I wanna calculate the natural frequency of OC3-monopile with apparent fixity foundation model. And conputing the PSD by performing a FFT of a FAST output time series could do that, right?
Then, I set WaveMode to 3 and WvDiffQTF, WvSumQTF to False, after 630s FAST simulation and through Jason_PSD.m, I got this result which I think is a little bit strange and don’t know how to get the natural frequency.
Yes, you can use white-noise waves excitation to compute natural frequencies, although the results are not always easy to interpret for complicated multiple DOF models.
What FAST output are you computing the PSD of? Are you plotting the bending moment at the mudline?
When using white noise, be mindful of the first-order low and high wave cut-off frequencies (WvLowCOff, WvHiCOff). This is the frequency range that your structure will be excited and this range should encompass the frequencies you are trying to identify.
To interpret the PSD plot, I think it helps to look at both the linear scale (as you have) and a semilog scale.