boundary conditions for the car in the atmosphere
 

To examine the exhaust of a car. i gave the enclosure of certain dimensions. what are the boundary conditions type for top surface and adjacent sides of the enclosure. The are not the walls. what boundary condition type should i use?

Can you show a picture of your domain. When dealing with low speeds and a pressure solver you can probably go with an pressure outlet (check that the inflow conditions match your "surrounding").

Attachment 96451
here is the geometry. i want to know what type of boundary condition should i give for top surface and side faces of the enclosure
thanks