Dear NREL-Team,
I´m a beginner using FAST (Version: v7.02) and I have been working on the NREL-5MW-Turbine using different types of onshore-towers.
My goal is to compare different kinds of onshore-towers. E.g. 100m tubular tower vs. 100m wood tower.
(Optionally I compare two 140m Hybrid-Systems.)
My first question is about the BModes-output-data.
In Modes it is simple to generate the mode shapes for FAST-Input from Modes-output. The fore-aft and side to side mode shapes are the same
[url]Tower fore-aft modes shapes] and the output Shape 1/Shape 2 is the input-data for mode 1/mode2 in the FAST-file “tower.dat”.
Because the BModes-output is more accurate I switched to BModes (Version: [url]National Wind Technology Center's Information Portal | Wind Research | NREL) . My problem is to generate the mode shapes from BModes-output.
For example the “Test.bmi” and “Test.dat”-files. In my understanding the “test.out”-file is the input-data for the mode shapes in FAST- “tower.dat” -file.
Can someone explain how can I get the mode-shapes from BModes- “test.out”?
I have another question regarding the mode shapes & eigenfrequencies for different types of onshore-towers.
For using BModes I need the” tower-top mass properties”. I get the properties for the NREL 5MW from Chapter 2 in “Modal Dynamics of Large Wind Turbines with Different Support Structures” respectively
[url]Tower Eigenfrequencies of NREL 5MW Turbine]
The properties are:
3.500003109E+005 tip_mass
-0.4137754432 cm_loc
1.9669893542 cm_axial
4.370E7 ixx_tip
2.353E7 iyy_tip
2.542E7 izz_tip
0.0 ixy_tip
1.169E6 izx_tip
0.0 iyz_tip
Can I use these properties for every type of onshore-tower? ( by steady using the NREL-5MW-turbine!)
My third question is about the ”Tower Parameters” in FAST- “Tower.dat”-file.
I want to validate the tower-systems with different damping-ratios. To regulate/control the damping ratio I want to use different values for the
TwrFADmp(1)
TwrFADmp(2)
TwrSSDmp(1)
TwrSSDmp(2)
How is the definition of these damping ratios?
It is
• δGes. =Sum of δ1,δ2,δ3
whereas (δ1=material damping, δ2=tower with tower-fittings, δ3=tower-construction on basement/foundation)
• Lehr´s damping factor (D=δGes./2π)
• Or completely different?
Thank you for your support.
Best regards,
Philipp
Onshore-Tower.dat-and-Test-files.zip (4.61 KB)
Dear Philipp,
While BModes doesn’t directly output the polynomial coefficients needed by FAST, we’ve supplied the “ModeShapePolyFitting.xls” spreadsheet in the FAST archive to aid in the effort of deriving the polynomial coefficients.
Yes, those tower-top mass/inertia properties are correct for the NREL 5-MW turbine and the same values can be used for any tower modeled within BModes for this turbine .
The tower damping ratios used by FAST are described in my Oct 08, 2013 post in the following forum topic: Natural frequency and damping ratio calculation.
Best regards,
Dear NWTC,
I need to use BModes for an eigenvalue analysis of an offshore wind turbine. However I am sitting on a Linux machine, and can not find BModes in any other form than a Windows executable. Does someone have one for Linux, or is there any archive I can use to compile it myself?
Best regards,
Asgeir H. Midthaug
Dear Asgeir,
I’m not aware of a Linux version of BModes. And, unfortunately, due to the departure of Gunjit Bir from the NREL and his use of a proprietary eigensolver in BModes, NREL is currently unable to recompile BModes.
Best regards,
Dear Jason
I have used BModes to regenerate mode shapes for the tower, to be updated in the ElastoDyn_Tower file for FAST8. I did this, to account for an Apparent Fixity depth in the OWT model. I have made use of the SubDyn derived mass and stiffness matrices as specified in the SubDyn manual. However, i have a doubt with the mode shapes generated using BModes.
BModes gives the tower frequencies and mode shapes of various modes. When i plot the first mode shapes of the S-S and F-A modes, they seem to match well. However, for the second modes, there seems to be a huge departure from this trend. Is this unusual? Am i doing something wrong in my calculations? I have attached the corresponding plots.
