The leading-edge vortex of yacht sails

It has been suggested that a stable Leading Edge Vortex (LEV) can be formed from the sharp leading edge of asymmetric spinnakers. If the LEV remains stably attached to the leading edge, it provides an increase in the thrust force. Until now, however, the existence of a stable and attached LEV has only been shown by numerical simulations. In the present work we experimentally verify, for the first time, that a stable LEV can be formed on an asymmetric spinnaker. We tested a 3D printed rigid sail in a water flume at a chord-based Reynolds number of ca. 104. The sail was tested in isolation (no hull and rigging) at an incidence with the flow equivalent to an apparent wind angle of 55◦ and a heel angle of 10◦. The flow field was measured with particle image velocimetry over horizontal cross sections. We found that on the leeward side of the sail, the flow separates at the leading edge reattaching further downstream and forming a stable LEV. The LEV grows in diameter from the root to the tip of the sail, where it merges with the tip vortex. We detected the LEV using the γ1 and γ2 criteria, and we verified its stability over time. The lift contribution provided by the LEV was computed solving a complex potential model of each sail section. This analysis showed that the LEV provides more than 10% of the total sail’s lift. These findings suggests that the performance of asymmetric spinnakers could be significantly enhanced by promoting a stable LEV.