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?


1 Answer 1

  • 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.

  • $\begingroup$ thanks for your response. Please continue to try to help me with this. If you were to track the motion of a few molecules of propane gas as they move from their propane tank to the burner of your propane grill, what fundamental concept moved them there, flow or expansion? $\endgroup$
    – Armadillo
    Commented Sep 9, 2015 at 0:00
  • $\begingroup$ Both. Even though those concepts are fundamentally different, one can cause the other. The expansion is restricted and can only happen in one direction. Instead of pushing neighbor particles in all direction giving no net motion, the are all pushed in one direction giving a net motion. $\endgroup$
    – Steeven
    Commented Sep 9, 2015 at 0:07
  • $\begingroup$ But to be clear on the wording, the flow itself is not the cause, only the result. It's not because of flow that they move. It is rather because they move uniformly that we call it a flow. And thus motion is caused by the expansion. $\endgroup$
    – Steeven
    Commented Sep 9, 2015 at 0:09
  • $\begingroup$ I think you said this, but just to clarify: "If expansion is restricted and can only happen in one direction, instead of pushing neighbor particles in all direction giving no net motion, they are all pushed in one direction, uniformly, resulting in a net motion which is by definition, "flow"."? $\endgroup$
    – Armadillo
    Commented Sep 9, 2015 at 0:34
  • $\begingroup$ Yes, that is correct. $\endgroup$
    – Steeven
    Commented Sep 9, 2015 at 5:40

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