Hi Varun (@pdarda)!
I think @varsey explained it very well here: Simulation - Porous Media
From one document listed above you can also find the following explanation:
“The global coordinate system of the geometry is set by default…Here it is specified with two vectors. The vector e1 is created as a linear combination of the global x- and y-axes so it is aligned with the angled duct and the vector e2 is set orthogonal to e1. The vector e3 is then created in a right handed order by the coordinateSystem class orthogonally to both e1 and e2.”
If that is not enough could you explain in detail what you are struggling with?
As an example, please have a look at this project: Gas Flow Through Catalytic Converter And Exhaust
Best,
Jousef