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Some basic theory about drag coefficients of spheres.

  1. Nov 19, 2009 #1
    We did a lab where we dropped three spheres into two different fluids, measured their velocities, and found drag coefficients. We were also asked some theory questions, and these are what I need help with.
    3.175 mm, 0.05 g Teflon
    6.35 mm, 0.375 g, aluminum
    12.7 mm, 8.36 g, steel
    Water (density 998 kg/m^3)
    Sugar water (density 1230 kg/m^3)
    1 - Why did some of the spheres of low density plastic follow an oscillating path as they fell?
    2 - Explain why some spheres tended to bounce along the sides of the wall as they fell. In particular, explain what drew the spheres into the wall, and pushed them away after contact.
    3 - Explain why a streamlined body can fall at an angle, rather than straight down.

    I would guess that vortex shedding has something to do with the first two, but don't really know. Any help (or links to some sites with relevant theory) would be much appreciated. Thanks.
  2. jcsd
  3. Nov 19, 2009 #2


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    Gold Member

    #1 Think about the pattern of Kármán vortex streets and the forces that would be created on a low density sphere.

    #3 Think about how lift is generated.
  4. Nov 20, 2009 #3
    Thanks, I got pretty good answers for those two now, just needed to prodded in the right direction.
    As an FYI for anyone interested, I found some good info on this wiki page http://en.wikipedia.org/wiki/Vortex-induced_vibration. We were introduced to vortex shedding, but never learned about the lateral forces it can generate (those it makes sense intuitively).

    Anyone have any help for question 2? It was a very pronounced effect, with the heavier balls at least. The steel sphere hit the wall before descending 15 cm, and hit hard enough to produce clicks audible across the room.
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