Creating volumes for analysis
 

I'm attempting to simulate the flow of water through a tank using the interfoam solver. When setting up my CAD geometry I have been using a large block to represent the volume of air available to vent the tank.

I'm using a large open volume for the area which the water will be released to. Please see the image attached, the red is water, the blue is air.

My questions is, is there a BC i can use at the vents of the tank/ the exit of the tank which would allow me to avoid meshing these massive volumes?