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Old   January 21, 2011, 05:12
Default boundary condition at the wall
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Hello,

I would like to model mass transport using convection-diffusion equation.
I want to apply a boundary condition of this type:
− D ∇c n + u c n = Js,
where c is the concentration, u the velocity vector and n the normal vector of the domain.

Any help?
Thanks in advance
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Old   January 21, 2011, 06:35
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Are you proposing solving for c (or the flux of c) at the boundary (or something like that)? Sounds tricky. I think you will probably need to write a fortran subroutine for the boundary condition to do this.
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Old   January 21, 2011, 08:10
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I want to solve for the concentration of a solute, but Js is a solute flux across the boundary (it is described by a simple CEL expression).
Can you give more details about what to program with fortran code, because i am not very familiar.

Thanks anyway
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Old   January 22, 2011, 05:02
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What is Js a function of?

What is D?

What is known and what is not known in your equation?
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Old   January 22, 2011, 05:14
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thanks again for your answer

Js is a function of wall shear stress (known from solving Navier-Stokes)
D is the diffusivity (known)
u is the fluid velocity given by the Navier-Stokes (unknown but it is solved)
c is the concentration (unknown and it is solved using the convection-diffusion equation)
I want to calculate c at the wall which is described by the above equation.
If you see − D ∇c n + u c n = Js is like convection-diffusion equation (transport equation in CFX: ∇(− D ∇c + u c) = 0)
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Old   January 22, 2011, 05:29
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If you can recast it into a form like the transport equation then you can simply use an additional variable with a flux boundary = Js (or some function like it). But if that is not possible you have a much harder problem on your hand and you will need to write a solver in fortran to do it. I don't think example fortran comes with CFX any more, you would have to talk to the support people to get one I guess.
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Old   January 22, 2011, 06:17
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Since Js describe the flux across the wall, could i use the flux in boundary condition?
But what is the mathematical formulation of this boundary condition? So that i could recast it in my form?

P.S. the positive flux in describes direction into the domain. If i want it to move out of the domain, should i use -Js?
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Old   January 23, 2011, 04:49
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For available boundary condition options see the documentation.

For what positive and negative does, give it a try and see what happens.
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