how to go in solidworks flow simulation for analysis of a building against wind?

I have used the flow simulation wizard and adjusted the references as per requirements, but something really confusing me, when the solver finishes its job and I need to check the trajectories, the air travels from the bottom side of the structure as well, while it should pass through the top n the sideways only.
Wizard never asks for any point (like brackets. joints etc) to be fixed? why it's so. please do guide me dear users, I shall be grateful to you for all that!

Regards,