Learning Forum

[AfshinSh]

How to enforce zero-gradient (in other words zero-diffusivity, nonzero penetration or fully-developed) condition on a boundary for a UDS?

I wrote a UDF to model the transport of some species in a domain. The physics is governed by a simple reaction-conv-diff problem: adv + diffusion + source terms in the equation. Therefore, I defined a UDS with the following terms: (1) Unsteady flux term, (2) steady flux term, (3) Diffusion term, (4) source term.

 Everything works fine except that I cannot enforce zero gradient boundary condition for this species. By zero gradient I mean perpendicular to the boundary the flow of this species is fully developed: Zero diffusive flux, NONZERO convective flux and the species penetrates and escapes the domain, it is not deposited. 

There are two choices for the UDS boundaries: Specified Flux and specified value. Obviously zero value doesn’t work for me because the value is not zero and unknown, but its gradient is known to be zero. However, when I try to enforce zero flux boundary, the solution is anything but zero flux or zero gradient. It is as if FLUENT takes zero flux as zero flow of species and therefore zero penetration. The solution in this case takes a very high gradient at the wall as if all the species are getting stuck at the wall. Therefore, as time goes on, this high concentration of species jammed before the wall increases until the floating-point error! The physics tells that the species migrate toward the wall, however, don’t get deposited, but simply scape the wall like on the back of the wall is fully developed out of the domain.  The species escape. 

It looks like FLUENT is incapable of enforcing the zero-gradient BC for the UDS you defined, and it takes it as ill-posed, unlike COMSOL! 

I need a boundary condition like the readily built FLUENT BCs for its Species model (called zero diffusivity flux). But I won’t activate and use this option since I need to enforce it on my own UDS. I also tried to set the boundary value equal to the neighbor cell value using the define profile macro hoping to get something like zero gradient and scaping BC, but it didn’t work!

Now I have three related questions:

1-     How to enforce a truly zero gradient boundary condition for your own defined UDS?

2-    Is this definition affected by what you choose as the boundary? For example, being a Wall, an Outflow, a Pressure Outlet, etc. makes a difference or not? (logically it shouldn’t because they are defined for the fluid flow and Navier-Stokes, not my own defined UDS, right? I won’t solve the momentum or continuity equations here; I only solve my own equations defined by the UDS, almost all boundaries you pick have a section for setting BCs for the UDS you defined.)

3-    To enforce a truly zero gradient successfully, do I also need to change the fluxes in the steady UDS macro where I defined the steady term of my convective flux term for the UDS or not?

[Surya Deb]

Hello,
It is not very clear about the type of boundary condition here.
If you are modeling a wall, then not only the species but the flow itself stops there considering the wall to be impenetrable.
So you cannot expect a convective flux of the species across the wall in that scenario What you are referring to might be a permeable membrane kind of situation where the flow along with the species crosses it.
So yes, the type of boundary condition will make a difference here as far as I understand.
Regards SD