For flow simulations we need to capture the fluid volume from the solid assembly. Depending on what you want to analyze you might need to simulate a flow inside your geometry, like pumps or heat exchangers, or the outside of your geometry, like the aerodynamic behavior of a car, or around a building.
The flow volume is the region where you have a liquid or gas. The image below shows the flow volume that we extracted theom a pipe.
To simulate the flow around a vehicle, you need to create a volume around your geometry. This flow volume is our virtual wind tunnel.