CFD Online Discussion Forums - How to include porosity in porous zones

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How to include porosity in porous zones - is there a way?

Hi Foamers,

I am working on a benchmark case of a porous cube in a wind tunnel. I am considering the flow incompressible. I have tried to use pimple foam with a very standard porous zone added through fvOptions.

Two issues have come up that seem to be really destroying my results:

1) There doesn't seem to be anywhere to specify porosity! It seems like to include porosity a new solver would need to be written since porosity would multiply various terms in the NSE that I don't think can be modified by fvOptions. If this is the case, why do we have porous zones in fvOptions - is there a physically defensible situation where you would have momentum extraction due to porous media without porosity effects? In my case, about 50 percent of the flow needs to be diverted around the cube regardless of the darcy-forchheimer coefficients. I looked at porousSimpleFoam, and it seems to have the same problem. Am I missing something here? I think the equations I should be solving should be somthing like equations (1) and (2) in this paper: https://static1.squarespace.com/stat...01452-main.pdf

2) There doesn't seem to be a place to specify density - the flow may be incompressible, but doesn't the affect the total momentum extraction? I measured the pressure drop in paraview to do a dummy check, and concluded that openFoam was operating with rho=1.

Here is an example of the porous zones I have tried:

Code:

porosity1

{

    type            explicitPorositySource;

    active          true;



    explicitPorositySourceCoeffs

    {

        type            DarcyForchheimer;

        selectionMode   cellZone;

        cellZone        cube_C1;



        DarcyForchheimerCoeffs

        {

            d   d [0 -2 0 0 0 0 0] (0 0 0);

            f   f [0 -1 0 0 0 0 0] (.151565 .151565 .151565);



            coordinateSystem

            {

                origin  (0 0 0);

                e1      (1 0 0);

                e2      (0 1 0);

            }

        }

    }

}

https://www.openfoam.com/documentati...r.html#details

I am entertaining any suggestions - the results I am getting are off by an order of magnitude, so.... please help me.

Thanks

Hi!

I think, pimpleFoam has got explicit porous formulation:
https://www.openfoam.com/documentati...leFoam_8C.html
and should work. If you don't mind, please upload your case. I will check it if I have time for that.

Thanks for the reply. The attached casefile is just a simple, very coarse testcase. There is a small porous cube hovering in space. If you run it, you will see that the flow slows down by about 1 m/s, but there is almost no deflection of flow around the porous element.

I am not sure, but I think the term "explicity porosity" might be used a little liberally in the documentation. In the casefile I attached, the "explicitPorositySource" in fvOptions, but as far as I can see, the porosity can not be specified there. pimpleFoam includes the p and U equations .H files, but I can't don't see any way that porosity can be included without writing a new solver.

Hi!

It looks that you forgot a few things in the testcase.zip. Could you, please, make an Allrun script to let me focus only on the particular problem without other bugs.

Thanks in advance!

Its hard to get a case small enough to upload here! Here is another version without the function object that was causing issues.

At this point, I think this is really a matter of figuring out what the solver is actually intended for. I can't think of any example where there would be significant momentum extraction, but not a correction for volume fraction. I would be really interested to hear from anyone who has had any success with this fvOptions or porousSimpleFoam.