# Diffusion of Additional variables in solid

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 October 18, 2010, 12:16 Diffusion of Additional variables in solid #1 New Member   AlemayehuA Join Date: Oct 2010 Posts: 7 Rep Power: 8 Sponsored Links What is the matter? I am simulating diffusion of a gas both in fluid and solid domain. I used additional variable to represent the diffusing gas but, the solution from ansys cfx doesn't look okay, the concentration in the solid domain is significantly higher expected to diffuse from the fluid domain to the solid domain. Is their something that I missed?

 October 22, 2010, 12:00 #2 Senior Member   Michael P. Owen Join Date: Mar 2009 Posts: 196 Rep Power: 10 Check volumetric vs. specific definition of your Additional Variable. The former is per unit volume, the latter per unit mass.

October 24, 2010, 15:16
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AlemayehuA
Join Date: Oct 2010
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Quote:
 Originally Posted by michael_owen Check volumetric vs. specific definition of your Additional Variable. The former is per unit volume, the latter per unit mass.
Thanks for your reply. In fact I did a check on the volumetric vs specific definition. That I think was not my problem. There is one other parameter that I simply play with and got somewhat resonable result. The parameter is contact resistance that should be defined during interface definishion. A very big value of this parameter gives a result that physically expected. But still I need to clearly understand the physical meaning of this 'contact resistance'...have you any info on this parameter?

 October 25, 2010, 08:15 #4 Senior Member   Michael P. Owen Join Date: Mar 2009 Posts: 196 Rep Power: 10 The contact resistance represents a geometricly then layer between the two domains that is not physically modeled. It has a lower diffusivity which, integrated through the thickness, creates a resistance to diffusion across the boundary. Physically this might be something like a coating, or in an example I've modeled in the past, a paper covering around a porous medium (I was modeling oxygen diffusion). I had a similar problem where my results made no sense until I realized I had left the Additional Variable definition on the default of "specific" (per unit mass) by mistake.

 October 27, 2010, 05:29 #5 New Member   AlemayehuA Join Date: Oct 2010 Posts: 7 Rep Power: 8 Dear michael_Owen; thanks so much. Will update you soon with my progress.

 September 9, 2014, 03:58 #6 New Member   Niels Bessemans Join Date: Apr 2013 Posts: 12 Rep Power: 6 Dear, What version of Ansys do you use? I am also trying to implement the skin resistance of a sphere's coating to diffusion of a mass species (additional variable), but I cannot manage yet.. When setting the interface, there seems not to be such an option to set a contact resistance directly. I am using ANSYS 14.5. Kind regards

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