- instead of sticking straight out? I would like to understand it better to apply that to a fluid dynamics issue. Any input appreciated, Jim, Minnesota
how about shedding vortices, off the flag pole ? if the flag was rigid (sheet metal, rather than fabric), it'd oscillate as the vorticies travel over it
I'm no expert on this, but if you blow a steady wind over a flat ocean, then you get waves don't you? Rotating that through 90° about an axis in the direction of the wind, I'd expect the air passing either side of the flag to create ripples in its surface.
Why should a steady wind over a flat sea create waves? Well, the slightest lump on the surface would represent a shape like an aeroplane wing and the lump would get sucked up further, making it larger. So while flatness might theoretically represet an equilibrium position, it'd be unstable equilibrium, and like a ball on the top of a hill, it doesn't want to stay like that.
Now imagine the effect on the flag if the wind were blustery too ...
My guess: A small perturbation in the shape of the flag creates a pressure differential side to side, which deforms the flag more. This curve builds up until you get separation when the whole thing collpases flat and starts all over again.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
Hi Nobog & fellows,
This question lies close to an area of expertise in which I aspire to become a specialist in future and in which I have a theory regarding the nature and cause of Tolmien-Schlichting waves.
First, whilst eddy currents around the flagpole prior to sweeping over the flag's fabric will certainly cause the flag to flap, I think you would discover that even if the flag could be suspended in airflow without the pole; the leading edge of the flag were perfectly steady; and the leading edge were in no way thicker than the fabric of the flag throughout its area; it would still flap, nonetheless.
TS Waves occur in the boundary layer and cause the boundary layer to expand progressively along its length. Yes, there is a relationship with vortices, so consider the phenomenon as having a TS Wave and a Vortical component. Note that the amplitude of flap is greatest at the trailing edge, where it has been amplified by both components and where the fabric has the most freedom to flap. So while the oscillations should be perceived to originate at the leading edge under momentary pressure differentials either side of the fabric and progressively increasing in magnitude, the fabric itself manifests a corresponding wave pattern and it propagates in both directions through the flag, thus reactively influencing the airflow responsible for initiating the wave incidence in the first place.
Eliminating any prior flow disturbance such as that caused by the pole; and even by the fact that the surface of woven fabric itself will set up a complex matrix of small pressure differentials as each thread weaves from one side of the fabric to the other; and also initiates Reynolds flow conditions on different scales through the full spectrum from laminar to turbulent separation; flapping will still occur and the final otherwise inexplicable causality can be attributed to molecular shear in the boundary layer.
Though yet to be conclusively proven and presently not formally regarded as a part of the body of knowledge in this field, I maintain that this molecular shear is responsible for the occurrence of Tolmien-Schlichting Waves in boundary layers. These TS waves WILL make the flag flap in wind, regardless of anything else that may be happening with eddies and vortices.
Try and find THAT one in the textbooks, gentlemen - but I think you'll find it isn't.
Regards, Kerry (Mad Prof).
It's a well known phenomenon the wake vortex behind a cylinder.
So I think the flag flutters is mainly due to the interaction with the pole than anything else.
Gentlemen:
1. With or without the flag pole, the metal sheet would oscillate and the fabric flag would flutter. That is a certainty.
2. Vortices MUST have a cause which relates directly to the presence of the flag in the airflow - forget sheeding vortices off the pole, because although it would certainly cause the flag to flap, it will still flap in total absence of any disturbance to the flow preliminary to the flag, itself; and moreover, it will still flap if the wind were dead steady in both speed and direction - let's call that a perfectly constant vector velocity.
Therefore - something is generating the vortices, either within the flag or within the boundary layer, or both; and without which there'd be none, and the flag would NOT flap, external influences like blustery conditions or a flagpole notwithstanding and likewise, surface imperfections regardless, too.
The flag itself disturbs the airflow, by causing it to slow down as it passes over the flag's surface. The closer to the flag, the slower that airflow travels and conversely, the farther from the flag's surface, the more nearly the flow matches that of the greater mass of air. This zone of flowing air, the velocity of which is equal neither to the flag, nor the greater airflow mass, but ranging through the full spectrum of velocities between that of flag and greater air mass is known as the Boundary Layer and within this layer is where it all happens.
It starts with sound resulting directly from molecular shear within the boundary layer and velocity differentials between adjacent "sub-layers". The waves of that sound propagate, including into and through the length and breadth of the flag. The velocity differentials generate pressure changes which curve the flow and set up the vortices.
