Why is the force on the fluid not equal to the change in momentum per unit time?

Thread starter andyrk
Start date Apr 20, 2015
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Change Flow Momentum

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The discussion centers on the confusion regarding the relationship between the force on a fluid and the change in momentum per unit time in a bent pipe. Participants clarify that the force acting on the fluid is indeed related to the rate of change of momentum, but they emphasize that it involves the flow of momentum rather than just momentum itself. The calculations presented demonstrate that the rate of change of momentum for fluid entering and exiting a control volume results in forces acting on the fluid due to the pipe walls. Additionally, the conversation touches on the assumptions of incompressibility and laminar flow, which are crucial for accurate calculations. Ultimately, understanding the distinction between flow of momentum and change in momentum is key to resolving the confusion.

May 11, 2016

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Vibhor said:

Chet , I understand your point . Just help me understand why in the books it is written that horizontal force exerted by fluid on pipe is ##\rho v^2A## . Are there any assumptions involved or am I misinterpreting something ?

Maybe they are assuming that the fluid pressure in the pipe is just atmospheric pressure (zero gage), or maybe they just inadvertently left out the pressure force.