The leading edge vortex of yacht sails

A 1:115th model-scale spinnaker sail of a modern America’s Cup yacht was tested in the University of Edinburgh water flume. Experiments were carried at a Reynolds number of 1.7 x 104. The rigid model of the sail was 3D printed in ABS and tested at an angle of attack equivalent to an apparent wind angle condition of 55o [1]. The velocity fields on four cross sections of the sail’s span were measured with Particle Image Velocimetry (PIV). We found that the flow separates at the leading edge followed by turbulent reattachment, forming a leading edge vortex (LEV). The LEV is stably attached to the leading edge and its diameter grows from the foot to the tip of the sail. On the lower half of the sail, the LEV has a negligible diameter while large trailing edge separation occurs from a quarter of the chord. At ¾ of the span, the diameter of the LEV is a quarter of the chord and trailing edge separation does not occur. The existence of a stable LEV on yacht sails was predicted numerically on the same geometry at Reynolds number 6 x 105 using Detached Eddy Simulation [1]. To the knowledge of the authors, this is the first time that it is observed experimentally. Ongoing and future works include the derivation of the pressure field from the PIV measurements [4], the analysis of the LEV with the gamma-2 criteria [3], and the development of a low-order model based on potential flow of the measured flow field [5].