Wind tunnel measurements on the influence of turbulence on polars and flow separation of an airfoil

The influences of turbulence on the performance of the airfoil and on the appearing forces are of great interest in wind energy since fluctuations and gusts are common in the atmospheric boundary layer1. To analyse and understand the effects, measurements in laminar and turbulent inflow conditions on a DU91-W2-250 airfoil are performed in a Göttingen type wind tunnel. An adjustable and reproducible turbulent flow is generated with an active grid2. Different experimental methods are used and compared with each other. With this knowledge active and passive control strategies to alleviate loads can be envisaged. \nPolar lines are measured with 3 different methods. Two force sensors and a moment sensor are attached to the airfoil that is arranged in a closed test section with rotatable end plates and allow an analysis of the force dynamics under turbulent conditions. First results of the force balance under laminar conditions are in good agreement with the coefficients gained with wall pressure measurements in the wind tunnel (Figure 1a) and with comparative CFD simulations applying the DLR structured flow solver FLOWer3. The local pressure distribution on the airfoil (Figure 1b) is obtained by pressure tabs which are sampled with a 32-channel pressure scanner. This allows a direct measurement of the local flow separation and the stall effect on the suction side. The integrated pressure distribution yields lift and drag curves that are also in good agreement with the results of the other methods (Figure 1a). The side walls of the wind tunnel out of Plexiglas allow optical access to the turbulent flow around the airfoil. Measurements with a high speed stereo PIV system are possible to investigate the local flow separation with a high temporal resolution. This presentation is an overview of first measurements with the mentioned experimental methods in laminar and turbulent conditions.