Performance enhancement of downwind sails due to leading edge flapping: A wind tunnel investigation

Abstract

This work presents a wind tunnel experimental study on the effect of the leading edge flapping on the aerodynamic performance of a spinnaker. Four J80-class spinnaker models, combining two different assembling structures (panel layout) and two different sail materials are tested at various wind speeds and wind angles in a wind tunnel. Results show that, for the wind angle range the spinnaker is designed for, the sustained periodic flapping of the sail leading edge has a significant benefit on performance, with 10% increase in drive force. In these model-scale tests, the sail structural properties did not show significant differences in performance, but affect the point where flapping sets in: a model with a stiffer material and a cross-cut panel layout starts flapping for a longer sheet length, compared to a lighter cloth and a tri-radial layout. Finally, it is shown that the nondimensional flapping frequency is rather constant 0.4 in the design range of wind angle, but it varies with the wind speed and sail structural properties on a smaller wind angle where the spinnaker is more stretched.

Recent Advances in Numerical and Experimental Downwind Sail Aerodynamics

“…Indeed, the greater availability of computer power now allows to run more cost-effective virtual wind tunnel tests with FSI, modelling the changes between the moulded and flying shape depending on the point of sail, wind speed and trim. An area where more research is certainly needed is, for example, luff flapping (Viola & Flay, 2009;Deparday, 2016;Aubin et al, 2018).…”

Experimental analysis of a strong fluid–structure interaction on a soft membrane—Application to the flapping of a yacht downwind sail

“…This flapping corresponds to a folding oscillation of the leading edge of the sail and is commonly associated to the best aerodynamic performance trimming. The benefit of the flapping has been recently measured during a wind tunnel campaign (Aubin et al, 2017(Aubin et al, , 2018. Even if the flapping instability may look similar to previously cited literature, it is of different nature: high Reynolds number (10 5 -10 6 ) of the flow, highly cambered 3D structure only held by 3 points with a free leading edge, 3D flow structure due to the atmospheric wind gradient and low aspect ratio of a 3D geometry, fluid added mass up to 100 times the structural mass, dynamic behavior inherent to sailing yacht.…”

Vortex flow of downwind sails

“…In contrast, only few and recent works consider the instantaneous vorticity field. These studies (Arredondo-Galeana & Viola, 2018; Aubin et al., 2018; Deparday, Augier, & Bot, 2018; Viola et al., 2014; Young, Morris, Schutt, & Williamson, 2019) show that the flow on the suction side of the sail is only intermittently attached. It remains to be determined whether the high sweepback of the leading edge could result in a leading-edge vortex (LEV) that remains steadily attached to the sail, as opposed to vortices shed downstream and that result, in the time-averaged sense, in an LESB.…”

“…Symmetric and asymmetric spinnakers are often trimmed by letting the leading-edge fold periodically (Aubin, Augier, Deparday, Sacher, & Bot, 2018; Viola & Flay, 2010, 2011a). This unsteady trim maximises the driving force at high , e.g.…”