Boundary-Layer Evolution over the Lower Surface of a Wind-Turbine Airfoil

The boundary-layer evolution over rotor-blade airfoils can be significantly affected by changes in the flow conditions. Because this sensitivity would have a major impact on the wind-turbine performance, it is crucial to evaluate it at the large Reynolds numbers characteristic of modern wind-turbine rotor blades. This work extends earlier wind-tunnel investigations on a DU 91-W2-250 airfoil by examining the transition behavior on its pressure surface at angles-of-attack from −14 to 20 deg and chord Reynolds numbers up to 12 million. Large regions of laminar flow were identified at positive angles-of-attack using temperature-sensitive paint, which also enabled an estimation of the development of turbulent separation at negative incidence. The global surface temperature distributions were supplemented by pressure tap data, which also provided an input for linear stability computations of the laminar boundary layer that supported and complemented the experimental findings. This systematic analysis elucidated not only the effect of Reynolds number and airfoil incidence on laminar-turbulent transition, but also a localized, facility-specific variation in the transition front. The observed boundary-layer evolution substantiated the discussion of the development of the aerodynamic coefficients, which were measured in previous investigations and were well reproduced in the present work.