On the Effect of the Leading-Edge Separation Bubble on the Aerodynamics of Spinnakers

The spinnaker is the most powerful and one of the most used sails both in racing and cruising - yet its complex aerodynamics governed by flow separation is still not fully understood. While the flow around a spinnaker is unsteady and highly tridimensional, locally the governing fluid mechanics may be represented by the quasi-steady bidimensional flow around a cambered circular arc with a sharp leading edge. The spinnaker is typically trimmed such that the stagnation point is at the leading edge with the sail streamline separating on the suction side and reattaching within the first 10% of the chord length, forming a leading-edge separation bubble (LESB). This flow feature sets the beginning of the boundary layer, whose separation further downstream is paramount for the global aerodynamic forces on the sail. This study investigates the effect of the LESB on the boundary layer regime and downstream flow separation through particle image velocimetry on a circular arc. The existence of the combination of a critical Reynolds number and a critical angle of attack to trigger turbulent separation is demonstrated. A turbulent LESB followed by a laminar boundary layer is observed in sub-critical regime. Conversely, in a post-critical condition, a turbulent LESB ensued by a turbulent boundary layer is detected, the latter continuing all the way to trailing-edge separation. This behaviour ultimately yields a sharp lift increase and drag reduction. These findings reveal the critical effect of the leading-edge vortical structures on the global flow field and forces experienced by cambered wings with leading-edge separation, including high performance spinnakers. It is envisaged that these results will contribute to improve the design and performance of downwind yacht sails.