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the result of building half model to modal ansys(or eigenfrequency ansys ) is different with the whole model?

Posted 19.10.2012, 09:32 GMT-4 Geometry, Parameters, Variables, & Functions, Results & Visualization, Structural Mechanics Version 4.3, Version 4.3a 2 Replies

Hao Yongcun
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I build two models to modal ansys, one is half model and the other is whole model(the model is symmetrical),but the results i got is different. frist , the number of whole model`s modal Shape under 20KHz is more than the half model`s. second, in the same modal Shape, the two model`s eigenfrequency is different, at a difference of a few Hz
i want to know why ? who can help me ,thanks a lot!

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2 Replies Last Post 19.10.2012, 14:25 GMT-4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 19.10.2012, 13:59 GMT-4
Hi

if you apply one symmetry to an object (via a plane) you need to run 2 eigenfrequency runs, one with symmetry, one with antisymmetry conditions, else your re estraining yourself to roughly half the modes, as the symmetry mode is fixed in the normal direction. If you have 2 symmetry plane (1/4 model) then you need to do 4 cases with all combinations of symmetry / antisymmetry loads tocover all modes

Also the mode frequency might vary depending on the mesh density/quality (and the mesh symetry) so some & difference is not astonishing, uit the differenece should reduce with a higher mesh density

--
Good luck
Ivar
Hi if you apply one symmetry to an object (via a plane) you need to run 2 eigenfrequency runs, one with symmetry, one with antisymmetry conditions, else your re estraining yourself to roughly half the modes, as the symmetry mode is fixed in the normal direction. If you have 2 symmetry plane (1/4 model) then you need to do 4 cases with all combinations of symmetry / antisymmetry loads tocover all modes Also the mode frequency might vary depending on the mesh density/quality (and the mesh symetry) so some & difference is not astonishing, uit the differenece should reduce with a higher mesh density -- Good luck Ivar
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 19.10.2012, 14:25 GMT-4
I just want to repeat Ivar’s point about geometric symmetries and mode shapes. Even if a part is symmetric it will have roughly as many non-symmetric modes as symmetric ones. Only if you are sure that your loading is also symmetric can you ignore the non-symmetry modes.
I just want to repeat Ivar’s point about geometric symmetries and mode shapes. Even if a part is symmetric it will have roughly as many non-symmetric modes as symmetric ones. Only if you are sure that your loading is also symmetric can you ignore the non-symmetry modes.

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