Abstract
Wind turbines partly operate in stalled conditions within their operational cycle. To simulate these conditions, it is also necessary to obtain 2-D airfoil data in terms of lift and drag coefficients at high angles of attack. Such data has been obtained previously, but often at low aspect ratios and only barely past the stall point, where strong wall boundary layer influence is expected. In this study, the influence of the wall boundary layer on 2D airfoil data, especially in the post stall domain, is investigated. Here, a wind turbine airfoil is tested at different angles of attack and with two aspect ratios of AR = 1 and AR = 2. The tests are conducted in a wind tunnel that is pressurized up to 150 bar in order to achieve a constant Reynolds number of Rec = 3 • 106, despite the variable chord length.
Highlights
Even a pitch regulated wind turbine operates for some time in stalled conditions
The Reynolds number is held constant in order isolate the effect of the end-walls and draw a conclusion about the wall boundary layer influence on 2-D airfoil tests
The end-walls are represented by endplates of a constant size for all aspect ratios
Summary
Even a pitch regulated wind turbine operates for some time in stalled conditions. These load cases must be included when using an aeroelastic program to prove that the structure does not fail due to ultimate or accumulated fatigue loads within the design lifetime of typically 20 years. The width of the test section for such a tunnel will typically not be more than 1 to 2 m giving a maximum aspect ratio of approximately 4 for Rec = 3 · 106, and in practice often not more than AR = 2 In this case the boundary layer and friction from the lateral walls holding the blade section can have a severe influence on the separated low momentum flow past the airfoil at high angles of attack distorting the flow from what should be an infinitely long section. In this case, one is not measuring a 2-D flow past an airfoil and the walls heavily influence the lift and drag forces. One may try applying boundary layer suction on the lateral walls or use compressed air to increase the momentum
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