Dear Mr. Jonkman.
A new question has come up that I hope you can help me with.
I am wondering why the natural frequencies from BModes for the original model (rigid foundation, no modification done in any of the FAST input files provided by NREL) is not expected to be the same as the natural frequencies given in “Definition of a 5-MW Reference Wind Turbine for Offshore System Development”. What is the difference between these “full system natural frequencies” and the ones from BModes? The first fore-aft frequency calculated from BModes is 0.2804 Hz, while the one given in the report is 0.3240 Hz.
I computed the PSD from the fore-aft tower top displacement (excited the structure with white noise wave spectrum only, no wind, as explained in this forum post: AeroDyn Feedback), the PSD figure is attached, the rigid (original) foundation model vs the apparent fixity model. The frequency peaks seem to be approximately similar to the fore-aft natural frequencies from BModes, but not exactly. Does this PSD seem correct? What have I misunderstood about the “full system natural frequencies?”, what do they represent and why are they not the ones present in the PSD plots?"
Thank you in advance.
Best regards,
Ingrid
