Convergence and pressure issues with HT-NS-Br coupling at fluid-porous interface

Dear FEATool Support Team,

I am writing to request assistance with a Conjugate Heat Transfer (HT), Navier-Stokes (NS), and Brinkman (Br) coupled model that is experiencing solver divergence.

Model Geometry and Domains:

My model consists of five domains labeled R1 through R5:
- Solids: R1 and R3
- Fluid: R2
- Porous Media: R4 and R5

Boundary Conditions and Flow Path:
-Inlet: Located at the R1-R2 interface, where fluid enters the fluid domain (R2).
- Internal Interfaces: Fluid-porous interfaces occur at the R2–R4 and R2–R5 boundaries.
- Interface BCs: Fluid-porous continuity is set to v_ns=v_br and P_ns = P_br.
- Outlets: Located at the outer boundaries of R4 and R5, with a fixed pressure of 1200 Pa.

Issue Description:
HT-NS-Br Coupling Case: When running the full HT-NS-Br physics, the solver "blows up" and fails to converge. The issue appears localized at the fluid-porous interfaces (R2-R4 and R2-R5). When I try different BC the solver converges but the results for the pressure are way off what I expect.

HT-Br Simplified Case: If I isolate the physics to just HT-Br, the model converges successfully. However, the resulting pressure distribution does not match established literature data.

Could you please provide guidance on the correct interface boundary condition setup or solver settings needed to stabilize this specific Brinkman-Navier-Stokes coupling in FEATool?

Please see attached files.
HT-BR_Test.feaHT-NS-BR-Test.fea

Thank you for your time and assistance.

Sergio wrote

When I try different BC the solver converges but the results for the pressure are way off what I expect.

The resulting pressure distribution does not match established literature data.

Hi Sergio, could you also provide the "pressure that you expect", and the "established literature data" that you are trying to match.

The actual FEA sim that I'm trying to solve is that of a 2D heat transfer of the human eye. I've used Comsol 6.3 and was able to get matching results with literature data. I'm trying to replicate those results with FEATool. I've tried different BC at the fluid-porous interface but none come close to the expected results.

You can find the technical document and my FEATool model in the google drive link below. The FEA file is too big to attach it here. The expected pressure in domain #2 is in the range of 1500-2000 Pa when the outlet pressure at domain 12 and 11 is set to 1200 Pa. The BC at the domain interface #2-11 and #2-12 are: vns = vbr and Pns = Pbr.

Any suggestions are appreciated. Thank you.

 https://drive.google.com/drive/folders/1i-C6r9-v0SZ6DPj1XYgcB6JDZ-oQq-W7?usp=drive_link



https://drive.google.com/drive/folders/1i-C6r9-v0SZ6DPj1XYgcB6JDZ-oQq-W7?usp=drive_link