Problems with Navier-Brinkman multiphysics

Good morning. When doing the Navier-Brinkman multiphysics simulation i have the following two issues:
when doing the Navier-only solution (as per the "porous media flow" tutorial) i end up with negative pressure values


I tried also changing the values, the boundary conditions and removing the rectangle that i need as porous object, but it does not solve the issue.
Regarding the Brinkman part, it gives me this error

here are the .fea files:
NavierOnlyPlottedNoForceSimplified.fea
Navier_and_BrinkmanERRORSimplified.fea

I am unsure on whether it's possible to solve this issue, but i'd like to know at least if it's a technical isue or a mistake on my part (es. i put the wrong boundary conditions).
Thank you in advance

Timmy098 wrote

when doing the Navier-only solution (as per the "porous media flow" tutorial) i end up with negative pressure values

The equations actually do not prohibit negative values, and the "Neutral outflow/stress boundary" technically only prescribes the the normal stress on the boundary should be zero, may also allow for flow into the domains.

Timmy098 wrote

Regarding the Brinkman part, it gives me this error

Actually, looking at your K value 10^-34 causes an enormous reaction force, which is why you get non valid NaN (not a number) values in the solution.

Thank you for the answer and sorry for answering late.
I considerably increased the permeability as suggested and now the simulation no longer gives me errors. Regarding the negative pressure values, i understand that the equation does not prohibit negative values, meaning that the pressure values are, as such, based on a reference value (i imagine in this case the atmospheric pressure). Did i get it right? Nevertheless, i would like to ask on how to minimize these areas of negative pressure (of course within the realm of possibility), because i also tried to substitute the neutral outflow with outflow-pressure with pressure 0 and it still gives me negative values in roughly the same areas.
Navier_Brinkman_negative_pressure.fea

Thank you in advance.

Some typical things to check for CFD and flow simulations is in general:

- that the mesh is not too coarse (especially in regions where flow gradients may be high)
- add artificial stabilization for convective terms if needed or a turbulence model if the flow is expected to be in the turbulent regime
- apply in- or out-flow profiles for the velocity instead of pressure
- sharp >= 90 degree corners can cause a singularity in the pressure, and it might therefore help to fillet such corners to smooth them out
- switch to the time dependent solver if the flow is expected to be instationary