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Internal boundary conditions for diffusion equation

Posted Jun 23, 2009, 1:20 p.m. EDT Chemical Reaction Engineering Version 4.3a, Version 4.3b 10 Replies

Joe Vallino
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Hi All,

I'm modeling simple diffusion in a 2D geometry; however, the geometry contains some internal boundaries that I would, at times, like to be able to represent as walls. Consequently, I need to specify a zero flux across these internal boundaries. While it is easy to specify a flux discontinuity, specifying a zero flux on both sides of the internal boundary is not obvious.

I suspect there is a simple answer, but I have yet to be successful in finding it.

cheers,
-joe
10 Replies Last Post Feb 22, 2017, 10:42 a.m. EST
Adam Davidson
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Posted: 2 decades ago Jun 24, 2009, 5:04 a.m. EDT
Hey,

I think the best way to do this is to turn off 'active in this domain' for the adjacent subdomain. This will make the internal boundary effectively become an external boundary. If you need to model diffusion on the other side of the wall, you can add another diffusion equation system for that subdomain. Hope this helps.

Adam
Hey, I think the best way to do this is to turn off 'active in this domain' for the adjacent subdomain. This will make the internal boundary effectively become an external boundary. If you need to model diffusion on the other side of the wall, you can add another diffusion equation system for that subdomain. Hope this helps. Adam
Joe Vallino
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Posted: 2 decades ago Jun 24, 2009, 9:35 a.m. EDT
Hi Adam,

That's a good idea, but unfortunately, the internal boundaries do not subdivide the domain, so there is only one domain. I can solve the problem by making the internal boundary an external one (give it some width), but then I have two different geometries and two different model files, which is what I'm currently doing.

The overall goal is to be able to easily turn internal baffle walls on and off so that I can easily examine their effect.

cheers,
-joe
Hi Adam, That's a good idea, but unfortunately, the internal boundaries do not subdivide the domain, so there is only one domain. I can solve the problem by making the internal boundary an external one (give it some width), but then I have two different geometries and two different model files, which is what I'm currently doing. The overall goal is to be able to easily turn internal baffle walls on and off so that I can easily examine their effect. cheers, -joe
Linus Andersson COMSOL Employee

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Posted: 2 decades ago Jun 26, 2009, 5:12 a.m. EDT
Hi,

I'd suggest the idea shown in the model linked to at the bottom of this entry. The concept is pretty general, so I will outline it here.

Some boundary conditions are available only on exterior boundaries. This is because on interior boundaries, they would cause a discontinuous solution, which is not allowed. Insulation is one such boundary condition. My model shows how to use an assembly in order to enable all boundary conditions everywhere.

Step one is to create the geometry such that it consists of one object on one side of the boundaries where you want to apply your condition, and another object on the other side. Although hidden when you first open the model, there is a line connecting the two insulating walls. This is what makes the division into two geometry objects possible.

With the use of an assembly, the walls will automatically consist of two exterior boundaries each. This makes it possible to apply the Insulation/Symmetry boundary condition. To reconnect the solution across the boundaries between the walls, create an Identity Pair and use the (default) Pair Continuity condition.

Note that the boundaries between the walls were suppressed in postprocessing. You can see them by clicking the Show All Hidden Objects button when in boundary mode.

Linus

-------------------------------------------
cds.comsol.com/mg/b4a44d998329d6.zip
Estimated size: 0.1 MB
This link expires July 3, 2009. Please make sure to download before that date.

