Damping and shift in natural frequency

Hello everyone,

I am doing a MDOF model for the IEA 15 MW. I obtained good agreement with the natural frequency in the technical report. However, when I add the damping matrix and doing a free decay for the tower in FA direction, the PSD shows a shift in the frequency around 4.5 % . I repeated the same test but for the undamped system and the PSD gives me the exact value !

Has anyone encountered this problem and succeeded in solving it?

Best regards,


Dear @Riad.Elhamoud,

If I understand what you are saying, the frequency you derive from a tower free-decay simulation shifts by 4.5% when adding damping; is that correct? Can you clarify what you mean by “add the damping matrix”? Are you referring to ElastoDyn tower input TwrFADmp(1)? What level of damping did you add?

Best regards,

Dear @Jason.Jonkman,

Thanks for your reply !
I swear to God i don’t know what happened !!!
When i am writing to reply, i want to attach some figures and here is the surprise. It works !!!

In fact, you are right . I was extracting the natural frequencies and the eigenvectors using
|-omega_square*[M] + [K]| = 0. And to be more sure, i was running a decay test without damping and doing a PSD for the response. I was doing this for the tower in FA and i obtained nearly the same results. But, when i added damping matrix then doing a decay test for the tower in FA and doing a PSD, the frequency that i obtained was shifted a little bit.
But now the problem is solved !!! :grin:

Sorry for bothering you !

Kindly find attached the frequency obtained using det(-omega_square*[M]+[K]) = 0 :

Also, kindly find attached the PSD in dB in the presence of damping :


Thank you again and sorry for bothering you

Best regards,