Why Do Flags Flutter: Explained Technically

[jonbonraki]

could anybody be technical enough to explain why do flags flutter

 

[brewnog]

Super discussion here. It was the first google result for "flag flutter".

http://www.eng-tips.com/viewthread.cfm?qid=180705&page=1

 

[boneh3ad]

Don't read that thread unless you want the wrong answer.

The real answer is a fairly complicated aeroelasticity and stability problem.

 

[brewnog]

There's no conclusion in the quoted thread to disagree with, just the level of discussion which you wouldn't get here on the matter.

 

[boneh3ad]

The discussion mostly consists of MadProf spouting nonsense while the people who are on the right track get drowned out. MadProf clearly has a good breadth of general fluid dynamics topics but he is lacking in the depth department, as he applies a lot of it poorly.

It doesn't have to do with von Kármán shedding or the Kelvin-Helmholtz instability or Tollmien-Schlichting waves. It is a fairly poorly understood phenomenon that is more complicated than all of the answers given there.

 

[minger]

I wouldn't say that it's a poorly understood phenomenon.

The flag flutters because there's a time-varying, asymmetric load on the sides.

Now, what causes the load, I would agree that it's a relatively complicated aeroelastic issue. Is it solvable? Absolutely. I think it would be interesting to see, at the very least, a 2D analysis of an infinitely thin surface of low stiffness in streamwise flow.

Anyone got time to run this?

 

[Studiot]

Interesting discussion in referred thread with some good points made, even by Madprof.

That discussion does, however, largely ignore the characteristics of the flag itself and attribute fluttering solely to the characteristics of the medium itslef.

Flag material is very rough insofar as surface variations are large in comparison to its thickness.
All the discussion has been limited to 'large' flags. Small flags were once used in wind tunnels to indicate stream/streaklines. Do these flutter?

 

[boneh3ad]

There is a threshold length where flags will flutter if they are larger and just follow the streamlines if they are smaller. A quick google search can get you the paper I got that from.

 

[Studiot]

Woven materials are also permeable and cannot long support pressure differences across (through) them.

Does this make a difference?

 

[boneh3ad]

Just to cite my previous post, the paper that I was referring to is here:
http://www.seas.harvard.edu/softmat/downloads/2005-01.pdf

As for permeability, I doubt that would have much, if any effect. The pressure gradient from one side to the other would be changing so fast (as evident by the rate of flapping) that I doubt much mass would have the time to pass through a flag. I could be wrong, but that is what my intuition tells me.

At any rate, von Kármán (http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_vortex_street" ) but it is actually a fundamentally different physical situation.

Read the paper if you want a really in-depth answer; it goes into far greater detail on the aeroelasticity problem than I ever could. Ask me about the nature of T-S waves or something, and I can help you, but aeroelasticity is something I have never touched.

 

[Studiot]

Thank you for the links, they will require careful consideration.

I note that whilst the Stanford article introduces some properties of the flag and its material, it concentrates on fluid dynamics. Regardless of the East European named vortices, it reduces the flag fluttering to a traditional forced oscillation which can be stiffness (compliance), mass or resistance controlled.

 

[boneh3ad]

The paper isn't 100% complete since it is a complicated problem, but they do a good job of modeling it linearly. Then again, they make a bunch of simplifications to make the problem solvable, but that is the name of the game in engineering.

The references it cites would also be good places to look.