# How can you do work on a control volume with a fixed boundary?

I am having trouble understanding the concept of flow work. All of the texts and online sources I have read so far claim that flow work is always done on or by a control volume where matter flows across the boundary. That is, work is done by matter outside the CV as it pushes new mass into the CV. But for a control volume (CV) with a fixed boundary, how can mass outside the CV exert a force on mass inside the CV through a distance without moving into the CV itself, in which then the work should be considered internal?

I have found a similar question here but I didn't quite understand what the answer was getting at and hence I thought it was appropriate to post this question.

The control volume is an open system. When you apply the first principle to a control volume, you are actually applying it to the closed system which coincides at time $$t$$ with the control volume. Between $$t$$ and $$t + dt$$, a part of the closed system exits and another advances at the entrance. So work on this closed system is indeed work performed by the external fluid.
• Thanks for the answer. So even after some mass leaves the CV during the time between $t$ and $t+dt$ (thereby doing work on external fluids during this time interval), it's still considered as being part of the closed system although it's no long inside the CV? And conversely, the mass that enters the CV during $dt$ is not considered part of the system? Sep 6, 2021 at 16:45