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Homework Help: Fluid Mechanics: Dimensionless Groups Question, explain an outlier

  1. Oct 21, 2014 #1
    1. The problem statement, all variables and given/known data
    Hi guys, I need to explain the outlier point here, which has been shaded in the excel spread sheet when comparing the two dimensionless groups,

    The dimenionsless group, drag-coefficient is given by Drag/(density*V^2*D^2)
    and dimensionelss group, spin parameter, is given by omega/VD



    3. The attempt at a solution

    The question explicitly asks to, "explain the outlier(s)". My friend thinks that it is because the flow for the outlier point, shaded, is extremely turbulent, whereas the others all have a roughly similar reynolds number of ~20,000.

    I can't explain it but I don't think this is correct. I think it has to do with the way we vary the product of the velocity and diameter of the ball.
  2. jcsd
  3. Oct 21, 2014 #2
    What is the experiment, and what are the definitions of the parameters V, D, drag, and omega (including units).

  4. Oct 21, 2014 #3
    The experiment is passing air through a wind-tunnel and keeping a ball of diameter D in the center and then plotting the relevant dimensionless groups against each other.

    The parameters have units:
    V = ms^-1
    D = m
    Drag = Newtons
    omega = rad/s
  5. Oct 21, 2014 #4
    I guess the ball is rotating? Did you calculate the Reynolds number for each case?

  6. Oct 21, 2014 #5


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    Homework Helper

    What length and speed scales are you using to define the reynolds number?

    If you use the diameter of the ball and the velocity of the oncoming fluid, then for the same fluid the reynolds number is proportional to the product of the velocity and diameter (with the constant of proportionality being the fluid density divided by the dynamic viscosity). For the run with V = 10.34 and D = 0.03 this reynolds number is indeed of the order of 20,000, whereas for the run with V = 50 and D = 0.4 the reynolds number is of the order of 1,300,000.
  7. Oct 21, 2014 #6
    You should calculate the Reynolds number for each and every case. The drag coefficient is a function of both the Reynolds number and ωV/D.

  8. Oct 21, 2014 #7
    Thanks a lot Chestermiller, this makes sense.

    Thanks for the help!
  9. Oct 13, 2015 #8
    what should i do after finding reynolds number? what is the meaning of re number in this case?
  10. Oct 13, 2015 #9


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    Staff Emeritus
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    Homework Helper

    Well, your friend had a theory about explaining the outlier based on the Reynolds No. of the flow.

    If you didn't calculate the Reynolds No. originally, then how do you know Re ~ 20,000?

    If you can eliminate your friend's theory as an explanation, then you are free to explore a different theory.
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