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-   -   Porous Media models in commercial CFD (http://www.cfd-online.com/Forums/fluent/29655-porous-media-models-commercial-cfd.html)

Clif Upton April 18, 2002 12:30

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!

Evan Rosenbaum April 19, 2002 13:11

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.

Greg Perkins April 20, 2002 23:23

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

Clif Upton April 22, 2002 11:40

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.

Clif Upton April 22, 2002 11:41

Re: Porous Media models in commercial CFD
 
Could you explain what's udf? thanks.

Erwin April 23, 2002 13:56

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.

Greg Perkins April 24, 2002 04:04

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

Erwin April 24, 2002 10:48

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

Greg Perkins April 24, 2002 20:49

Re: Porous Media models in commercial CFD
 
Thanks - you can turn off the solids flow in Fluent 4 if you like.

Greg


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