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]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.[/code]
Output of Modes:
[code] 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[/code]
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.
My problem occurs when I want to plott them. I’m plotting with Matlab the shapes with the polynomial functions:
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:
For comparison I scaled the x-axis with my model but still the shapes don’t look right.
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
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,
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!
Best regards,
Natalia
Hi everyone!
I want to do a modal analysis for the blade and tower for the 15 MW IEA Wind turbine and i am gonna do it in Modes. Is it advisable to use Modes for this? Or is it better BModes or even other?
Apart from this, I am scrolling down through the files from FAST VERSION 7 and i cannot find anything related to the input list that is said in this thread conversation. Could you help me find this input, please?
The input that i am saying is this one:
[code]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.[/code]
Dear @Alberto.Utrera,
I would say that it is reasonable to use the old Modes code for computing blade mode shapes for ElastoDyn, but I would not apply Modes for deriving tower mode shapes. The tower-top mass and inertia, as well as the substructure boundary condition, can have a strong effect on the tower mode shapes, and Modes cannot capture these.
For the tower mode shapes, I would recommend using BModes (or BModes_JJ discussed on this forum if the tower is installed on a flexible foundation or offshore substructure), or Frame3DD, which is called within WEIS. BModes can also be used to derive mode shapes for spinning blades.
Best regards,
Thank you very much @Jason.Jonkman . I have downloaded BModes and it is much better. I have a doubt regarding HUB_RAD in the tower file. I suppose the tower rigid-base height is the tower height, right? Plus, bl_thp could e the blade pitch angle in the rated(nominal) condition?
BR
Dear @Alberto.Utrera,
HubRad in BeamDyn is the distance to the base of the blade from the hub center (for blades) or base of the tower from the MSL/ground (for towers), i.e., the point in the blade or tower where flexibility first starts.
Yes, bl_thp can be used to specify the pitch angle for a blade, although if you are using BModes to compute blade mode shapes for ElastoDyn, we generally recommend setting bl_thp, as well as str_tw in the beam section properties file to zero, as mentioned in other BModes-related forum posts.
Best regards,
