360° Polar Extrapolation

使用Qblade对airfoils的Cl、Cd做计算，基于Xfoil的计算方法一般限于-20° to +20°的AOA，而导入WInDS或Qblade，使用BEM方法对叶片整体计算都需要360°的数据。此时可以使用Qblade对数据进行计算。使用传统的Viterna方法，或Montgomerie方法。个人推荐后者，见文末图。虫洞

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. *

• I prefer the Viterna method for its robustness. The Montgomerie methods will give some weird error as Fig1 if the initial Xfoil data is not very good.

Fig 1 Cd plot gross error

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Relevant Literature

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