Coupling of External and Internal Aerodynamics of Car
 

I want to couple the internal and external aerodynamics of a car. I want to understand the effect of external air flow over the air flow inside the car. The car is moving at 20 m/s. The air from outside is not allowed to enter the car directly but only through a air vent . Air vent speed is 3 m/s. How can this problem be solved?
Note : here by effect of flow means both velocity and temperature.