Flagpole Vortex: Wave Motion Causes Flag Flutter

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The discussion explores the relationship between vortex shedding from a cylinder and the fluttering motion of flags on poles, suggesting that Karman vortices may contribute to this effect. It posits that while smaller flags are influenced by these periodic vortices, larger flags are likely affected more by aerodynamic instabilities inherent to their structure. The conversation also touches on the persistence of asymmetry in fluid flow around symmetrical objects and questions how altering the shape of the cylinder could eliminate vortex oscillation. Additionally, it considers whether the oscillation period of a flag is proportional to the speed of a car, and mentions the role of turbulence in creating instability in flexible objects like flags. The discussion concludes by speculating on the behavior of low-viscosity fluids, such as liquid helium, in relation to periodic turbulence around a cylinder.
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I was looking at this animation of vortex being shed by a cylinder http://ctr.stanford.edu/gallery/cyl3900.html
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?
 
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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.
 
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?
 
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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.
 
Would liquid helium, which has very low viscosity, flow with periodic turbulence around a cylinder?
 

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