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Drag coefficient of a sphere (please help!)

  1. Jan 21, 2013 #1
    While writing a physics report, I obtained a data that
    for balls of rough surfaces, there is a higher drag force and thus
    the ball can stay stable at a much smaller angle when put up in an airstream.
    However, while analyzing this result, I found out that the drag coefficient is not always
    bigger for rough spheres : it depends on the reynolds number of the flow.
    I would really like to know whether the flow past a sphere
    (in my experiment, styrofoam balls) is attached flow (Stokes flow) and steady separated flow, separated unsteady flow, separated unsteady flow with a laminar boundary layer at the upstream side, or post-critical separated flow, with a turbulent boundary layer.
    Put simply, what is the reynolds number of the air coming out of an air supply?
    For further information, the air supply used in our lab was SF-9216, PASCO.
  2. jcsd
  3. Jan 26, 2013 #2
    After discussion with an intimate physics professor, we reached contradicting results:
    like in the case of golf balls, the rough surface can make air pockets,
    or when the air meets a certain condition (some sort of Reynolds number boundaries)
    the drag coefficient is bigger for rougher spheres.
    Also, we concluded that the air flow from the supply is turbulent.
    Could you please help us out? Thank you!
  4. Jan 26, 2013 #3


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    Last edited by a moderator: May 6, 2017
  5. Jan 26, 2013 #4


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    The reynolds number is not determined by your air supply. It is based on the size of your object and the velocity of the air around it. Assuming that your supply gives a reasonably smooth flow (not always a guarantee), you can calculate the reynolds number using the equation Re = ρvL/μ, where ρ is the density of the fluid (air, in this case), v is the velocity of the fluid, L is a characteristic dimension of your object (for a sphere, this would be the diameter), and μ is the viscosity of the fluid. Density and viscosity can be calculated or looked up on a table based on the temperature and pressure of the air in your lab, and you should be able to measure your flow velocity directly (or if you do not have that capability, it may be specified in the manual of your air supply).

    Once you know the reynolds number, then you will have a better idea of what kind of flow conditions your sphere will have around it, and that will determine the effect that dimples will have.

    (Wiki even has a fairly nice image showing the relevant flow regimes: http://upload.wikimedia.org/wikipedia/commons/3/3f/Reynolds_behaviors.png)
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