Minus flagpole, surface imperfections and changeable wind velocities, a metal sheet suspended magically in the airflow will still vibrate and vortices will still occur - they'll merely be of far lesser amplitude, because the metal permits far less amplitude of oscillation, due to its greater rigidity.
Thanks for the input. I think MadProf has it right "The velocity differentials generate pressure changes which curve the flow and set up the vortices" with that quote. I work for a "major" heart valve manufacturer and I just mention the flag flutter scenario as a means to an end as certain heart valves, either mechanical or tissue, will have their occluder flutter. We are just trying to better understand the phenomena. Jim
personally, i think that's what just about all the replies say when we say "vortex street".
also, i think the analogy of a falg for a heart valve is possibly a bit misleading ... for the flag it is the flag pole that is the dominant cause of the vortices, possibly for a flag supported by a lanyard (ie no flag pole, the leading edge is held is tension by ropes attaching to fixed points above and below the flag) is a closer analogy for your heart valve ... here i think the tension is one of the critical features of the design, that and the stiffness of the membrane.
Yes, Sailoday & IRstuff, an interesting point.
Invariably, whether flying or hanging, the force of gravity is operating. However, my case claims that the flag will flutter even in the absence of gravity, were such a thing to be possible. This was made clear enough already.
Therefore, a more ideal test to verify this would place the flag and the entire envelope of air flowing around it in free fall, so that the effect of such a force is neutralised.
In the end analysis however, you cannot avoid the reality that in the absence of ALL other perturbations, the flag will still slow the air flow, thus causing the creation of the boundary layer. This ALONE is enough to flutter the flag. It then remains to determine in detail precisely what mechanism operating within the boundary layer is responsible. I believe I have also clearly cited that base-level cause.
Either you agree, or you don't.
Regards, Mad Prof.
I'm guessing flags flutter in same way that airplane wings flutter. A self-excited instability in which the structure extracts energy from the air. In nonlinear dynamics terminology, a Hopf bifurcation. The resulting limit cycle which you see is probably due to a combination of nonlinearity due to fluid (the above mentioned vortex street and flow separation along the chord of the wing ie flag) and structure(stretching of the membrane inducing some bending stiffness). Because the stiffness of the flag is so low, the critical flutter speed, ie the critical value of the bifurcation parameter(velocity) is quite low compared to aircraft.
Google papers on axial flow and flutter and you will see work done by myself and others on this topic.
PJA,
A worthy comment. The factors you mention bring the obvious considerations of fluid density, viscosity and the rigidity and elasticity of material the body - in these cases, the flag and wing.
Bringing consideration of a wing into the discussion throws up more dynamical variables, however, because it's an asymmetric entity usually designed to force the creation of pressure differentials in order to generate lift.
An inherent property of this asymmetry is the tendency to create vortices, again through curving the airflow. Per the vortex street phenomenon, oscillations will be triggered in the body at the trailing edge to propagate in waves through the body against the direction of the flow. Then, if the wing is swept, tapered, anhedral or dihedral etc., the effect may become more pronounced if damping is not induced in some way.
Flow condition also influences this very strongly, particularly when it is non-laminar - ie: one of the different separated flow conditions. Pronounced low pressure zones and dramatically accelerated airflows in non-laminar directions get eddies and vortices happening big-time. There's an unavoidable velocity relationship here that hooks straight back into the displacement, density and viscosity (Reynolds) equation.
These considerations influence overall wing design, particularly with respect to vortices and I would draw your attention to that of the very beautiful wing design of the BAC/SUD Concorde.
However, much of this is macro-scale stuff.
I have been speaking of the micro-scale stuff.
One thing we can agree upon is periodicity and amplitude of oscillation within either flag or wing.
Many interesting guesses, but the truth is that this phenomenon is called Von Kármán vortex street.
It´s not tue to the flag, but to the mast.
nveneno and rb1957 are right.
Answering to madprof, the frequency of the oscillations depends on the thickness of the mast. Of course the pmaterial of the flag matters downwater, but not affects the movement (assuming the material is homogeneous and there´s no difference between one side and the other. If it´s not the situation you will not have a Von Karman vortex street, but other kind of vortex street).
You could put an example. If you dont use a mast and instead use a cable there should be no fluttering.
False. The size of the wire (or even the same leading edge of the flag) give thickness enough as to make it flutter.
If you want a demonstration is a very complicated to put it here, but search "Von Karman vortex street" and you will have the answer.
You can see the same phenomenon in rivers with water flowing through columns.
By the way, to madprof, your mistake is to assume that the flag is the creator of the oscillations. The vortex are there even with no flag (assuming there is a mast). Look at the experiences with wind tunnels and cylinders. The vortex are the same as the flag fluctuations.
