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I Pipe-Flow Momentum Balance

  1. Oct 19, 2016 #1


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    Hi PF!

    Suppose we have an incompressible UNSTEADY fluid passing through a level pipe. Let station 1 have area, velocity, and pressure ##A_1##, ##V_1(t)## and ##P_1(t)##. Station 2 is defined similarly. I know the unsteady Bernoulli equation could solve this, but if I wanted to make a momentum balance I would have $$\partial_t\iiint_v \vec{V} \rho \, dv + \iint_{\partial v} \rho \vec{V} (\vec{V} \cdot \hat{n}) \, dS = \sum \vec{F}$$ I'm not worried about any specifics here except for one detail, the volumetric time rate of change integral. Since velocity ##\vec{V}## monotonically changes from station 1 to station 2, this integral ##\partial_t\iiint_v \vec{V} \rho \, dv## is definitely not zero; then how do we solve for it? Would we have to look at Navier-Stokes for the fluid to get the fluid velocity profile to solve? I know NS is a momentum balance and takes identical form to the equation I posted, but I'm not sure how to proceed here. Any idea?
  2. jcsd
  3. Oct 19, 2016 #2


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    Well the usual approach, if the goal is to do this analytically, would be to use the divergence theorem to remove the integrals and solve the integrands as a system of differential equations.
  4. Oct 19, 2016 #3

    Have you checked out BSL, Chapter 7 like I suggested. They show how to do what you want for an inviscid fluid. It involves using the rate of change of kinetic energy within the control volume.
  5. Oct 19, 2016 #4


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    I don't have the book on me right now. I moved a little while ago and left my book at my old school. I am picking it up this November though, so I was planning on studying it then! I'll be sure to give it a good read. Perhaps I'll check and see if our library has it now though. Then I can read before asking a bunch of questions.
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