Fluid mechanics

so the momentum balance equation goes rate of P in - rate of P out +F acting on the fluid=dP/dt
where dP/dt is the net force acting on the control volume but i though the controlled volume has a fixed position and shape so how can a force act on it?

The force arises from changes in pressure around the control volume. If you have a converging nozzle for example, then the area at the end is smaller than the area at the beginning so by mass flow rate conservation the flow flows faster as it exits, hence the pressure at the end of nozzle is greater than start, if you draw a square control volume around the nozzle then F = pressure at end - pressure at beginning. Due to pressure changes, there is an resultant force. Control volumes just represent the region in which we're interested, it's imaginary. So the force is acting on the physical object in real world such as jet engine,, the control volume just helps us calculate it. Idk if this even makes sense lol


Posted from TSR Mobile

i understand what you said about the nozzle. When it comes to the control volume the net force acting on that volume is the net force acting on that volume of fluid right so what can you do with that? can we determine the net force on the whole system?

By equilibrium, the net force acting on the whole system is opposite to the force acting on the fluid.

Thanks for that

another question i had was in thermodynamics am i suppose to assume the system volumes of compressors, turbines and pumps are always constant? so gas does no work on these systems?

Thank you