I'm doing experiment measurements on porous composite materials to determine the Darcy and Forchheimer coefficients. My question is about the values that I found. I have observed that the determined coefficients depend on the flow direction, i.e., they depend on the sample face crossed by the gas flow. How this phenomenon could be explained ?
Regards,
Here is a thesis on this issue, it may help you:
http://www.hydrosys.uni-stuttgart.de/institut/hydrosys/publikationen/paper/2012/JambhekarMaster.pdf
The Darcy and Forchheimer permeability coefficients are material intrinsic properties. If the topology of the porous structure is anisotropic, the permeability coefficients could be different for each direction. If the measurements are taken according to the provided schematic, where the pressure differential is only in one direction, one would measure the permeability coefficients only for that direction through the material. Whether or not they are the same in every direction will depend on the porous topology. You can imagine the coefficients as a measurement of the impedance that the flow experiences whilst flowing through a porous material. In other words, these account for the losses, i.e. the viscous and kinetic losses, respectively. A higher value of the coefficients means the material is more 'permeable', hence a lower pressure differential would be needed to pass a particular amount of mass flow. A lower coefficient on the other hand means the material is less 'permeable.' Note that a material could have a low Darcy coefficient but a high Forchheimer coefficient, and vice-versa -- depending on the porous topology.
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