Porous Media models in commercial CFD
 

What porous model is in Fluent? We have been using another commercial CFD code with Brinkman model and it is pretty bad in many circumstances. Of the results it produces are 100% off!

Re: Porous Media models in commercial CFD
 

FLUENT uses an extra sink term on the momentum equation to account for the extra resistance. The sink term has the form: (mu x v / alpha) + (C2 x 1/2 x rho x abs(v) x v) where mu is viscosity, v is superficial velocity alpha is a permeability factor, C2 is an inertial resistance factor and rho is density.

We have benchmarked single-phase gas flow using this model and gotten decent results. It doesn't work for multiphase or phase change, because you are limited to single entries for alpha and C2.

It can also use a power-law function of the velocity magnitude, but I'm not very familiar with that approach.

Re: Porous Media models in commercial CFD
 

You can easily write a udf to compute this term or use another term that you prefer. In this case, you can then use more complex relations since you don't need to rely on the assumptions used by Fluent.

Greg

Re: Porous Media models in commercial CFD
 

Thanks, Evan. What you described is the Brinkman's model. As I mentioned, I completely fails in some circumstances, such as when the inflow is very non-uniform and at an angle, while the media's resistance is not very high.

Re: Porous Media models in commercial CFD
 

Could you explain what's udf? thanks.

Re: Porous Media models in commercial CFD
 

Simple models like that will also fail when turbulence dependent phenomena like dispersion (thermal, species) become important. Porous media need a specific turbulence model below porosities of say 0.8 to account for the pseudo-vortices around particles and size limited interstitial vortices between particles.

Re: Porous Media models in commercial CFD
 

Have you tried any in particular?

What about the granular model of Fluent4? It has some turbulence models for the mutliphase flow, but I'm not aware if they account for dispersion as you point out. Do you know?

Regards Greg

Re: Porous Media models in commercial CFD
 

I am very unfamiliar with Fluent 4, but I'd guess that in a granular model Fluent it is assumed that the solids also flow. So Fluent offers a primary and secondary turbulence option to account for that. Maybe there's a way to modify it for porous media flow, I dunno.

I have not yet tried other turbulence models, since in our cases dispersion effects etc. have not yet been important. I did find two literature articles describing such models:

1) Turbulence model for flow thorugh porous media - T.Matsuoka - Int. J. Heat Mass Transfer - Vol.39, No.13, pp2803-2809

2) A general two-equation macroscopic turbulence model for incompressible flow in porous media - B. Antohe - Int. J. Heat Mass Transfer - Vol.40, No.13, pp3013-3024

Re: Porous Media models in commercial CFD
 

Thanks - you can turn off the solids flow in Fluent 4 if you like.

Greg