This article discusses the results of wind tunnel measurements performed on a modified DU 97-W-300 aerofoil at Reynolds numbers between 1 ¥ 10 6 and 10 ¥ 10 6 in the cryogenic wind tunnel of DNW in Köln, Germany. The aerofoil was modified by reducing the trailing edge thickness from 1·74% to 0·49% of the chord. Although the measurements showed large scatter when flow separation occurred on the model, it was possible to establish the variation with Reynolds number of the maximum lift coefficient, the maximum lift/drag ratio and the design lift coefficient for a Mach number of 0·2. Furthermore, the effect of wraparound Carborundum 60 roughness and zigzag tape of 0·4 mm thickness on the upper and lower surfaces was studied. The experimental results were compared with RFOIL calculations. The measurements indicate that there was no significant variation in the maximum lift coefficient with Reynolds number for the clean aerofoil. In contrast to the RFOIL calculations, the experimental maximum lift/drag ratio decreased with increasing Reynolds number from an average of 95 at R = 3 ¥ 10 6 to about 85 at R = 10 ¥ 10 6 .The Carborundum 60 roughness had a larger negative effect on the aerofoil performance than the zigzag tape, but in both cases the aerofoil performance improved drastically with increasing Reynolds number.

Knowledge about laminar–turbulent transition on operating multi megawatt wind turbine (WT) blades needs sophisticated equipment like hot films or microphone arrays. Contrarily, thermographic pictures can easily be taken from the ground, and temperature differences indicate different states of the boundary layer. Accuracy, however, is still an open question, so that an aerodynamic glove, known from experimental research on airplanes, was used to classify the boundary-layer state of a 2 megawatt WT blade operating in the northern part of Schleswig-Holstein, Germany. State-of-the-art equipment for measuring static surface pressure was used for monitoring lift distribution. To distinguish the laminar and turbulent parts of the boundary layer (suction side only), 48 microphones were applied together with ground-based thermographic cameras from two teams. Additionally, an optical camera mounted on the hub was used to survey vibrations. During start-up (SU) (from 0 to 9 rpm), extended but irregularly shaped regions of a laminar-boundary layer were observed that had the same extension measured both with microphones and thermography. When an approximately constant rotor rotation (9 rpm corresponding to approximately 6 m/s wind speed) was achieved, flow transition was visible at the expected position of 40% chord length on the rotor blade, which was fouled with dense turbulent wedges, and an almost complete turbulent state on the glove was detected. In all observations, quantitative determination of flow-transition positions from thermography and microphones agreed well within their accuracy of less than 1%.

This paper discusses the findings from a measurement campaign on a rotating wind turbine blade operating in the free atmosphere under realistic conditions. A total of 40 pressure sensors together with an array of 23 usable hot-film sensors (based on constant temperature anemometry) were used to study the behavior of the boundary layer within a specific zone on the suction side of a 30 m diameter wind turbine at different operational states. A set of several hundreds of data sequences were recorded. Some of them show that under certain circumstances, the flow may be regarded as not fully turbulent. Accompanying Computational Fluid Mechanics (CFD) simulations suggest the view that a classical transition scenario according to the growth of so-called Tollmien-Schlichting did not apply.