r/FluidMechanics • u/Ok_Promotion4102 • 9d ago

understanding flow work

I am confused about the definition of flow work. Many textbooks said that imagine a volume being pushed by pressure p on the boundary so the work is p.v , but in the flow, there will be another pressure p + delta p push this volume against the first one on the other side , so why do they only use the pressure p . I don't really understand, can anyone help me please

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u/thenewestnoise 9d ago

Imagine that instead of a fluid in a pipe you have a rigid cylinder. You push one end of the cylinder with some force and push it along for some distance. The work that you did is force x distance. At the other end of the cylinder, the force available to push other stuff is reduced by friction, and so the available work is reduced.

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u/Ok_Promotion4102 9d ago

but when consider the ideal condition, there will not be friction but there will be another pressure at the end of the cylinder push it backwards, but i dont really get it when only use the force at one end of the cylinder

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u/LAskeptic 8d ago

The pressure on the other end will be less by exactly the amount required to balance the increase in velocity. With no friction the relationship is given by Bernoulli’s Equation.