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Maxwell Stress Tensor question

Posted 10.02.2011, 18:41 GMT-5 Low-Frequency Electromagnetics Version 3.5a 5 Replies

Ricardo Aguilar
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So I'm running into a problem finding the maxwell stress force acting on the surface of a nanowire. I chose boundary integration, and then chose the maxwell stress tensor on the z direction, and when I hit ok I get the following error message:


Error: 6099
Evaluation of post processing expression failed
Failed to evaluate variable.
- Variable: unTzav_emqvw
- Geometry: 1
- Boundary: 8
Failed to evaluate variable.
- Variable: mts_nTz_emqvw
- Defined as: unTzav_emqvw
Failed to evaluate expression.
- Expression: 2*pi*r*mts_nTz_emqvw


Any help would be useful. This if for a lab that I'm currently TAing, and so the students are now kind of stuck on this one. It's supposed to show you the integration value in the output window at the bottom but it won't show you the value. The version I'm using is 3.5a....when I ran this simulation on version 3.5 it worked, but now it doesn't. Thanks.

Ricardo

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5 Replies Last Post 15.02.2011, 02:05 GMT-5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 11.02.2011, 02:10 GMT-5
Hi

I'm not sure whats happening with your BC's several are dimmed and the equations are bizarre for me (and I'm forgetting the way of 3.5 ;) For me, you should have "isolation" all around, and "axial symmetry" all along the axis. and finally when you apply 5V on a boundary, I would leave an isolation slab to avoid "shortening it out to GND at the points/edges. When looking into the physics boundary conditions, these do not all seem logical. Thant could be the case why your variable is not defined correctly on boundary 8

--
Good luck
Ivar
Hi I'm not sure whats happening with your BC's several are dimmed and the equations are bizarre for me (and I'm forgetting the way of 3.5 ;) For me, you should have "isolation" all around, and "axial symmetry" all along the axis. and finally when you apply 5V on a boundary, I would leave an isolation slab to avoid "shortening it out to GND at the points/edges. When looking into the physics boundary conditions, these do not all seem logical. Thant could be the case why your variable is not defined correctly on boundary 8 -- Good luck Ivar
Ricardo Aguilar
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Posted: 1 decade ago 14.02.2011, 15:40 GMT-5
by isolation do you mean electric insulation? I believe I have it all around. The reason some of the boundaries are greyed out is because I deactivated those subdomains (the bottom right and left electrodes), that way they don't have an affect on the overall DEP force. Their only purpose is to generate the voltage potential. Now, for the wire itself (boundaries 6, 8, and 10) I'm not sure why they would be greyed out...they light up when check the "Interior boundaries" check box, but again, I'm not sure what would be a valid boundary condition to place on these boundaries in order for the maxwell stress tensor to actually be defined.
by isolation do you mean electric insulation? I believe I have it all around. The reason some of the boundaries are greyed out is because I deactivated those subdomains (the bottom right and left electrodes), that way they don't have an affect on the overall DEP force. Their only purpose is to generate the voltage potential. Now, for the wire itself (boundaries 6, 8, and 10) I'm not sure why they would be greyed out...they light up when check the "Interior boundaries" check box, but again, I'm not sure what would be a valid boundary condition to place on these boundaries in order for the maxwell stress tensor to actually be defined.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 14.02.2011, 15:46 GMT-5
Hi

ifI remember right (3.5 is getting oldish for me) greyed boundaries are the interiours, or those linked to not-active domains. And these can be activated via the "selection" check boxes.

For me the Maxwell tensor use requires an "air gap" all around the boundary to be integrated, as its derived from the gradient of the field. And the maxwell tensor is very very sensitive to the gradient distribution w,r,t mesh density, sharp edge singularities etc. One should always check fo a symmetric mesh, and test 2-3 mesh densities to ensure a stable result

--
Good luck
Ivar
Hi ifI remember right (3.5 is getting oldish for me) greyed boundaries are the interiours, or those linked to not-active domains. And these can be activated via the "selection" check boxes. For me the Maxwell tensor use requires an "air gap" all around the boundary to be integrated, as its derived from the gradient of the field. And the maxwell tensor is very very sensitive to the gradient distribution w,r,t mesh density, sharp edge singularities etc. One should always check fo a symmetric mesh, and test 2-3 mesh densities to ensure a stable result -- Good luck Ivar
Ricardo Aguilar
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Posted: 1 decade ago 14.02.2011, 16:35 GMT-5
So I just figured out the difference between when it worked and when it didn't. All those steps to make that geometry and to have those boundary conditions to work out and actually give me the maxwell stress tensor were made 2 and half years ago on version 3.4 of COMSOL. I just rand the geometry (both 2D and 3D) on that version and they spit out the answer. I wonder if there's some subtle change between versions that may be the reason for the error.
So I just figured out the difference between when it worked and when it didn't. All those steps to make that geometry and to have those boundary conditions to work out and actually give me the maxwell stress tensor were made 2 and half years ago on version 3.4 of COMSOL. I just rand the geometry (both 2D and 3D) on that version and they spit out the answer. I wonder if there's some subtle change between versions that may be the reason for the error.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 15.02.2011, 02:05 GMT-5
Hi

That could well be, but I do not know about anything in particular

--
Good luck
Ivar
Hi That could well be, but I do not know about anything in particular -- Good luck Ivar

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