# Natural frequency of every DOF

dear,Jason
I do the modal analysis about the 5MW onshore wind Turbine by set the GENDOF =FALSE,PCMODE=0,VSCONTRL=0,ROTORSPEED=12.0.ANALMODE=2,and STEADY and EQUIL in the *.ipt .I get the CampbellDiagram_5MW .xls:

Maybe something is wrong about the natural frequency,for example the 2ND tower for-aft and sid-sid DOF,there are no red marked data in the row the two DOF. I think something is wrong .
Ruiliang
CampbellDiagram - 5mw.xls (252 KB)

Dear Railiang,

I don’t see a problem with your results. The number highlighted in red simply identifies the degree-of-freedom (DOF) with the highest magnitude in a given full-system mode. The tower-bending DOFs couple with the blade-bending DOFs in the full-system tower-bending modes, and it is not guaranteed that a tower-bending DOF will have a higher magnitude than the blade-bending DOF in a tower-bending mode. In your results, I would identify modes 12 and 13 as the 2nd tower fore-aft and 2nd tower side-to-side bending modes, respectively.

Best regards,

Dear,Jason

``````In the FAST CertTest,In the Test03.lin,there are five DOF in the system,but the mode number is six,I think the DOF correspond to one mode in the system.why is the number of DOF is smaller than the mode. In the 5MW,they have same size.
``````

Yours
Ruiliang.Wang

Dear,Jason
I have another question about the CambellDiagram_5MW.xls,As you see,the same DOF of the three blades (the three 1stDOF_BF) have different natural frequency,and have a large gap,Is it normal ?Do we choose whicn one? Do you have some tips about how to get accurate natural ferquency. I really don’t identify their natural fequency. I am not sure of the natural frequency of DrTrDOF too.

Do we use the marked red and green data to make sure the natural frequency of DOF?

Ruiliang.Wang

Dear Ruiliang,

As described in this forum topic: http://forums.nrel.gov/t/learizing-baseline-5mw-wind-turbine-with-fast/494/1, rigid-body modes show up in MBC3 as a pair of zero-valued (or near-zero-valued) frequencies with +/- inf damping (i.e., eigenvalues with real values only). That is, each rigid-body mode will introduce an additional mode beyond the number of enabled DOFs.

When the rotor is spinning, MBC3 changes the description from individual blade DOFs to rotor DOFs (collective, cosine, sine), as described in my May 16, 2011 post in the forum topic found here: http://forums.nrel.gov/t/eigenanalysis-fast/362/1. When the rotor is spinning, each rotor mode is likely to have a different natural frequency, which are also different from the corresponding mode of the isolated, nonspinning blade. I suggest that you first linearize a mode with zero rotational speed to understand how the system behaves when the rotor is not spinning; the mode shapes are easier to interpret in this case.

Please see the following forum topic where I helped a user interpret the mode shapes of a spinning rotor–the results are nearly identical to yours: http://forums.nrel.gov/t/linearization-in-fast-with-unbalanced-rotor/721/1.

Best regards,

Dear Jason,
I have read your report named"RotatingBeamFinalReport",I find that the natural frequencey of the rotating beam vary with the rotating speed.I can see the frequencey increase when the rotating speed increase.
But in my simulation about 2.3MW wind turbine,there are no difference on natural frequecey within 0.0rpm and 12.0rpm.The frequencey value of the two speed have little change.I want to know why .
I don’t know if the setting is wrong ,GenDOF =false,PCContrl and VScontrl=0.

Best Regard !
Ruiliang

Dear Ruiliang,

In a linearization analysis wth FAST v7, PCMode must be 0, but there are no restrictions on setting of GenDOF or VSContrl.

I’m not sure why you are not seeing a frequency change with variations in rotor speed in your model. I suggest simplifying the model first – disable all DOFs except for the FlapDOF1, linearize across a variety of rotational speeds and see if the frequencies change.

Best regards,

Dear Jason
I simplify the model by only set the FlapDof1=True.But I am sorry to see the natural frequencey have no change (0.70at 0rpm,0.75at 12.0rpm),I upload the information of my primary file ,hope you can help me find the error.
CampbellDiagram -2.3MW_v0.xls (250 KB)
Linear.txt (2.07 KB)
2.3MW.txt (17.6 KB)

Dear Ruiliang,

I skimmed your input files and don’t see any problems.

You say that the natural frequencies don’t change with rotor speed, but then you identify a small change. Are you referring to the natural frequencies of the collective blade-flap mode? How do the frequencies change with rotor speed for the progressive and regressive (sine/cosine) blade-flap modes?

Best regards,

Dear Jason,
Yes ,You are right about small change of collecive flapDOF I identified.

About the progressive and regressive (sine/cosine) blade-flap modes,I really don’t understand the meaning ,But I get the CampbellDiagram of rotorspeed=12.0 ,Can you help me find the problem?CampbellDiagram - 2.3_v12.xls (250 KB)

Best Regard
RUIliang

Dear Ruiliang,

I don’t see any problem. Here’s how I interepret your linearization output:

Mode → Description (Hz)
1 → 1st Tower Side-to-Side (0.28)
2 → 1st Tower Fore-Aft (0.28)
3 → 1st Blade Flap Regressive (0.54)
4 → 1st Blade Flap Collective (0.75)
5 → 1st Blade Edgewise Regressive (0.84)
6 → 1st Blade Flap Progressive (0.92)
7 → 1st Blade Edgewise Collective (0.98)
8 → 1st Blade Edgewise Progressive (1.25)
9 → 2nd Blade Flap Regressive (1.86)
10 → 2nd Blade Flap Collective (2.17)
11 → 2nd Blade Flap Progressive (2.25)
12 → 2nd Tower Side-to-Side (2.74)
13 → 2nd Tower Fore-Aft (2.84)

The rotor progressive and regressive (sine/cosine) modes have clearly changed with rotor speed compared to your results at 0 rpm.

Best regards,

Hello,Jason

``````I want to use the FAST to simulate the seismic load use your 5MW onshore wind turbine .Maybe I have to compile the FAST. Can you give me the source code in order to  compile them by fortran or give me the website I can download myself.
``````

Thank you very much.

BEST REGARD
Ruiliang.Wang

Dear Ruiliang,

A seismic module was compiled with FAST v7.00.01a-bjj and is available here: wind.nrel.gov/designcodes/simulators/seismic/. We will soon be updating this for FAST v7.02.00d-bjj.

Best regards,

Hello ,Jason
When I use the 5MW onshore wind turbine to do the modal analysis,I get the Campbell file .CampbellDiagramWITH.xls (251 KB)
In the file ,The last naturl frequency is 3.677.but in your paper "Definition of a 5-MW Reference
Wind Turbine for Offshore System Development " there are no so large frequency.I think the DOF set is same to yours.

I don’t know what happened.Can you give me some help?

Best Regard
Ruiliang.Wang

Dear Ruiliang,

Actually, I’ve calculated that 3.68 Hz mode also, but didn’t report it in the Table.

The blade-bending modes of a three-bladed rotor come in groups of three (or the number of blades if different from three). For example, the three first flapwise-bending modes of the parked rotor of the NREL 5-MW turbine all have natural frequencies around 0.67 Hz (all three frequencies are not identical because each blade couples with the flexible turbine support structure differently). However, the edgewise-bending modes couple strongly with the drivetrain torsion, and so, the third frequency (3.68 Hz) is very different than the other two (around 1.08 Hz).

I hope that helps.

Best regards,

Dear Jason
I am studying about natural frequencies related to a wind turbine structure. I chose AWT-27CR2, which is a constant speed wind turbine (GenDOF=False).
At first, I enabled FlapDOF1, FlapDOF2, and EdgeDOF. I obtained the following frequencies (Hz):
2.6871
2.687
8.4727
8.4725
6.9335
6.9336
when I enabled DrTrDOF, I obtained:
2.7146
2.6871
8.3113
8.4726
6.9335
21.265
1.7077
I found that 1.7077 belongs to DrTrDOF, and two frequencies 6.9335 and 21.265 to EdgeDOF.
why is there a big difference between two frequencies of EdgeDOFs (i.e., 6.9335 and 21.265)?
regards

Dear Hamid,

I repeated your two linearizations and obtained similar results, although not identical. In my results the blade edgewise modes change from 6.9 Hz in the first case to 4.1 Hz, 6.7 Hz, and 25 Hz in the second. I performed my linearization with the rotor parked (GenDOF = False and RotSpeed = 0) and without aerodynamics (CompAero = False).

In the first case, both blades are independent because the tower through to the hub are rigid.

In the second case, the blade edgewise modes get coupled together with the enabled drivetrain-torsion DOF. The 4.1-Hz mode is dominated by drivetrain torsion and–due to the flexibility of the rotor–is a bit less than the 5.5-Hz rigid-body drivetrain torsion mode when the rotor is made rigid. The 6.7-Hz mode is asymmetric whereby the blades bend in opposite directions (effectively uncoupled from drivetrain torsion), which is why the frequency is similar to the uncoupled edgewise-bending frequency. The 25-Hz mode includes further coupling between the edgewise-bending and drivetrain-torsion DOFs.

Best regards,