Why does wind, even a uniform wind, make flags flap? What determines the frequency of the flapping?

Answer 1

Wow! This is an amazing question. Almost everyone takes this sort of thing for granted, without a moments thought. Myself included.

The flaping is caused by fluid dynamics. (a clever-dick way of saying moving air in this case, though it could be moving liquids too)

The air will never hit the flag exactly edge on when the wind first starts to blow. As the flag rises, it will have the air hitting one side more than the other. That pushes the flag one way, then it gets pushed back again, and so forth. This is the easy part.

More complex is the fluid dynamic part akin to how a plane wing works. When the flag is too far left, the air passes over the right side faster (fluid dynamics) generating a lift force to the right . This also makes the flag move to the right, not just the air pushing it. Again, this is a self reversing process. Flap, flap, flap, flap, etc...

As for the frequency...This is beyond me. I know that wind speed and weight of the flag are important but the fabric the flag is made from and how flexible it is will be important.

A flag can be made from rigid material (an aluminium sheet, say). This too would flap in the wind but I assume has far more predictable properties than a flag, (determined by mass and wind speed) hence their use in weather vanes, (which wobble also)

Answer 2

I think its becuase of the width of the flag - the wind hits the side nearest the pole first, then travels along it so it opens the flag as it goes causing it to flap. I also think its impossible to have a uniform wind for speed and direction.

Again, I would imagine the fluctuations of the wind, and the flag stiffness would affect the flap.

All a bit of educated guess work, but nothing scientific!

Answer 3

I think the flagpole sets up vortices, just as any round surface moving in air does, and the flag conforms to these vortices, which expand and degrade as they move away from their point of origin. The vortices are probably produced in the normal staggered manner, alternating vortex direction and side of the flagpole. Thus when they get to the end of the flag, the end has to quickly conform to one clockwise vortex on one side of it, then a counter-clockwise vortex on the other side, etc., which would produce the flapping.

I also suspect that the flag itself helps maintain the stability of the turbulence by acting as a barrier between successive vortices. Without the flag, the vortices would probably "mix" and degenerate faster.

The frequency of the flapping would likely be determined by the frequency of vortex formation, which should mostly be a function of wind speed, somewhat modified by the flagpole profile.

I'd love to see an experiment comparing the flapping of flags hung from flagpoles with different profiles.

Answer 4

The principles of the flapping flight of all flying creatures realized either by birds or by insects are just the same. THE FLAPPING WING HAS AN ELASTICITY BOTH ALONG THE SPAN AND THE CHORD. The elasticity along the span conduces to the smoothness and high effectiveness of the flapping flight. The adaptive elasticity of the wing along the chord make it possible to achieve a maximum traction in a wide range of frequencies and amplitudes of the flaps. THE TRACTION IS CREATED BY THE OUTER PART OF THE WING WHILE THE INNER ONE GENERATES THE MAIN PART OF THE LIFT. When the inner part of the wing is moving upward the outer part lags and bends downward in relation to the inner part because of the air drag and inertial forces. In this way the drag of the wing as a whole diminishes. When the inner part is nearing the top dead center the elastic forces raising the outer part upward and latter continues to create traction while the reaction aids the inner part and the wing as a whole to pass across the top dead center and to begin moving downward.THE WING ELASTICITY ALONG THE CHORD IS SUCH THAT THE WING PRACTICALY DO NOT BEND IN GLIDING-EQUILIBRIUM POSITION.
The direction of the wing flapping motions is not perpendicular to the flight direction but with declination backwards which decrease by increasing of the flight velocity.

So therefore a combined dimensionless parameter from the wing geometry (length L, thickness \delta and chord c) and flapping amplitude, \alpha, uniquely determines the forward flight speed.

Answer 5

flags are made with a light materialand even the smallest of wind will make a flag flap

Answer 6

When the wind blows the flag, it is parallel to the flag's length.
As the flag is fixed to the pole, the wind is curving round both sides of the pole causing the waving motion due to Bernoulli's Principle
First one side then the other gets a change in velocity due to the pole and the flag's motion and you have a waving flag.
The stronger the wind, the faster the waving.

Answer 7

Because the notion of "uniform wind" across the entire surface area of the flag, over any length of time is not reality.