Sincerely
Abhinav
CS_Monopile.txt (8.49 KB)
mode_shapes_bmodes.txt (37.3 KB)
Dear Abhinhav,
It is not uncommon for the tower-bending side-to-side modes to be different from the fore-aft modes because the inertia of the tower top (particularly the rotor overhang and rotor inertia) is different in the two directions. I would expect more effect for the higher modes. What is odd to me in your results is that first tower-bending modes are identical between the directions; you may want to double check that you grabbed the correct modes from BModes.
Best regards,
Dear Jason
Thanks for the reply. I am confused about the modes that i grabbed from BModes. For instance, i have pasted the first two modes from the Bmodes output below. I believe that they represent the first S-S and F-A modes respectively. And the values look almost identical. Am i missing something, sir?
-------- Mode No. 1 (freq = 0.230153E+00 Hz)
span_loc s-s disp s-s slope f-a disp f-a slope twist
0.0000 0.001139 0.011572 0.000000 0.000000 -0.000335
0.0035 0.001181 0.012402 0.000000 0.000000 -0.000337
0.0104 0.001273 0.014063 0.000000 0.000000 -0.000341
0.0174 0.001377 0.015728 0.000000 0.000000 -0.000345
0.0244 0.001492 0.017396 0.000000 0.000000 -0.000349
0.0313 0.001619 0.019067 0.000000 0.000000 -0.000353
0.0383 0.001758 0.020741 0.000000 0.000000 -0.000357
0.0453 0.001908 0.022419 0.000000 0.000000 -0.000361
0.0522 0.002070 0.024101 0.000000 0.000000 -0.000365
0.0592 0.002244 0.025786 0.000000 0.000000 -0.000369
0.0662 0.002429 0.027474 0.000000 0.000000 -0.000373
0.0731 0.002627 0.029166 0.000000 0.000000 -0.000377
0.0801 0.002836 0.030862 0.000000 0.000000 -0.000381
0.0870 0.003056 0.032562 0.000000 0.000000 -0.000385
0.0940 0.003289 0.034266 0.000000 0.000000 -0.000389
0.1010 0.003534 0.035974 0.000000 0.000000 -0.000393
0.1079 0.003790 0.037685 0.000000 0.000000 -0.000397
0.1149 0.004059 0.039398 0.000000 0.000000 -0.000402
0.1219 0.004339 0.041114 0.000000 0.000000 -0.000406
0.1288 0.004631 0.042832 0.000000 0.000000 -0.000411
0.1358 0.004936 0.044552 0.000000 0.000000 -0.000416
0.1399 0.005121 0.045568 0.000000 0.000000 -0.000419
0.1497 0.005580 0.048001 0.000000 0.000000 -0.000426
0.1567 0.005920 0.049730 0.000000 0.000000 -0.000432
0.1636 0.006273 0.051461 0.000000 0.000000 -0.000438
0.1706 0.006637 0.053195 0.000000 0.000000 -0.000444
0.1776 0.007014 0.054932 0.000000 0.000000 -0.000451
0.1845 0.007402 0.056672 0.000000 0.000000 -0.000458
0.1915 0.007803 0.058415 0.000000 0.000000 -0.000466
0.1985 0.008216 0.060160 0.000000 0.000000 -0.000474
0.2054 0.008641 0.061909 0.000000 0.000000 -0.000483
0.2124 0.009078 0.063659 0.000000 0.000000 -0.000492
0.2194 0.009528 0.065409 0.000000 0.000000 -0.000501
0.2263 0.009989 0.067162 0.000000 0.000000 -0.000510
0.2333 0.010463 0.068915 0.000000 0.000000 -0.000519
0.2403 0.010949 0.070670 0.000000 0.000000 -0.000528
0.2472 0.011447 0.072426 0.000000 0.000000 -0.000538
0.2507 0.011701 0.073305 0.000000 0.000000 -0.000543
0.3203 0.017431 0.090960 0.000000 0.000000 -0.000642
0.3797 0.023286 0.106022 0.000000 0.000000 -0.000734
0.4248 0.028326 0.117439 0.000000 0.000000 -0.000810
0.4596 0.032569 0.126181 0.000000 0.000000 -0.000872
0.4866 0.036071 0.132918 0.000000 0.000000 -0.000922
0.5137 0.039754 0.139610 0.000000 0.000000 -0.000975
0.5407 0.043616 0.146215 0.000000 0.000000 -0.001030
0.5677 0.047656 0.152729 0.000000 0.000000 -0.001087
0.5947 0.051869 0.159145 0.000000 0.000000 -0.001146
0.6217 0.056255 0.165445 0.000000 0.000000 -0.001209
0.6488 0.060809 0.171596 0.000000 0.000000 -0.001273
0.6758 0.065528 0.177585 0.000000 0.000000 -0.001341
