Abstract
The aerodynamic performance of airfoils and blades designed for modern wind-turbine rotors, which have diameters of the order of hundred meters, must be examined at chord Reynolds numbers matching those of practical applications. In general, such high Reynolds numbers cannot be achieved in conventional wind tunnels. Moreover, knowledge on the boundary-layer transition location is essential to evaluate airfoil and blade performance at these flow conditions. This work presents an experimental methodology that can be applied at flow conditions reproducing those of real wind-turbine rotor blades and simultaneously provides aerodynamic coefficients and transition locations. The experimental methodology consists of: the Temperature-Sensitive Paint (TSP) technique for global, non-intrusive and reliable transition detection; conventional pressure measurements for the determination of the aerodynamic coefficients; and the High Pressure Wind Tunnel Göttingen (DNW-HDG) to run the experiments at Reynolds numbers matching those of real applications. The obtained results can be used to verify airfoil and blade performance and to validate numerical predictions. In the present work, the experimental methodology was applied to systematically investigate the aerodynamic performance of an airfoil designed for the mid-span sections of modern wind-turbine rotor blades. The examined chord Reynolds numbers were as high as 12 million and the angle-of-attack ranged from -14° to +20°. The presented methodology was here demonstrated to be mature for productive testing.
Highlights
Rotor blade aerodynamics has an essential role in overall wind-turbine performance and has continuously improved in the last decades
These Temperature-Sensitive Paint (TSP) results show the wind-tunnel model as it would be seen from the top wall of the DNW-HDG test section; they were achieved after the TSP images, acquired by the camera mounted at the turntable of the testsection side wall, had been mapped onto a three-dimensional grid representing the model upper surface
The regions at y/b ≤ 19 % and y/b ≥ 78 % were masked white, since these regions were not completely visible in the TSP images and/or their spatial resolution was too low. (These limitations were due to the restricted optical access in the DNW-HDG test section – see [19].) In Figure 3, the flow is from the left; bright and dark areas correspond to regions of lower and higher heat flux, respectively, and to regions of lower and higher wall shear stress
Summary
Rotor blade aerodynamics has an essential role in overall wind-turbine performance and has continuously improved in the last decades. The airfoils for mid-span and outboard sections, are subjected to Reynolds number and roughness effects [1,2,3]; both these effects are related to the occurrence of boundary-layer transition on the airfoils under practical conditions. It is of fundamental importance to design airfoils for mid-span and outboard sections that present low sensitivity to roughness [1,2,5] and experimentally verify their performance at Reynolds numbers relevant for modern wind turbines with large rotor diameters (which can be of the order of 100 m).
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