For fluid flow simulations, we need to capture the fluid volume from the solid geometry. Depending on what you want to analyze you might need to simulate a flow inside the geometry, like pumps or heat exchangers, or the outside of the geometry, like the aerodynamic behavior of a car, or around a building. Hence, for CFD simulation in SimScale, we need to extract this very volume region which is called flow volume.
Internal flows involve flow through or within a geometry. The geometry need not be completely blocked from all sides. It can be open with inlets and outlets like a pipe or just a completely closed room. Hence, simulating the physics of fluid under such scenarios can be classified as an internal flow simulation.
The figure below shows the internal flow volume extracted from a non-return valve casing.
As we are only interested in the fluid inside the valve rather than the valve itself fluid volume extraction is mandatory.
External flows involve flow around the geometry. This involves flow around a vehicle, a golf ball, a wind turbine, or a large city. Hence, simulating the physics of fluid under such scenarios can be classified as an external flow simulation.
To simulate you need to create a volume around your geometry. This resulting flow volume captures the negative of the geometry acting as a virtual wind tunnel. See figure below: