360° Polar Extrapolation
Airfoil polars measured in the wind tunnel or simulated with XFOIL or other CFD software are usually limited to small and medium inflow angles (e.g. -20° to +20° AoA). However the blades of both VAWTs and HAWTs often operate at high inflow angles. In order for the BEM routines to converge and produce reasonable results it is essential that the airfoil performance polars (Lift, Drag...and Moment coefficients) are extrapolated to a 360° inflow angle range.
Traditionally the method of Viterna has been extensively used for this purpose. However B. Montgomerie of the Swedish Defense Research Agency has also developed a polar extrapolation method for wind turbine applications. QBlade includes both methods thus allowing the user to use the full extrapolation flexibility of both.
If you have the chance to play around with various extrapolation parameters within QBlade you will notice how critical the airfoil 360° polar extrapolation is for the validity and accuracy of the BEM simulation results.
- MONTGOMERIE, B: Methods for Root Effects, Tip Effects and Extending the Angle of Attack Range to +-100deg, with Application to Aerodynamics for Blades on Wind Turbines and Propellers, FOI Swedish Defense Research Agency, Scien-tific Report FOI-R-1035-SE, 2004
- VITERNA, L.A.; C ORRIGAN, R.D.: Fixed Pitch Rotor Performance of Large Horizontal Axis Wind Turbines, NASA Lewis Research Center, Cleveland, Ohio,1982