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?
Best Answer
A gas can expand by filling more volume than before. Like a balloon in a pressure chamber where the pressure is suddenly lowered. No net motion (no flow) happens here.
A water stream can flow continously without simultaneous expansion. Consider a circular stream that ends where it starts. As a bathtub where there is a big plastic bucket in the center. By twirling around in the water a flow can be established. No expansion happens.
The definition of flow is about a net motion of fluid particles. The definition of expansion is about the relative size or space taken up by the fluid particles. These concepts are fundamentally different.