Optistruct Complex Eigenvalue Analysis
Hallo, i need some help because i cant find any solution to my problem.
I've a Project where i have to work with dynamic stiffness (pbusht element) of a structure and want to see the Eigenvalues and normal modes of my structure.
To get into the theory i set up a simple model: A CBUSH element with a pbush property and with a 1D Mass (m=1kg) on top and a BC on the bottom. I specified in pbusht property with elastic stiffness of K=100 (N/mm) and a loss factor of GE=0.3
Then i did an normal mode analysis and i know that the 'Damping is neglected' (see link (1)) and i have just my K=100. The solution of my normal mode analysis is a normal mode frequency of omega = 50,33 Hz. This is correct, because the formular for free, undamped oscillations is f = sqrt(k/m)/2PI = sqrt(100 N/mm / 0,001 tonnen) / 2PI = 50,33 Hz
(i use N, mm, tons as units)
Now i want to include the damping, so i use the complex eigenvalue analysis. In theory i learned that for free, undamped oscillations with the damping factor D<1 the cycle frequency omega_d = omega * sqrt(1-D²). Because of the relation that 2*D = tan(phi) = GE i can fill in the my loss factor of GE=0.3. Then i got my omega_d = 49.76 Hz. This is also logically, because for D<1 the oscillations of damped systems are always slower than the ones of the undamped system. ...so omega_d < omega is okay. Now i did the complex eigenvalue analysis (just defined a load collector EIGC card and connected it to the CMETHOD of the complex eigenvalue load step (of course the SPC and EIGRA card to the SPC and Method(Struct) too like i did for the normal modes). The result of the simulation is a frequency of 50,88 Hz and a loss factor of 0,2935. Now im confused - first: why is my lossfactor not 0,3? Yes its just a little difference but its a really simple model. But okay, maybe some numeric accuracy. But i defenetly do not understand the frequency of 50,88 Hz. Where is the mistake? Its higher than before whats is wrong in theory and not my calculated value.
Anyone got some ideas? I already thank you
Regards
Rene
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