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Difference in shape of eigenmode by size of meshes

Posted Dec 30, 2011, 2:39 p.m. EST Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers, Structural Mechanics Version 4.2a 1 Reply

Sungwan Cho
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Hello.

I'm studying MEMS and NEMS in university.

To see the resonant frequency and shape of eigenmode at that frequency,

I used COMSOL 4.2a with solid mechanics.

Geometry of device is square membrane (thickness = 200nm, length of each side = 40 micron) and material

is Silicon Nitride.

When first tried with normal mesh size(free tetrahedral mesh), Comsol showed shape of normal mode which

is symmetric to axis across the center as shown in the second file attached.

But when I changed to the extremely fine mesh size(free tetrahedral mesh), shape of normal mode was

rotated to clockwise direction.

Are there any reason to rotation of mode shape?

When I made real sample and tested that device, shape of normal mode absolutely symmetric without rotation.
1 Reply Last Post Dec 31, 2011, 12:59 a.m. EST
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Dec 31, 2011, 12:59 a.m. EST
The geometry you specified has several equal eigenvalues dues to its symmetry. If there are two eigenvalues at the frequency you showed in the figures then any linear combination of the eigenmodes is also an eigenmode. In that case both solutions are correct!

Nagi Elabbasi
Veryst Engineering
The geometry you specified has several equal eigenvalues dues to its symmetry. If there are two eigenvalues at the frequency you showed in the figures then any linear combination of the eigenmodes is also an eigenmode. In that case both solutions are correct! Nagi Elabbasi Veryst Engineering

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