Dear Jason:
Thank you so much for your replay on 24. October. Sorry for answering one month later because was out of my Account. Finally i linearized with FAST 7 about 36 Azimut Steps (NAzimStep). Now it is working well.
Best regards
Dear Jason,
referring to your post on Tue Nov 15, 2016 and the following I wonder when you can publish a stepwise procedure for a FFT (or PSD) to get the Campbell Diagram of a floating wind turbine with FAST v8.xx.
Or even better:
Will you implement full system linearization functionality for (floating) offshore turbines in the near future in FAST v8.xx?
Best regards,
Simon Wiedemann
Dear Simon,
We at NREL are not currently funded to work on either, so I can’t comment on when these will become available.
Perhaps someone else on the forum can respond.
Best regards,
Dear Jason,
can you explain why the GenDOF splits in two modes when applying MBC?
In my case one of these is increasing little with Wind speed and a lot with increasing pitch angle while the other is staying near zero. I attach a pic of the curve.
It don’t understand why it behaves like this.
Best regards,
Simon
Dear Simon,
As described in this forum topic: Learizing Baseline 5MW Wind Turbine with FAST, rigid-body modes (i.e. modes without stiffness) 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 and the damping is unphysical. When aerodynamics are enabled, perhaps there is a small amount of aerodynamic-induced stiffness that causes the rigid-body generator mode to have a slightly positive frequency.
Best regards,
Dear Jason,
I want to know how to calculate the 3rd FA frequency and 3rd SS frequency of the tower, or even higher order frequency? If you know the method or the third order results, can you share them with me?
Best wishes!
Dear @Xu.Pengfei,
I’m not sure I really understand your question. What do you mean by 3rd order? Are you referring to the 3rd fore-aft or side-side bending modes of the tower? For which system are you analyzing? Are you using FAST / OpenFAST?
Best regards,
Dear Jason,
What I mean is the 3rd fore-aft or side-side bending modes of the 5MW onshore wind turbine tower like the Table 9-1 in “Definition of a 5-MW Reference Wind Turbine for Offshore System Development”. I am using FAST, but it seems that it is unable to calculate the eigenanalysis higher than 2nd fore-after or side-side bending modes of the tower.
Bset wishes!
Dear @Xu.Pengfei,
If you are modeling the tower via the ElastoDyn module of OpenFAST, you are limited to two bending modes in each direction. If you are modeling the tower via the SubDyn module of OpenFAST, you can enable as many modes as you want (including modes other than bending like shear, torsion, and axial).
That said, in many wind turbines, only the lowest two bending modes are important in loads analysis applications.
Best regards,
Dear Jason,
I tried to use the linearization function in FAST_v7 and processed the results with mbc, and then pasted the corresponding results into the CampbellDiagram-pengfei.xls file. It appears that most of the frequency results are similar to those described in “Definition of a 5-MW Reference Wind Turbine for Offshore System Development”. But I have a doubt. In CampbellDiagram_pengfei.xls, the frequency order of the Natural (Undamped) Frequencies (Hz) row does not seem to correspond to the frequency order of the States column, so if I determine the Natural (Undamped) Frequencies (Hz) Which State does a certain frequency of the row correspond to? In other words, which parts of the vibration correspond to the 14th and 15th order frequencies? Sorry, I suddenly noticed that the forum doesn’t allow me to upload files.
Natural (Undamped) Frequencies (Hz)：
0.3136585860 0.3242010842 0.6188439335 0.6662559338 0.6672781122 0.6991227166 1.0791658789 1.0899244483 1.9210241590 1.9334641427 2.0205247347 2.9185184660 2.9527926795 3.6759704970 6.1382215693
States
‘Row/column 1 = 1st tower fore-aft bending mode DOF (internal DOF index = DOF_TFA1)’
‘Row/column 2 = 1st tower side-to-side bending mode DOF (internal DOF index = DOF_TSS1)’
‘Row/column 3 = 2nd tower fore-aft bending mode DOF (internal DOF index = DOF_TFA2)’
‘Row/column 4 = 2nd tower side-to-side bending mode DOF (internal DOF index = DOF_TSS2)’
‘Row/column 5 = Nacelle yaw DOF (internal DOF index = DOF_Yaw)’
‘Row/column 6 = Drivetrain rotational-flexibility DOF (internal DOF index = DOF_DrTr)’
‘Row/column 7 = 1st flapwise bending-mode DOF of blade 1 (internal DOF index = DOF_BF(1,1))’
‘Row/column 8 = 1st flapwise bending-mode DOF of blade 2 (internal DOF index = DOF_BF(2,1))’
‘Row/column 9 = 1st flapwise bending-mode DOF of blade 3 (internal DOF index = DOF_BF(3,1))’
‘Row/column 10 = 1st edgewise bending-mode DOF of blade 1 (internal DOF index = DOF_BE(1,1))’
‘Row/column 11 = 1st edgewise bending-mode DOF of blade 2 (internal DOF index = DOF_BE(2,1))’
‘Row/column 12 = 1st edgewise bending-mode DOF of blade 3 (internal DOF index = DOF_BE(3,1))’
‘Row/column 13 = 2nd flapwise bending-mode DOF of blade 1 (internal DOF index = DOF_BF(1,2))’
‘Row/column 14 = 2nd flapwise bending-mode DOF of blade 2 (internal DOF index = DOF_BF(2,2))’
‘Row/column 15 = 2nd flapwise bending-mode DOF of blade 3 (internal DOF index = DOF_BF(3,2))’
Bset wishes!
Dear @Xu.Pengfei,
I’m sorry, but I’m not sure I understand your question. Perhaps the question would be more clear if you shared your spreadsheet some way?
