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Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

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One basic and fundamental question about boundary conditions

Posted 2012年2月10日 GMT-5 05:40 Modeling Tools & Definitions, Parameters, Variables, & Functions Version 3.5a 4 Replies

Pu
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I feel confused about the introduction of Lagrange multipliers in generalized Neumann boundary condition. Why do we need to have extra dependent variables? I checked relevant items in documentation, and found the following description:

"When Dirichlet boundary conditions are introduced, the finite element algorithm makes a corresponding change to the Neumann boundary conditions so that the resulting model becomes solvable."

It sounds like there exists certain inconsistency within Neumann and Dirichlet BC's, making the equation system unsolvable.

If anyone knows about these details, could u please explain it? Thanks a lot! I really feel unsafe to run simulations with doubts in underlying systems.
4 Replies Last Post 2012年4月5日 GMT-4 13:14
Pu
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Posted: 1 decade ago 2012年2月11日 GMT-5 04:42
Finally I got something in the documentation.

In the Modeling Guide, it is claimed the content on page 362 explains where the particular form of constraint force for ideal constraint comes from. However, the documentation is not aimed to beginners. I cannot understand it. Could anyone explain it in plain language or point out any reference?

Thanks a lot!
Finally I got something in the documentation. In the Modeling Guide, it is claimed the content on page 362 explains where the particular form of constraint force for ideal constraint comes from. However, the documentation is not aimed to beginners. I cannot understand it. Could anyone explain it in plain language or point out any reference? Thanks a lot!
Pu
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Posted: 1 decade ago 2012年2月12日 GMT-5 10:06
Perhaps my questions is too simple for most people, especially advanced people here. But please anyone give me some hint, or reference. Thanks!
Perhaps my questions is too simple for most people, especially advanced people here. But please anyone give me some hint, or reference. Thanks!
Pu
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Posted: 1 decade ago 2012年2月14日 GMT-5 15:13
After so long time searching, it's found the sections on weak form in Reference Guide of Version 4.0a are helpful for understanding my concerns.

Hope this information can help more people who have the same question in mind.
After so long time searching, it's found the sections on weak form in Reference Guide of Version 4.0a are helpful for understanding my concerns. Hope this information can help more people who have the same question in mind.
Luis Roca
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Posted: 1 decade ago 2012年4月5日 GMT-4 13:14
hi, i work with 4.2 version and too have problems with the boundary conditions, you aren't alone, good luck!
hi, i work with 4.2 version and too have problems with the boundary conditions, you aren't alone, good luck!

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