Aerodynamic study of a two-elements wingsail for high performance multihull yachts

Abstract

This paper is devoted to the numerical study of a 1:20th model-scale wingsail typical of America’s Cup yachts like AC72, AC62, AC45 or any C class catamaran to gain insight in its complex aerodynamic behavior and to prepare a wind-tunnel campain. This rigging has still not been much studied and needs more knowledge. This study is based on CFD simulations of the flow around the wingsail by resolving Navier-Stokes equations. Two modeling issues are investigated: the Unsteady Reynolds Average Navier-Stokes (URANS) and the Large Eddy Simulation (LES). These numerical approaches are used to characterize the wingsail aerodynamic behavior and variations with some key design and trim parameters (camber, slot width, angle of attack, flap thickness). Unsteady modeling are used to characterize the stall behavior and improve our understanding of the flow physics that may occur in such configurations. The analysis of the results shows transition phenomena due to a laminar separation bubble and strong interaction of boundary layers of main and flap in the slot region. LES results give flow physics understanding and qualitative elements of validation of the URANS simulations. Both URANS and LES results emphasized the central role of the slot geometry and its associated leakage flow in the onset of stall. Some key parameters of the wingsail are identified. The stall behavior for low and high camber (through flap deflection) is characterized showing differences in both configurations. These differences are related to the leakage flow in the slot where a non-linear coupling between flap deflection, slot width and flap thickness takes place. These results illustrate the complex aerodynamic of multi-element wingsail and that a better knowledge of its behavior is necessary to open roads for better design and enhanced performances for future multihull yachts and foilers.