Best regards,
Dear Jason,
Here is my spreadsheet.
Though I have reviewed the posts where you have helped others interpret the modes to get a sense of how it works… My confusion still lies in not knowing how these two sets of data correspond. The Mode Number and Natural (Undamped) Frequencies (Hz) corresponding to Green Background are the natural frequencies corresponding to this state. Is my understanding correct? But how to find the DOF that belongs to a certain natural frequency if there are many mode shape magnitudes with green background in the natural frequency colum? For example, how to identify modes 12 and 13 as the 2nd tower fore-aft and 2nd tower side-to-side bending modes? How to identify which DOF mode number 14 and 15 belong to?
Another question is what you mentioned last time, If I am modeling the tower via the SubDyn module of OpenFAST, how can I enable as many modes as I want?
Last qustion is maybe I can input the white noise excitation by inputting the seismic wave at the bottom, extract the acceleration time history result at the tower, do the spectrum analysis, find the frequency corresponding to the peak and obtain the higher-order mode?
Best wishes!
Dear @Xu.Pengfei,
Each mode generated through the linearization + MBC3 post-process is a full system mode involving a combination of all degrees of freedom in the model. The green and red highlighting do not identify specific modes, but they likely help you pick out the most important DOFs prominent to each mode.
In your case, modes 12 and 13 correspond to the 2nd fore-aft and side-side bending modes of the tower. Mode 15 corresponds to the nacelle-yaw mode and Mode 14 is an extraneous edgewise bending mode.
When SubDyn is enabled, the number of finite-element DOFs and number of retained Craig-Bampton modes is specified by the user and can be as high as you want. But please note that SubDyn is only available in FAST v8 and OpenFAST and linearization with SubDyn enabled was not introduced until OpenFAST v2.6 and newer.
I’m not sure I fully understand your last question, but you can input white noise excitation through the seismic functionality of FAST v7. However, SubDyn is not part of FAST v7.
Best regards,
1 Like
Dear Jason,
Thank you for your detailed answer. I think only the mode shape can tell which degree of freedom a certain order frequency corresponds to. In fact, the mode shape may also be the result of the coupling of multiple degrees of freedom. But in CampbellDiagram_pengfei.xls, can the specific Mode14 and Mode15 only be determined by experience as an extraneous edgewise bending mode and the nacelle-yaw mode?
The last question I want to express is that by inputting white noise excitation at the bottom, extracting the response of the structure, and then performing Fourier transform, the mode of the structure can also be identified, although mainly the mode of the corresponding part of the response.
Best wishes!
Dear @Xu.Pengfei,
In mode 15, the nacelle-yaw DOF is highlighted green, which implies that DOF is most relevant to this mode; I also know that the nacelle-yaw actuator was designed with this frequency in mind. Mode 14 includes a strong blade-edgewise bending component (seen via the red highlighting) and two other edgewise modes appear in modes 7 and 8. So, mode 14 can be ascertained knowing that all blade modes in a three-bladed turbine come in sets of three, and with edgewise modes, one mode typically has a much higher frequency than the other two. But I agree, it takes some experience to interpret the eigensolution correctly.
Yes, white-noise excitation can be used to identify natural frequencies of the structure (assuming the excitation is applied in a way that will excite the modes), but it is often difficult to deduce which modes correspond to which frequencies with white-noise excitation alone.
Best regards,
1 Like
Dear Jason,
I would like to ask you, if I want to get the frequency and mode shape of the tower vibration of the first 5 orders of the wind turbine, as well as the modal participation coefficient or modal effective mass of each order vibration shape, how can I get it? As far as I know, this can be calculated by modal analysis in ANSYS software but a finite element model that equivalently treat the top rotor-blade-nacelle as lumped mass is nesscrssary to create firstly. Is there any other way?
The crux of the matter is that I only care about the frequency and mode shape of the tower. The mass matrix and stiffness matrix extracted after FAST linearization can also be used for eigenvalue analysis to obtain the various order modes and natural frequencies of the system, but that includes blades.
In addition, I would like to ask the specific parameters of the tower radius and wall thickness of the baseline 5MW wind turbine, can you tell me?
Best wishes!
Dear @Xu.Pengfei,
Regarding the model participation factor and modal effective mass factor, these are not directly computed by OpenFAST or by a post-processing script provided by NREL (as far as I know), but I’m sure you could implement a script to calculate these things from the OpenFAST solution.
Please note that for a tower modeled in the ElastoDyn module of OpenFAST, only the first two bending modes in each direction (fore-aft, side-side) are accounted for. You can capture higher bending modes, or other modes such as axial and torsion, when modeling the tower in the SubDyn module of OpenFAST.
The tower diameter (radius) and thickness of the NREL 5-MW baseline wind turbine are documented in its specifications report: https://www.nrel.gov/docs/fy09osti/38060.pdf.
Best regards,
Dear @Jason.Jonkman
I have a question about the different meaning of DOF. What is the difference between these descriptions? Could you please help me to figure it out? Thanks in advance!!!
Dear @Yinghan.Liu,
“Collective” means all blades deflect in phase. “Cosine” and “Sine” are asymmetric modes. where one blade has the opposite phase of the other two.
For an operating (spinning) rotor, the natural frequencies of the asymmetric modes can increase faster than collective (called “progressive”) or decrease slower than collective (called “regressive”).
For a stationary (not spinning) rotor, the asymmetric modes can couple with tower fore-aft (called “pitch”) or couple with tower torsion (called “yaw”).
Best regards,
I got it! Thanks again for your quick reply and detailed answer!!!