# Fluid mechanics watch

1. 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?
2. (Original post by jonnypdot)
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

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3. (Original post by bigboateng_)
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

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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?
4. (Original post by jonnypdot)
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.
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
6. (Original post by jonnypdot)
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
A compressor does work to the gas. A pump isn't typically used with gas, as it wouldn't be very effective. Pumps are generally used for liquids, with compressors being used instead for gas.

A turbine extracts "work" from a fluid. Think of a turbine as kinda the opposite of a pump or compressor: in the later, energy is supplied to the impeller so that it can perform work on the fluid, e.g.to move it (think of a pump that pumps water to the top of a building). Whereas the former extracts energy from a moving fluid so that it can be used to, e.g. generate electricity (think of a wind turbine, or the turbines in a dam that generate electricity from the water).

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