Free decay analysis and natural frequency

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Abhinay.Goga
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Re: Free decay analysis and natural frequency

Postby Abhinay.Goga » Mon Sep 28, 2020 3:25 am

Dear Akheel,

Addressing your first question: In the ElastoDyn input file you can find initial platform displacements (6DOF's). Give an offset value onetime for each DOF and then can verify how the system decays before it finds a new equilibrium position. Hope this helps

Kind regards

Jason.Jonkman
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Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Mon Sep 28, 2020 10:51 am

Dear Akheel,

To make the RNA a rigid body, set CompElast = 1 in the OpenFAST primary (*.fst) input file, and disable in ElastoDyn the blade (FlapDOF1=FlapDOF2=EdgeDOF = False), hub (TeetDOF = False, for 2-bladed turbines only), and drivetrain (DrTrDOF = GenDOF = False) DOFs.

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
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Re: Free decay analysis and natural frequency

Postby Simone.Covre » Fri Nov 27, 2020 3:29 am

Dear All,

I performed free decay simulations for a 10MW OffShore RWT in order to verify the mode frequencies(in particular Pitch and Surge) of the model. The simulations were performed with CompElast, CompHydro and CompMooring active only, where HydroDyn reproduced still water enviroment. For what concern CompElast, I perfomed the decay simulations for completely rigid model, completely flexible model and completely flexible model with the rotor modelled by BeamDyn.

The results obtained for Pitch were consistent according to my opinion, while the results obtained for Surge were not; in particular, the parts of the Surge results that I can't understand are related to the difference between the damping of rigid model, flexible model and flexible model with rotor modelled by BeamDyn.

In order to be clearer, I specify that the DOFs activated in rigid model are the platform's DOFs only, the DOFs activated for flexible model are platform's DOFs+(TwFADOF1+TwSSDOF1) for tower+(FlapDOF+EdgeDOF) for blades and the DOFs activated for flexible model with BeamDyn are platform's DOFs+(TwFADOF1+TwSSDOF1) for tower and blade's DOFs associated with BeamDyn modelling.

I attach the results of the free decay analysis, I hope someone will answer about the damping question I'm asking.

Thanks in advance.
Attachments
NREL1.PNG
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NREL2.PNG
NREL2.PNG (41.89 KiB) Viewed 1165 times

Jason.Jonkman
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Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Fri Nov 27, 2020 7:06 am

Dear Simone,

I agree that the results with BeamDyn look different. I'm not familiar with the model you are running. What solver options have you set in BeamDyn and could numerical damping inherent in the BeamDyn integrator be playing a role? (You could reduce the numerical damping by increasing rhoinf; setting rhoinf = 1 eliminates numerical damping, but you'll likely need to drop the time step when increasing rhoinf.) What structural damping have you set in BeamDyn? If indeed structural or numerical in BeamDyn is having any effect, it is odd to me that it appears to be reducing the overall level of damping.

Also, does your BeamDyn model give similar results to your ElastoDyn model with only blade DOFs enabled?

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
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Organization: Politecnico di Milano
Location: Italia

Re: Free decay analysis and natural frequency

Postby Simone.Covre » Fri Nov 27, 2020 8:55 am

Dear Jason,

I performed free decay simulations with flexible rotor and rigid tower without BeamDyn, hence with platformDOFs+(FlapDOF+EdgeDOF) activated, and the results are extremely close to flexible and rigid model for both pitch and surge.

I will leave as attachment the input file I used for the simulations. The structural damping set in BeamDyn individual blade input file is equal to 0.001. I will try to perform the simulation setting rhoinf equal to 1 and I will update with the results.

Thanks for helping.
Attachments
NREL Forum.7z
(15.89 KiB) Downloaded 41 times

Simone.Covre
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Re: Free decay analysis and natural frequency

Postby Simone.Covre » Fri Nov 27, 2020 11:01 am

Dear Jason,

I verified numerical damping question and I can say that numerical dampng is not the source of the problem; I performed the simulation using rhoinf=1 and the results is equal to rhoinf=0.0.
I have no idea about possible cause right now.

Thanks for your help.

Jason.Jonkman
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Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Fri Nov 27, 2020 12:26 pm

Dear Simone,

Do you see any effect when you eliminate the structural damping in BeamDyn?

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
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Organization: Politecnico di Milano
Location: Italia

Re: Free decay analysis and natural frequency

Postby Simone.Covre » Fri Nov 27, 2020 3:26 pm

Dear Jason,

I modified BeamDyn individual blade input file fixing damp_type equal to 0 and the undamped configuration gave same output of the damped one. So I increased the structural damping, first to 0.005 then to 0.01, and the output are still equal to the undamped configuration.
I did not try higher values of damping because they require quite small time step.

I can't understand why flexible model without BeamDyn is correctly damped and suddenly, modelling the rotor with BeamDyn, the overall damping goes down.

Thanks for helping.

Jason.Jonkman
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Location: Boulder, CO
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Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Sat Nov 28, 2020 6:52 am

Dear Simone,

I'm glad that the structural and numerical damping are not playing a role...I would not have expected that anyway, but there have been underlying issues with BeamDyn, and so, I want to rule out all options.

I took a brief look at your files, but I don't see the primary FAST / OpenFAST input (*.fst) file. So, I can't run this myself. Besides, the HydroDyn file that was attached includes incident waves, which I wouldn't expect for a free-decay simulation, so, I'm not sure you sent me the correct file(s) anyway.

