Drag Prediction for Blades at High Angle of Attack Using CFD

Abstract

In the present paper it is first demonstrated that state of the art 3D CFD codes are capable of predicting the correct dependency of the integrated drag of a flat plate placed perpendicular to the flow. This is in strong contrast to previous 2D investigations of infinite plates, where computations are known to severely overpredict drag. We then demonstrate that the computed drag distribution along the plate span deviate from the general expectation of 2D behavior at the central part of the plate, an important finding in connection with the theoretical estimation of drag behavior on wind turbine blades. The computations additionally indicate that a “tip effect” is present that produces increased drag near the end of the plate, which is opposite of the assumptions generally used in drag estimation for blades. Following this several wind turbine blades are analyzed, ranging from older blades of approximately 10 meter length (LM 8.2) over more recent blades (LM 19.1) around 20 meters to two modern blades suited for megawatt size turbines. Due to the geometrical difference between the four blades, the simple dependency on aspect ratio observed for the plates are not recovered in this analysis. The turbine blades behave qualitatively very similar to the flat plates and the spanwise drag distributions show similar “tip effects.” For the turbine blades this effect is even more pronounced, because the tapering of the blades makes the tip effect spread to a larger part of the blades. The findings are supported by visualizations of the wake patterns behind the blades.