Tower Eigenfrequencies of NREL 5MW Turbine

[Jason.Jonkman]

Dear Hamid,

BModes can be used to derive mode shapes and frequencies of blades or towers. When used together with FAST, it is only the blade and tower mode shapes that are needed. For the NREL 5-MW turbine atop the MIT/NREL TLP, we've already created mode shapes for FAST using ADAMS in place of BModes. So, we don't have the BModes input files for this system. You should be able to make them yourself, but there really is no need to. If all your interest is in obtaining the natural frequenices of the TLP, you can use the linearization feature of FAST to derive them. Otherwise, these frequencies are published in Table 6 of the following report: viewtopic.php?f=4&t=921.

Best regards,

[Hamid.Hokmabady]

Dear Jason
Thanks for your great answers and solutions!

I ran the linearization of FAST for the frequency;
I got the result; thanks for that; but now I need Eigenanalysis.m for MATLAB but there isn't any file like that in CertTest folder! how can I get to that! is the file in somewhere else that I have to download it?

Regards
Hamid

[Monika.Blackman]

Dear Hamid,

I'm fairly sure the Eignanalysis.m file you are looking for has been superceded by the mbc3 package, where I think cce.m gives the information you're looking for. Take a look at this post to see if it helps you viewtopic.php?f=4&t=475.

Regards,

Monika

[Hamid.Hokmabady]

Dear Monika;
Thanks for your post

Yeah, your right; I've read that post but I forgot that, thanks for reminding.

Regards
Hamid

[Hamid.Hokmabady]

Hello everyone;
I got the Natural Frequency (Rad/S and Hz) for 5MW TLP as following, USING CCE function in MATLAB:

AvgNaturalFrequency
1: 4.255
2: 4.513
3: 2.749
4: 1.300
5: 1.354
6: 0.617
7: 0.104
8: 0.104

and by Hz:
AvgNaturalFrequencyHz
1: 0.677
2: 0.718
3: 0.437
4: 0.206
5: 0.215
6: 0.098
7: 0.016
8: 0.016

Using CambellDiagram that Jason had been sent for the first mode I mean Surge it is 0.104 rad/s and 0.016Hz but I thinks it is not right!
what is the form of these outputs? is (number 1:) natural frequency of the first DOF (SURGE) or numbers are upside down I mean number 8 is for the first DOF (SURGE)? I have 8 DOF: 6 for structure and 2 first bending modes!
I need structures natural frequency in each DOFs mostly in Surge and Pitch.

I really confused!
Any suggestions would be so great!

[Jason.Jonkman]

Dear Hamid,

I'm not really sure I understand your question, but MATLAB's eigensolver (used within CCE) does not sort the eigensolution in the order of increasing frequency. However, the CampbellDiagram.xls workbook, for aid in interpretation, sorts the eigenvalues and eigenvectors by frequency and highlights the dominant components of each mode.

If you are referring to the NREL 5-MW turbine atop the MIT/NREL TLP, the first surge natural frequency is 0.0165 Hz. You can find other natural frequencies for this system documented in Table 6 of Denis Matha's MS thesis-turned NREL report: http://www.nrel.gov/docs/fy10osti/45891.pdf.

Best regards,

[Francesca.Taddei]

Dear all,
I am working with BMODES and I have a question about the input for the 5MW Turbine:
Hier
http://wind.nrel.gov/public/jjonkman/BM ... nopile.bmi
in the tower-top mass properties I reda the following:

3.500003109E+005 tip_mass blade-tip or tower-top mass (kg)
-0.4137754432 cm_loc tip-mass c.m. offset from the tower axis measured along x-tower axis (m)
1.9669893542 cm_axial tip-mass c.m. offset tower tip measures axially along the z axis (m)
4.370E7 ixx_tip blade lag mass moment of inertia about the tip-section x reference axis (kg-m^2)
2.353E7 iyy_tip blade flap mass moment of inertia about the tip-section y reference axis (kg-m^2)
2.542E7 izz_tip torsion mass moment of inertia about the tip-section z reference axis (kg-m^2)
0. ixy_tip cross product of inertia about x and y reference axes(kg-m^2)
1.169E6 izx_tip cross product of inertia about z and x reference axes(kg-m^2)
0. iyz_tip cross product of inertia about y and z reference axes(kg-m^2)

But then in the article:
"Modal Dynamics of Large Wind Turbines with Different Support Structures" from G. Bir and J. Jonkman I read the following values

Table1.JPG (94.88 KiB) Viewed 10480 times

which referring to the BMODES tower axes systems must be transformed from the the cm coordinate system to the tip-section reference axis . Is that right?

