Tower (and blades) mode shapes BMODES vs. FAST

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Claudio.Pedrazzi
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Tower (and blades) mode shapes BMODES vs. FAST

Postby Claudio.Pedrazzi » Thu Nov 29, 2007 7:16 am

Hi everybody,
here I am again after a long silence :-) with a new question:

In my understanding, BMODES calculates and outputs modal shapes as adimensional plots of normalized displacement vs. normalized beam length. And every mode shape can be coupled, in the sense that it can contain both fore-aft and side-to-side components.

FAST, that I am approaching for the first time, does require for the same input polynomial coefficients, and moreover it requires that the polinomials be already categorized as "fore-aft" or "side-to-side".

Is there a right way to go from BMODES output to FAST input? since the BMODES manual states that ultimately BMODES is a preprocessor for FAST, I am unsure if my interpretation is correct... I would have expected an optional polynomial coefficient output, or something similar.

Of course I can interpolate the data and calculate the polynomial coefficients myself, and try to decide if a given shape is fore-aft or side to side, but I wanted to be sure that this is expected from the user, that I am not missing some "piece" of software, or misinterpreting the manual.

Best regards and thanks a lot in advance!
Claudio Pedrazzi
Claudio Pedrazzi

Senior Project Engineer
Technical Dept.
TEROM S.p.A.
Bologna, Italy
www.terom.it
http://www.terom.it/ASP/Pages/eng/News.asp

Jason.Jonkman
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RE: Tower (and blades) mode shapes BMODES vs. FAST

Postby Jason.Jonkman » Thu Nov 29, 2007 9:46 am

Dear Claudio,

You are not missing a "piece" of software or "misinterpreting the manual." BModes generates coupled modes, and FAST currently uses uncoupled modes (input as equivalent polynomial coefficients). We have plans to upgrade FAST to use coupled modes, but unfurtunately, we haven't had time yet to add this feature. But when we get around to upgrading FAST we will enable it to directly read-in the output from BModes.

As it is now, you must identify the "prominent component" of each coupled mode generated by BModes and then pass a best fit polynomial throught that component (ignoring the other, less dominant, components of the mode). I use the MS Excel array function "LINEST" to calculate the best fit polynomial, but you could calculate it anyway you want.

I hope that helps.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Claudio.Pedrazzi
Posts: 47
Joined: Wed Nov 09, 2005 12:05 am
Location: Bologna, Italy
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Postby Claudio.Pedrazzi » Fri Nov 30, 2007 12:53 am

Dear Jason,
thanks for the quick answer. It does help, in the sense that now I know I am going the right way. Actually, I planned to use MathCAD or EXCEL to find the polynomial coefficients.

Since, in my understanding, this forum is also to be used for posting suggestions, requests, wishes :-) for any Design Codes, I would like to add my suggestions for BModes:

1) provide a way that the Nacelle Center of Mass could be Offset in any three directions from the tower top. X, Y and Z (now it is possible only with X).

2) interface more directly to the FAST input. This of course could imply quite a bit of programming, but at the moment this is a significant "break" in the code chain (think what one has to do if he/she wishes to experiment with different thicknesses of a segment of the tower)

Best regards and thanks again
Claudio
Claudio Pedrazzi



Senior Project Engineer

Technical Dept.

TEROM S.p.A.

Bologna, Italy

www.terom.it

http://www.terom.it/ASP/Pages/eng/News.asp

Claudio.Pedrazzi
Posts: 47
Joined: Wed Nov 09, 2005 12:05 am
Location: Bologna, Italy
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FAST Polynomial coefficients

Postby Claudio.Pedrazzi » Wed Dec 05, 2007 5:54 am

Hi,
I would have a sequel :-) to the previous question, now specifically concerning FAST input. Sorry for bothering you, I appreciate very much your help.

I would like to confirm the statement, to be found in page 72 of the FAST user guide, that the polynomial coefficients should have sum = 1. The reason I ask is that I have a mode like this:

