Water Tunnel Testing of Downwind Yacht Sails

Downwind yacht sails, such as spinnakers, are low-aspect-ratio, highly-cambered wings with a sharp leading edge. In this talk, we will present water tunnel tests, including particle image velocimetry, of the downwind sails of a racing yacht. We will consider six model-scale rigid models at average-chord-based Reynolds numbers ranging from 5 870 to 61 870. We will discuss the critical Reynolds number above which the forces remain constant, and we will demonstrate that the sharp force increase with increasing Reynolds number is associated with the suppression of the relaminarization rather than with the laminar-to-turbulent transition. We will also discuss the upwash of the mainsail on the spinnaker and the excellent agreement between the overall performances of the sails measured in the water tunnel and those estimated by the sail designers with computational fluid dynamics.