Hi
See our courses in harmonic analysis for details on how to set that up and how to create a FRF.
As for the other question what is the drag force (is there a fluid flow past this device), and what do you mean when drag force changes so does the frequency? Also what is the excitation here (force), you are only showing the modal system, include harmonic. so many more facts so forum members can provide some feedback.
All the best
Erik
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