Different output variables for rotor averaged wind speed

Hi all,

I am trying to determine the undisturbed averaged wind speed over the rotor plane for my OpenFAST simulation with the NREL 5MW turbine (AeroDyn V15).
I tried different output variables from AeroDyn and am getting slightly different results for every one of them, so that I am wondering about the difference:

  • I computed the average of BαNβVUndx over 27 possible nodes (9 nodes at each blade). This should give me the undisturbed wind speed. Is this the same output that I would get when averaging over the same set of points in the wind field from InflowWind?
    Would I get a very similar output for the WindDiskVelX variable in InflowWind? In the current version of OpenFAST, I am somehow getting the error message
FAST_InitializeAll:InflowWind_Init:InflowWind_SetParameters:SetOutParam:WindDiskVelX is not an
available output channel.
  • The analogous average of BαNβVDisx gives me a disturbed wind speed. If I understand the source code correctly, the difference to the undisturbed one are shadowing effects of the tower - is that correct?

  • RtVAvgxh gives me the disturbed average wind speed minus the structural motion of the wind turbine in axial direction. So the average of BαNβVDisx minus the average of TwNβSTVx should give me RtVAvgxh, but still I find a small discrepancy. Is this is only coming from the discretization of nodes?

  • Is it correct that all of these wind speeds do not include the effect of induction? When eventually computing the inflow velocity to the blade elements, induction should be accounted for, right?

Thanks in advance for your help!


Dear @Martin.Wagner,

Here are my responses:

  • Spatial averaging AeroDyn output BαNβVUndx as you propose won’t result in InflowWind output WindDeskVelX exactly, because BαNβVUndx involves nodes along each blade rotating with the rotor and WindDeskVelX includes an annulus averaging at 70% span. But I would expect the comparison to be reasonable close.
  • Regarding the error you are receiving selecting output WindDeskVelX from InflowWind; which version of OpenFAST are you using? Does upgrading to the newest version (currently v3.5.3) solve the issue?
  • I agree that AeroDyn output BαNβVDisx includes the effect of tower influence/shadow.
  • AeroDyn output RtVAvgxh is computed by spatial averaging the relative wind velocity (wind minus structural velocity) at each aerodynamic analysis across each blade, and so, accounts for blade deformation in addition to tower deformation.
  • I agree that all of the aforementioned velocities don’t include induction. And indeed, induction is important when computing the relative velocity for dynamic pressure and aerodynamic loads.

Best regards,