Hi @pdarda!
For the case you described e1 has coordinates (1, 0, 0) and e3 has ccordinates (0, 0, 1) (I’ve described simulation with exactly the same configutation of flow on screenshot here - link)
So the vector e1 represents the direction in which the material will be porous, right?
Vectors e1 and e3 create a coordinate system, so you cant define just one of them to set direction of your porosity. You have to define at least two vectors among e1, e2 and e3.