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-   -   Different diffusivity for additional variable in Fluid and porous domain (http://www.cfd-online.com/Forums/cfx/103730-different-diffusivity-additional-variable-fluid-porous-domain.html)

ftab June 25, 2012 08:23

Different diffusivity for additional variable in Fluid and porous domain
 
Hi all,

I am trying to model a mass transport in a fluid and porous domain.
The mass is released from an obstacle wall inside a fluid domain and being transported inside fluid domain and the same tansported material infiltrated through the wall into porous medium.

I do this defining the released material concentration as Additional variable (AV) and giving it the kinematic diffusivity. The problem is that obviously the diffusivity is not the same inside the fluid domain and solid domain, but CFX only asks for it in the "fluid models" tab of fluid and porous domain, and for the single AV you have only one diffusivity.
How I can have two different diffusivity in fluid and porous domain for the same AV?

Do I need to define different AVs for the same material inside fluid and porous domain? Then how I should handle the continuity of momentum and mass?

Thanks

ghorrocks June 25, 2012 19:09

I am not sure how CFX handles diffusivity for AVs in porous domains.

An alternative approach would be to define diffusivity as a function of location, using the inside() function. Then you can make the diffusivity one value in the fluid region and another in the porous region.

ftab June 26, 2012 04:58

Thanks Glenn,
This inside function definitely solves the problem.

I have another theoretic problem if you have some idea on porous media.
Why CFX is asking for porosity when it is also getting the permeability? Aren't they kind of interrelated? I have seen several papers regarding simulation of mass transfer inside artery without any information on porosity. They solely report the permeability. I could not even find the porosity explicitly reported in all those simulations, or any other paper.

Thanks for your kind help

ghorrocks June 26, 2012 19:33

I am not an expert in porous flows so cannot answer your question. This is discussed in the CFX documentation but I recall a similar discussion taking place recently on this forum so the doco might not be completely clear. Do a search on the forum to see if you can find that previous thread, it might answer your question.

Chander June 27, 2012 13:03

Porosity is simply the fraction of area available for flow or more simply you can also say it is the volume of fluid/total volume.
Permeability is a function of porosity and representative pore size and it quantifies the bulk resistance of porous medium to flow of fluid through it. For more details, refer the textbook on porous medium by Kaviany.
In CFX, you can either use the full porous medium model or you can use the fluid model with a momentum source term. The full porous medium model accounts for porosity in all terms of momentum equation and that is why CFX asks for it. You can use the second approach where i think you will not need porosity value separately and you can specify a momentum loss term. For more details, refer CFX theory manual.


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