper presents a comparison between wind tunnel and full-scale aerodynamic sail force measurements using enhanced wind tunnel testing techniques to model the full-scale sailing conditions more accurately. The first comparison was conducted by Hansen et al., 2003a, in the Twisted Flow Wind Tunnel (TFWT) at The University of Auckland and followed standard testing procedures. Since then enhancements have been made and two aspects not considered in the original comparison are highlighted here. The interaction of the hull and sail forces is now considered and trim changes of the sails due to wind strength are included. For the enhanced comparison the interaction between the hull/deck and the sails is investigated by installing a secondary force balance inside the model to measure the hull/deck forces and by pressure tapping the hull/deck to determine the surface pressure distribution. It is found that the presence of the sails significantly affects the forces on hull/deck when sailing upwind, which should be accounted for consistently in comparisons of full-scale, wind tunnel, and computational fluid dynamics (CFD) data. In the original comparison the sails were trimmed in the wind tunnel to the aerodynamically optimal shape by maximizing the drive force. Trim variations due to wind strength were however noted in full-scale data so that depowering is considered in the enhanced comparison. The sails in the wind tunnel were trimmed based on the fullscale wind strength and the yacht performance by employing a Real-Time Velocity Prediction Program (VPP) to achieve realistically depowered sail shapes. Utilising the enhanced wind tunnel techniques a generally good qualitative and quantitative agreement with the full-scale data was achieved, but a conclusive judgment of the accuracy of the comparison cannot be made.