Local Tower Loads output

I just want to ask for the clarification of the local tower load output. I output tower loads at the specific tower sections. TwHt1FLxt, TwHt1FLyt, TwHt1FLzt TwHt1MLxt, TwHt1MLyt, and TwHt1MLzt are requested to be output. However, I am doubtful about the definition of these parameters. According to the “OutListParameters.xlsx”, TwHt1FLxt is “Local tower roll (or side-to-side) force of tower gage 1”. TwHt1FLyt is “Local tower pitching (or fore-aft) force of tower gage 1”. TwHt1FLzt is “Local tower yaw (or torsional) force of tower gage 1”. TwHt1MLxt is “Local tower roll (or side-to-side) moment of tower gage 1”. TwHt1MLyt is “Local tower pitching (or fore-aft) moment of tower gage 1”.Finally, TwHt1MLzt is “Local tower yaw (or torsional) moment of tower gage 1”.

However, I am not quite sure if these parameters refer to the local axis. I checked by running simulations with no external conditions (no wind and wave, only static equilibrium) but many nacelle yaw directions. For the case of nacelle pointing at 0 degree, TwHt1MLyt is larger than TwHt1MLxt which makes sense and I understand that the Fore-aft moment is TwHt1MLyt while the Side-side moment is TwHt1MLxt. However, when the nacelle yaw direction is at 90 degree, TwHt1MLxt is larger than TwHt1MLyt. So, TwHt1MLxt is the Fore-aft Moment which is strange to me because the local axis in my understanding should rotate as nacelle rotates and the fore-aft moment should always be TwHt1MLyt.

So, could you please clarify?

Dear Danupon,

The (Lxt,Lyt,Lzt) local tower coordinate system is fixed within the tower and displaces/deflects with the platform and tower. But it does not rotate with the nacelle. The terms “fore-aft” and “side-to-side” in the output descriptions really only apply when the nacelle-yaw angle is zero or 180 degrees.

Just FYI: It looks like the local tower forces–TwrHt1FLxt, TwrHt1FLyt, and TwrHt1Flzt etc.–are not described correctly in the OutListParameters.xlsx spreadsheet. They should be described as “Local tower fore-aft shear force of tower gage 1”, “Local tower side-to-side shear force of tower gage 1”, and “Local tower axial force of tower gage 1” etc., respectively. I’ve now marked this as an issue to be fixed in OpenFAST: github.com/OpenFAST/openfast/issues/413.

Best regards,