Deriving lift/drag coefficients

Dear Readers,

I wish to get the angle of attack, lift and drag coefficients for each blade section node in the time domain. I believe this can be done using the aerodyn input file. I also wish to get the lift and drag forces on the sections. How can this be done in FAST ?

Thanks
Subham

Dear @Kashyap.Subham,

If I understand correctly from other posts, you are using FAST v7, in which case, the local aerodynamic data that you want is available by setting the PrnElm column in the AeroDyn input file to PRINT at each element that you want these data for.

In AeroDyn v15 within OpenFAST, these aerodynamic data are available by adding the desired outputs to the OutList–see: 4.2.1.9. Appendix — OpenFAST v3.2.0 documentation.

Best regards,

Dear Sir,
I am using FAST v7 indeed. Where do i find the description of each column of the file generated when the print is enabled in Aerodyn input file. I get the following output channels in the file generated. Also, the generated file is really large which makes it difficult for post processing. Any suggestions in handling the same?

This file was generated by FAST(v7.02.00d-bjj, 20-Feb-2013) on 24-Jul-2022 at 01:54:08.
Time VX17 VY17 VZ17 Alpha01 DynPres01 CLift01 CDrag01 CNorm01 CTang01 CMomt01 Pitch01 AxInd01 TanInd01 ForcN01 ForcT01 Pmomt01 ReNum01 Alpha02 DynPres02 CLift02 CDrag02 CNorm02 CTang02 CMomt02 Pitch02 AxInd02 TanInd02 ForcN02 ForcT02 Pmomt02 ReNum02 Alpha03 DynPres03 CLift03 CDrag03 CNorm03 CTang03 CMomt03 Pitch03

Thanks
Subham

Dear @Kashyap.Subham,

The element output file generated with PrnElm set to PRINT is documented in the old AeroDyn User’s Guide: https://www.nrel.gov/wind/nwtc/assets/text/aerodyn.pdf.

I don’t believe anything can be done to the file size other than changing the time step or reducing the number of elements written to the file.

Best regards,

Dear Sir,

Thank you for the assistance. With every assistance of yours, I am discovering the immense effort you and the NREL team have put in to develop a tool like FAST.

Another query though:
VX(NELM) m/sec
ft/sec
The global X velocity of the wind at the most outboard element location.
This includes all shear and tower shadow effects, but no induced velocity
effects.

What is meant by the phrase “no induced velocity effects” ?

Subham

Dear @Kashyap.Subham,

The induced velocity is the change in wind velocity caused by the interaction of the wind with the rotor (the impact of aerodynamic loads on the wind).

Best regards,

Dear Sir,
Thanks for the reply. I would also like to confirm if Fn and Ft obtained by aerodyn for each blade node represent the lift and drag force, respectively at the node under consideration.

Thanks
Subham

Dear @Kashyap.Subham,

Are you referring to old AeroDyn outputs ForcN and ForcT or AeroDyn v15 outputs Fn and Ft? ForcN and ForcT from the old AeroDyn are normal and tangential to the rotor plane. Fn and Ft from AeroDyn v15 are normal and tangential to the chord. Neither are lift and drag forces, which would normal and tangential to the local relative wind vector.

Best regards,

Dear Sir,

Is there a way I can derive the local lift and drag forces at the blade nodes? I am using the FAST v7 Seismic executable distributed in the Seismic archive zip file.

Thanks
Subham

Dear @Kashyap.Subham,

The PRINT option of AeroDyn v13 within FAST v7 can output the local aerodynamic quantities at each blade element. To calculate the local lift or drag forces, you can multiply Fl = CLift * DynPres * Chord and Fd = CDrag * DynPres * Chord.

Best regards,

Dear Jason Sir,

Throughout the literature, I have seen that the lift and drag coefficients are plotted with the angle of attack of the local airfoil sections. And similar data in a tabular format is given as input to the aerodyn code in the blade airfoil files.

At the same time, I wish to make similar plots between lift and angle of attack for an operational wind turbine using FAST. By printing the output file with results at various nodes, I get the time series for Cl, Cd, Alpha, etc. How do I get curves Cl vs. Alpha from these time series? I am trying to illustrate the dynamic stall condition.

Thank You
Subham

Dear @Kashyap.Subham,

Once you output the angle of attach and lift coefficient time series, you can plot these series against each other to illustrate dynamic stall. Such a plot should match what you input when unsteady airfoil aerodynamics is disabled (AFAeroMod = 1).

Best regards,

Dear Sir,

The output file using the Aerodyn, gives data with 25 entries for each time step. Why do I get 25 data points for the time step? Is it some printing error in the source code, or is it a manifestation of the semi-empirical model utilized in Aerodyn? I am using the Aerodyn version in the seismic module package.

Thanks
Subham

Dear @Kashyap.Subham,

I’m not sure I understand your question. By “25 entries”, are you referring to the columns of tabular time-series output? For AeroDyn within the Seismic version of FAST v7, which uses AeroDyn v13, the AeroDyn outputs are dictated by the PRINT/NOPRINT flags specified at each aerodynamic analysis node in the AeroDyn v13 input file.

Best regards,