AeroDyn output channels coordinate system

Wind & Water Computer-Aided Engineering Software Tools
1

I am using OpenFAST to precompute the aerodynamic loads as input to my dynamic model. I am trying to use the new output channels available in AeroDyn: RtAeroFxi, RtAeroFyi, …, RtAeroMzi, and wanted to ask about their definition.

By the letter i I take these loads should be defined in the global inertial frame coordinate system. However, the magnitude of the results suggests they could instead be defined in the yaw (tower top) coordinate system, as additional translation to the platform reference point (i.e., the origin of the global system) is required to obtain the right order of magnitude of the moments. Could you confirm in which coordinate system these outputs are defined, and whether additional transformations for the platform/tower motion and shaft uptilt are required, please?

For reference, these are the mean values for the six aerodynamic load components I obtained for the case of the 5MW rotor, hub height of 90m, wind along the global x direction:

RtAeroFxi_[kN] RtAeroFyi_[kN] RtAeroFzi_[kN] RtAeroMxi_[kN-m] RtAeroMyi_[kN-m] RtAeroMzi_[kN-m]
543.78 -11.59 -84.67 3949.99 1656.90 -234.53

It seems like the loads are rotated to a system with x in the horizontal plane, judging by a significant value of RtAeroFzi, but not yet translated to the global origin (platform reference point), judging by RtAeroMxi larger than RtAeroMyi. I would appreciate your help.

Kind regards,
Kasia

2

Dear @Kasia.Patryniak,

The coordinate systems in the AeroDyn outputs (and most OpenFAST outputs in general) refers to the orientation of the vector components. The coordinate systems can be considered free floating, and for the integrated rotor aerodynamic loads, the origin is at the hub center rather than at the origin of the inertial-frame coordinate system. That is, the only difference between RtAeroMyi and RtAeroMyh is the orientation of the y axis, not a change in moment arm.

I hope that clarifies things.

Best regards,

3

Dear @Jason.Jonkman ,

Thank you very much for the clarification, all clear now!

Kind regard,
Kasia