What is Control volume; open system? — Physics

Definition 5.1 University Physics — Year 2 · Chapter 5 — Momentum and Energy Balances in Flows

A control volume (or control surface) is a fixed region of space $\Sigma$ through which fluid flows; the fluid inside it at any instant is an open system, which exchanges mass with the outside through the inlets and outlets of $\Sigma$ . The closed system to which Newton’s laws apply is the fluid that is inside $\Sigma$ at time $t$ , followed to $t + \dd t$ : it then occupies $\Sigma$ minus what has left plus what has entered.

Left: a control volume in a steady flow — the momentum leaving through S_2 minus that entering through S_1 equals the sum of the external forces on the fluid inside, pressure forces on the two sections included. Right: a jet stopped by a wall pushes it with D_mv; a jet turned back by a Pelton bucket moving at u pushes with up to 2D_m(v - u). — Left: a control volume in a steady flow — the momentum leaving through $S_2$ minus that entering through $S_1$ equals the sum of the external forces on the fluid inside, pressure forces on the two sections included. Right: a jet stopped by a wall pushes it with $D_mv$ ; a jet turned back by a Pelton bucket moving at $u$ pushes with up to $2D_m(v - u)$ .

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