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Error in eigenfrequency study

Posted 20.01.2014, 01:41 GMT-5 2 Replies

Aravindhan V
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Hi,
I am trying to run a vibration analysis on a part. I used the Solid mechanics module and the Eigenfrequency study. I specified the boundary condition, the loading and the material. But when i compute the study, two things occur. Firstly, some of the computed eigenfrequencies are imaginary, like 3.78i.What does that mean? Secondly the mode shape plot is completely confusing. I am attaching a screenshot for reference, as the model is a bit too large( 26MB). How do i get it it to work?

Any help would be appreciated.
Thanks.

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2 Replies Last Post 20.01.2014, 04:57 GMT-5
RVN
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Posted: 1 decade ago 20.01.2014, 04:37 GMT-5

Hi,
Firstly, some of the computed eigenfrequencies are imaginary, like 3.78i.What does that mean?

An imaginary part should be interpreted as a loss in the system. Either there is some materialdamping or your model is radiating energy towards the surroundings through a radiation boundary condition or a PML (perfectly matching layer). The larger the imaginary part is, the higher the loss is at that frequency. It is a fact that certain modes radiate energy better than others, this is geometry dependent.


I am attaching a screenshot for reference, as the model is a bit too large( 26MB).

Is the model still to large if you clear the solutions and clear the mesh?


--
Hope this helps
--
/RVN
[QUOTE] Hi, Firstly, some of the computed eigenfrequencies are imaginary, like 3.78i.What does that mean? [/QUOTE] An imaginary part should be interpreted as a loss in the system. Either there is some materialdamping or your model is radiating energy towards the surroundings through a radiation boundary condition or a PML (perfectly matching layer). The larger the imaginary part is, the higher the loss is at that frequency. It is a fact that certain modes radiate energy better than others, this is geometry dependent. [QUOTE] I am attaching a screenshot for reference, as the model is a bit too large( 26MB). [/QUOTE] Is the model still to large if you clear the solutions and clear the mesh? -- Hope this helps -- /RVN
Aravindhan V
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Posted: 1 decade ago 20.01.2014, 04:57 GMT-5
I'm afraid so. Let me try to explain what's happening. The coloured contour should be on the surface of the part. But it is appearing completely disjoint from the part, floating around in thin air...
Have you ever faced anything like this? And if so how did you resolve it?
I'm afraid so. Let me try to explain what's happening. The coloured contour should be on the surface of the part. But it is appearing completely disjoint from the part, floating around in thin air... Have you ever faced anything like this? And if so how did you resolve it?

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