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Calculating Reynolds Number

  1. Apr 26, 2009 #1
    1. The problem statement, all variables and given/known data
    Hi everyone,

    This problem concerns calculating Reynold's Number of flow after exiting a duct. The duct that the flow exits is rectangular. The flow is measured directly underneath the center of the duct at a distance of 10 cm below the exit (Y-axis). Flow is measured at 11 m/s. It is determined that the velocity becomes 0 at a distance of 8 cm outward from the center (X axis). I am assuming that the flow is symmetric and thus the flow is esentially in a pipe with a radius of 8 cm.

    2. Relevant equations

    Re = "rho"*V*d/"mu"

    3. The attempt at a solution

    "rho" = density = 1.20 kg/m^3
    V = velocity of air = 11 m/s
    d = diameter of pipe = 2*0.08m = 0.16 m
    "mu" = viscosity of air = 1.8418e-05 kg/m*s

    plugging those into the equation above yields: Re=1.147e+05

    I guess my question is regarding my assumtion of the pipe. Was I correct in my assumption?
  2. jcsd
  3. Apr 28, 2009 #2


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    Science Advisor

    After the fluid leaves the pipe, the concept of a Reynolds number is kind of meaningless. Furthermore, the number can be based upon anything. Sure, pipe flow Reynolds numbers based on diameter are well known and can be a sure-fire way to indicate laminar vs turbulence. However, once the fluid leaves the pipe, you no longer have simple pipe flow. You now have a jet, and the concepts of walls and such no longer apply.

    You certainly can measure the velocity and viscosity. After that, the characteristic length can be whatever you want it to be, there is no wrong answer. Having said that, you can also no longer use the pipe flow Moody diagram and so on and so forth.
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