Turbsim OutputTime and Tmax in FAST

I’m also generating FF turbulence with Turbsim, I have two quesions:
(1)Turbsim FF turbulence OutputTime must be no less than Tmax in FAST, if OutputTime is greater than Tmax,then which part of the OutputTime will be used?
(2)This question is not about time parameters:Turbsim codes employ Cholesky decomposition, does this suggest that harmony superposition method is used in Turbsim?
Thank you!

(1) FAST uses the the first part of the wind file. It always starts at the beginning of the simulation and will not produce an error until it reaches a point in the simulation where it has exceeded the boundaries of the full-field wind file.

(2) TurbSim uses the Sandia Method (also called Veers Method). You can read about it here.

Dear Minxi,

Just to clarify Bonnie’s answer to Question 1, FAST will make is so that at T=0, the part of the wind file at the center of the tower is equal to 0.5GridWidth/MeanWindSpeed seconds into the wind file. The first grid is actually located at 0.5GridWidth downstream of the center of the tower at T=0. This is to prevent a turbine that is yawed 90 degrees from the wind from having parts of the rotor outside to wind “box.”

Newer versions of TurbSim allow the use of periodic wind files that do not have such logic. At T=0, the first plane is at the tower center line and any parts of the turbine that are downstream of the tower start with the last part of the wind file.


Thank you, Bonnie and Marshall. Your answer is very informative.

I noticed that FAST8 is now able to calculate wind drag force on tower, but according to your reply in the post “FAST and wind loads on tower”, current version of AeroDyn cannot output the tower drag to a file because of I/O routines.

So I am planning to output wind velocity series on each tower grid in formatted full field files. Then I would use FAST8 to output local tower node motion with NewTower features on(i.e including wind loads on tower in my simulation).

With wind velocity series and tower motion added up, I can calculate relative wind velocity series. Together with Re vs Cd table available in the file “NRELOffshrBsline5MW_AeroDyn_DT5_Tower”, I should be able to calculate wind loads on tower myself.

But I don’t know the grid numbers correspond to each tower node in formatted full field. :unamused:

I would be glad if you can offer me some suggestions.

Best regards!

Dear Minxi,

The grid points in the wind file do not align with the analysis nodes on the tower. AeroDyn interpolates the wind file for the tower nodes. If I needed to know the tower loads, I would modify AeroDyn to write them to a file. I think the calculation is done in AD_CalcOutput(). You would probably want to open the file in AD_Init().


Dear Marshall,

Thanks a lot for your quick reply.

I did find a loop calculating tower aerodynamic loads in subroutine AD_CalcOutput() in file “Aerodyn.f90”. But compiling Aerodyn is now a challenge to me because C is the only computer language I can master.

And I heard that you are writing new I/O routines for AeroDyn now and the new version will be able to output tower-element data similar to how it currently generates blade-element data. I will look forward to the new version, and the need to output tower drag is not urgent.

Kind regards,