"Double" modes in modal analysis of simple circular membrane clamped in its boundary

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  • Last Post 09 February 2019
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kravky posted this 08 February 2019

Hello, 

i am trying to find normal modes of simple ideal circular membrane.
I chose strecutral steel as material, and geoemtry is: diamater is 0,391mm, thickness is 0,0035mm.


However, i got "double" modes: every two modes are very very similar - with the almost same value of frequency.
i got same answer with other commercial program. 
I tried to chcange mesh, i tried do model my geomtry as solid and also as a shell/surface but i got the same problem.

see pictures: mode 4 and mode 5 is only ONE mode in reality. Why i got this result?

 

mode_4

 

mode_5

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peteroznewman posted this 08 February 2019

To a mathematician, there is only one mode since they generalize that the orientation of the mode in the circle is arbitrary.

To ANSYS, there are two modes since it treats X and Y axes as separate and it can't recognize that these two modes are just offset by some rotation about the center.

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kravky posted this 08 February 2019

Thank You for explanation.

Should i define polar coordination system in order to get mathematically correct solutions? (not to have double modes)

peteroznewman posted this 09 February 2019

No, you just have to recognize the pairs are the same mode separated by a rotational angle.

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