i would like to ask if there is a firm rule for choosing the stall (initial) angle of attack together with its coressponding lift and drag coefficients which are used in Viterna’s model to extend the airfoil characteristics up to 90deg,
because i noticed that increasing the initial lift and drag coefficents choise produce a continous increase in the predicted power using the bem code.
i also need explanation for the following:
the stall delay model like that one used in the airfoilprep spreedsheet use the 2-d airfoil tables as the inpult values to correct for stall delay phenomenon, but actually the wind turbine blade has finite aspect ratio, its not 2-d, so is using the 2-d airfoil data to correct for stall delay up to an angle like 20deg and then using the Viterna’s model up to 90 deg would be reasonable.
thank you all
The Viterna method is at least partially explained in the AeroDyn theory manual - wind/designcodes/simulators/aero … Theory.pdf
The rule of thumb is not firm, but depends on what data you have available. If you have data available above stall, you may want to use all the data you have. But this also depends on how the Viterna method fits the data above those you put in the table (see the theory manual). If you get unrealistic values for Cl or Cd you may want to use a different portion of the data to fit. Unfortunately, this is more of an art than science - we call it “tuning”.
Your second proposal sounds reasonable. Initially,I would only correct data for stall delay that I had available from measurements and not Viterna extrapolated values. But again, most often we are trying to match a measured power curve from a turbine and we may increase Cl or Cd depending on the final answer. How this is done varies between designers.