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

- February 8, 2019 at 8:09 pm
  
  kravky
  Subscriber
  
  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?
- February 8, 2019 at 9:34 pm
  
  peteroznewman
  Subscriber
  
  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.
- February 8, 2019 at 9:55 pm
  
  kravky
  Subscriber
  
  Thank You for explanation.
  
  Should i define polar coordination system in order to get mathematically correct solutions? (not to have double modes)
- February 9, 2019 at 12:51 am
  
  peteroznewman
  Subscriber
  
  No, you just have to recognize the pairs are the same mode separated by a rotational angle.
- April 7, 2022 at 2:23 pm
  
  LEOLEE
  Subscriber
  
  .
  Hi! can i ask you a favor that if its possible to tell me how did you got this modal? its a project that i have that needs this modal and i dont know how to do it with ansys
  .