Code: Select all

 -------- Mode No.   4  (freq = 0.10110E+02 Hz)               
               
span_loc   s-s disp   s-s slope   f-a disp   f-a slope   twist
               
0   0   0   0   0   0
0.0017   0   0   0   -0.000117   0
0.0019   0   0   0   -0.000142   0
0.0908   0   0   -0.003674   -0.075573   0
0.1798   0   0   -0.012481   -0.114779   0
0.2687   0   0   -0.022941   -0.11431   0
0.2689   0   0   -0.022962   -0.114302   0
0.2723   0   0   -0.023352   -0.114228   0
0.2725   0   0   -0.023374   -0.11422   0
0.3632   0   0   -0.032501   -0.085446   0
0.4542   0   0   -0.038758   -0.054295   0
0.5262   0   0   -0.042065   -0.039923   0
0.598   0   0   -0.044805   -0.038513   0
0.5982   0   0   -0.044812   -0.038515   0
0.6017   0   0   -0.044948   -0.038539   0
0.6019   0   0   -0.044955   -0.038541   0
0.6928   0   0   -0.048778   -0.043658   0
0.693   0   0   -0.048786   -0.043649   0
0.7838   0   0   -0.051631   -0.007355   0
0.8745   0   0   -0.046378   0.14377   0
0.9691   0   0   -0.018785   0.463748   0
0.9693   0   0   -0.018697   0.463878   0
0.971   0   0   -0.017908   0.463939   0
1   0   0   -0.004432   0.464512   0

As you can see, it has a negative value in x/L=1. Moreover, the value in x/L=1 is small, relative to the values in x/L = 0.8.

Image

Of course I did the mathematical transformation (with MathCAD linfit function), and it works fine, but I am not satisfied because I see "wild" :-) polynomial coefficients, namely:

Code: Select all

x^2 = 211.19
x^3 = -435.953
x^4 = 84.86
x^5 = 512.075
x^6 = -371.172

I am not particularly happy with such big numbers, even if the resulting polynomial behaves quite appropriately. It reaches around 17.3 around x/L = 0.8. Is this the intended normalization? I am unsure if the correct normalization wouldn't be to have a maximum value of 1.0 (instead of final).

Thanks a lot
Best regards
Claudio
Claudio Pedrazzi



Senior Project Engineer

Technical Dept.

TEROM S.p.A.

Bologna, Italy

www.terom.it

http://www.terom.it/ASP/Pages/eng/News.asp

Jason.Jonkman
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RE: FAST Polynomial coefficients

Postby Jason.Jonkman » Thu Dec 13, 2007 4:59 pm

Hi Claudio,

The approach you have taken to get the coefficients is correct. You could also try to use both the displacement and slope output from BModes in your curve-fitting routine to compute the polynomial coefficients, but I'm not sure if the curve fit would be much better.

FAST in its current form needs to have the polynomial coefficients sum to unity. The polynomial coefficients must sum to unity because the mode shapes are normalized by the tip displacement. This normalization scheme is not perfect, however, because there may be instances were the 2nd mode has zero tip displacement (through a suitable choice in tip mass). Luckily in your case, the tip displacement is not zero--it is just close to it.

When we upgrade FAST so that it uses coupled flap-lag-twist modes instead of uncoupled modes, we also plan to modify the mode shape normalization scheme to eliminate this potential problem.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Claudio.Pedrazzi
Posts: 47
Joined: Wed Nov 09, 2005 12:05 am
Location: Bologna, Italy
Contact:

Postby Claudio.Pedrazzi » Fri Dec 14, 2007 12:33 am

Jason,
thanks. You have completely answered my question.
Best regards and happy Christmas / New Year to you and all the NWTC team from Italy.
Claudio
Claudio Pedrazzi



Senior Project Engineer

Technical Dept.

TEROM S.p.A.

Bologna, Italy

www.terom.it

http://www.terom.it/ASP/Pages/eng/News.asp

Yinping.Yang
Posts: 55
Joined: Tue Jul 28, 2009 1:36 am
Location: Germany

Postby Yinping.Yang » Fri Aug 14, 2009 10:02 am

Hi,

If I want to get the Polynomial parameters for the FAST blade input data, the BModes gives me 20 Modes. From each Mode, I get one set of prominent component so as to interpolate the polynomials. But so many modes. which can be taken as the input polynomial? The first several low frequencies?

Regards

Jason.Jonkman
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Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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RE: FAST Polynomial coefficients

Postby Jason.Jonkman » Fri Aug 14, 2009 10:17 am

Hi Yinping Yang,

Of course, you can choose any of the modes from BModes to use in the FAST model. However, the lowest frequency modes are likely the modes that are most important to the aero-elastic response of the wind turbine. So, typically one would use the lowest frequency modes.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Yinping.Yang
Posts: 55
Joined: Tue Jul 28, 2009 1:36 am
Location: Germany

Postby Yinping.Yang » Mon Aug 17, 2009 8:32 am

Hi,

I tried to calculate the mode Polynomials of Baseline_Blade.dat, which was used in Test11.fst of FAST Certest.

