boundary condition at the wall
Hello,
I would like to model mass transport using convectiondiffusion 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 
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.

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 
What is Js a function of?
What is D? What is known and what is not known in your equation? 
thanks again for your answer
Js is a function of wall shear stress (known from solving NavierStokes) D is the diffusivity (known) u is the fluid velocity given by the NavierStokes (unknown but it is solved) c is the concentration (unknown and it is solved using the convectiondiffusion 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 convectiondiffusion equation (transport equation in CFX: ∇(− D ∇c + u c) = 0) 
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.

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? 
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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