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Why does a flag ripple/wave?

  1. Dec 4, 2008 #1
    Hey guys,
    My psych teacher claimed that there is no physical reason as to why a flag ripples/waves in the wind. I got to thinking and i really coudln't think of why. Afterall, the wind could be blowing at it at a constant angle and rate for a long period of time but it continues to ripple. Is this really a mystery to physicist? or is it just beyond my current thinking...????

    Also... a carpenter bee supposedly does not have the capabilities of the propulsion it would need to fly, based on its wing size, yet, it flies. This also true or a myth?

    thanks in advance guys.
     
  2. jcsd
  3. Dec 5, 2008 #2

    vanesch

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    I suppose it is somehow similar to the way wind over a water surface generates waves, although I have no idea about the detailled dynamics.
     
  4. Dec 5, 2008 #3

    Dale

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    Your psych teacher is wrong. See this abstract for one example (and look at other manuscripts by the last author, Jun Zhang, for more). The bee is also an urban legend.
     
  5. Dec 5, 2008 #4
    Let's hope your psych teach is better at psych than physics....

    A flag flaps in the breeze for the same reason a boat swings on its anchor....they are both relatively unstable and so is the wind...the wind NEVER blows steadily and uniformly from a pefectly exact direction, there are always, eddies,changes in direction and velocity...assume the wind was steady for a few moments...nearby trees, for example, would still move a bit and still create eddies,etc...and even if they did,with one end tethered and once moving you'd be hard pressed to eliminate ALL movement...
     
  6. Dec 5, 2008 #5

    HallsofIvy

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    I suspect your teacher is pulling your leg and just wanted you to think about this (in which case your teacher was sucessful!). If there were no physical reason for it to wave- but it does wave- would that imply "non-physical" reasons for a physical event?

    Air speed is not uniform. The speed of air on the two sides of the flag, at different points on the flag, is not the same so produces differences in air pressure which causes the ripples.
     
  7. Dec 5, 2008 #6

    Danger

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    Even if the wind could be perfectly uniform (which, as the others have noted, it can't), the flag is always down-wind from the pole. The wind therefore has to split into two streams in order to go around it. That alone would disturb the flow enough to cause flapping.
    True, that applies only to vertical poles.
     
  8. Dec 5, 2008 #7
    Well, even if the wind were not disturbed by the pole when going to the flag, it would still flap.

    Imagine the wind going uniformly into a flag, and displace (curve) the flag to one side. There will be force coming from the wind hitting it on its displaced side.

    Basically, in the wind, the flag exhibits characteristics of harmonic oscillation in the sense that then is curves one way, there is a restoring forces causing it to go the other way.
     
  9. Dec 5, 2008 #8

    Danger

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    But if the wind was perfectly even, and there was no pole or other obstruction, there wouldn't be a displacement to either side.
     
  10. Dec 5, 2008 #9
    The flag is never perfectly alligned with the wind, and the system is much more erratic than the ideal situation.
    My point is that a flapping flag has qualities of harmonic motion (as erratic as it is).
     
  11. Dec 5, 2008 #10

    Danger

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    I understood that, Jeans, and did not mean to demean your explanation. It was merely my intent to show that even under the unattainable ideal conditions, it would still flap. Even if the nonexistent perfect airflow split around the pole evenly, the vibrations that it set up in that pole would cause an imbalance.
    Basically, the damned thing will flap in the presence of an atmosphere that isn't at perfect rest. (That includes the elimination of Brownian motion.)
    Sorry if I seemed to be contradicting you.
     
  12. Dec 5, 2008 #11
    No problem. I completely agree with you. This is just one system that has no possible chance of equilibrium.
     
  13. Dec 5, 2008 #12

    rcgldr

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    The flag flaps because friction and viscosity lead to turbulence, including eddies (like tiny tornadoes) that flow along the surface and cause the pattern. You also have gravity pulling down on the flag, so parts of the flag are trying to "flop" over. The turbulence is related to wind speed, it gets worse as speeds increase. If a flag were suspended from a horiztonal line, and a very slow wind was blowing downwards, there would be little flapping.

    Regarding the flying bee urban legend, although most articles refer to some 1934 book, I also recall a lecture where the professor was explaining that the simplified approach in introductory aerodynamics classes wouldn't explain how a bumble bee could fly, and some reporter twisted that into a newspaper story that scientist prove bumble bees can't fly.

    There are two seperate issues regarding bumble bee and humming bird flight. First neither can glide, so they have to flap wings. For the bumble bee, it's small and speeds are slow, operating in a very low Reynolds number state, so viscosity and turbulent effects are significantly large. The actual flapping motion of insects and humming birds is somewhat complex. Some rely on wing clapping, like butterflies. Others, like humming birds, flap at different angles quickly to hover. Plus the wings need to flex along the wing axis, so the motion produces a net lift force.

    The other issue is how the flapping can be done without huge consumption of energy. For some insects, there is a semi flexible membrane that acts a bit like harmonic spring, reducing the energy losses of flight. For humming birds, the spring like mechanism is in their muscles, the muscles are very elastic (small energy loss from movement).
     
  14. Dec 5, 2008 #13

    epenguin

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    Last edited: Dec 6, 2008
  15. Dec 5, 2008 #14

    Dale

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    Even so, the energy consumption is very large. Many flying insects boost their metabolism by a factor of 100 or more when flying.

    The science behind things like that is much more interesting than the "bees fly because they don't know they can't" nonsense. I don't understand why that myth is so persistent.
     
  16. Dec 6, 2008 #15

    Ranger Mike

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    having spent a lot of time doing tuft tests with yarn and wind tunnels, i have observed that air at speed splits when it hits a round bar (control arm axle half shaft and the like). there i significant low pressure immediately opposite the air impact area. This low pressure area causes drag. The air reattaches itself after hitting the round bar. am off to Germany today so do not have vortex pics showing drag and Coefficent of friction stuff on this lap top computer.
     
  17. Dec 6, 2008 #16

    Danger

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    Safe journey, Mike. Don't play too rough. :biggrin:
     
  18. Dec 7, 2008 #17

    Ranger Mike

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    Thank You, Danger..i got to frankfurt too late for the Discos..they were closed. on plane ride, i was thinking my input on Drag was minimal Ref: the flag thing...but any time you are moving, you are pushing air. and this is what led me to this web site to begin with.
     
  19. Dec 7, 2008 #18
    What is holding up the flag in the first place?
    For a piece of the flag to be held up against gravity, something needs to give it a force upwards. Even if you could reach the limit of a perfectly out-stretched flag, the wind would not be proving an upwards force and so the flag would bend / winkle in order to start falling. The wind can then hit these wrinkles / bends and provide force to keep the flag mostly out-stretched.
     
  20. Dec 7, 2008 #19
    You are correct that the force that the wind acts on the flag is not directly upward. However, remember that the flag is fixed to the pole, so the horizontal force causes a torque that raises the flag.
     
  21. Dec 8, 2008 #20
    I believe the real answer lies in the propagation of a Kármán vortex street behind a cylindrical surface in a fluid flow. Said cylindrical surface is the flagpole. The waving action of the flag has nothing to do with the flag, but the pole itself.

    More information:
    http://en.wikipedia.org/wiki/Von_Kármán_vortex_street
     
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