Included files:
- Interior_Insulation.mph
-------------------------------------------
Hi, I'd suggest the idea shown in the model linked to at the bottom of this entry. The concept is pretty general, so I will outline it here. Some boundary conditions are available only on exterior boundaries. This is because on interior boundaries, they would cause a discontinuous solution, which is not allowed. Insulation is one such boundary condition. My model shows how to use an assembly in order to enable all boundary conditions everywhere. Step one is to create the geometry such that it consists of one object on one side of the boundaries where you want to apply your condition, and another object on the other side. Although hidden when you first open the model, there is a line connecting the two insulating walls. This is what makes the division into two geometry objects possible. With the use of an assembly, the walls will automatically consist of two exterior boundaries each. This makes it possible to apply the Insulation/Symmetry boundary condition. To reconnect the solution across the boundaries between the walls, create an Identity Pair and use the (default) Pair Continuity condition. Note that the boundaries between the walls were suppressed in postprocessing. You can see them by clicking the Show All Hidden Objects button when in boundary mode. Linus ------------------------------------------- http://cds.comsol.com/mg/b4a44d998329d6.zip Estimated size: 0.1 MB This link expires July 3, 2009. Please make sure to download before that date. Included files: - Interior_Insulation.mph -------------------------------------------
Joe Vallino
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Posted: 2 decades ago Jun 26, 2009, 1:12 p.m. EDT
Thanks Linus, that does solve the problem I'm trying to solve!

-joe
Thanks Linus, that does solve the problem I'm trying to solve! -joe
Ganesh Samdani
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Posted: 1 decade ago Jan 24, 2011, 5:42 a.m. EST
may i get that file which shows how to use insulation at internal boundaries ?

please help.
may i get that file which shows how to use insulation at internal boundaries ? please help.
Ganesh Samdani
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Posted: 1 decade ago Jan 24, 2011, 5:42 a.m. EST
may i get that file which shows how to use insulation at internal boundaries ?

please help.
may i get that file which shows how to use insulation at internal boundaries ? please help.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jan 24, 2011, 5:56 a.m. EST
Hi

these files are only awailable a short time, but the explanations of Linus are straightforward. use an assembly mode and link part of the dedoubled boundary as continuity, another part with your specific physics

--
Good luck
Ivar
Hi these files are only awailable a short time, but the explanations of Linus are straightforward. use an assembly mode and link part of the dedoubled boundary as continuity, another part with your specific physics -- Good luck Ivar
Chao Liu
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Posted: 1 decade ago Apr 7, 2014, 1:20 p.m. EDT
Hi Linus:

I am just wondering if you could send the example zip file again. I was trying to follow your description but still cannot achieve the interior boundary conditions in COMSOL. I know it has been a long time since you uploaded the file. Sorry for this request from a COMSOL rookie.
Hi Linus: I am just wondering if you could send the example zip file again. I was trying to follow your description but still cannot achieve the interior boundary conditions in COMSOL. I know it has been a long time since you uploaded the file. Sorry for this request from a COMSOL rookie.
Regina da Fonseca
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Posted: 8 years ago Feb 22, 2017, 10:07 a.m. EST
Hello Linus,

Please, you can make the file available again below?

-----------------------------------------------------------------------------------------------
cds.comsol.com/mg/b4a44d998329d6.zip
Estimated size: 0.1 MB
This link expires July 3, 2009. Please make sure to download before that date.
------------------------------------------------------------------------------------------------------
I I can not download.
Thank you!

Regina
Hello Linus, Please, you can make the file available again below? ----------------------------------------------------------------------------------------------- cds.comsol.com/mg/b4a44d998329d6.zip Estimated size: 0.1 MB This link expires July 3, 2009. Please make sure to download before that date. ------------------------------------------------------------------------------------------------------ I I can not download. Thank you! Regina
Jeff Hiller COMSOL Employee

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Posted: 8 years ago Feb 22, 2017, 10:42 a.m. EST
Hello Regina,
While the approach that Linus described should still work, the internal boundary conditions called Thin Impermeable Barrier, which was introduced after Linus's post, is a much more direct solution. It saves you the need to form an assembly and work with pairs.
It looks like the Thin Impermeable Barrier condition was introduced at version 4.3b, see www.comsol.com/blogs/this-just-in-comsol-multiphysics-4-3b/ .
Best,
Jeff
Hello Regina, While the approach that Linus described should still work, the internal boundary conditions called Thin Impermeable Barrier, which was introduced after Linus's post, is a much more direct solution. It saves you the need to form an assembly and work with pairs. It looks like the Thin Impermeable Barrier condition was introduced at version 4.3b, see https://www.comsol.com/blogs/this-just-in-comsol-multiphysics-4-3b/ . Best, Jeff

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