Definition of parameters

Discuss theory and modeling of wind-turbine structures.

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Jason.Jonkman
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Re: Definition of parameters

Postby Jason.Jonkman » Mon Jun 17, 2019 5:19 pm

Hi Rocío,

What I said is that ElastoDyn assumes the neutral axis and pitch axis are coincident. ElastoDyn also assumes the center of gravity of each cross section is coincident with the pitch axis.

Obviously, the blade structural model of ElastoDyn is quite limited (bending only, no offsets of the mass or elastic axis from the pitch axis, etc.). We use the BeamDyn module of FAST / OpenFAST for modeling the blade structural dynamics with higher fidelity.

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

Rocio.Torres
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Re: Definition of parameters

Postby Rocio.Torres » Mon Aug 26, 2019 4:50 am

Hi again Jason,

I wrote some time ago because I was trying to adapt an V27 OpenFAST model to the new turbine Orcaflex feature and I had some doubts about the definition of certain parameters you very kindly clarified.
In that time you told me the neutral axis in ElastoDyn is assumed to lie on the pitch axis. As I had the corresponding "..._ElastoDyn_Blade.dat" file containing the pitch axis list I took that data. However, even then the distribution of the pitch axis there specified seemed a bit weird to me, since it placed the pitch axis for the root cylinder and thicker, near to root profiles, already at ~26% of the chord from the leading edge instead of 50% for the cylinder and near to 50% values for the inmmediate following sections, which is what would have seem reasonable to me.
After that, checking on the Orcaflex adaptation of the 5 MW NREl tubine Orcina did, I realized they had taken Pitch/netral axis values from the list you provided in a post of the following thread: viewtopic.php?f=2&t=440&p (Fri Jan 28), but these values are different from those other specified in the NRELOffshrBsline5MW_Blade.dat file , downloaded with the F8/OpenFAST archive. The values on the list you provide in the forum post make sense to me, while similarly to the case of the V27 FAST model I am trying to adapt, the list on the "..blade.dat" file don't.
I was wondering if there is some transformation here needed that I am missing...Why are the values for the pitch axis distribution different on the list you provided on the post compared to those in the 5MW baseline files?
I am attaching the files I make reference to.

Thank you in advance for your time.
Best regards,
Rocío Torres
Attachments
5MW_Blade_posted.txt
(1.52 KiB) Downloaded 4 times
SNLV27_ElastoDyn_Blade.txt
(5.09 KiB) Downloaded 3 times
NRELOffshrBsline5MW_Blade.txt
(6.57 KiB) Downloaded 3 times

Jason.Jonkman
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Re: Definition of parameters

Postby Jason.Jonkman » Mon Aug 26, 2019 7:12 am

Dear Rocio,

Yes, indeed this is confusing. Unfortunately, the two definitions of PitchAxis are not the same. The forum post you reference used the following definition (see viewtopic.php?f=2&t=440&p for details):

PitchAxis = 0.25 - AeroCent - AeroRef

But in ElastoDyn within FAST v8, PitchAxis is defined as (from the FAST v8 ReadMe file: https://wind.nrel.gov/nwtc/docs/README_FAST8.pdf):

PitchAxis = 0.5 – AeroCent

Regardless, the PitchAxis variable in the ElastoDyn module of FAST v8 is not used when AeroDyn v15 is enabled. This input in ElastoDyn is used to define the aerodynamic center offset from the pitch axis for AeroDyn v14. But when AeroDyn v15 is enabled, the aerodynamic center is defined within AeroDyn v15 instead of ElastoDyn. Once AeroDyn v14 is removed from OpenFAST (AeroDyn v15 is meant to replace AeroDyn v14), the PitchAxis input from ElastoDyn will be eliminated as well.

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

Rocio.Torres
Posts: 12
Joined: Thu Aug 30, 2018 7:50 am
Organization: X1WIND
Location: Spain

Re: Definition of parameters

Postby Rocio.Torres » Tue Aug 27, 2019 6:12 am

Hi Jason,

Thank you. But I am afraid I am still confused...Then the "PitchAxis" variable in the ElastoDyn module of FAST v8 is NOT the fraction of the chord from the leading edge to the actual pitch axis, is it?

Is there any way I can calculate the actual pitch axis from the information contained in the FAST v8 model files I have for the V27? I guess the only way would be to have the "Aeroref" values and use the first equation, but I don't have those for the first blade span sections...

Thank you!
Best regards,
Rocío

Jason.Jonkman
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Joined: Thu Nov 03, 2005 4:38 pm
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Re: Definition of parameters

Postby Jason.Jonkman » Tue Aug 27, 2019 7:56 am

Dear Rocio,

Correct. PitchAxis in ElastoDyn is not necessarily the fraction of the chord from the leading edge to the actual pitch axis.

The PitchAxis input to ElastoDyn of FAST v8 / OpenFAST, as well as the old AeroCent input in the structural model of FAST v7, only determine where the aerodynamic center is relative to the pitch axis. The actual leading and trailing edge locations are not needed by the software, and so, are not derivable from the data specified in the FAST input file(s).

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

Rocio.Torres
Posts: 12
Joined: Thu Aug 30, 2018 7:50 am
Organization: X1WIND
Location: Spain

Re: Definition of parameters

Postby Rocio.Torres » Wed Aug 28, 2019 4:30 am

Hi Jason,

Crystal clear now. Thank you very much.

Best regards,
Rocío


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