See subject.
The weight of the flag is continuously being pulled down by gravity, while the breeze keeps it aloft. Those two forces working against each other give the fluttering or rippling effect.
There may also be minor pockets of air turbulence involved.
Gravity.
Nah, it’s not gravity. A flag held vertically in a vertical breeze would still flutter. The question is why a flag in the breeze is inherently unstable.
Not that I’m doubting you, but how do you know this? Is there a flag hanging somewhere that is subject to only a steady downward breeze (with no obstacles in the way or people walking around it)?
And adding to my first post, I think the ripples in the air currents have a lot to do with it. Especially if there are any buildings or trees around, they will create swirls and little pockets of turbulence that we may not be able to perceive at ground level.
Bernoulli’s principle, maybe? :dubious:
So you put a flag on a treadmill…
I think a hypothetical smooth, uniform flag, subjected to a hypothetical perfectly laminar breeze would not flutter.
In reality, flags aren’t perfectly smooth and uniform and the wind is turbulent - even at a small scale, and especially after being perturbed as it passes by the flagpole. Any small irregularity in the flow of air across the flag displaces the lightweight fabric and in so doing, it causes further disturbance elsewhere - amplifying the effect.
All of the above plus “vortex shedding”
This is a well studied phenomenon in Fluid dynamics and there are multiple simulations available to solve this. It was first addressed in 1879. Here is an old publication discussing this : http://www.ecs.umass.edu/mie/faculty/perot/mie821/Reviews/annurev-fluid-flapping.pdf
If you look around on youtube you can find some simulations too.
Yeah? Well why does the US flag on the moon ripple? :dubious:
Thanks.
the flagpole creates turbulence which affects the flag in the downstream wake.
It doesn’t anymore. It did initially because the astronauts were moving the pole/frame about and that made the material of the flag flap around. After a bit, internal friction would have damped the motion, and, in the absence of further disturbance by time-varying external forces (like wind) it should remain quite still. I think Mythbusters put a flag in a big vacuum chamber to test this out a while back.
-Rick
Well, first I’d say there’s no such thing as a “steady breeze”, the wind always has small but continuous changes in velocity.
But that said, two reasons - boundary layer and the flagpole.
- The aerodynamic effect called the “boundary layer.”
which causes Boundary Layer Separation
The wiki articles are pretty mathy, but basically, airflow along a flat surface (like a wall, or a perfectly stiff flag) will eventually get turbulent - no longer smooth airflow. The flag has this happening on both sides, but since it would almost never happen exactly identically, the flag ripples in the breeze.
- The flagpole is an obstruction to the flow of the air (wind). The air spills around the flagpole and creates small-scale turbulent whirlpools and eddys of air. Thus the flag ripples.
Note that 1) would happen even if you didn’t have a flagpole, so the flag would still ripple.
That’s what they want you to believe.
A flag extended by a breeze is a thin flexible membrane under tension and will therefore act to transmit waves, i.e. ripple, especially if not loaded in a perfectly symmetrical way. There really isn’t a reasonable way for it to be loaded symmetrically by the wind. A non rippling flag extended by a laminar flow of wind would probably not have enough viscous forces to maintain an extended configuration and it will flop down, but it will wrinkle asymmetrically as it drops causing asymmetrical forces and rippling.
Of course a waving flag will interact with the flow and may create vortices, but I wouldn’t say that turbulence and vortices cause the waving. Imagine hold a towel at 2 corners and letting it hang under its own weight in a vacuum. If you lightly shake that towel it will ripple much like a flag waving in the wind, because you would have the thin flexible membrane under tension.
Isn’t that called a transmission wave? When I snap the bullwhip and the force travels over less and less mass–propelling it as the the force barely attenuates on the thinning whip–till past sound speed.
What is the initial “flip” energy? How can that be maintained over a flag?–eg, when you whip up high a sheet to cover your bed, the flip is always difficult because you have to figure in the air that the flip moves, and how it affects the travel of that sheep (the course of its transmission wave–right?).