Regardless, which version of FAST / OpenFAST are you running? There have been many issues in BeamDyn that have been solved over the past few years; when running simulations with BeamDyn, I would recommend upgrading to the newest version of OpenFAST--at this time, the master branch of OpenFAST is v2.4. Does OpenFAST v2.4 give the same results you are showing above?

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 » Sun Nov 29, 2020 4:52 am

Dear Jason,

I am sorry about past missing files, I attached two folders, the former includes complete FASTv8 model, the latter includes complete OpenFAST v2.4.0 model(latest release I found).

I followed your advice and I performed free decay simulations using latest model, but I'm in trouble with the update included in the new model.
Even using a rigid model with ElastoDyn only, the simulation stops due to fatal error; the fatal error is related to too large platform's angle and displacement detected by HydroDyn and ElastoDyn respectively, it is also related to NaN state detected in MoorDyn.
I leave an attachment about the output which is displayed with fatal error.

I think I should probably work on MoorDyn and HydroDyn models in order to solve previous fatal error but I am not so skilled with latest release.

Thanks for your assistance.
Attachments
NREL Forum.png
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NREL Forum-OpenFASTv2.4.0.7z
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NREL Forum-FASTv8.7z
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Jason.Jonkman
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Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Mon Nov 30, 2020 4:32 pm

Dear Simone,

Looking briefly at your OpenFAST v2.4 model, it appears that the platform mass and inertias in ElastoDyn have been zeroed out. I'm not sure how you converted the model, but this is likely the cause of the numerical instability you are seeing.

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 » Tue Dec 01, 2020 3:53 am

Dear Jason,

I fixed the errors you noticed, now the simulation runs correctly.

I inspected the results with the new model compatible with OpenFAST 2.4.0 and they are the same I posted with my initial question about the damping decrease after BeamDyn introduction. Hence structural damping, numerical damping and software implied to run the simulation are not the problem that causes damping decrease.

I attach the current OpenFAST correct model I used for simulations.

Honestly I have no idea to explain or solve this damping decrease.

Thanks for your assistance.
Attachments
NREL Forum-OpenFASTv2.4.0.7z
(138.73 KiB) Downloaded 35 times

Jason.Jonkman
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Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Tue Dec 01, 2020 5:35 pm

Dear Simone,

Given all of the improvements made to BeamDyn over the past several years, I'm very surprised that you are saying the BeamDyn solution does not change between FAST v8 and OpenFAST v2.4. Can you share the OpenFAST v2.4 results for the ElastoDyn-only and ElastoDyn+BeamDyn solutions?

You said earlier that enabling blade flexibility in the ElastoDyn-only solution has very little impact on the free-decay response. Do you see similar levels of blade-tip deflection between the ElastoDyn-only solution (with blade flexibility enabled) and ElastoDyn+BeamDyn solution?

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 » Thu Dec 03, 2020 4:14 pm

Dear Jason,

I repeated simulations using exactly the same model for both FAST8 and OpenFAST2.4, the differences are only related to the updates introduced with latest versions; in particular ElastoDyn file,ED blade input and ED tower input files, HydroDyn file, MoorDyn file are equal for both softwares, BeamDyn differences are only related to the lines introduced in latest versions.

The simulatios were carried out considering floating(6 platform's DOFs) completely rigid model, floating(6 platform's DOFs) flexible[(1st Edgewise+Flapwise mode)+(1st fore aft+side to side tower bending mode)] model using CompElast=1, floating(..) flexible(..) using CompElast=2(hence BeamDyn for rotor).

Most clear differences in the simulations results are related to tip blade deflection during pitch free decay simulations. I attached the figure related to previusly mentioned differences and two folders which includes all the significative figures of the two decay simulations performed by each software.

For what concern surge and pitch free decay, in my opinion pitch decay does not highlight any significative differences between the two softwares neither between the 3 structural models (rigid,flex,flex+BeamDyn); while surge decay highlights damping differences in between the two softwares related to rigid and flexible models, instead flexible+BeamDyn model gives same results for both softwares. In particular, surge free decay with BeamDyn is still less damped than other solutions.

In order to have another comparison parameter, I considered tower base fore aft shear force too. The result related to this parameter highlight high coherence between the 3 models and the two softwares, results are almost equal for all the considered cases indeed.

If you need more data, more parameter inspections or whatever, feel free to ask for that.

Thanks for your assistance, I hope I give you useful material.
Attachments
Forum Figure OpenFAST.7z
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Forum Figure F8.7z
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NREL1.PNG
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Jason.Jonkman
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Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Fri Dec 04, 2020 2:24 pm

Dear Simone,

Indeed, the BeamDyn solution is surprising. it definitely looks there is more energy / less damping when BeamDyn is enabled. I can't really explain this right now.

Have you tried to quantify the different level of damping (using the local minima and maxima) and determine if this is more difference in linear or quadratic damping (via a PQ analysis)? In a free-decay simulation, quadratic damping tends to dominate at large amplitudes and linear damping tends to dominate at low amplitudes. Do the two damping levels approach each other at low amplitudes?

One way to dig into this more is to calculate the natural frequencies and damping through a linearization analysis rather than through free-decay simulations. Linearization functionality for floating wind wind turbines was introduced in OpenFAST v2.4. While this would take some work, I'd hope it could predict the change in damping level between these two models by examining the eigensolution and through the various linearized matrices (mass, stiffness damping), help you better isolate where the change in damping is coming from.

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


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