Plotting tower shapes with Modes v.2.2

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Natalia.Claros
Posts: 3
Joined: Thu Nov 29, 2018 6:47 am
Organization: TU Berlin
Location: Germany

Plotting tower shapes with Modes v.2.2

Postby Natalia.Claros » Thu Apr 11, 2019 9:53 am

Dear Jason,

I'm currently doing a modal analysis of a 63m tower. I used the program Modes v.2.2 to calculate the modes. Below the input for Modes.

Code: Select all

False     Blade Switch:  True = Blade, False = Tower
   0.0    Steady state angular velocity of rotor (rpm)  [Ignored for towers]
   0.0    Pitch angle for blades (degrees)  [Ignored for towers]
  63.0    Total beam length (m)
   0.0    Rigid beam length (m)
28850.0   End mass (kg)
   5      Number of modes shapes or coefficients
   2      Order of first coefficient
   10     Number of input stations for distributed parameters
   1.0    Factor to adjust mass
   1.0    Factor to adjust tower stiffness
   1.0    Factor to adjust in-plane stiffness  [Ignored for towers]
0.00E+00  1.8797984E+03  6.4919605E+10 
1.11E-01  1.7732607E+03  5.4495781E+10 
2.22E-01  1.3267888E+03  3.2870718E+10 
3.33E-01  1.1738875E+03  2.2766165E+10 
4.44E-01  1.0259185E+03  1.5197022E+10 
5.56E-01  8.8288178E+02  9.6859372E+09 
6.67E-01  8.5328798E+02  8.7442928E+09 
7.78E-01  7.0137313E+02  5.9951282E+09 
8.89E-01  5.0270006E+02  2.7937987E+09
1.00E+00  6.4997856E+02  3.1947576E+09
                                   
For the lines of distributed parameters, the first column is the fractional
distance along the flexible portion of the tower.  It must go from 0 to 1.
The second column is the mass/length in kg/m, and the third column is the
stiffness in N m^2.


Output of Modes:

Code: Select all

               Shape 1     Shape 2     Shape 3     Shape 4     Shape 5
               -------     -------     -------     -------     -------
 Freq (hz)      0.4459      2.8752      8.0611     19.6116     26.3147

    x^2         0.5963    -15.8018     48.2225   -621.2538    631.6574
    x^3        -0.1676      8.9338    327.6914   3204.8274  -4760.0376
    x^4         1.7325    -32.1423  -1542.1295  -5478.7134  12354.1416
    x^5        -1.4457     82.9072   1838.8284   3606.8958 -13313.1475
    x^6         0.2845    -42.8969   -671.6127   -710.7558   5088.3857


I'm trying now to compare now the shapes 1&2 with a simple MATLAB script that I wrote. The first and second frequencies look very plausibel and the discrepancy between both models are very low.
Comparison of 1st and 2nd natural frequencies.PNG
Comparison of 1st and 2nd natural frequencies.PNG (13.22 KiB) Viewed 333 times


My problem occurs when I want to plott them. I'm plotting with Matlab the shapes with the polynomial functions:

Code: Select all

Shape_1=-0.5963*x.^2+0.1676*x.^3-1.7325*x.^4+1.4457*x.^5-0.2845*x.^6
Shape_2=-15.8018*x.^2+8.9338*x.^3-32.1423*x.^4+82.9072*x.^5-42.8969*x.^6


And I get the following functions:
Shape1_2_Modes.png
Shape1_2_Modes.png (27.98 KiB) Viewed 333 times


For comparison I scaled the x-axis with my model but still the shapes don't look right.
Modes_Matlab_Shape1_2.png
Modes_Matlab_Shape1_2.png (37.19 KiB) Viewed 333 times


I already looked up to BModes but I didn't find something that could help me to plot them right. Am I missing some correction factor or something similar?

Thank you in advance!!

Best regards,

Natalia

Jason.Jonkman
Posts: 4104
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Plotting tower shapes with Modes v.2.2

Postby Jason.Jonkman » Thu Apr 11, 2019 4:00 pm

Dear Natalia,

In your polynomial equation, x must be between 0 (at the tower base) and 1 (at the tower top). Thus, the value of polynomial at the top (x=1) should be 1. Is this what you've done?

As suggested many times on this forum, I suggest using BModes in combination with the ModeShapePolyFitting.xls spreadsheet to compute tower modes for ElastoDyn. This spreadsheet will plot the mode shape polynomial for you.

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

Natalia.Claros
Posts: 3
Joined: Thu Nov 29, 2018 6:47 am
Organization: TU Berlin
Location: Germany

Re: Plotting tower shapes with Modes v.2.2

Postby Natalia.Claros » Fri Apr 12, 2019 3:00 am

Dear Jason,

first, thank you so much for the quick response. With the correction you mention of tower base (=0) and tower top (=1) the graphics work very well!
Shape2_Modes_CORRECT.png
Shape2_Modes_CORRECT.png (32.27 KiB) Viewed 317 times


Best regards,

Natalia


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