# Setting different diffusivities for two domains (fluid+porous)

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 November 25, 2014, 13:14 Setting different diffusivities for two domains (fluid+porous) #1 New Member   Leonardo Join Date: Nov 2014 Posts: 27 Rep Power: 9 Hi there, I need some help setting up a CEL expression to define different values of kinematic diffusivity for two different domains: a fluid and a porous domain. I'm aware you can use the inside() function, but I'm not sure how to write it. I tried the script below but it didn't work. 1.58e-5 [cm^2 s^-1] * inside()@flow; 0.46*1.58e-5 [cm^2 s^-1] * inside()@porous Thanks in advance.

 November 25, 2014, 14:45 #2 Senior Member   Join Date: Jun 2009 Posts: 1,539 Rep Power: 27 I assume you are defining the Kinematic Diffusivity parameter for an Additional Variable, or mass fraction Component Kinematic Diffusivity = 1.58e-5 [cm^2 s^-1] * ( inside()@flow + 0.46 * inside()@porous) That is all you need to do. I wonder what the 0.46 means. It is not the volume porosity, is it ?

November 25, 2014, 14:48
#3
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Leonardo
Join Date: Nov 2014
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Quote:
 Originally Posted by Opaque I assume you are defining the Kinematic Diffusivity parameter for an Additional Variable, or mass fraction Component Kinematic Diffusivity = 1.58e-5 [cm^2 s^-1] * ( inside()@flow + 0.46 * inside()@porous) That is all you need to do. I wonder what the 0.46 means. It is not the volume porosity, is it ?
Thanks a lot, I'll try that.

What is the meaning of the asterisk symbol in the CEL syntax? I did a quick check on the documentation and couldn't find much about syntax.

And no, it's just the "correction" factor for the permeability at the porous media.

 November 25, 2014, 15:26 #4 Senior Member   Join Date: Jun 2009 Posts: 1,539 Rep Power: 27 * is for multiplication between two terms in the expression. Not sure I catch on the "correction factor" for the permeability. There is no permeability in the diffusion terms of the transport equations I have seen. I have only seen the permeability in the drag force term in the momentum equation.. Did you mean porosity ? The only time I have seen permeability into a transport equation is when Darcy's law is substituted into the continuity equation, and a pure diffusion equation for Pressure is obtained. Is that what you are solving for ?

November 25, 2014, 15:30
#5
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Leonardo
Join Date: Nov 2014
Posts: 27
Rep Power: 9
Quote:
 Originally Posted by Opaque * is for multiplication between two terms in the expression. Not sure I catch on the "correction factor" for the permeability. There is no permeability in the diffusion terms of the transport equations I have seen. I have only seen the permeability in the drag force term in the momentum equation.. Did you mean porosity ? The only time I have seen permeability into a transport equation is when Darcy's law is substituted into the continuity equation, and a pure diffusion equation for Pressure is obtained. Is that what you are solving for ?
Sorry about that, I meant diffusivity.
According to the paper I'm basing my simulations on, they estimate the effective diffusivity of ethanol in the porous media using this factor, which indeed happens to include the porosity:

D_eff = (porosity)^1.5*D

 Tags cel expression, diffusivity, kinematic diffusivity, porous