Natural frequency DeepCWind and Random Seeds

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Andre.White
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Organization: University of Plymouth
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Natural frequency DeepCWind and Random Seeds

Postby Andre.White » Thu Mar 11, 2021 1:46 am

Dear Sir or Madam:

I have two queries about FASTv8:
1a) Is there a way to find the natural frequency of the floater using FAST? In particular, the DeepCwind floater.
1b) I had initially thought that since the tower is connected to the floater they would act as one and hence the natural frequency of the combined system would be required(similar to a fixed-based OWT). However from the literature, I realize that the natural frequency of the tower is calculated separately from the floater. Is the concept that you would determine the natural frequency for the floater and then attach tower and determine natural frequency and that frequency of combined system is taken for the tower? This is not clear please explain
2) The different random seeds used to get in design load cases to get multiple sets of results for the same sea state variables, does this occur naturally when more than one simulation is run or do you have to change a value is FAST to activate the different seeds?

Regards,
AOAW

Jason.Jonkman
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Re: Natural frequency DeepCWind and Random Seeds

Postby Jason.Jonkman » Thu Mar 11, 2021 6:47 am

Dear Andre,

Here are my answers to your questions:

1a) The most direct way of computing full-system natural frequencies is through the linearization functionality of FAST / OpenFAST. But in FAST v8, full-system linearization was only available for land-based turbines (for the ElastoDyn, InflowWind, AeroDyn, and ServoDyn modules). Linearization capability for floating offshore wind turbines (including for the HydroDyn, MAP++, BeamDyn, and SubDyn modules) has been added to OpenFAST; I would suggest upgrading from FAST v8 to OpenFAST to use this capability. Alternatively, you can use white-noise waves excitation to compute natural frequencies (as discussed several times on this forum), although the results are not always easy to interpret for complicated multiple DOF models.

1b) I'm not sure what literature you are referring to, but I would say that the analysis of the combined system is required; the floating boundary condition can impact the tower natural frequencies and the flexible tower can impact the floating platform natural frequencies (particularly for TLPs).

2) You need to change the WaveSeed input in HydroDyn to get different sea states for the same wave conditions.

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

Andre.White
Posts: 37
Joined: Wed Jan 29, 2020 3:16 am
Organization: University of Plymouth
Location: United Kingdom

Re: Natural frequency DeepCWind and Random Seeds

Postby Andre.White » Tue Mar 16, 2021 3:38 am

Hi Jason:
Many thanks for the prompt feedback. With regard to OpenFAST I would very much like to upgrade to it. I had tried using it prior to FAST v8 but had problems setting it up. Now that I have a gained a bit more understanding I would like to find out:
- Is it possible to install the binaries for OpenFAST to my computer in a similar manner like for FAST v8 or do I have to go through the compilation process with CygWin and CMake etc? I am working with a Windows OS.

I am assuming that I have to change both random seeds for each simulation and that the value of each can be any number within the range specified. Is that correct?

I have checked another paper and it does refer to the natural frequency of the whole system. Thanks for the clarification.

Regards,
AOAW

Jason.Jonkman
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Re: Natural frequency DeepCWind and Random Seeds

Postby Jason.Jonkman » Tue Mar 16, 2021 9:54 am

Dear Andre,

Precompiled binary executables for Windows have been provided with each release of OpenFAST--see: https://github.com/OpenFAST/openfast/releases. To run these, if you don't already have the Intel Fortran compiler installed, you must install the free redistributable libraries from Intel as discussed here: https://github.com/OpenFAST/openfast/issues/663.

If you need to change the source code, you must recompile, but in my opinion, on Windows, it is easiest to compile OpenFAST through Visual Studio using the provided solution files (without CMake).

You can change one or the other or both wave seeds. Having two seeds provides for many different realizations.

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

Andre.White
Posts: 37
Joined: Wed Jan 29, 2020 3:16 am
Organization: University of Plymouth
Location: United Kingdom

Re: Natural frequency DeepCWind and Random Seeds

Postby Andre.White » Wed Mar 17, 2021 1:26 pm

Jason:

Thank you. I will install the compilers and binaries and see how it goes.

Regards,
AOAW

Andre.White
Posts: 37
Joined: Wed Jan 29, 2020 3:16 am
Organization: University of Plymouth
Location: United Kingdom

Re: Natural frequency DeepCWind and Random Seeds

Postby Andre.White » Fri Mar 19, 2021 3:10 am

Hi Jason:

Going back the natural frequency of DeepCwind. In openfast I see that there is a file called "5MW_OC4Semi_Linear.FST".

1) Is this the file that is required to determine the natural frequency of the system? Please also advise me on anything else that may be required to carry out this process.

Thank you.

Regards,
AOAW

Jason.Jonkman
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Re: Natural frequency DeepCWind and Random Seeds

Postby Jason.Jonkman » Fri Mar 19, 2021 9:35 am

Dear Andre,

Yes, the r-test file 5MW_OC4Semi_Linear.fst is used to perform a linearization analysis of the OC4-DeepCwind semi in still water without aerodynamic loads while spinning at 12.1 rpm, from which the full-system natural frequencies and mode shapes can be extracted. The process and results are described in our DeepWind 2019 paper: https://iopscience.iop.org/article/10.1 ... 6/1/012022.

