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Fluid Mechanics Internal Flow Question

  1. Nov 11, 2009 #1
    Hey all,

    I know that it is forbidden to post homework questions here, this is a question off a past exam, so I'm hope asking this is Kosher.

    A question says:
    "For a circular pipe of 6cm diameter, what is the expected pressure drop per unit length due to frictional losses, at a Reynolds number of 10000"

    Obviously this is a turbulent flow question, so the Poiseulle flow friction factor can't be used. But given that no density, viscosity or friction factor information is given (I know density and viscosity are buried in the Reynolds number) how the hell do you work this out ?

    Does anyone have any ideas ? because I'm starting to think that some additional information was left off ...

    Also - can someone verify this for me:
    For Laminar flow you can work out the friction factor just with knowledge of the Reynolds number
    For Turbulent flow you need to know the roughness, viscosity & density to work out pressure drops

  2. jcsd
  3. Nov 11, 2009 #2


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    Science Advisor

    The friction factor for turbulent flow is dependent on the relative roughness as well, so you will need to know that.

    The frictional losses are proportional to the velocity squared. So you'll need to know the flowrate as well.

  4. Nov 11, 2009 #3


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    Science Advisor

    For laminar flow, yes, you can use f = 64/Re

    For turbulent flow you need the relative roughness and Reynold's number.

    Note that the Reynold's number can use either the dynamic viscosity and density or just the kinematic viscosity (both need the velocity and diameter though).

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