Wind tunnel investigation of dynamic trimming on upwind sail aerodynamics

Abstract

An experiment was developed at the Yacht research Unit’s Twisted Flow Wind Tunnel (University of Auckland) to test the effect of dynamic trimming on three 60 IMOCA inspired main sails models in upwind configuration. This study presents dynamic fluid structure interaction results in well controlled conditions (wind, sheet length) with a dynamic trimming system. First the optimum optimization target CFobj coefficient with a steady trim for AWA = 60 degrees using the car traveler position and main sail sheet length is located. Oscillation are then done around this optimum value using the main sheet length Lsheet oscillation. Different oscillation amplitudes and frequencies of trimming are investigated. Measurements are done with a 6 components force balance and a load sensor giving access to the unsteady main sail sheet load. The driving CFx and optimization target CFobj coefficient first decrease at low reduced frequency fr for quasi-steady state then increase, becoming higher than the steady state situation. The driving force CFx and the optimization target coefficient CFobj show an optimum for the three different design sail shapes located at fr = 0:255. This optimum is linked to the power transmitted to the rig and sail system by the trimming device. The effect of the camber of the design shape is investigated too. The flat mainsail design benefits more than the other mainsail designs from the dynamic trimming compared to their respective steady situation. This study presents dynamic results that cannot be accurately predicted with a steady approach. These results are therefore valuable for future FSI numerical tools validation in unsteady conditions.