Coordinatesystem.JPG (46.71 KiB) Viewed 10480 times

In this case, the input data of the CS_Monopile.bmi are not correct, I guess.

Any idea?

Thanks

[Jason.Jonkman]

Dear Francesca,

The BModes inputs for tower-top inertia (ixx_tip, etc.) are specified about the center of mass (CM) of the tower top. The BModes input file you linked to and Table 1 from the paper you referenced are consistent.

Best regards,

[Daniel.Kim]

Dear Dr. Jason.

Could you let me know how you obtained those tower-head mass moment of inertia in above table 1? I thought that those was calculated by using FAST linearization, but I could not make the same moment of inertia Ixx as values in above table 1 when I used linearization.

Sincerely,
Daniel Kim.

[Jason.Jonkman]

Dear Daniel,

We obtained the data presented in Table 1 using the aggregate mass tool of MSC.ADAMS -- see my post dated Jun 07, 2013 in the following forum topic for more information: viewtopic.php?f=3&t=842&p=3426.

You could also obtain these properties using the FAST v7 linearization if you set up your model to eliminate the platform and tower and locate the platform reference point at the yaw bearing.

Best regards,

[Daniel.Kim]

Dear Dr. Jason.

When I used the FAST linearization, I obtained following results regarding tower top mass properties. The first column is given in NREL, and the second column is values calculated by me using FAST linearization (v7.00)

Title 1st column 2nd column
Topmass 3.50000E+05 3.50000E+05
cm_loc -4.13775E-01 -4.13774E-01 (From Tower center line)
cm_axi 1.96699E+00 1.96699E+00 (From Tower Top)
ixx_tip 4.37000E+07 3.86484E+07 (w.r.t. CM of RNA)
iyy_tip 2.35300E+07 2.35557E+07
izz_tip 2.54200E+07 2.54085E+07

In order to calculate this, I used very small tower and platform mass and located platform reference point* at 89.56699 from MWL (z coordinate CM of RNA). Also, I used default angle of azimuth value and initial azimuth. (0.0 AzimB1Up & 0.0 Azimuth ) . When FAST linearization is used, smaller Ixx_tip is obtained than when Adams is used. I don't know why this difference is generated. I am not sure whether Ixx = 3.86484 is correct or I make some mistakes in linearization input files. Could you check this? I used NRELOffshrBsline5MW_OC3Hywind.zip in http://wind.nrel.gov/public/jjonkman/NR ... Bsline5MW/ I changed some values like followings for linearization.

In NRELOffshrBsline5MW_Floating_OC3Hywind.fst
2 AnalMode
0 PCMode
0.0 RotSpeed

In NRELOffshrBsline5MW_Linear.dat
False CalcStdy
2 MdlOrder

In NRELOffshrBsline5MW_Platform_OC3Hywind.dat
0.0 TwrDraft
0.0 PtfmCM
-8.9566990E+01 PtfmRef
0.0 PtfmMass
0.0 PtfmRIner
0.0 PtfmPIner
0.0 PtfmYIner
0 PtfmLdMod

In NRELOffshrBsline5MW_Tower_OC3Hywind.dat
Very small mass density 0.0000001kg/m

* If PtfmRef is less than TwrDraft, an error is occured, but I modified the source code so that the error is ignored.

Sincerely,
Daniel Kim.

[Jason.Jonkman]

Dear Daniel,

I'm not sure. I'm quite confident that ixx_tip = 4.370E7 kg*m^2 is correct for the NREL 5-MW turbine.

