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-   -   Porous Zone coordinate system (http://www.cfd-online.com/Forums/openfoam/92930-porous-zone-coordinate-system.html)

 Rapha September 29, 2011 09:53

Porous Zone coordinate system

Hi everybody,

Could somebody please explain in a simple way how the porous zone in OpenFOAM works. From what I have gathered, I have calculated the viscous and inertial forces using equations related to the sphericity, pebble diameter and porosity of the material, which give the values for d and f, however this is a single value rather than a vector which we must provide.

For my system of a pebble bed, it is the same porosity in each direction, x, y and z. So does that mean that each of the x, y, z values in the d and f vectors are the same values as which I calculated from the equations? Or is it purely in the direction which the velocity is going?

Thanks,
Rapha

 olesen September 30, 2011 02:28

Quote:
 Originally Posted by Rapha (Post 326081) Hi everybody, Could somebody please explain in a simple way how the porous zone in OpenFOAM works. From what I have gathered, I have calculated the viscous and inertial forces using equations related to the sphericity, pebble diameter and porosity of the material, which give the values for d and f, however this is a single value rather than a vector which we must provide. For my system of a pebble bed, it is the same porosity in each direction, x, y and z. So does that mean that each of the x, y, z values in the d and f vectors are the same values as which I calculated from the equations? Or is it purely in the direction which the velocity is going? Thanks, Rapha
From your description, you have an isotropic porosity. Thus the resistance values are identical in all directions. If you don't want to type the same value three times, you can use the "multiplier" short-cut. For example,

Code:

``` d  d [0 -2 0 0 0]  (5.3756e+07 -1 -1);```
See the doxygen (or source code) for porousZone, where it states:
"Since negative Darcy/Forchheimer parameters are invalid, they can be used to specify a multiplier (of the max component)."

Since the porosity is isotropic, you don't need any particular coordinateSystem for it and you can just leave out specifying anything there and OpenFOAM should default to the global system.

 Rapha September 30, 2011 04:21

Thank you very much Olesen, that's crystal clear.

Cheers,
Rapha

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