Free decay analysis and natural frequency

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

Moderators: Bonnie.Jonkman, Jason.Jonkman

Andy.Platt
Posts: 10
Joined: Thu Dec 15, 2011 4:53 pm
Organization: NREL
Location: Boulder, Colorado

Re: Free decay analysis and natural frequency

Postby Andy.Platt » Mon Dec 07, 2020 12:58 pm

Dear Simone,

I noticed a couple of things in your input files.

First, the timestep in the main .fst file is too large for a simulation using BeamDyn. I would expect this to be on the order of 0.0025 seconds or smaller. This may be causing numerical instability in the coupling of BeamDyn to ElastoDyn -- this is noticable in the low speed shaft (LSShftMxa channel).

Second, the twist distribution in the BeamDyn input file is not smooth. BeamDyn is not be able to represent it well with the Legendre Spectral Finite Elements (LSFE) that it uses in calculations. This may lead to some numerical instabilities in the calculations and a poor representation of the blade.

After reducing the timestep for the BeamDyn simulation, I see closer agreement for the Surge decay (I'll post a plot once the simulation completes).

Regards,
Andy
Twist.png
Twist.png (18.3 KiB) Viewed 392 times

Simone.Covre
Posts: 22
Joined: Thu Jul 30, 2020 6:41 am
Organization: Politecnico di Milano
Location: Italia

Re: Free decay analysis and natural frequency

Postby Simone.Covre » Wed Dec 09, 2020 7:10 am

Dear Andy,

I appreciated your advices. I performed a surge free decay simulation using time step equal to 0.001s which is half the time step you mentioned and it is one order of magnitude less than the previous one I used.

From the new simulation the result obtained is the same I obtained with larger time step.
The only parameter I changed in the simulation was time step, I didn't modify twist distribution.

Did you change more parameters or did you modify only the time step?
I'll wait until you post your results before post the PQ analysis performed on the previous data obtained with larger time step.

Thanks for your help and I apologize for the time without giving answer to Dr. Jonkman but I wasn't familiar with PQ analysis, so I needed more time to make myself clear.

Simone Covre

Andy.Platt
Posts: 10
Joined: Thu Dec 15, 2011 4:53 pm
Organization: NREL
Location: Boulder, Colorado

Re: Free decay analysis and natural frequency

Postby Andy.Platt » Wed Dec 09, 2020 2:06 pm

Dear Simone,

I forgot to mention that I did modify the twist distribution. I set the first 6 keypoints to a twist of 42.8040 degrees (same as keypoint #6 originally). This really isn't a good fit either and won't be well represented by the Legendre Spectral elements either due to the discontinuity.

I also increased the order of the element from 4 to 5. This helps a little in achieving a better fit to the twist. With the blade better represented by the Legendre functions, the result is much closer to the ElastoDyn results.

SurgeDecay.png
SurgeDecay.png (20.34 KiB) Viewed 371 times


We are in the process of making several updates to BeamDyn that should make this easier for end users:
  • Better fitting of the keypoint line using a least squares fit of Legendre spectral functions (https://github.com/OpenFAST/openfast/pull/576)
  • Summary file information with details on the fit to the keypoint line. This will allow for quick plotting to compare how BeamDyn is modeling the blade vs. what was given.

Regards,
Andy

Jason.Jonkman
Posts: 5771
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Wed Dec 09, 2020 2:31 pm

Thanks, Andy! This is very informative.

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

Simone.Covre
Posts: 22
Joined: Thu Jul 30, 2020 6:41 am
Organization: Politecnico di Milano
Location: Italia

Re: Free decay analysis and natural frequency

Postby Simone.Covre » Wed Dec 09, 2020 4:06 pm

Thank you very much for the explanation Andy and thank you all for the support.

Simone Covre


Return to “Computer-Aided Engineering Software Tools”

Who is online

Users browsing this forum: No registered users and 1 guest