A low-Reynolds-number, high-angle-of-attack investigation of wind turbine aerofoils

Abstract

This article describes an experimental, aerodynamic investigation of four aerofoils intended for small wind turbine applications. The aerofoils of these small machines (both horizontal and vertical axes) normally experience conditions that are quite different from large-scale machines due to smaller chord length and lower wind speed, resulting in significantly lower Reynolds numbers. They also operate with an unusually wide range of incidence angles (0° to 90° for horizontal axis and 0° to 360° for vertical axis). Four appropriate aerofoils were chosen for testing at three Reynolds numbers (65 000, 90 000, and 150 000) through 360° incidence to cover almost all possible conditions that might be encountered by both types of turbines. The investigations were conducted in terms of lift, drag, and surface static pressure coefficients. The experimental results show that both geometry and Reynolds number had significant effects on aerodynamic lift, not only when unstalled but particularly in the post-stall region from 20° to 50° incidence. These effects were also seen at other incidences but to a lesser extent. By contrast, the drag characteristics were similar for all blade geometries. Static pressure measurement revealed that, at these low Reynolds numbers, separation bubbles always form near the leading edge of the suction surface at moderate incident angles and increase in size with decreasing Reynolds number. Comparisons of force and static pressure measurements showed that the aerofoil stalling behaviour is closely related to the presence of a separation bubble at the leading edge of the suction surface. Discrepancies between the experiments and predictions using the AERODAS model confirm the continued need for accurate wind tunnel testing.