Experimental investigation on the aerodynamics of a windsurfing sail under steady sailing conditions

The sail is the primary source of propulsion in windsurfing, making it crucial to understand its aerodynamic mechanisms for optimal performance. However, due to its unique shape and characteristics, different from widely investigated yacht sails, the aerodynamic features of windsurfing sails have remained largely unexplored. This study aims to investigate the flow characteristics around a steady windsurfing sail under various conditions, including Reynolds numbers, angles of attack, and rig-back. The study presents the aerodynamic force, moment and flow features of a windsurfing sail model. The flow features were observed through wind tunnel tests by means of tuft flow visualisations, oil flow visualisations, and wake pressure scanning. The study reveals a flow separation bubble downstream of the leading edge on the suction sail surface. In addition, another separation bubble was found on the pressure surface at the concave region between the luff and the sail panel. At a lower rig-back angle, a larger wake total pressure deficit region is created by the sail foot vortex. At the studied angle of attack alpha(A) = 3 degrees to 23 degrees, the major sources of pressure drag in windsurfing sails are the sail tip flow separation and vortex, flow separation from mid-span, and the sail foot vortex. The study finds that the sail foot vortex is the primary drag source under most angles of attack and rig-back. This information can be used to improve the design and optimisation of windsurfing sails.