Post stall studies of untwisted varying aspect ratio blades with an NACA 4415 airfoil section. Part I

TLDR: In this article, the results of the wind tunnel investigation of untwisted, constant chord blades having four aspect ratios, with an NACA 4415 series airfoil section, at angles of attack ranging from -10 to 100 degrees are discussed.

Abstract: Wind Turbine blades operate over a wide angle of attack range. Unlike aircraft, a wind turbine's angle of attack range extends deep into stall where the three dimensional performance characteristics of air foils are not generally known. Peak power predictions upon which wind turbine components are sized, depend on a good understanding of a blade's post stall characteristics. The results of the wind tunnel investigation of untwisted, constant chord blades having four aspect ratios, with an NACA 4415 series airfoil section, at angles of attack ranging from -10 to 100 degrees are discussed. Tests were conducted for aspect ratios of 6, 9, 12 and infinity at four Reynolds numbers ranging from one-quarter million to one million. The results on the same family of airfoil section but with varying thickness ratio are given. Results of force and pitching moment measurements over the angle of attack range for all combinations of Reynolds numbers and aspect ratios, and the effects of boundary layer tripping, are presented. Both initial and secondary stall are presented. The maximum drag coefficient is found to occur an an angle of attack of 90 degrees. The pitching moment is unstable beyond stall. The lift and post-stall drag-coefficients decrease with decreasing aspect ratio. The boundary layer tripping is observed to decrease the lift curve slope and stalling angle of attack. The drag coefficient (with tripping) is significantly affected only at low aspect ratio. Beyond secondary stall, the lift to drag ratio is independent of aspect ratio. The maximum lift to drag ratio for the infinite aspect ratio blade is roughly twice that of the blade with an aspect ratio of six. This effect is independent of Reynolds number in the range studied.