
#1
Oct1111, 03:01 PM

P: 107

Hi
I am using Ansys to find the natural frequency of a structure but this has meant that my understanding of vibrations has become unstuck. When I last studied natural frequencies, it was under free vibration. I formed the impression that one structure can have only one natural frequency which would correspond with a mode shape. However, in Ansys, for a continous structure, you can find an infinite number of mode shapes (default of 6). So my question is, how can a structure have so many natural frequencies and therefore corresponding mode shapes. Confused me! Any help much appreciated! 



#2
Oct1111, 03:55 PM

Sci Advisor
PF Gold
P: 11,352

I could ask why there should be only one mode shape. The simple model of two nearly identical masses on nearly identical springs, coupled by a third, weak spring, has two modes. A taught string can have a vast number of overtones. So why shouldn't a complex structure have loads of possible modes of vibration?




#3
Oct1111, 05:05 PM

P: 107

You could yea but that would only make me ask more questions and not answer my OP?




#4
Oct1111, 05:19 PM

Sci Advisor
PF Gold
P: 11,352

Natural Frequency and mode shapes
I have at least given you a reason to think that there can be many modes. It's acceptable to answer a question with a question because, in my question, there is a route to the answer to yours. You didn't want spoonfeeding did you?
Do you not watch any of those Mystical Eastern Combat films? They never get straight answers there either but they end up being able to fly. 



#5
Oct1111, 05:42 PM

Engineering
Sci Advisor
HW Helper
Thanks
P: 6,342

If a structural model has N degrees of freedom, it will have N natural frequencies and mode shapes. For a finite element model, N is usually equal to the number of unconstrained mesh points in the model, times the number of degrees of freedom at each mesh point, which might be 3 or 6 depending whether or not the finite elements have rotation variables. Actually that's a bit oversimplified, but the bottom line is that most reallife FE models will have a very large number of natural frequencies and mode shapes, Only the lowest frequencies will be accurate enough to be useful, so it would be pointless (and it would take an unfeasilbly long time) to try to find all of them. Mathematically, the equations of motion of a continuous flexible structure have an infinite number of vibration modes. 



#6
Oct1111, 06:07 PM

P: 107





#7
Oct1211, 04:59 AM

Sci Advisor
PF Gold
P: 11,352

Even 'Karate Kid' has some of that nonsense in it, I think. I wouldn't suggest you take time to see it if you haven't but Crouching Tiger is worth seeing. Never a straight answer in the whole film. Of course, if your system has only one DOF, you couldn't expect more natural frequencies but you asked in general so you got the general answer ( / question). 


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