Experimental Force Measurements and Flow Visualisation around Downwind Yacht Sails

The spinnaker is the most powerful yacht sail, yet its complex aerodynamics governed by flow separation remains to be fully understood. Only recently, quantitative measurements of the flow fields have been achieved with Particle Image Velocimetry (PIV) in water tunnels. PIV might lead to new breakthroughs in downwind sail aerodynamics, but high spatial and time resolution are achieved by increasing the model size and decreasing the stream velocity. In this paper, we explore to which extent spinnakers can be accurately tested at high blockage ratios (defined as the ratio of the frontal sail area to the tunnel’s cross-sectional area) and low Reynolds numbers, and whether the effect of the mainsail can be accounted for without it being physically present during experiments. Rigid models of a spinnaker are tested, with and without a mainsail, at average-chord-based Reynolds numbers ranging from 5 870 to 61 870, and at blockage ratios between 0.036 and 0.094. The measured flow fields are compared with industry-standard computational fluid dynamics simulations. The experiments reveal that a blockage correction can be devised; that the critical Reynolds number is independent of the blockage ratio; and that the effect of the mainsail’s upwash can partly be accounted for by applying a rotation angle to a spinnaker tested in isolation. These findings provide new insights into the experimental testing of highly-cambered wings at high blockage ratios and low Reynolds numbers, and may contribute to improving the design and performance of spinnakers.