Aerofoil behaviour at high angles of attack and at Reynolds numbers appropriate for small wind turbines

Abstract

The aerodynamic characteristics of a NACA0018 aerofoil have been investigated experimentally for incidence angles ranging from Formula to Formula in closed-jet and open-jet wind tunnels with different blockage coefficients at Reynolds numbers from 60,000 to 140,000. The results provide a comprehensive data set for studying the performance of typical, small-scale Darrieus wind turbine blades which mainly operate at relatively low Reynolds number and experience extreme angles of attack, particularly during start-up. Measurements in both very high and very low blockage, open-jet wind tunnels capture a “second-stall” phenomenon at high angles of attack, but this behaviour is not observed in the closed-jet wind tunnel confirming the sensitivity of aerofoil performance at extreme incidence to wind tunnel configuration. Surface flow visualisation suggests that the “second-stall” occurs when the flow separation point near the leading edge of the aerofoil moves from the suction side to the pressure side which leads to a sudden change of wake structure. In the closed-jet wind tunnel, the tunnel walls constrain the wake and prevent the flow from switching from one regime to another. The measured data are also used to demonstrate that established wind tunnel blockage corrections break down under these extreme, post-stall angles of attack.