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Fluid Mechanics Theoretical Pipe Diameter

  1. Jan 23, 2017 #1
    1. The problem statement, all variables and given/known data
    I need to find the pipe diameter or mean velocity.
    Q= 0.01m^3s
    Density= 960kg m^3
    Viscosity = 0.081 Pa s
    Max Velocity = 1.8 m s

    2. Relevant equations
    Mean velocity= 4Q/pi.D^2

    3. The attempt at a solution
    I have two unknowns in this equation so I'm unable to find D or Mean Velocity. Am I right in thinking the mean velocity is max velocity/2? And does this work for laminar/Turbulent/transitional flow? Any help appreciated. Thanks
  2. jcsd
  3. Jan 24, 2017 #2


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    Staff: Mentor

    Even I know that velocity does not have units of m s :eek:

    Your first priority should be to show units correctly. Not only will it help discipline your thinking, but units correctly incorporated into equations allow an independent check of formulae.

    Please fix all the units, and show them correctly in a follow-up post.
    Another example of sloppiness. :mad: This desperately needs some parentheses to make it correct.
  4. Jan 24, 2017 #3
    They must be referring to laminar flow, where the maximum velocity is twice the average velocity. In turbulent flow, the ratio is much closer to 1. As a check on this, once you find the diameter for laminar flow, you should calculate the Reynolds number to make sure that the laminar-turbulent transition hasn't been exceeded. If it has, then you need to start looking at the case of turbulent flow.
  5. Feb 1, 2017 #4
    Thanks for your reply, much appreciated. I have divided by 2 to get the mean velocity. I've then plugged it into the Reynolds Number formula to get 1280. Which means it is laminar flow.
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