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Friction in pipe

  1. Apr 20, 2016 #1
    1. The problem statement, all variables and given/known data
    what is the meaning of n here ? i only know the formula of head loss is f(L/ D ) (V^2) / (2g)

    2. Relevant equations


    3. The attempt at a solution
     

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  3. Apr 20, 2016 #2

    SteamKing

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    Apparently, n is the exponent of the flow velocity V as shown on the diagram.

    For laminar flow, the friction factor f is directly proportional to V.

    For the transition zone, the friction factor is proportional to a certain power of V, which is expressed as f ∝ Vn, where n = 1.75 - 2.0.

    For fully turbulent flow, f ∝ V2.
     
  4. Apr 20, 2016 #3
    so , the formula of head loss still f (L)(v^n) / (2gD ) ????? at turbulent region ?
     
  5. Apr 20, 2016 #4

    SteamKing

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    Yes, if you make n = 2.
     
  6. Apr 20, 2016 #5
    the book gave that for fully turbulent flow , n = 1.75-2.0 , THE CORRECT SHOULD BE for transition zone flow , n = 1.75-2.0 ???
     
  7. Apr 21, 2016 #6

    SteamKing

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    The roughness of the pipe wall also has an influence.

    Look, the book is trying to establish an approximate relationship between the velocity of the flow and the friction factor, as illustrated by the diagram. For whatever reason, they would like this relationship to be a smooth curve throughout the flow regimes from purely laminar flow to purely turbulent flow.
     
  8. Apr 24, 2016 #7
    for transition region , the relationship between the velocity and friction factor is not constant , it may vary .... so there's is discontinuous line on the graph , am i right ?
     
  9. Apr 24, 2016 #8

    SteamKing

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    The dashed line in the transition region appears to be an extrapolation of what happens after fully laminar flow has stopped and when fully turbulent flow is established.
    When flow is laminar, the friction factor is directly proportional to flow velocity; when flow is fully turbulent, friction factor is proportional to flow velocity raised to the power indicated by the exponent on the graph. In the transition zone, there is some non-linear relationship between friction factor and flow velocity; it's not entirely linear, but it's not fully quadratic, either.
     
  10. Apr 24, 2016 #9
    Darcy -weisbech equationgave that hf = fL(v^2) / 2gD , and Darcy -weisbech equation is for both laminar and turbulent , am i right ? why according to the graph , the friction factor is directly proportional to hf / shouldn't it be n= 2 ?
     
  11. Apr 24, 2016 #10

    SteamKing

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    If you look at the formulations of the D-W friction factor, you'll see that f for laminar flow is very different for f for fully turbulent flow.

    https://en.wikipedia.org/wiki/Darcy–Weisbach_equation

    You also shouldn't confuse how f behaves with flow velocity with how head loss behaves with flow velocity. The Moody diagram, for example, establishes that for turbulent flow, f is independent of flow velocity.

    The diagram in the OP documents Reynold's measurements of friction versus pipe length. It's not clear how well Reynold's measurements correlate with the work of others based on this one diagram.
     
  12. Apr 24, 2016 #11
    i know that , when the flow is fully turbulent , the friction factor is independent of velocity , but for both laminar and fully turbulent flow , the head loss has the formula of fL(v^2) / 2gD , so for both flow , the hf is directly proportional to v^2 , am i right ? but the diagram for v and hf is proportional, is the diagram wrong ?
    P/s: in the first diagram , it's a graph if hf vs velocity , not friction factor vs velocity
     
    Last edited: Apr 24, 2016
  13. Apr 24, 2016 #12

    SteamKing

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    There's only one diagram: a plot of head loss versus flow velocity made using data recorded by Reynolds.

    Apparently, Reynolds chose not to use the modern formula, HL = f (L/D) v2 / 2g to plot his results. That's why there's a range of exponents. IDK why Reynolds chose this method; if you want to know, you'll have to research Reynolds work on the matter.
     
  14. Apr 24, 2016 #13
    So, this is for to show expermintal data only? In real life, we wouldq use the formula hf = f(L/D)(V^2)/2g ???
     
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