# Boundary conditions for the transport of a new variable

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 February 9, 2020, 00:47 Boundary conditions for the transport of a new variable #1 Member   katty parker Join Date: May 2018 Posts: 30 Rep Power: 3 Hi All, I want to model the transport of some species in a kind of plant. The problem setup is shown in the figure below: CFD11.png Water enters the main tube (white rectangle) and then a part of it enters the side porous domain and the rest will leave the domain at the outlet. As is shown in this figure, on the lateral wall of the porous media, an opening boundary condition with the pressure of 0 Pa is set. The value of the species on this boundary condition is also set to a fixed value. The problem is that after that the solution converges, the value of species on the lateral wall of the porous domain is not equal to the specified value (0 mol/m3). My question is that why the solution does not follow the value of the species that is enforced to be equal to 0 mol/m3 on this wall?

 February 9, 2020, 05:04 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 16,061 Rep Power: 123 When an opening has outflow the boundary value of an AV is convected from inside the domain. In other words, you do not set the value (or the gradient). Providing the fluid velocity is much faster than the diffusion velocity then this approach is valid. If your diffusion velocity is faster than the fluid velocity this approach is not valid and you are going to have to use another approach. In other words, whether this is suitable depends on what you are doing and what the properties of the fluids are. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 February 9, 2020, 10:34 #3 Member   katty parker Join Date: May 2018 Posts: 30 Rep Power: 3 Dear Glenn, Thanks for your help. So if I understood correctly, using this approach I need to adjust the fluid properties in the porous domain (let say using a trial and error approach) to make the value of AV equal to zero there. May I ask you to please let me know about the alternative approachs? Thanks.

 February 9, 2020, 18:22 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 16,061 Rep Power: 123 No, you do not appear to have understood my comment. Please re-read my comment and also look at the CFX documentation about boundary conditions so you understand what you are doing. Can we check what approach is correct first? What is the fluid velocity in the porous material? What is the diffusion velocity scale in the porous material? You need to figure that out to determine the correct approach. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.