Aerodynamics of headsails: a review of measured surface pressures and expected flow fields

The general flow features observed on yacht sails can be predicted with well-established fluid dynamic theory. Headsails are thin wings with a sharp leading edge and thin-airfoil theory can predict the general flow and pressure fields around these sails. However, a closer look shows viscous flow features, such as the leading edge bubble, that cannot be modelled with inviscid flow and, indeed, cannot be predicted accurately with analytical fluid dynamics. Moreover, the available literature lacks of quantitative flow measurements (e.g. particle imaging velocimetry) and high-fidelity numerical simulations (e.g. wall-resolved large eddy simulations) on yacht sails due to the difficulties related to the high Reynolds numbers and the complex sails’ geometries. Therefore there is limited knowledge of the viscous flow field near sails. Here we provide a detailed description of the pressure distributions measured in a wind tunnel on model-scale headsails and we describe the expected flow field with analogies with other viscous fluid flows. We show that while the general pressure distribution trend is dominated by potential flow, the viscous flow features have a significant effect on the pressure distributions near the leading and trailing edges. In particular, we propose that a long-type laminar separation bubble occurs at the leading edge while trailing edge separation may occur at high angles of attack; we discuss how these features vary with the sail geometry, the angle of attack, the Reynolds number and the turbulence level of the onset flow. We think that the proposed description of these flow features will enhance the interpretation of sail pressure distributions and, in turn, of the aerodynamic forces experienced by a sailing yacht.