direction vector in porous media
 

Hello everyone.
I have a Problem is simulating a 3D-porous-Bend in Fluent!
My Bend is 3D and Porous. and my porous zone is ISOTROPIC!
My problem is when i want to set the porous properties in fluent, i should set direction vectors in x-y-z coordinates! But the flow in bend IS NOT in x-y or z! ( Because its a Bend!)
i know i have to write a UDF for direction vectors but i don't know how to write it.
any one can help me?
As my porous zone is ISOTROPIC, Can i set direction vectors to (1,0,0) and (0,1,0) Or i should write UDF and set the direction vectors parallel to my flow??
Thanks in advanced.