airfoil polar data during post stall stages (high AOA)
Table of Contents
1 airfoil polar during post stall state
why do we need airfoil polar during post stall state?
because 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.
cl cd vs. AOA data when AOAs are high
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).
at a givend Re
- for low AOAs before stall angle, lift and drag vs AOA,
can be gained from experimental data or Javafoil/Xfoil
- after stall angle, lift and drag profile need to be extrapolated
AOA from experiental data normally stalled after 15 degree, the AOA data need to extrapolated casue wind turbine blade station has higher local AOA
Why is it difficult to get polar of airfoil after stall angle based on experimental wind tunnel test? How to validate the extrapolated polar data?
1.1 polar extrapolation methods
- Vitenas‘ method
- extrapolation can be done using Javafoil, or NREL‘s AirfoilPrep 1 based on Viternas method2
- extensively used
- Snel‘s Method
- extrapolate cl, cd up to the stall delay angle using the method proposed by Snel et al.3.
Further extrapolations beyond the stall delay angle were achieved using Viterna and Corrigan’s methodology for post stall predictions 4.
- empirical correction
- used by batten 2008, University of Southampton
- Montgonerie‘s method
author: B. Montgomerie of the Swedish Defense Research Agency 5 application: used by Qblade
Footnotes:
SA Ning. AirfoilPrep.py Documentation. Technical report, National Renewable Energy Laboratory, 2013.
Larry A Viterna and David C Janetzke. Theoretical and experimental power from large horizontal-axiswind turbines. Technical Report NASA TM-82944, National Aeronautics and Space Ad- ministration, 1982.
Snel H, Houwink R, Bosschers J. Sectional prediction of lift coefficients on rotating wind turbine blades in stall. Report ECNC– 93-052, Energy research Centre of the Netherlands, 1994.
Viterna LA, Corrigan RD. Fixed pitch rotor performance of large horizontal axis wind turbines. DOE/NASA workshop on large horizontal axis wind turbines, Cleveland, Ohio, 1981.
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
