Direction vector in porous media panel
 

Hi users, I am trying to model a gas flow through porous media (a catalytic layer in a cylinder --> 3D model where the gas is flowing in negative y-direction according to gravity direction). Could anybody please tell me what I have to define as Direction-1 Vector and Direction-2 Vector? Is one vector for the flow direction? Or is it the "plate" plane on which the catalyst lays (in my case on x-y-plane). Thanx for advice. Claud