Blade twist definition in AeroDyn/OpenFAST

Wind & Water Computer-Aided Engineering Software Tools
1

I’m running some simulations just using AeroDyn to get some information about local angle of attack at different TSRs. I want to make sure my blade data file has the correct information in the twist column and would greatly appreciate it if you could clarify the blade twist definition!

From OpenFAST readthedocs (Section 4.2.3.19.2. Blade Data Input File):

“BlTwist specifies the local aerodynamic twist angle (in degrees) of the airfoil; it is the orientation of the local chord about the vector normal to the plane of the airfoil, positive to feather, leading edge upwind; the blade-pitch angle will be added to the local twist;”

Is “vector normal to the plane of the airfoil” referring to the blade’s rotational axis/pitching axis (defined at z_b in AeroDyn driver section)? If so, is this angle referenced from the rotor plane to the local chord line? Additionally, is the blade twist angle defined about the blade root or tip? For example, in Hansen’s Aerodynamics of Wind Turbines and Manwell’s Wind Energy Explained, blade pitch angle and twist angle are defined with respect to the tip chord.

Additionally, at the end of Section 4.2.3.19.2, the documentation references a figure specifically for twist. I’m not sure if it’s a typo, but shouldn’t theta (local pitch) = pitch angle + twist in this figure?

Thank you so much for your help and response,

Megan Andersen

2

Dear @Megan.Andersen,

Just a few comments:

  • AeroDyn can support blades that have built-in curvature and sweep. If the blade is straight, then the undeflected blade will lie along the pitch axis (z_b), and then BlTwist will be defined about z_b as you state. But if the blade is not straight–with build-in curvature or sweep–the axis about which BlTwist is defined will differ from z_b, which is why the definition of BlTwist is written the way it is.
  • The pitch angle is not defined with respect to a given blade station such as the root or the tip. Rather, AeroDyn uses the definition that the orientation of the local chordline is the sum of the pitch + twist. In other words, you can say that the pitch angle is defined at the blade station where the twist is zero.
  • I agree that theta = pitch + local twist, but that is what is plotted in Fig. 4.3. I’m not sure what typo you are referring to.

Best regards,

3

Thanks for your response!

The typo I’m seeing is that the figure says theta: local pitch + twist. Theta is the local pitch though, so I think it should say theta (local pitch) = (blade) pitch angle + twist.

4

Dear @Megan.Andersen,

Thanks for clarifying. I think it may be more clear to say that

local theta = blade pitch + local twist

But, regardless, it sounds like we are in alignment and I don’t think the figure as is too misleading.

Best regards,