Fluids (both liquids and gases) will move from one point in space to another due to a potential gradient. Some examples may be:
1) horizontal pipe flow, a fluid will move from a region of high pressure to a region of low pressure.
2) inclined open channel flow, a fluid will flow from high elevation to low elevation.
Both liquids and gases have a degree of compressibility, gases more so than liquids. Fluids expand or compress when subjected to a change in pressure, volume, and/or temperature.
In the first example concerning horizontal pipe flow, if the fluid was gas moving from a high pressure to low pressure, would it be said that the gas was flowing or expanding through the pipe? What features about the gas' movement is different than the liquid's movement described in the second example -- open channel movement of a liquid down an incline?
What are the definitions of flow and expansion and how does that distinguish the two when describing fluid movement from one point in space to another?