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Friction factor at turbulent region

  1. Apr 28, 2016 #1
    1. The problem statement, all variables and given/known data
    in my book , the author stated that the friction factor become maximum when the flow become turbulent ...However , according to the Moody Chart , we can know that the friction factor decreases from laminar to turbulent and then constant .... is the statement in the book wrong ?

    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Apr 28, 2016 #2
  4. Apr 29, 2016 #3

    SteamKing

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    It would appear to be so, based on the Moody diagram.

    Can you provide a snap shot of the text where the author makes this statement?
     
  5. Apr 29, 2016 #4
     

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  6. Apr 29, 2016 #5

    SteamKing

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    I think the author should have said that the friction factor reaches a minimum when the flow becomes fully turbulent.
     
  7. Apr 29, 2016 #6

    haruspex

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    Yes, but from the text leading up to it I would say it was not just a slip of the pen. The author really does think it reaches a maximum there.
    It does seem paradoxical because, as the author says, the pumping force required is greater at higher velocities. But looking at the v2 term in the Darcy-Weisbach equation, it is clear that some small reduction in friction factor as the velocity increases will not counter that much.
     
  8. Apr 29, 2016 #7
    so , it is true that the friction factor will increase when the flow become fully turbulent ? so , the author 's statement that the friction factor will become maximum is wrong ? hwo about the pumping power?
     
  9. Apr 29, 2016 #8

    SteamKing

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    While I agree that the total friction varies as flow velocity squared, the author doesn't say that the total friction is a maximum for fully turbulent flow; instead he says the friction factor is a maximum. That distinction is the one thing which operates in the author's favor.

    The Moody diagram, which plots friction factor versus Reynold's No., and by extension flow velocity, shows that the friction factor assumes a constant value which is independent of the Reynold's No. of the flow and hence the flow velocity for fully turbulent flow. That's what all those horizontal lines indicate on this diagram:


    Moody_diagram.jpg

    For a given value of relative pipe roughness, the friction factor takes on a relative maximum value somewhere in the transition zone between fully laminar and fully turbulent flow. Granted, the Reynold's No. at which the different types of flow appear seems to be somewhat arbitrary, having a minimum friction factor in the turbulent zone is quite obvious from the Moody diagram.
     
  10. Apr 29, 2016 #9

    haruspex

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    No.

    Yes.
     
  11. Apr 29, 2016 #10

    SteamKing

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    No. Read my Post #8 above and look at the Moody diagram.

    Yes, obviously.
    The pumping power is still directly proportional to the pressure drop and the flow rate.
     
  12. Apr 29, 2016 #11

    haruspex

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    No, it operates against the author. The total friction increases as the velocity increases, but the friction factor decreases.
     
  13. Apr 29, 2016 #12
    so
    so , correct statement should be , the total friction become maximum ,while the frictional factor become minimum ?
     
  14. Apr 29, 2016 #13

    haruspex

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    Yes.
     
  15. Apr 29, 2016 #14
    so , beside the friction factor that cause the friction force , there are other fricition forces acting on it ?
     
  16. Apr 29, 2016 #15

    SteamKing

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    You'll have to be more specific.
     
  17. Apr 29, 2016 #16
    what do you mean ?
     
  18. Apr 29, 2016 #17

    SteamKing

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    The pressure drop formula is:

    PD = f (L/D) v2 / 2g

    Except for g, everything else can be a variable.

    Other friction forces acting on what exactly? That's what is not clear.
     
  19. Apr 29, 2016 #18
    on fluid and pipe
     
  20. Apr 29, 2016 #19

    SteamKing

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    Doesn't seem to be for simple incompressible flows.

    That's not to say that you could wind up with some wild, compressible flow situation with a lot of externally applied forces, temperature changes, phase changes, etc., but why go looking for trouble?
     
  21. Apr 29, 2016 #20

    haruspex

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    No. The friction force is a product of various terms. One of them is the friction factor. As the flow velocity increases, the friction factor reduces a bit, but the other factors increase more, so the friction force overall increases.
     
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