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Determining if Maxwell Stress Tensor is correct for DEA model

Posted 28 gen 2011, 16:56 GMT-5 MEMS & Nanotechnology, MEMS & Piezoelectric Devices, Geometry, Mesh, Studies & Solvers, Structural Mechanics Version 3.5a 1 Reply

Aleksandar Nacev
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Hello all,

I have been trying to solve a crenelated DEA model using the electrostatics module, plane stress, and the ALE module. I have been going back and forth on the meshing and the boundary conditions necessary to make the Maxwell Stress Tensor work correctly. I have looked at previous threads and most say that the boundary and mesh needs to be symmetric with respect to the boundary at which the force is applied. I have tried to accomplish this by using a boundary layer mesh and a far away / large exterior boundary to remove edge effects. The problem I have is that I am not sure if the deflection amount that I am seeing is correct. I try to do a mesh convergence analysis to see if changing the mesh size actually makes the solution converge but am getting nowhere. Has anyone run into a similar issue and can assist or has words of advice? I simply cannot trust the solutions that I am getting.

I have attached my unmeshed model.

Thank you in advance,

Alek

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1 Reply Last Post 29 gen 2011, 11:39 GMT-5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 29 gen 2011, 11:39 GMT-5
Hi

you have several things not "clean" in there.

First I would say that you should start with a far smaller "air" infinite region, use only a diameter of 3-4 times the total length of your system, you can always make it bigger later.

then in the structural you should not use nu=0.5, hat is for fully incompressible material and gives singularities, you can use 0.49 if this is relevant, but ideally you should then also solve for the pressure i.e. "nearly incompressible solids" options, I would expect someting between 0.2 to 0.35 for the Poisson anyhow, but that depends on your material

Then I believe there are some crossing on some of the boundary selections, not sure it matters as not all are active in the ALE

another poin in EMES, why set the ground on both sides of the small boundaries, arent they electric conducting so the voltage would solve anyhow ?
All your emes thickness are per "m" OK if you do not use absolute forces to translate into the other physics but use per denisty figures. Else you should use the same thickness as the structural

Finally I do not really understand why you use the ALE and where you use it, if yu do not allow the air mesh to follow you will try to induce a "gap" and that will not go.

I can only propose, build one physics at the time, start simple, use i.e. a boundary pressure load to mimic the electrostatic load, then add the EMES, and finally if you really need it the ALE. You can also do quite nice things with only the structural ad the properties "deformed frame" on

--
Good luck
Ivar
Hi you have several things not "clean" in there. First I would say that you should start with a far smaller "air" infinite region, use only a diameter of 3-4 times the total length of your system, you can always make it bigger later. then in the structural you should not use nu=0.5, hat is for fully incompressible material and gives singularities, you can use 0.49 if this is relevant, but ideally you should then also solve for the pressure i.e. "nearly incompressible solids" options, I would expect someting between 0.2 to 0.35 for the Poisson anyhow, but that depends on your material Then I believe there are some crossing on some of the boundary selections, not sure it matters as not all are active in the ALE another poin in EMES, why set the ground on both sides of the small boundaries, arent they electric conducting so the voltage would solve anyhow ? All your emes thickness are per "m" OK if you do not use absolute forces to translate into the other physics but use per denisty figures. Else you should use the same thickness as the structural Finally I do not really understand why you use the ALE and where you use it, if yu do not allow the air mesh to follow you will try to induce a "gap" and that will not go. I can only propose, build one physics at the time, start simple, use i.e. a boundary pressure load to mimic the electrostatic load, then add the EMES, and finally if you really need it the ALE. You can also do quite nice things with only the structural ad the properties "deformed frame" on -- Good luck Ivar

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