I am posting a question for a fellow student, as she is still in process of requesting a forum account.
Her question regards the calculation of natural frequencies with FAST v8, I hope you can help her out. Thank you in advance!
Linearization functionality has not yet been added to FAST v8. However, we’ve recently initiated the effort and plan to release it in 2016.
Until then, FAST v7 (and its linearization functionality) is still available. Alternatively, another way to identify natural frequencies of a FAST model is by computing the PSD (or FFT) of FAST time series and identifying the frequency peaks. It is best if you can excite the structure with white noise (e.g., broadband turbulence). However, it may be difficult to identify which frequencies correspond to which modes without running multiple simulations with various degrees of freedom (DOFs) enabled/disabled. But I have not written up a detailed stepwise procedure on the approach on the process.
I’m new in FAST and I decided to use the last version FAST v8.16. I am also working on the Test #19, which is the 5MW Baseline Wind Turbine with OC3 Monopile. My target is to derive the natural frequencies, but also the related damping of the modes of the full system.
I need to include the platform in the linearization, since I am looking at the dynamic characteristics of the whole structure. The only way to derive the natural frequencies of the structure is, as I can see, by looking in the FFT of time histories. However, in that way I cannot determine the related damping.
Is it possible to determine the natural frequencies and also the damping of the whole system, including the platform, using FAST v8.16, or should I try to shift into FAST v7?
Since the original forum post, we’ve added full-system linearization capability in FAST v8.16 with the AeroDyn, InflowWind, ServoDyn, and ElastoDyn modules. More recently, we are working on additional linearization capability in OpenFAST with BeamDyn, as well as with HydroDyn and MAP++ for floating offshore wind turbines, but we haven’t yet been funded to add this functionality to SubDyn for fixed-bottom offshore wind turbines.
Until this linearization capability with SubDyn is added, using the linearization functionality of FAST v7 is likely the most direct way of obtaining the natural frequencies and damping of full-system wind turbines atop fixed-bottom monopile substructures.
You may be able to do free-decay simulations in FAST v8.16 or OpenFAST to calculate the damping of some modes (e.g. perturb the tower-top displacement and calculate the damping from the resulting decay of the time series), but this process may be difficult for all modes.
The documentation for FAST / OpenFAST is still a work in progress. While new documentation is in the works, we don’t currently have detailed step-by-step guide to performing linearization analysis and interpreting the eigensolution. That said, there have been many questions posed over the years both on this forum and on github issues (github.com/OpenFAST/openfast/issues) and much guidance has been provided based on those questions. I would encourage you to use the search functionality and browse the related forum topics and github issues.