Perhaps there is a problem because you overwrote the error that PfmRef < TwrDraft? Do you get the correct result if you set PtfmRef = 0.0, linearize to obtain the mass matrix, then transform the inertias from the tower base to the rotor CM?

Best regards,

[Daniel.Kim]

Dear Dr. Jason.

Even though I calculated Ixx w.r.t the tower base (0.0 TwrDraft, 0.0 PtfmCM , 0.0 PtfmRef) without modifying source code and transformed Ixx w.r.t CM of RNA, the result was the same as 3.8648381E+07.

Ixx w.r.t. tower base: 2.8464364E+09
Ixx w.r.t CM of RNA: 2.8464364E+09 - 35,000*89.566990^2 = 3.8648381E+07

I used the NRELOffshrBsline5MW_OC3Hywind.zip in http://wind.nrel.gov/public/jjonkman/NR ... Bsline5MW/ , and I didn't change other variances except for values that I listed above. I have to make the consistent input files regarding tower-top between BModes and FAST in other case, so I am first checking the existing NREL 5MW wind turbine, but the results are not matched to the given values, which is calculated by ADAMS. How can I confirm it? Have you obtained inertia results of NREL 5MW tower-top by using FAST linearization and compared them to the results obtained from ADAMS?

Sincerely,
HyoungChul Kim.

[Jason.Jonkman]

Dear HyoungChul,

Good question. I was able to reproduce your results using a linearization analysis with FAST v7. While I have the old ADAMS models, unfortunately, I no longer have a working MSC.ADAMS license to check the aggregate mass results we've used in the past. There must be something set differently between these models, but I don't recall seeing these differences before. I'm now just as confused as you.

Best regards,

[Rene.Mebus]

Dear Jason,

I have a question about the izx_tip (cross product of inertia about z and x reference axes) value of the BModes v1.03.01 Input File.
When I calculate the first 10 modes of the NREL 5MW Tower, there is no influence of the value on the BModes results. The modes and the frequencies are exactly the same. I tested ixz_tip with a value of 0, 1.169E6 and 1.169E7.

So, when is it necessary to enter the izx_tip value?

Attached the results of BModes in the command window.

Code: Select all

with ixz_tip = 0:
      eigenvalue(450) =  0.390794D+11        mode  1 frequency =      0.329475
      eigenvalue(449) =  0.398465D+11        mode  2 frequency =      0.332693
      eigenvalue(448) =  0.126954D+13        mode  3 frequency =      1.877897
      eigenvalue(447) =  0.187182D+13        mode  4 frequency =      2.280244
      eigenvalue(446) =  0.774666D+13        mode  5 frequency =      4.638803
      eigenvalue(445) =  0.921857D+13        mode  6 frequency =      5.060349
      eigenvalue(444) =  0.459755D+14        mode  7 frequency =     11.300868
      eigenvalue(443) =  0.471883D+14        mode  8 frequency =     11.448961
      eigenvalue(442) =  0.169404D+15        mode  9 frequency =     21.692580
      eigenvalue(441) =  0.170534D+15        mode 10 frequency =     21.764779

with ixz_tip = 1.169E6:
      eigenvalue(450) =  0.390794D+11        mode  1 frequency =      0.329475
      eigenvalue(449) =  0.398465D+11        mode  2 frequency =      0.332693
      eigenvalue(448) =  0.126954D+13        mode  3 frequency =      1.877897
      eigenvalue(447) =  0.187182D+13        mode  4 frequency =      2.280244
      eigenvalue(446) =  0.774666D+13        mode  5 frequency =      4.638803
      eigenvalue(445) =  0.921857D+13        mode  6 frequency =      5.060349
      eigenvalue(444) =  0.459755D+14        mode  7 frequency =     11.300868
      eigenvalue(443) =  0.471883D+14        mode  8 frequency =     11.448961
      eigenvalue(442) =  0.169404D+15        mode  9 frequency =     21.692580
      eigenvalue(441) =  0.170534D+15        mode 10 frequency =     21.764779


Thank you for your answer.

Best regards,
René