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