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How could I define a customized 'Flux/Source' boundary condition in 'Coefficient form PDE' interface?

Posted 1 ago 2021, 05:49 GMT-4 Version 5.6 4 Replies

Runzi Wang
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Hi

I'm trying to define a customized 'Flux/Source' boundary condition. The default flux equation in the 'Coefficient form PDE' interface is

However, the expression I need in my model is

Due to the structure of my governing equation, I couldn't define the constant 'c' as 0. So I'm wondering is there a way to define a customized flux boundary condition in the PDE interface?

4 Replies Last Post 5 ago 2021, 17:47 GMT-4
Alexis Prel
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Posted: 3 years ago 1 ago 2021, 06:02 GMT-4

Hello,

your boundary condition is unusual because it does not depend on the variable solved for, unless or depend on .

May I ask what physical conditions you are trying to model?

Hello, your boundary condition is unusual because it does not depend on the variable solved for, unless \gamma or g depend on u. May I ask what physical conditions you are trying to model?
Runzi Wang
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Posted: 3 years ago 2 ago 2021, 08:16 GMT-4

Hi Alexis

Thank you for your reply! Your guess is correct, is defined as an expression that depends on .

I'm dealling with a phase field model. I'm trying to model the flux of the phase field variable . However, the expression for the flux is non-standard, I couldn't use the flux/source node directly. I want to define my flux expression by myself. I know there is a boundary node called 'weak constraint' that allows me to define my own boundary condition. But when I applied this condition to my boundary, I kept getting an error telling me 'There are 293 void equations (empty rows in matrix) for the variable comp1.lm', where 'comp1.lm' is the Lagrange multiplier variable. I'm not familiar with weak consrtaint, so I don't know what this means.

Do you have any ideas on this problem? Or do you have any better ideas on how to implement this boundary condition?

Any advice will be much appreciated!

Hi Alexis Thank you for your reply! Your guess is correct, \gamma is defined as an expression that depends on u. I'm dealling with a phase field model. I'm trying to model the flux of the phase field variable \phi. However, the expression for the flux is non-standard, I couldn't use the flux/source node directly. I want to define my flux expression by myself. I know there is a boundary node called 'weak constraint' that allows me to define my own boundary condition. But when I applied this condition to my boundary, I kept getting an error telling me 'There are 293 void equations (empty rows in matrix) for the variable comp1.lm', where 'comp1.lm' is the Lagrange multiplier variable. I'm not familiar with weak consrtaint, so I don't know what this means. Do you have any ideas on this problem? Or do you have any better ideas on how to implement this boundary condition? Any advice will be much appreciated!
Alexis Prel
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Posted: 3 years ago 5 ago 2021, 11:34 GMT-4
Updated: 3 years ago 5 ago 2021, 11:38 GMT-4

Hello Runzi,

Have you tried including into the definition of ?

Hello Runzi, Have you tried including - \vec{n} \,\cdot\,(-c\vec\nabla u) into the definition of g ?
Runzi Wang
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Posted: 3 years ago 5 ago 2021, 17:47 GMT-4
Updated: 3 years ago 5 ago 2021, 17:47 GMT-4

Hi Alexis

Do you mean I do ,

where (some other parameters)?

Well, that's a good idea! I'll have a try.

Thank you!

Hi Alexis Do you mean I do -n\cdot( -c\nabla u-\alpha u + \gamma)=g -q u, where g=- n \cdot (-c\nabla u)+ (some other parameters)? Well, that's a good idea! I'll have a try. Thank you!

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