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

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


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?





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peteroznewman posted this 09 February 2019

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

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 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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