In setting up the boundary conditions for a porous zone, we need to specify the direcion vectors, how do we do that and what does it mean physically.(Fluent Manual wasnt elaborative)

Thanks to all. Shankar.

I also have the same problem from last 6/7 months, but could not found the solution. FLUENT engineers only can help.

If you get it pl. let me know the solution.

All the Best!!

Shankar,

I will assume you are using FLUENT 5.x and are referring to the Boundary Conditions input screen. The direction vectors are used if the porous medium has orthotropic hydraulic resistance properties AND if the coordinate system for specifying these resistance properties is not aliigned with the global coordinate system.

If for example, the porous medium coordinate system is rotated 45 degrees from the global coordinate system, then you would specify the Direction 1 vector as X = 0.707, Y = 0, Z = 0.707.

By default, the Direction 1 vector is aligned with the global X-axis and the Direction 2 vector is aligned with the global y-axis. You only need to change these settings if your porous medium doesn't align with the global axes correctly.

Hi Evan,

Thanx for the reply.

I want to simulate the turbulent flow in a channel, whose bed is porous(sand) and there is upward seepage through the bed along with a turbulent flow in the channel. I have the following questions...

1. Is it possible to simulate the laminar flow in the porous sand bed and turbulent flow above the bed at the interface simultaneuosly to get velocity and shear stress distributions? i.e. How to use porous media Boundary condition in this case?

2. How to set the boundary condition at the sand & water interface which will take into account the roughness of sand and the upward seepage velocity?

Please help at the earliest. Thanks in advance.

Sharad Dugad

Thanks a lot Evan. Shankar.

Thanks a lot Sharad. Shankar.

FLUENT can't do what you want. The porous medium is simply a fluid with and additional, possibly orthotropic, momentum sink term. Because the porous medium is a fluid and not a solid, you will not get proper shear stresses at the interface and you cannot include the effects of any surface roughness.

Hi Evan,

If it is not possible with FLUENT, do you know if there is any alternative solution/other software which can do this?

Is it not possible to treat the sand-water interface as solid surface with some porosity and make it a 'Velocity Inlet' B.C. there?

Waiting for Reply.

Thanks.

Sharad Dugad

Hi Sharad,

I have not done this in any project yet, but it is something that you can try and see if it works. I will write refering to Fluent 4, fluent 6 might have the capability but I don't know.

Turn on your Eulerian Granular Multiphase model, with a continuous primary phase and sand as your granular phase. Patch the void fraction for your granular phase in the part of your domain you have the sand. Under solve-> controls->Equations, deselect the granular phase, this means you have frozen your granular phase and only the primary phase is solved.

This is just a suggestion that may or may not work. Let me know if it works.

Hi Amadou,

Thanx a lot for your reply.

Could you please explain in detail step by step procedure to do what you suggested. I am using Fluent 5.5 & Gambit for grid generation.

Waiting for your reply.

Sharad Dugad

Hi Sharad,

When I have the time I will give you a step by step explanation of what to do in fluent 4.

Regards, Amadou

[Akim679]

Hi,

I am having problems with specifying the boundary condition of a bottom wall of a 3D rectangular channel as a "porous jump" . In GAMBIT I select Boundary type as "Porous Jump" however, when I import my mesh in FLUENT I receive an error stating that: "Cannot change porous to porous-jump because
there is only one adjacent cell thread". I am not sure what that means, can anyone please help me with this case. Any suggestion is appreciated.

Thanks,
Akim

[delaneyluke]

Hi Akim,
This means that the boundary you are specifying as a porous jump is an exterior surface, a porous jump BC can only be specified to an interior surface

Regards
Luke