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

Andre.White
Posts: 37
Joined: Wed Jan 29, 2020 3:16 am
Organization: University of Plymouth
Location: United Kingdom

Re: Natural frequency DeepCWind and Random Seeds

Postby Andre.White » Tue Mar 23, 2021 5:54 am

Many thanks Jason.

Regards,
AOAW

Andre.White
Posts: 37
Joined: Wed Jan 29, 2020 3:16 am
Organization: University of Plymouth
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Re: Natural frequency DeepCWind and Random Seeds

Postby Andre.White » Wed May 05, 2021 6:36 am

Jason:
I have obtained the eigenvalues of the DeepCwind system using A=eig(Amatrix) in MATLAB. However, I am not sure which is the first natural frequency of the different degrees of freedom, surge, yaw etc. Also the output is of the format A+Bi, I do not remember how to interpret or convert to the natural frequencies in Hz as I did this a long time ago.

Would be grateful for some clarification.

Thank you.

Regards,
AOAW

Andre.White
Posts: 37
Joined: Wed Jan 29, 2020 3:16 am
Organization: University of Plymouth
Location: United Kingdom

Re: Natural frequency DeepCWind and Random Seeds

Postby Andre.White » Wed May 05, 2021 7:47 am

Jason:
If my understanding is correct I believe the real part is the natural frequency, so if eigenvalue = A + Bi, the natural frequency is A. While the imaginary part gives information about the damping for that particular mode. However, I do not know which frequency is for which degree of freedom.

Regards,
AOAW

Jason.Jonkman
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Re: Natural frequency DeepCWind and Random Seeds

Postby Jason.Jonkman » Wed May 05, 2021 7:58 am

Dear Andre,

Interpretation of the eigensolution has been discussed many times on this forum, including a discussion of the scripts from the MATLAB Toolbox (https://github.com/OpenFAST/matlab-toolbox)--such as MBC3 and runCampbell--that can aid in the interpretation of the MATLAB eig() function. The derivation of the natural frequencies, damped frequencies, and damping ratios from real and imaginary components of the eigenvalues has been discussed explicitly, e.g. here, which is the approach used by MBC3.

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

Andre.White
Posts: 37
Joined: Wed Jan 29, 2020 3:16 am
Organization: University of Plymouth
Location: United Kingdom

Re: Natural frequency DeepCWind and Random Seeds

Postby Andre.White » Wed May 05, 2021 10:57 am

Many thanks Jason.

Regards,
AOAW

Andre.White
Posts: 37
Joined: Wed Jan 29, 2020 3:16 am
Organization: University of Plymouth
Location: United Kingdom

Re: Natural frequency DeepCWind and Random Seeds

Postby Andre.White » Thu May 06, 2021 6:03 am

Jason:
Tried to carry out the eigenanalysis using mbc. I carried out free decay test in OpenFAST and got the following file:
5MW_OC4Semi_Linear.1.lin.

However on executing the GetMats command in MATLAB I am getting the following error:
Running Eigenanalysis.m using "5MW_OC4Semi_Linear.1.lin"
Please wait...
Index exceeds the number of array elements (50).

Error in GetMats (line 56)
RotSpeed = str2num( line(55:68) ); % in (rad/s)


Not sure what is happening here. If I need to modify the .lin file before executing the GetMats command. I realize that in the .lin file the number of continuous states is 138. So my AMatix is 138 x 138.
1) Could this be the source of the error? Also, not sure why 138 states are used here.

I also have another query:
In the .fst file under linearization, CalcSteady was set to False. However, in a particular forum I saw where you advised that it should be set to true when carrying out the free decay test.
2) Should CalcSteady be set to true when carrying out the free decay test?

Would be very grateful if you could assist.

Thank you.

Regards,
AOAW

Jason.Jonkman
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Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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Re: Natural frequency DeepCWind and Random Seeds

Postby Jason.Jonkman » Thu May 06, 2021 6:41 am

Dear Andre,

You appear to be running the old MBC scripts for FAST v7. In FAST v8 and OpenFAST, you should use fx_mbc3.m, followed by campbell_diagram_data.m, as documented at the bottom of the page here: https://github.com/OpenFAST/matlab-toolbox.

OpenFAST allows you to linearize a model at any point in time, but the linear system is most useful (and able to predict a representative eigensolution) only when the model is in steady state. Enabling CalcSteady is an automated way to ensure that the solution is in equilibrium before linearizing.

A free-decay simulation is a transient event and the solution is not an equilibrium until the transients have subsided.

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

Andre.White
Posts: 37
Joined: Wed Jan 29, 2020 3:16 am
Organization: University of Plymouth
Location: United Kingdom

Re: Natural frequency DeepCWind and Random Seeds

Postby Andre.White » Thu May 06, 2021 11:52 am

Thanks Jason. Will try that one.

Regards,
AOAW


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