0.7028 0.070406 0.183399 0.000000 0.000000 -0.001412
0.7298 0.075438 0.189001 0.000000 0.000000 -0.001486
0.7568 0.080618 0.194358 0.000000 0.000000 -0.001563
0.7839 0.085940 0.199452 0.000000 0.000000 -0.001644
0.8109 0.091395 0.204259 0.000000 0.000000 -0.001729
0.8379 0.096976 0.208722 0.000000 0.000000 -0.001818
0.8649 0.102671 0.212808 0.000000 0.000000 -0.001911
0.8919 0.108471 0.216484 0.000000 0.000000 -0.002009
0.9189 0.114366 0.219705 0.000000 0.000000 -0.002111
0.9460 0.120340 0.222408 0.000000 0.000000 -0.002219
0.9730 0.126380 0.224544 0.000000 0.000000 -0.002332
1.0000 0.132469 0.226057 0.000000 0.000000 -0.002451
-------- Mode No. 2 (freq = 0.231784E+00 Hz)
span_loc s-s disp s-s slope f-a disp f-a slope twist
0.0000 0.000000 0.000000 0.001148 0.011658 0.000000
0.0035 0.000000 0.000000 0.001190 0.012493 0.000000
0.0104 0.000000 0.000000 0.001283 0.014165 0.000000
0.0174 0.000000 0.000000 0.001388 0.015840 0.000000
0.0244 0.000000 0.000000 0.001504 0.017519 0.000000
0.0313 0.000000 0.000000 0.001632 0.019201 0.000000
0.0383 0.000000 0.000000 0.001771 0.020886 0.000000
0.0453 0.000000 0.000000 0.001922 0.022574 0.000000
0.0522 0.000000 0.000000 0.002086 0.024266 0.000000
0.0592 0.000000 0.000000 0.002260 0.025961 0.000000
0.0662 0.000000 0.000000 0.002447 0.027660 0.000000
0.0731 0.000000 0.000000 0.002646 0.029362 0.000000
0.0801 0.000000 0.000000 0.002856 0.031068 0.000000
0.0870 0.000000 0.000000 0.003078 0.032778 0.000000
0.0940 0.000000 0.000000 0.003313 0.034492 0.000000
0.1010 0.000000 0.000000 0.003559 0.036209 0.000000
0.1079 0.000000 0.000000 0.003817 0.037930 0.000000
0.1149 0.000000 0.000000 0.004087 0.039652 0.000000
0.1219 0.000000 0.000000 0.004369 0.041377 0.000000
0.1288 0.000000 0.000000 0.004664 0.043105 0.000000
0.1358 0.000000 0.000000 0.004970 0.044834 0.000000
0.1399 0.000000 0.000000 0.005156 0.045855 0.000000
0.1497 0.000000 0.000000 0.005618 0.048301 0.000000
0.1567 0.000000 0.000000 0.005961 0.050038 0.000000
0.1636 0.000000 0.000000 0.006315 0.051778 0.000000
0.1706 0.000000 0.000000 0.006682 0.053520 0.000000
0.1776 0.000000 0.000000 0.007061 0.055265 0.000000
0.1845 0.000000 0.000000 0.007452 0.057013 0.000000
0.1915 0.000000 0.000000 0.007855 0.058764 0.000000
0.1985 0.000000 0.000000 0.008270 0.060518 0.000000
0.2054 0.000000 0.000000 0.008698 0.062274 0.000000
0.2124 0.000000 0.000000 0.009137 0.064031 0.000000
0.2194 0.000000 0.000000 0.009589 0.065789 0.000000
0.2263 0.000000 0.000000 0.010054 0.067548 0.000000
0.2333 0.000000 0.000000 0.010530 0.069309 0.000000
0.2403 0.000000 0.000000 0.011019 0.071071 0.000000
0.2472 0.000000 0.000000 0.011520 0.072833 0.000000
0.2507 0.000000 0.000000 0.011775 0.073715 0.000000
0.3203 0.000000 0.000000 0.017536 0.091424 0.000000
0.3797 0.000000 0.000000 0.023419 0.106514 0.000000
0.4248 0.000000 0.000000 0.028482 0.117938 0.000000
0.4596 0.000000 0.000000 0.032742 0.126677 0.000000
0.4866 0.000000 0.000000 0.036257 0.133404 0.000000
0.5137 0.000000 0.000000 0.039953 0.140081 0.000000
0.5407 0.000000 0.000000 0.043828 0.146665 0.000000
0.5677 0.000000 0.000000 0.047880 0.153151 0.000000
0.5947 0.000000 0.000000 0.052104 0.159531 0.000000
0.6217 0.000000 0.000000 0.056499 0.165788 0.000000
0.6488 0.000000 0.000000 0.061062 0.171887 0.000000
0.6758 0.000000 0.000000 0.065788 0.177814 0.000000
0.7028 0.000000 0.000000 0.070671 0.183555 0.000000
0.7298 0.000000 0.000000 0.075706 0.189075 0.000000
0.7568 0.000000 0.000000 0.080887 0.194338 0.000000
0.7839 0.000000 0.000000 0.086207 0.199324 0.000000
0.8109 0.000000 0.000000 0.091657 0.204009 0.000000
