Fluctuations in the rotor swept area

Dear all,
I am using openFAST3.5.1 to simulate a 5MW semi-submersible wind turbine, I tried to get ‘RtAeroFxh’ through the ExtPtfm module by giving the platform forced vibration(I only give GenDOF and PtfmSgDOF ‘TRUE’),
but the results were not what I expected, so I checked ‘RtArea’, ‘RtAeroCt’, and ‘RtVAvgxh’,
Among them, the fluctuation of ReArea makes me more confused. I tried to turn off the subdyn module and turn off the PtfmSgDOF, and RtArea no longer fluctuates, so I think the current fluctuation is related to surge.

As shown in the figure, I think the change of ‘RtArea’ should be consistent with surge.
Could you please help me analyze the possible reasons for the fluctuation of ‘RtArea’?
In addition, if I need to add any simulation information to help judge, I will reply soon!


Dear @He.Li,

I would not expect AeroDyn output RtArea to change with time unless blade structural degrees of freedom are enabled. I’m not sure I understand the units on the blue y-axis. Is the variation you are showing simply a numerical precision issue?

Best regards,