Natural Frequencies with FAST v8

Provide feedback, request enhancements, and get help with wind-turbine computer-aided engineering tools.

Moderators: Bonnie.Jonkman, Jason.Jonkman

Randi.Widjojo
Posts: 8
Joined: Sun Jul 26, 2015 2:25 am
Organization: University of Stuttgart
Location: Germany

Natural Frequencies with FAST v8

Postby Randi.Widjojo » Fri Oct 23, 2015 6:25 am

Dear Jason,

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!

Dear Dr. Jonkman,

I've been reading the forum and other documents to find out a way to calculate natural frequencies with FAST v8.
I'd like to know whether the linearization functionality has been added to FAST v8 and if not would you please tell me if there is any predictions for release time of it?

Would you also please introduce some documents on PSD-procedure to compute full-system natural frequencies with FAST v8?

Thank you very much in advance for your reply.

Regards


Regards,
Randi

Jason.Jonkman
Posts: 4862
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Natural Frequencies with FAST v8

Postby Jason.Jonkman » Fri Oct 23, 2015 10:10 am

Dear Randi,

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.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Georgios.Gasparis
Posts: 9
Joined: Thu Sep 20, 2018 9:19 am
Organization: self
Location: Netherlands

Re: Natural Frequencies with FAST v8

Postby Georgios.Gasparis » Thu Sep 27, 2018 3:15 pm

Hello Everybody,

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?

Thank you very much!

Best regards,

Georgios

Jason.Jonkman
Posts: 4862
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Natural Frequencies with FAST v8

Postby Jason.Jonkman » Thu Sep 27, 2018 3:47 pm

Dear Georgios,

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.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Georgios.Gasparis
Posts: 9
Joined: Thu Sep 20, 2018 9:19 am
Organization: self
Location: Netherlands

Re: Natural Frequencies with FAST v8

Postby Georgios.Gasparis » Fri Sep 28, 2018 4:53 am

Dear Jason,

Thank you very much for your help and your ideas!

Kind regards

Georgios

Mustafa.Vardaroglu
Posts: 19
Joined: Sat Jan 06, 2018 11:45 am
Organization: University of Campania
Location: Italy

Re: Natural Frequencies with FAST v8

Postby Mustafa.Vardaroglu » Tue Sep 17, 2019 4:39 am

Dear Jason,

In addition to the platform natural frequencies, is there a way to conduct a free decay test on tower oscillations? Such as an application of unit impulse to the tower?

Sincerely,
Mustafa Vardaroglu

Jason.Jonkman
Posts: 4862
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Natural Frequencies with FAST v8

Postby Jason.Jonkman » Tue Sep 17, 2019 6:29 am

Dear Mustafa,

Instead of providing a unit impulse, you can set an initial tower-top displacement (in the ElastoDyn module) to conduct a free-decay test of the tower oscillation.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Mustafa.Vardaroglu
Posts: 19
Joined: Sat Jan 06, 2018 11:45 am
Organization: University of Campania
Location: Italy

Re: Natural Frequencies with FAST v8

Postby Mustafa.Vardaroglu » Tue Sep 17, 2019 11:05 am

Jason.Jonkman wrote:Dear Mustafa,

Instead of providing a unit impulse, you can set an initial tower-top displacement (in the ElastoDyn module) to conduct a free-decay test of the tower oscillation.

Best regards,


Dear Jason,

Thanks a lot for your time and help...

Sincerely,
Mustafa Vardaroglu


Return to “Computer-Aided Engineering Software Tools”

Who is online

Users browsing this forum: No registered users and 1 guest