Tower Eigenfrequencies of NREL 5MW Turbine

[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

[Jason.Jonkman]

Dear René,

I'm a bit surprised izx_tip has no effect on the eignensolution, although I haven't tried your test before. A nonzero izx_tip (when the other cross moments of inertia are zero) implies that the principle axes of inertia are rotated about the y axis. This would lead me to expect that a nonzero izx_tip would effect modes that involve rotation of the tower-top about the x and z axes, unless izx_tip is small relative to the ixx_tip and izz_tip.

Best regards,

[Rene.Mebus]

Dear Jason,

Thank you for your answer.
After your reply I have tested when izx_tip has an influence on the results. With realistic data for tor_stff (from Table 6-1 of the NREL 5 MW Definition document) instead of a "very large number" (I have 1.0E+60) in the sec_props_file you can see an influence.
Can your recommend me a "very large number" for tor_stff and axial_stff for mimic the FAST model correctly?

Thank you again for your support.

Best regards,
René

[Jason.Jonkman]

Dear René,

To make tor_stff a "very large number", I would recommend setting tor_stff to be a couple orders of magnitude larger than the corresponding bending stiffness. You can use the same values for the axial_stff (despite the different units). Setting these stiffness values too large may lead to numerical problems, and thus inaccurate solutions, in BModes.

Best regards,

[Christos.Lathourakis]

Dear mr Jonkman,
I am trying to ran the exact simulation of the 5MW OWT with distributed springs (OC3 Phase II), in order to check the accuracy of my recompiled version of FAST. My question refers to BModes and the eigenfrequencies. My results are the following:
freq 1: 0.2448 (s-s)
freq 2: 0.2464 (f-a)
freq 3: 1.278 (s-s)
freq 4: 1.357 (s-s)
freq 5: 1.511 (f-a)

The first 2 are pretty close to the right ones. I got a little confused that there were two consequtive side to side frequencies following, but as I read in this post, the 3rd is the tortional (is that right?). However, the 2nd s-s frequency (freq 4) differs from the correct value, i.e. 1.5459. I am attaching both the input files and the output, in case you could kindly spot some mistake.
Thank you in advance for your help.

Best regards,
Christos Lathourakis

[Jason.Jonkman]

Dear Christos,

Your BModes input files look fine with me.

Yes, I would guess that mode 3 is the first torsion mode and mode 4 is the second side-to-side bending mode. But you could verify that further and better isolate the tower side-to-side bending modes by increasing the torsional stiffness of the tower in BModes (by an order of magnitude of two).

Regardless, the second tower-bending modes tend to have a lot of coupling with blade bending, which of course cannot be captured by BModes. You'll only see this effect in your FAST simulation i.e. I'd expect you'd get different frequencies from FAST for the second tower-bending mode than you would in BModes.

Best regards,

[Christos.Lathourakis]

Dear mr Jonkman,
thank you very much for your time and your precious advice. Your answers were of great help for me.

Best wishes,
Christos Lathourakis

[Zhou.Lin]

Dear Jason,

I'm using BModes calculating model of the Tower , but I want to calculate the higher order model in the FAST (more than the second order), so I got the BModes calculation results, and calculate the six order model, then import to ModeShapePolyFitting. XLSX was calculated in the Fore and aft side - to - side six modal coefficient, and verified, addition of each set of coefficients is 1, and I have modified the ElstoDyn - Tower, the corresponding coefficient of the input file you can see in the attachment.

NRELOffshrBsline5MW_OC3Monopile_ElastoDyn_Tower.rar

(1.71 KiB) Downloaded 14 times

Then I ran FAST, but there was an error, and the error message is shown below.

NRELOffshrBsline5MW_OC3Monopile_ElastoDyn_Tower.rar

(1.71 KiB) Downloaded 14 times

What I want to ask is whether the higher-order (more than second-order) modes of Fore-Aft and side-to-side can be calculated in FAST, whether they can be implemented in FAST and, if so, how should I modify them.

Looking forward to your reply, thank you

Best regards
Zhou.Lin

[Jason.Jonkman]

Dear Zhou.Lin,

The archives you attached appear to be corrupted, so, I can't review what you attached. But regarding the invalid numeric input error, as with any input file format problem, I suggest that you use the Echo option from the input file to debug errors in the input-file processing.

I'm not sure what you mean be "higher order". I assume you are referring to bending modes of the tower higher than the first and second modes--is that correct? Without customization of the FAST source code, the structural model of FAST v7 and the ElastoDyn module of FAST v8/OpenFAST are limited to two fore-aft and two side-to-side bending-mode DOFs in the tower. If you’ve looked in the source code, you’ve likely noticed that most things are parameterized into loops through enabled DOFs. As such, it would not be too difficult to customize ElastoDyn to allow for additional bending-mode DOFs in the tower. However, we’ve not done this ourselves because as you introduce higher-frequency modes/DOFs, likely modes other than bending will become important (e.g., torsion). To capture effects other than bending requires a change to the beam formulation used for the tower. Another option would be to model the tower with the SubDyn module instead of ElastoDyn. SubDyn allows for any number of modes and includes torsion (and other effects), but SubDyn is purely a linear formulation whereas ElastoDyn captures some geometric nonlinearities, which may be important for a tubular tower.

Best regards,

[Zhou.Lin]

Dear Jason,
Thank for your reply,I see what you mean. Thank you for your kindness.

Best regards,
Zhou.Lin

[Yun.Lu]

Dear Dr.Jonkman,
Could you please send me "ModeShapePloyfitting.xls". Thank you very much.

My email: luyun86674838@163.com

[Jason.Jonkman]

Dear Yun.Lu,

The spreadsheet ModeShapePolyFitting.xls is included in the FAST v8 archive (https://nwtc.nrel.gov/FAST8) in the Utilities directory.

Best regards,