Improving Airfoil Drag Prediction

An improved formulation of drag estimation for thick airfoils is presented. Drag underprediction in XFOIL-like viscous-inviscid interaction methods can be quite significant for thick airfoils used in wind turbine applications (up to 30%, as seen in the present study). The improved drag formulation predicts the drag accurately for airfoils with reasonably small trailing-edge thicknesses. The derivation of drag correction is based on the difference between the actual momentum loss thickness based on freestream velocity and that based on the velocity at the edge of the boundary layer. The improved formulation is implemented in the most recent versions of XFOIL and RFOIL (an aerodynamic design and analysis method based on XFOIL, developed by a consortium of the Energy Research Centre of the Netherlands, the National Aerospace Laboratory/NLR, and the Delft University of Technology after the Energy Research Centre of the Netherlands acquired the XFOIL code; after 1996, the Energy Research Centre of the Netherlands maintained and improved the tool), and the results are compared with experimental data, results from commercial computational fluid dynamics methods like ANSYS CFX, as well as other methods like the Technical University of Denmark Aeroelastic Design Section (AED)'s EllipSys2D and the National Renewable Energy Centre (CENER)'s Wind Multiblock. The improved version of RFOIL shows good agreement with the experimental data.