Learning Forum

[1213kkw]

Hello, I am currently working on a membrane separation CFD model in ANSYS Fluent, where I implemented CO₂ transport across a membrane using a UDF based on a pressure-driven flux model. And now I am encountering strong numerical instability in the species eld near the membrane. (it applied to wall-shadow wall)

CO₂ mass fraction near the membrane alternates between: a finite value → almost zero → finite value → …, and this oscillation persists across iterations. 

It appears that: In one iteration, CO₂ is almost completely removed from the adjacent cell due to high flux, in the next iteration, the driving force collapses (since CO₂ ≈ 0), so flux drops, then CO₂ is replenished via convection/diffusion, finally the cycle repeats. I think this results in a non-physical “on-off” oscillation of flux and species. 

Is it acceptable to introduce flux limiting (capping) in a membrane UDF?

[Rob]

How are you defining the flux? In reality there's a limiter based on either membrane concentration gradient (how much can be pushed through) or diffusion/convection in the fluid. In CFD we can do what we want, it's then a case of explaining why we did it.