Wind Tunnel Wall Corrections for Two-Dimensional Testing up to Large Angles of Attack
WA Timmer (TU Delft - Wind Energy)
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Abstract
An accurate representation of two-dimensional airfoil characteristics measured in a wind tunnel generally requires the inclusion of corrections for interference effects that exist due to the presence of the wind tunnel walls. This chapter discusses the most commonly used correction schemes both for streamlined and separated flow regimes. The classical correction method based on small velocity perturbations gives very good results up to angles of attack of about 20 degrees for chord-to-tunnel height ratios c/h up to 0.36. Even with separation of the boundary layer at a chord location of 30% the corrected pressure distribution matches that of a much smaller model with c/h = 0.15. In the deep-stall range of angles of attack, where the flow separates from the leading edge, the method based on the wake analysis by Maskell with a blockage factor of 0.96 seems to give good results for two-dimensional models up to c/h values of 0.27. A comparison with measurements corrected with the matrix version of the pressure signature method, which uses the pressure distribution on the tunnel walls, shows that the latter leads to slightly larger corrections. Maskell’s method, for which the blockage parameter of 0.96 apparently is based on a single measurement of a two-dimensional flat plate, seems to give better results when a value of 1.03 is used.