A study of the vortex flows of downwind sails

Spinnakers are thin, curved and twisted sails used to sail downwind, i.e. in the direction of the wind. Analogue to wings, spinnakers provide lift, but they use it to drive the boat forward. According to recent computational studies [1], such sails can promote the formation of a leading-edge vortex (LEV). The LEV grows in diameter from the bottom to the top of the sail and it can remain attached to the surface of the sail near the leading-edge. If this happens, the vortex could increase the lift and the thrust force generated by the spinnaker. However, the presence of the LEV in downwind sails has only been shown numerically and not experimentally. Therefore, it is not certain whether the LEV remains stationary at the leading-edge or is shed downstream. Additionally, the role of the LEV in sail aerodynamics has not yet been assessed. It is known that an LEV either enhances lift or delays stall and this thesis investigates the role of the LEV in the context of downwind sailing. Based on the numerical evidence provided by Viola [1], the hypothesis of this work is that there is an LEV attached to the leading-edge of a downwind sail and that it increases lift at the maximum driving force angle. The flow around a scale model spinnaker, in the absence of a hull, is studied with Particle Image Velocimetry (PIV). The existence of an LEV is confirmed and it is revealed that the LEV can remain trapped but can also shed downstream for finite periods. The contribution of the LEV to the lift is computed at different cross-sections parallel to the flow. It is found that the LEV is responsible between 10 to 20% of the sectional lift at the top of the sail [2]. The LEV is a major contributor to lift generation and gains in sail aerodynamics can be achieved by controlling it.[1] Viola, I.M., Bartesaghi, S., Van-Renterghem, T., Ponzini, R., 2014. Detached eddy simulation of a sailing yacht. Ocean Engineering 90, 93–103.[2] Arredondo-Galeana, A., Viola, I. M., 2018. The leading-edge vortex of yacht sails. Ocean Engineering 159, 552-562.