# boundary condition at the wall

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 January 21, 2011, 06:12 boundary condition at the wall #1 Member   Join Date: Apr 2009 Posts: 34 Rep Power: 9 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

 January 21, 2011, 07:35 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,832 Rep Power: 100 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.

 January 21, 2011, 09:10 #3 Member   Join Date: Apr 2009 Posts: 34 Rep Power: 9 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

 January 22, 2011, 06:02 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,832 Rep Power: 100 What is Js a function of? What is D? What is known and what is not known in your equation?

 January 22, 2011, 06:14 #5 Member   Join Date: Apr 2009 Posts: 34 Rep Power: 9 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)

 January 22, 2011, 06:29 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,832 Rep Power: 100 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.

 January 22, 2011, 07:17 #7 Member   Join Date: Apr 2009 Posts: 34 Rep Power: 9 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?

 January 23, 2011, 05:49 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,832 Rep Power: 100 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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