What is the significance of the input vectors (e1, e2, d, f) in porousSimpleFoam
I'm solving a flow through a radiator using porousSimpleFoam. I understand Darcy's law to the extent of what I read in wikipedia, but what is the coordinate system for and why does d and f have vectors associated with them?

The d f coefficients are relative to the porousZone and the resistance is described in each direction
so it is better to set the porousZone parallel to the e1 e2 plane to ease the entry for d and f that is what i have understood 
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The d is darcy law and f is the Forchheimer coeff. If you have an isotropic porosity you can take any arbitrary local coordinate system (eg, take the global system) and use the same d/f coefficients for each direction. For convenience, you can set one coffecient direction and use a negative coefficient (eg, 1) as a multiplier for the other two directions. We often have porosities that only allow flow in onedirection. Our convention is to specify the coordinate system so that this is the local 'z' flow direction. 
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Here is the code taken from angleDuct tutorials in $FOAM_RUN/tutorials/incompressible/porousSimpleFoam Code:
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You might then want to have 'e3' (ie, local zdirection) being the defined flow direction and add 'e1' (ie, local xdirection) to orient about this axis. 
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