0.8379 0.000000 0.000000 0.097229 0.208336 0.000000
0.8649 0.000000 0.000000 0.102912 0.212269 0.000000
0.8919 0.000000 0.000000 0.108695 0.215775 0.000000
0.9189 0.000000 0.000000 0.114568 0.218807 0.000000
0.9460 0.000000 0.000000 0.120515 0.221301 0.000000
0.9730 0.000000 0.000000 0.126522 0.223205 0.000000
1.0000 0.000000 0.000000 0.132572 0.224461 0.000000
Sincerely
Abhinav
Dear Abhinav,
I didn’t plot them up, but your side-to-side and fore-aft tower-bending modes look close, but not identical.
Best regards,
Dear Sir
Thanks for the reply. I realize that the difference in the values for the S-S and F-A modes is very low. Does it mean that my model is not taking into consideration, the aforementioned difference in inertia at the top of the tower? Is the difference in the values for the S-S and F-A modes considerable for the first modes in the two directions?
Sincerely
Abhinav
Dear Abhinav,
As I said, I would expect more influence of the tower-top inertia on the higher modes. I guess in your case, the tower-top inertia only has a minor effect on the first tower-bending modes.
Best regards,
Dear Jason
I’m still confused as to whether or not i’m using the right modes from BModes to get the polynomial coefficients for FAST. My doubt is regarding the third mode. My first 5 modes are as follows:
mode 1: s-s
mode 2: f-a
mode 3: s-s
mode 4: s-s
mode 5: f-a
the third mode (s-s) has very low values of displacement along the tower. but the fourth mode (s-s) and fifth mode (f-a) show better comparison. please take a look at the figures. is the third mode representative of a torsion mode? is it possible to skip the third mode and use the fourth mode as 2nd s-s mode for the FAST AeroDyn file?
Sincerely
Abhinav
Dear Abhinav,
It looks like either could be the second side-to-side tower-bending mode. Whether one is actually a torsion mode depends on the magnitude of the twist, which you haven’t plotted.
As I discussed in my Aug 29, 2014 post here, I recommend you set the distributed torsion stiffness high in BModes when creating modes for FAST anyway. In this way, I wouldn’t expect to see any low-frequency torsion modes.
You can use whichever modes you want in FAST, as long as they are representative of the response you are trying to simulate.
Best regards,
Dear Jason,
I have considered following points as the gage points along the tower.
Gages Heght above the tower base(m)
1 4
2 8
4 16
6 24
8 32
12 48
14 56
17 68
20 80
So, my question is when I pick values from BModes result corresponding to these points mentioned above (9 points), I am getting the following mode shapes where I can non linear/deflection at midpoint of mode shape. Could I know what could be reason for this?
Thanks,
Satish J
Dear Satish,
I’m sorry, but I don’t really understand your question. You seem to be referring to tower gages in the ElastoDyn output, but then mention the use of BModes to generate the mode shape input to ElastoDyn.
When using BModes to calculate mode shapes for ElastoDyn, BModes will output the deflection at all FE nodes of the BModes model.
Best regards,
Dear Jason,
Thanks for your reply.
Though I am getting s-s disp at all FE nodes, I am taking only s-s disp points in BModes results in such a way that it corresponds to the distance from tower base at which I considered for gage points in Elastodyn. (Just to understand I took only 9 s-s disp corresponding to the gage points)
Thanks,
Satish J
Dear Satish,
I’m sorry, but I still don’t really understand your question. Are you deriving the mode shapes for ElastoDyn from displacement of all FE nodes outputs from BModes? Is this curve smooth?
Best regards,
