Force generation mechanisms of downwind sails

The force generation mechanisms of a yacht sail are discussed with the aid of force and flow measurements on a model scale spinnaker. The velocity and vorticity fields are measured on five horizontal sections with particle image velocimetry. By comparing the forces measured with a balance to those computed from the vorticity field, we demonstrate how the force generation can be interpreted by the production and stretching of vortex rings. We consider vortex rings to be continuously generated and shed from the perimeter of the sail. The intersection of their vertical legs with horizontal planes are leading and trailing edge vortices. The sail force is due to the rate of change of the impulse of the vortex rings. Consequently, we show that the force can be computed from the time-averaged vorticity field using the Kutta-Joukowski lift formula, or from the strength and relative velocity of the leading and trailing edge vortices, or from the vorticity flux at the perimeter of the sail. The drag is estimated with Filon's drag formula. These results confirm experimentally the theoretical work of Viola et al. and pave the way to the development of design methodologies that improve sail performance by manipulating the local vorticity field.