Flagpole Vortex: Wave Motion Causes Flag Flutter

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Discussion Overview

The discussion explores the relationship between vortex shedding from cylindrical objects and the fluttering motion of flags. Participants examine the role of Karman vortices, aerodynamic instabilities, and fluid dynamics in this context, considering both theoretical and experimental aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that the periodic motion of vortices shed by a cylinder may cause a flag to flutter.
  • Another participant identifies Karman vortices as periodic and speculates that they might influence smaller flags, while larger flags may be affected more by aerodynamic instabilities.
  • A participant questions the persistence of asymmetry in fluid flow around a symmetrical cylinder and asks what shape could eliminate vortex oscillation.
  • There is a query about the relationship between the period of a flag's oscillation and the speed of a car, indicating a potential proportionality.
  • One participant introduces the concept of turbulence and its role in providing initial instability in fluid flow, mentioning the effects on flexible objects like flags.
  • A question is raised about the behavior of liquid helium, which has low viscosity, and whether it would exhibit periodic turbulence around a cylinder.

Areas of Agreement / Disagreement

Participants express varying viewpoints on the factors influencing flag motion, with some suggesting Karman vortices play a role while others emphasize aerodynamic instabilities. The discussion remains unresolved regarding the specific contributions of these factors.

Contextual Notes

Participants mention various fluid dynamics concepts such as the Strouhal number and the conditions under which turbulence occurs, but the discussion does not resolve the implications of these factors on flag motion.

verdigris
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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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