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Flagpole vortex?

  1. Jan 28, 2007 #1
    I was looking at this animation of vortex being shed by a cylinder http://ctr.stanford.edu/gallery/cyl3900.html [Broken]
    and was wondering if the side to side periodic motion of the vortex is what causes a flag to fly in a wavy way on a pole?
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Jan 28, 2007 #2


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    That looks like Karman vorticies. If they are then they are very much periodic. I think they would have something to do with smaller flags, but with a larger flag, I would guess that aerodynamic instabilities from the flag itself play the major role. That's just a guess though.
  4. Jan 28, 2007 #3
    The oscillation of the vortex from side to side is counterintuitive because
    the cylinder is symmetrical.So some initial asymmetry in the fluid flow persists throughout time. What shape could the cylinder be changed into to stop the oscillation of the vortex? Are there any fluids for which the asymmetry would disappear for a symmetrical cylinder? I often see people with flags flying on their cars.Is the period of the side to side oscillation of the flag proportional to the speed of the car?
    Last edited: Jan 28, 2007
  5. Jan 29, 2007 #4


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    Google for "Karman vortex streets", and "Strouhal number" (thats a nondimensional fluid dynamics parameter, like Reynolds number, etc).

    Turbulence in the flow is enough to provide the initial instability, so any fluid with turbulent flow and fairly low viscosity can show the effect - including water flow in rivers, round fixed objects like bridges etc.

    For a flexible object like a flag, there is another source of instability which is similar to the reason why a fire hose "whips" about if it has a high velocity flowing of fluid through it and it isn't restrained properly. If waves are trying to go upstream at the same speed as the fluid velocity downstream, the waves can form a stationary pattern which builds up in amplitude. This wave velocity can be much lower than the speed of sound, because the air is pushing the extra mass of the flag around.
  6. Jan 29, 2007 #5
    Would liquid helium, which has very low viscosity, flow with periodic turbulence around a cylinder?
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