# Tower natural frequencies

Dear Jason,

I would like to know how do i intepret the time histories of the shear force in the base of the tower, i mean fft of this would be giving me peaks of what?
also jason if i want to find out the tower natural frequencies what is the procedure for that, kindly reply in this regard,

regards

Dear Purushotham,

An FFT or PSD shows how the frequency dependence of the variation of a signal. The integral of the PSD across frequency equals the variance (standard deviation squared) of the signal.

Natural frequencies can by found through FAST linearization followed by eigenanalysis or by exciting the system with broadband excitation and examining the FFT/PSD of the time-series response, as has been discussed many times in this forum.

Best regards,

Dear Jason,

I have the understanding that the peaks of the fft gives us the natural frequencies, now given the fact i have the load history of tower base shear force variation in the 3 axes and the corresponding moments in the three axes, would it be rite to say that the peaks at which they are occuring would be their natural frequencies and when you say first mode tower side to side and second mode side to side natural frequencies what exactly do you imply, do u mean the occurence of two consecutive peaks and also the fore-aft natural frequencies,would the peaks of fft of twrbsfxt,fyt and fzt give the peaks of these,also can i apply the same logic to all the loading(yawbrfx etc). kindly reply as soon as posssible.

regards

Dear Puroshotham,

The peaks in the FFT result from excitation of natural frequencies or frequencies peaks in the wind/wave excitation loads. Without knowledge of the wind/wave excitation, it is difficult to say that the peaks in the FFT are purely natural frequencies, which is why we typically recommend the use of white-noise excitation if the goal is to identify natural frequencies through FFT analysis. That said, for an operational rotor, even if a white-noise wind spectrum is used, the wind excitation will still occur at harmonics of the rotor revolution (0P, 3P, 6P etc. in the nonrotating frame for a 3-bladed rotor).

By “first side-to-side mode” and “second side-to-side mode”, I’m referring to the 1st and 2nd bending modes of the tower e.g. as depicted in the following image.

Best regards,