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For a control volume, am I right in thinking it's just the boundary of the volume that is fixed? The volume of fluid inside the control volume can change, over time?

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  • $\begingroup$ the boundary is fixed, and thus so is the volume. The volume cannot change, but the pressure , mass, and other variables of the gas or fluid inside can $\endgroup$
    – user66432
    Commented Apr 7, 2015 at 15:23

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A control volume can move. It can do anything you want it to do. You introduce a control volume so that you can focus on what happens inside it, but you must take account of anything that passes through the control volume. It does not matter whether material flows through the control volume or the control volume passes through the material. A very useful control volume is one that moves with the material, so that there is no flow through the control volume and it always encloses the same material.

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  • $\begingroup$ A control volume that moves with the material is situationally useful, but not always. $\endgroup$
    – JMac
    Commented Apr 18, 2017 at 17:09
  • $\begingroup$ I never said always $\endgroup$
    – Philip Roe
    Commented Apr 18, 2017 at 18:26
  • $\begingroup$ "A very useful control volume is one that moves with the material, so that there is no flow through the control volume and it always encloses the same material." Just saying there are plenty of caveats to this statement. Sometimes that's actually the opposite of "very useful" and that's not clear from your answer. $\endgroup$
    – JMac
    Commented Apr 18, 2017 at 18:37
  • $\begingroup$ Its the basis for Lagrangean fluid dynamics; Id say that's useful. Should I have written an essay? It was just a comment. $\endgroup$
    – Philip Roe
    Commented Apr 19, 2017 at 4:17
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Yes That's right. for example You want to do an experiment inside a beaker. you put your fluid inside that control volume (Beaker's volume) and determine the volume of fluid.

in thermodynamic we always talk about a "System". and control volume is the volume of our system.

remember when we are talking about ideal gases, the volume of the gas is equal to the control volume.

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