# Additional Variable BC

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 May 22, 2010, 11:33 Additional Variable BC #1 New Member   Join Date: Mar 2009 Posts: 18 Rep Power: 16 Hi There How can I set such a Boundary condition on wall for an "Additional variable" C: K /= H (C-C0) K and H are constants. It's like heat transfer coefficient for wall but the variable is C. Thank you so much

 May 22, 2010, 12:42 #2 New Member   Jordi Join Date: May 2010 Posts: 26 Rep Power: 15 It looks like a diffusion, is that your system? You do not provide very much info on your problem, but if it's like a heat convection, where C is temperature, then you can fix both temperature (C) and transfer coeff. (H) at the same surface, can't you?

 May 23, 2010, 18:49 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,363 Rep Power: 139 Can do this with a gradient boundary with a CEL expression. Should be easy.

 May 24, 2010, 15:19 #4 New Member   Join Date: Mar 2009 Posts: 18 Rep Power: 16 Thank you so much Glenn Your advices have always helped me and other students. But would you please explain it more for me? or refer me to a tutorial or example? I don't know how to interpret a gradient boundary. Thanks again Last edited by Eli; May 24, 2010 at 15:47.

May 24, 2010, 15:26
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Quote:
 Originally Posted by Jordi It looks like a diffusion, is that your system? You do not provide very much info on your problem, but if it's like a heat convection, where C is temperature, then you can fix both temperature (C) and transfer coeff. (H) at the same surface, can't you?
Thanks Jordi for your attention,
C is a scaler variable and shows kind of concentration in the simulation, I just don't know how to define the Left hand of equation.

 May 24, 2010, 18:42 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,363 Rep Power: 139 Recast the equation as dC/dx=H/K(C-C0) then you have the gradient of the scalar at the boundary is a simple function. Set the function to a CEL expression and it is done. Set this for the additional variable boundary condition at the walls.

January 13, 2011, 08:57
#7
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Lance
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Quote:
 Originally Posted by Eli Hi There How can I set such a Boundary condition on wall for an "Additional variable" C: K /= H (C-C0) K and H are constants. It's like heat transfer coefficient for wall but the variable is C. Thank you so much
I have a similar problem, I want to set a BC on a complex 3D wall for my additional variable that looks like:
V*C-D*dC/dn = k*C,
where V, D and k are constants and C is my additional variable. The problem is how to calculate the dC/dn as it is the gradient in the normal direction from the wall. Any suggestions?

 January 13, 2011, 16:53 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,363 Rep Power: 139 Will the CEL variables normal, with extensions like _Coordinate_Direction help you? See the CFX reference guide, "Quantitative CEL functions in CFX"

January 14, 2011, 04:56
#9
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Lance
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Quote:
 Originally Posted by ghorrocks Will the CEL variables normal, with extensions like _Coordinate_Direction help you? See the CFX reference guide, "Quantitative CEL functions in CFX"
Yes, maybe. In POST I calculated the gradient of my AV in the Variables tab, so that I got AV.Gradient X, AV.Gradient y, and AV.Gradient Z. Then I made an expression: AV.Gradient X*Normal X+ AV.Gradient Y*Normal Y+AV.Gradient Z*Normal Z. Is this be the same as Av.Gradient Normal?
And can I define AV.Gradient X (...etc) in PRE as well?

And btw, how do I define the boundary condition at the wall? The units resolve to [kg/(s m^2)]. Transfer coefficient+additional variable or Flux in?

Last edited by Lance; January 14, 2011 at 11:16.

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