I used the BModes input file and changed some parameters as specified in Test11.fst.

20.0 rot_rpm
35.0 radius
1.75 hub_rad
0.0 precone
7.5 bl_thp


Other parameters are not changed. The data in blade_sec_props.dat are the same. So I ran the BModes to get 20 Modes for this input file. For the first four modes, I chose prominent component for each mode respectively ( the bigger displacement column compared to the other displacement) and interpolated these displacements to obtain the curve polynomials by using Excel.

Then I ran the Modes code, which was also provided by Nrel. Input parameters are like this:

20 Rotor angular velocity
7.5 Pitch angle for blades
35.0 Total beam length
1.75 Rigid beam length
0.0 End mass
5 Number of modes shapes
2 Order of first coefficient
21 Number of input stations
1 Factor to adjust mass
1 Factor to adjust out-of-plane stiffness
1 Factor to adjust in-plane stiffness
Properties are the same as example Test11.


Finally I have got all the results but they look seem different. The Modes results are closer to the input modes used in Test11.fst. The BModes result are different. Are the modes of Baselien_Blade coming from BModes? Or other Tools like Bladed?

Code: Select all

Results are like this:
 BModes,                       
Flap Mode 1(Mode 1)       
      f=1.2783 Hz               
         -0.0701                 
          0.7369                                         
         -1.2092                                         
          1.1878                                         
         -0.4591                                       

Edge Mode 1(Mode2)                     
      f=1.8298 Hz                     
         -0.1863                             
         -0.0716                                   
          0.0763                                   
         -0.0948                                   
          0.0637                                 

Flap Mode 2(Mode 3)                   
      f=3.5875 Hz               
          0.4580                             
         -1.9169                           
          4.9466                   
         -5.892                   
          2.3219                   

Edge Mode 2(Mode 4)         
      f=6.0728 Hz             
          0.3271               
         -0.5314                   
          0.9484                       
         -1.7856                   
          0.9240
Modes,                         

Out-of-Plane Shape 1               
f=1.3715 Hz         
0.0938                               
1.7722                               
-2.2002                             
2.5779                               
-1.2436                           

In-Plane Mode Shape 1         
f=1.919 Hz         
0.2975                         
3.3297                         
-6.4117                       
5.9343                         
-2.1496                       

Out-of-Plane Mode Shape 2     
f=3.9947 Hz         
 -0.3346                         
 -2.1447                         
 -3.8977                         
 16.0501                       
 -8.6731                         

In-Plane Mode Shape 2
f=6.4816 Hz         
-0.7686
-7.0411
17.252
-8.665
0.2227

Baseline_Blade.dat
Flap Mode 1
0.0838
1.6525
-1.5682
1.6947
-0.8628

Edge Mode 1
0.3165
3.2618
-6.4005
6.0367
-2.2146

Flap Mode 2
-0.3008
-1.9968
-4.6564
16.9661
-9.0121


Thanks in advance!

Best Regards
Last edited by Yinping.Yang on Wed Aug 19, 2009 3:32 pm, edited 2 times in total.

Jason.Jonkman
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RE: FAST Polynomial coefficients

Postby Jason.Jonkman » Tue Aug 18, 2009 7:13 pm

Hi Yinping Yang,

I calculated the polynomials in Baseline_Blade.dat for Test11.fst with Modes v2.22. The only differences between my model and yours, is that I used a rotor speed of 20.463 rpm and a pitch angle of 0.0 deg. I suspect you'll get the same values I get when you make these two changes to your Modes model.

I developed the FAST model of the WindPACT 1.5-MW baseline wind turbine (Test11) before BModes was ever available. Even though BModes is now available--and is recommended to be used in place of Modes--I have not had a chance to update this FAST model with mode shapes from BModes. I plotted up the mode shapes using your results, however, and the Modes and BModes predictions are quite close. Please note that you must normalize the polynomial coefficients so that they sum to unity. For example, your flap mode 1 from BModes:

-0.0701
0.7369
-1.2092
1.1878
-0.4591

should actually be:

-0.3763
3.9554
-6.4906
6.3757
-2.4643

when normalized. Even though these coefficents are not identical to those predicted by Modes, the mode shapes, when plotted, are quite similar.

I hope that helps.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Yinping.Yang
Posts: 55
Joined: Tue Jul 28, 2009 1:36 am
Location: Germany

Postby Yinping.Yang » Wed Aug 19, 2009 10:20 am

:)
Last edited by Yinping.Yang on Wed Aug 19, 2009 3:10 pm, edited 1 time in total.

