# AeroDyn Forces/Moments reference systems

Hi,

I am running the OC3-Hywind platform case (Certification test 24) with the purpose of extracting the aerodynamic loads (forces and moments) from AeroDyn.

The output channels that I have activated are referenced in hub and global coordinate systems: “RtAeroFxg, RtAeroFxh, RtAeroFyg, RtAeroFyh, RtAeroFzg, RtAeroFzh” “RtAeroMxg, RtAeroMxh, RtAeroMyg, RtAeroMyh, RtAeroMzg, RtAeroMzh”

I am assuming that the hub coordinate system rotates with the rotor having x pointing oposite to the wind direction as follows:

And the global coordinate system is static located at the tower base, with the x pointing to the wind direction. Am I right?

Irene

Dear @Irene.Berdugo,

Yes, AeroDyn uses the same hub coordinate system as ElastoDyn, but the Figure you show from the old FAST v6 documentation actually illustrates a downwind turbine. In AeroDyn and ElastoDyn, the x-axis of the hub coordinate system is nominally pointing in the direction of the wind.

Best regards,

Thanks Jason!
Since I’m interested in getting the aerodynamic forces in global coordinates, I did some comparisons regarding the extracted results from the Certification Test 24 (Hywind). The forces in X are quite similar for both references, where I agree given that X is the rotation axis on the hub. (Fxglobal=RtAeroFxg and Fxhub= RtAeroFxh)

On the other hand, I was expecting to get a global moment in Y mainly influenced by the X force applied on the hub. In other words, a Myglobal = Fx*Tower height(h=87.6m). But the provided by AeroDyn is much lower, as can be observed on the plot. (Myglobal=RtAeroMyg and Myhub=RtAeroMyh)

Therefore, I have checked the momentum distance by dividing Myglobal/Fxglobal obtaining a result of around 2 meters which differs considerably from the tower height. I have reduced the certification case to a much simpler one by fixing the platform degrees of freedom and using a constant wind velocity field of 10m/s, obtaining a similar distance of around 2 meters.

I’m not really sure where is the problem, or if I’m considering a wrong analysis of the physics.
Best regards,
Irene

Hi Irene,

It is only the orientation of the output that has changed (global versus local axes), not the reference point for the load calculation.

Best regards,

Jason Jonkman

Hi Jason,

Thank you for your answer, yes forces and moments are computed at the same point. What I was expecting to get, is a difference in the moments since global coordinates are supposed to be located at the tower base. Therefore, by changing to local to global, the tower height should have an influence, especially in “y” moment.

I checked the conversion from hub coordinates to fixed ones, by decomposing the rotation through sines and cosines and also taking into account the nacelle angle. This gave me as a result approximately the same values as the extracted in global coordinates.

If I’m not wrong, only the reference from rotation to fixed has changed and “RtAeroFxg, RtAeroFyg, RtAeroFzg, RtAeroMxg, RtAeroMyg, RtAeroMzg” global coordinate system is still placed in the hub, not in the tower base.

Best regards,

Irene

Dear @Irene.Berdugo,

Yes, your understanding is correct. The output coordinate systems can be considered as “floating reference frames”, where the loads output output in different coordinate systems are relative to the same point; it is only the orientation of the coordinate system that is changed.

Best regards,