model Validation

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Jason.Jonkman
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Re: model Validation

Postby Jason.Jonkman » Tue Nov 10, 2020 9:05 am

Dear Satish,

How many nodes are you using? I would expect the results to converge as the number of nodes is increased. You may also want to increase the structural discretization in SubDyn.

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
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Satish.Jawalageri
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Re: model Validation

Postby Satish.Jawalageri » Tue Nov 10, 2020 9:10 am

Dear Jason,

Thanks for your reply.

I have considered 25 nodes (got this from .HD sum file) from +3m to -20m and considered NDiv in Subdyn as 3.

Thanks,
Satish J

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Re: model Validation

Postby Jason.Jonkman » Tue Nov 10, 2020 9:17 am

Dear Satish,

And how does the result change when you increase the number of nodes?

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

Satish.Jawalageri
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Re: model Validation

Postby Satish.Jawalageri » Tue Nov 10, 2020 9:48 am

Dear Jason,

Thanks for your reply.

Sorry for the confusion, first case I have considered 25 nodes (got this from .HD sum file) from +3m to -20m and considered NDiv in Subdyn as 3; second case I have considered 33 nodes from +11 to -20m.

Here, the wave height of 6m is considered for both cases.

-When I plot against time, I can observe increase in BM for second case when compared to first case.

- But finally, in any of the cases I could not succeed to validate.

Thanks,
Satish J

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Re: model Validation

Postby Jason.Jonkman » Tue Nov 10, 2020 9:58 am

Dear Satish,

Are you running a case with linear waves (Airy) or with free surface nonlinearity? 3 m above MSL is OK for the former, but not for the latter, where the wave peaks would be higher than 3 m for a 6-m wave height.

Regardless, the solution may still not be converged. We typically recommend using a wave discretization of at last 0.5 m near the free surface. I suggest increasing resolution to see if results improve.

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

Satish.Jawalageri
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Re: model Validation

Postby Satish.Jawalageri » Wed Nov 11, 2020 2:36 am

Dear Jason,

Thanks for your reply.

Yes, I am running case with linear waves.
I have increased number of nodes i.e., from +3m to -20m with 0.5m node in hydrodyn. I could not see much change (attached plot).

Thanks,
Satish J
Attachments
LC5.1_Task23_Comparison 3 - -20 - 1 and 0.5 div.jpg
LC5.1_Task23_Comparison 3 - -20 - 1 and 0.5 div.jpg (50.63 KiB) Viewed 892 times

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Re: model Validation

Postby Jason.Jonkman » Wed Nov 11, 2020 9:08 am

Dear Satish,

It looks like the main difference between your results for load case 5.1 from OC3 Phase I and those from OC3 are in the mean value. This could come from differences in the mean thrust or wave nonlinearity (mean drift, from wave stretching). I would suggest running load case 3.1 (wind only) and load case 4.1 (wave only) to help isolate the difference in mean value.

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

Satish.Jawalageri
Posts: 117
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Re: model Validation

Postby Satish.Jawalageri » Wed Nov 11, 2020 10:00 am

Dear Jason,

Thanks for your reply.

As you suggested,

1. I have run the analysis LC 3.1 by considering water density = 0 and WavMod = 0 in hydrodyn file and considered TStart as 30sec. I am getting different values when compared (attached plot).

2. For the LC 4.1, I have considered air density = 0 in aerodyn module and RotSpeed = 0 in elastodyn , but I am getting following error,

Code: Select all

FAST_InitializeAll:FAST_Init:FAST_ReadPrimaryFile:1/VTK_fps is not an integer multiple of DT.
FAST will output VTK information at 15.385 fps, the closest rate possible.
FAST_InitializeAll:AD_Init:ValidateInputData:The air density (AirDens) must be greater than zero.

 FAST encountered an error during module initialization.
 Simulation error level: FATAL ERROR

 Aborting OpenFAST.


Thanks,
Satish J
Attachments
LC 3.1 Comparison.jpg
LC 3.1 Comparison.jpg (41.72 KiB) Viewed 880 times

Jason.Jonkman
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Re: model Validation

Postby Jason.Jonkman » Wed Nov 11, 2020 1:38 pm

Dear Satish,

LC 3.1 of OC3 Phase I involves a land-based turbine. So, you should not be comparing loads at the seabed.

For LC 4.1, it is better to disable aerodynamic loads altogether, rather than to set the air density to zero. Set CompAero = 0.

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

Satish.Jawalageri
Posts: 117
Joined: Sun May 31, 2020 4:57 am
Organization: University College Dublin
Location: Ireland

Re: model Validation

Postby Satish.Jawalageri » Wed Nov 11, 2020 3:02 pm

Dear Jason,

Thanks for your reply.

- For LC 3.1, I have analyzed by setting WaveMod and water density to zero and I have compared tower top FA BM and have attached plots as they are not matching well when compared.

- For LC 4.1, as you suggested I have considered CompAero = 0 and I have attached the plots of comparison (Mudline BM).

Thanks,
Satish J
Attachments
LC 4.1 Comparison.jpg
LC 4.1 Comparison.jpg (44.53 KiB) Viewed 870 times
LC 3.1 - Comparison.jpg
LC 3.1 - Comparison.jpg (67.39 KiB) Viewed 870 times

Jason.Jonkman
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Re: model Validation

Postby Jason.Jonkman » Wed Nov 11, 2020 4:12 pm

Dear Satish,

It looks like these results still have some start-up transients in them. How do the results compare after the start-up transient has passed?

For LC 4.1, how does the bending moment at the mudline look?

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

Satish.Jawalageri
Posts: 117
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Re: model Validation

Postby Satish.Jawalageri » Thu Nov 12, 2020 2:46 am

Dear Jason,

Thanks for your reply.

The results I have attached in the previous post and this post are after transient time of 30sec (TStart = 30sec).

I have attached the plot for BM of LC 4.1.

Thanks,
Satish J
Attachments
LC 4.1 Comparison.jpg
LC 4.1 Comparison.jpg (46.31 KiB) Viewed 863 times

Jason.Jonkman
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Re: model Validation

Postby Jason.Jonkman » Thu Nov 12, 2020 6:14 am

Dear Satish,

There still appears to be some transients in both LC 3.1 and LC 4.1. I would suggest increasing the start-up transient period to 60 s and only outputting after that.

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

Satish.Jawalageri
Posts: 117
Joined: Sun May 31, 2020 4:57 am
Organization: University College Dublin
Location: Ireland

Re: model Validation

Postby Satish.Jawalageri » Thu Nov 12, 2020 10:17 am

Dear Jason,

Thanks for your reply.

I have considered TStart = 60 sec and attached the plots. It looks LC 4.1 is matching well but not LC 3.1.

Thanks,
Satish J
Attachments
LC 3.1 - Comparison 60 sec transient.jpg
LC 3.1 - Comparison 60 sec transient.jpg (69.95 KiB) Viewed 854 times
LC 4.1 Comparison 60 sec trans.jpg
LC 4.1 Comparison 60 sec trans.jpg (44.23 KiB) Viewed 854 times

Jason.Jonkman
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Re: model Validation

Postby Jason.Jonkman » Thu Nov 12, 2020 1:01 pm

Dear Satish,

The difference in mean bending moment at the mudline from LC 5.1 that you showed was about 3000 kNm. The difference in mean value of tower-top moment for LC 3.1 that you are showing is about 200 Nm. So, there is still something that is not yet making sense. That said, the moment arm from the thrust to the tower-top force is rather smaller. The shear force at the tower top multiple by the height of the support structure is likely the bigger effect. How does the shear force at the tower-top compare?

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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