Air change rate in CFD post
 

Hello

I want to calculate ACH in my model. There is a room with two windows, aligned in the direction of the airflow. After simulation, I am trying to process the data in CFD-post.

So my questions are:

1) Does the u,v,w components of velocity correspond to the local x,y,z coordinates?
2) When calculating surface integral of a velocity component (say w), I picked the w-velocity from the Variable list. Then why is the Direction field (below Variable) available for selection? Have I not already selected a direction by going for w.
The Direction field allows me to pick either Global, X, Y or Z. What does that mean in this context then?

Your help will be appreciated.

Thanks

Somebody has mentioned that u,v,w correspond to x,y,z on a seperate post. So I guess Q.1 is resolved.
However, I still don't follow Q.2