Yinping.Yang
Posts: 55
Joined: Tue Jul 28, 2009 1:36 am
Location: Germany

Postby Yinping.Yang » Wed Aug 19, 2009 3:08 pm

Thanks for your help! Your advice is really helpful to me, especially for a beginner. :)

Does the Pitch angle influence the mode shapes ? If yes, should I use Pitch angle as specified in FAST primary input to calculate the Modes or just use 0 degree Pitch angle for all modes calculations ?

I normalize my flap mode 1 in such a way you mentioned before:
All the nodal displacements of the blade are divided by the tip displacement. Then I interpolate the normalized displacements by using Exel to get the polynomials, it looks a little different,maybe due to the interpolation method.

-0.3601
3.7878
-6.2155
6.1053
-2.36

Is there another method to normalize the mode shape as you did using my flap mode 1 ?

If the plotted mode shapes are quite close, nearly identical, but the coefficents are different. Does that mean, both are correct? Using my normalized coefficents in FAST input file will have the same final results as the original mode coefficents you used in test11.fst ?

If I have the whole range (-180 degree to 180 degree) of AOA CL,CD,CM Tables, is it necessary to run AirfoilPrep to calculate our AeroDyn input data? The AeroDyn manual said that AirfoilPrep is not necessary if the whole range of aerodynamics data are available.

Maybe my silly questions are too many. I hope that does not annoy you.

Thanks again!

Best regards

Jason.Jonkman
Posts: 4617
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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RE: FAST Polynomial coefficients

Postby Jason.Jonkman » Wed Aug 19, 2009 5:40 pm

Yinping Yang,

Mode shapes in real life change with operational parametes, such as rotor speed, pitch angle, and frequency of excitation. However, my experience has been that the accuracy of the mode shape (as long as they are close to correct) is not critical to the results of a loads analysis--the modal frequencies are far more important. So, we typically calculate the mode shapes only at one design point--typically the conditions at rated power. As wind turbines are getting larger and more flexible, the impact of the mode shapes will likely become more important.

I simply took your coefficients and scaled them so that they sum to unity. Again, as long as the mode shapes are close to correct and the system frequencies (and other inputs) are accurate, your simulation results should be good.

AirfoilPrep not only has a feature to extrapolate to the full range of AoA, but it also has features to (1) apply rotational augmentation (i.e., 3D corrections), (2) blend airfoil data (if needed), (3) calculate dynamic stall parameters, and (4) convert the airfoil data to AeroDyn format. So, AirfoilPrep is useful to use regardless of if the AoA range you have.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Gabriel.Maciel
Posts: 9
Joined: Mon May 08, 2017 1:44 pm
Organization: Instituto Superior Técnico
Location: Portugal

Tower mode shapes FAST

Postby Gabriel.Maciel » Sat May 20, 2017 10:08 am

Hi,

I´m conducting a study of the tower´s mode shapes. In FAST v8, the files "NRELOffshrBsline5MW_ (name of substructure) _ElastoDyn_Tower.dat" needs the polynomials' coefficients of tower´s fore-aft (FA) and side-to-side (SS) mode shapes (first and second orders) as input. I have noticed that these polynomials start at the origin (0;0) point and with end at (1;1) point. For the OC3 Tripod structure see figures 1 and 2.


I also noticed that the OC4Jacket structure has modes shapes very similar to those in the figures.

Analyzing the figures it is clear that the deformation at the tower’s base is zero, so I deduce that the Tripod substructure is rigid (i.e not flexible) in this analysis. Am I right?

I ´m studying a tripod-shaped structure, similar to OC3 Tripod structure and I evaluated the mode shpapes in ANSYS software. When I placed the tower’s base, on top of my substructure, (the piles are fixed, simulating the connection to the seabed), I noticed that the tower base has a non-zero deformation in first and second order mode shapes (as in Figure 3 for example). Thus, it is impossible to obtain a polynomial like the OC3Tripod ones, since the graphics never begin at (0;0) point.

Do you know if FAST v8 requires the polynomial to start at point (0;0) (i.e, zero deformation at the base)?

Regards,

Gabriel Maciel.
Attachments
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Jason.Jonkman
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Re: Tower (and blades) mode shapes BMODES vs. FAST

Postby Jason.Jonkman » Sat May 20, 2017 2:38 pm

Dear Gabriel,

I have answered a similar question in my Apr 10, 2012 post in the following forum topic: viewtopic.php?f=3&t=607&p=2255.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov


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