Out of plane blade 1 tip deflection

Dear @Jason.Jonkman ,

I did a pitch free decay test for OC3 Hywind and i’m interested in out of plane blade 1 tip deflection (oopdefl1).
I plot the PSD of the oopdefl1 and i found four peaks. I was expecting three peaks which correspond to pitch, tower top fore aft and 1st blade flapwise respectively. I think the fourth is for 2nd blade flapwise.

But, what i find a little weird, that the amplitude that corresponds to the frequency of the 1st tower fore aft is the highest. Frankly, i was expected that the amplitude that corresponds to 1st blade flapwise will be the highest.

What do you think ? Plz if you could explain.

Hereafter, the plot of the PSD :



Dear @Riad.Elhamoud,

Presumably your pitch free-decay begins by initializing nonzero platform pitch with zero tower and blade deflection. But because this state is not a natural full-system mode of the structure, the free-decay motion leads to motion in other full-system modes of the model. Apparently in this situation, it is the tower mode that has the largest response. The full-system tower mode likely has large motion of the blade out-of-plane DOF.

Best regards,