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-   -   Need help for easy modal analysis of a beam! (http://www.cfd-online.com/Forums/ansys/78128-need-help-easy-modal-analysis-beam.html)

yooy July 13, 2010 16:57

Need help for easy modal analysis of a beam!
 
Hi, Johan, french student based at TORONTO for a internship. I want to have the free vibration of a beam... So it seems easy!
I have a big mistake with any results given by ansys but I dont know what wrong in my input:

system: beam damped damped element
type: beam3
real constant: AREA =1 IZZ=1/12 HEIGHT=1
Material model: DENS=8E-6 EX=1E7 PRXY=0
Section: centroid B=1 H=1
Modeling: line from (0.0.0) to (10.0.0)
Loads displacement on keypoints (0.0.0) and (10.0.0)
Meshing with 100 nodes..

Results:
***** INDEX OF DATA SETS ON RESULTS FILE *****
SET TIME/FREQ LOAD STEP SUBSTEP CUMULATIVE
1 11434.
2 31088.
3 55904.
4 59692.



Using Brick model with the same geometry I have the same transverse results:

***** INDEX OF DATA SETS ON RESULTS FILE *****

SET TIME/FREQ LOAD STEP SUBSTEP CUMULATIVE
1 10925.
"2 10925."
3 28372.
"4 28372."
"5 36566."
6 51930.




So to analyze the result I undimension the frequency thank to the formula: (big_omega)^2=rho*A0/(E0*IZ0)*L0^4 *omega^2 with big_omega the free frequencies undimensioned
here I use
rho=8E-6
A0=1
E=1E+7
IZ0=1/12

L0=10
And I should also find the famous result:
big_omega1=22.39
big_omega1=61.67...
and not
big_omega1=3.542
big_omega1=9.63....

So I really dont understand and I'm lock for 3days now and nobody can help me!
Thanks for all Ps: ANSYS 12.0 used....


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