Why does the unsteady flow equation state that the RHS of the equation (change of energy in the system) is made up of internal energy, kinetic energy, and potential energy? Where is the flow energy? Shouldn't enthalpy (u+pv) be considered? Specifically, I am referring to the RHS of the equation. I understand how the energy components from mass in/out come about. Some people mentioned that the flow work is 0 in the system as there is no boundary work on the system, but pressure can change in unsteady flow, which means that the change in flow work (pv) should be accounted for, right? Can you help clarify my concept?