# AeroDyn v.14 (VT, CNN, CTT)

Dear NREL forum,

I have some question regarding the AeroSubs.f90 module:

1. lines 1801 - 1802:
``````O%ElOut%CNN    ( O%ElOut%ElPrList(J) )    = CLA * COS(O%Element%ALPHA(J,IBlade)) + CDA * SIN(O%Element%ALPHA(J,IBlade))

Why the angle-of-attack (ALPHA) is used here instead of the PHI angle in the above formulas for the normal and tangential force coefficients?

1. Is the tangential velocity component corrected twice with the tangential induction factor ( 1. + A2P ) in lines 1986 and 2090?

Best regards,
Koen

Dear Koen,

1. The normal and tangential force coefficients output by AeroDyn are normal and tangential to the chord, not the rotor plane, so, alpha is used in place of phi.

2. I don’t know the reason why the algorithm is programmed as it is, but the tangential induction factor is not applied twice. In SUBROUTINE AXIND(), the local variable VTA is equated to VT * ( 1. + A2P ), and in SUBROUTINE VIND(), the variable VT is equated to VT * ( 1. + A2P ); the later equation changes the value of VT.

Best regards,

Dear Jason,

thank you for the helpful support that you provide to the forum questions. It was easy to install and run simulations with FAST, but understanding some of the implementation details is not trivial, e.g. the different coordinate systems.

I was confused in the first question because DFN and DFT (lines 1780,1781 of AeroSubs.f90) are computed with respect to the PHI angle, so I thought that CNN and CTT should be given with the same orientation but then I saw that the elemental tangential and normal forces are further transformed with the AeroTwst angle (in AeroDyn.f90 , lines 1103,1104), so in the end they are also at the same system (normal/tangent to the chord).

But that leads me to another short question(s): a) do you transform these elemental forces to the rotor plane when you compute the rotor thrust and torque, or you integrate DFN and DFT in the chord system (i can’t find where in the code this operation takes place)? b) the outputs OoPDefl1, IPDefl1 are also given as deflections normal and tangent to the chordline? so technically they are not ‘in/out-of-rotor-plane’?

Best regards,
Koen

Dear Koen,

a) Actually, AeroDyn v14 never integrates DFN and DFT to calculate the total aerodynamic rotor thrust and torque. This is a new feature we’ve added to AeroDyn v15.

b) Outputs OoPDefl1 and IPDefl1 from the ElastoDyn module are normal and tangential to the rotor plane, not the chord.

I hope that helps.

Best regards,

Dear Jason,

yes it helped a lot. But for the first point, I was referring to the operations done in ElastoDyn.f90 that gets the aerodynamic forces from AeroDyn.f90 and then in subroutine CalculateForcesMoments it computes root shear forces and moments and also shaft torque. Are these shear forces/moments computed using the DFN, DFT orientation about the chord, or are they somewhere converted back to the rotor plane?

Is there a FAST output that gives the deflections about the principal structural axes instead of the OoP or flapwise direction?

Best regards,
Koen

Dear Koen,

The aerodynamic loads are transferred from AeroDyn to ElastoDyn in the global (inertial frame) coordinate system. ElastoDyn then integrates these loads, along with the inertia loads to calculate the blade-root loads, shaft loads etc.

You can output the blade deflections in the coned “c” system (OoP and IP) or the blade “b” system (flap/edge), the later of which is rotated with the blade-pitch angle about the z_c axis. However, there are no outputs that provide the blade deflection relative to the local principle axes of bending. I’m not sure these coordinates would be of much use as they rotate along the blade with the local structural-twist angle; when the structural twist varies along the blade, the blade flapwise and edgewise motions are always coupled anyway.

Best regards,

Dear all,

I’m using AeroDyn v.14.. Is it possible to print the outputs of aerodynamic quantities like angle of attack, lift coefficient etc at each airfoil section. It seems possible in AeroDyn v.15 . But is it possible using v.14? If yes, how? Thanks.

Regards,
Kumara

Dear Kumara,

Yes, this is possible via the PRINT/NOPRINT option for each element as specified in the the AeroDyn v14 primary input file. This feature is documented further in the old AeroDyn User’s Guide: nwtc.nrel.gov/system/files/AeroDyn.pdf.

Best regards,

Thanks, Jason. That was oversight from my side. I’ve attached figure here for making it more easy to see. The output is written to a file “filename.AD.out”.

Regards,
Kumara