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Coupling Chemical Transport Across Porous/Non-Porous Interface

Posted 24 mars 2025, 21:31 UTC−4 Microfluidics, Porous Media Flow, Molecular Flow Version 6.3 2 Replies

Carl K
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Hello all,

I have a model where I have two regions, one with freely flowing laminar flow and one with flow through porous media (using the free and porous media Brinkman physics). The two regions are touching along an interface.

I would like to model the concentration of a solute convecting and diffusing through both the non-porous and the porous media. I expect to need to use two physics models, the "transport of diluted species" physics for the nonporous region, and the "transport of diluted species in porous media" for the porous region.

I cannot seem to figure out how to match the concentrations and fluxes at the interface between the two regions, as the two physics models have self-defined separate concentration variables (c and c2). Is there a standard practice for equalizing the concentrations and solute fluxes at an interface such as this so that it does not cause the model to break?

I have attached my file. I would be very grateful for any guidance.

Cheers.

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2 Replies Last Post 25 mars 2025, 18:09 UTC−4
Henrik Sönnerlind COMSOL Employee

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Posted: 1 week ago 25 mars 2025, 09:34 UTC−4

The, by far, easiest way is to just rename the 'c2' to 'c' in the Dependent Variables section of the interface settings. This approach also makes plotting easier, since you have only one concentration field.

Alternatively, you can add a Concentration node in one of the interfaces with the common boundary as selection. Then, you set the prescribed value to 'c' or 'c2' depending on in which physics interface you placed the node.

Doing so will, however, cause a default segregated solver to fail. All variables that are connected using pointwise constraints must be placed in the same segregated group, so you would need to adjust the solver sequence. Alternatively, you can choose to use weak constraints. But then you need to switch to weak constraints for all constraints on the concentration, including Inflow.

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Henrik Sönnerlind
COMSOL
The, by far, easiest way is to just rename the 'c2' to 'c' in the *Dependent Variables* section of the interface settings. This approach also makes plotting easier, since you have only one concentration field. Alternatively, you can add a *Concentration* node in one of the interfaces with the common boundary as selection. Then, you set the prescribed value to 'c' or 'c2' depending on in which physics interface you placed the node. Doing so will, however, cause a default segregated solver to fail. All variables that are connected using pointwise constraints must be placed in the same segregated group, so you would need to adjust the solver sequence. Alternatively, you can choose to use weak constraints. But then you need to switch to weak constraints for all constraints on the concentration, including *Inflow*.
Carl K
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Posted: 7 days ago 25 mars 2025, 18:09 UTC−4

Thank you Henrik.

For my specific case, I was able to remediate the issue by simply assigning a fluid domain to the porous media, which made that region act as non-porous - something which I had not realized until after the post.

However, your recommendation has introduced me to some concepts that will surely assist me in the future. I will try to implement them into my own model to learn how they work. I thank you for your help.

All the best,

Carl

Thank you Henrik. For my specific case, I was able to remediate the issue by simply assigning a fluid domain to the porous media, which made that region act as non-porous - something which I had not realized until after the post. However, your recommendation has introduced me to some concepts that will surely assist me in the future. I will try to implement them into my own model to learn how they work. I thank you for your help. All the best, Carl

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