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Why do flags flutter?

Why do flags flutter?

Why do flags flutter?

(OP)
- 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

RE: Why do flags flutter?

I forget what the answer is - but a friend of mine had this question on a Fluid Mechanics test . . .

RE: Why do flags flutter?

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

RE: Why do flags flutter?

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 ...

RE: Why do flags flutter?

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.

RE: Why do flags flutter?

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).

RE: Why do flags flutter?


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.

Take a look at this picture
http://www.eng.fsu.edu/~shih/succeed/cylinder/cylinder.htm
http://www.fluent.com/about/news/newsletters/05v14i3/img/a21_1lg.jpg

Veneno

RE: Why do flags flutter?

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.

Thoughts, gentlemen?

RE: Why do flags flutter?

(OP)
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

RE: Why do flags flutter?

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.

RE: Why do flags flutter?

Won't gravity cause part of the flag to droop slightly to one side thus causing flow to flag to be non-symetric?
Regards

RE: Why do flags flutter?

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.

RE: Why do flags flutter?

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.


RE: Why do flags flutter?

maybe flags flutter 'cause if they didn't they'd look boring ?

RE: Why do flags flutter?

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.

RE: Why do flags flutter?

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.

RE: Why do flags flutter?

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.

RE: Why do flags flutter?

So you are claiming that an infinitely thin flag with no pole (just a tensioned forward edge)  would not flutter?

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Why do flags flutter?

I´m an engineer. I´ve never seen that.

RE: Why do flags flutter?

think maybe about the fore-sail on a boat, with the boat head-to-wind ...

RE: Why do flags flutter?

Would like to apologize for the bad temper of the answer before.
Of course the material matters. Air has viscosity, and any surface will produce a vortex. The question is that regardless of the material this vortex is created within the leading edge of the flag. If you had a wooden mast and a flag made of a material that couldn´t generate any vortex you would have fluttering.
The opposite case, a flag with the leading edge extremely thin and with no rugosity, and a normal flag attached to it would probably produce two different vortex streets not related at all. Downwater you would have different conditions depending on the material, size, etc. and may you have another VKVS generated by the flag, but the reason why flags flutter is because of the mast.
You can see this in the period of the oscillations of the flag with a thick and a thin mast and the same fabric for the flag.

RE: Why do flags flutter?

i thought you were questioning greg's proposition of a flag without a mast

RE: Why do flags flutter?

The point I was making was that there may be more than one mechanism that generates flutter. A Karman vortex street generated by the flagpole is certainly a possibility, but the mechanism proposed earlier, where psoitive feedback causes a ripple, needs no flagpole. I'm not saying either is right, I suspect both are contributors.

 

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Why do flags flutter?

Hi Greg, Saskimoto, Nobog, Rb1957;
Some responses to offer:
1. Greg is correct. There are many mechanisms and they operate on a variety of scales.
2. Saskimoto's vortex street operates on a macro scale, but I hold it to be a bit superficial. It's a mechanism that can cause the flag to flutter, but Vortex streets and indeed vortices themselves don't merely exist. They are caused.
3. Obviously, the presence of a body in a fluid mass flow will do this.
4. Its bulk is just as obviously a cause, if it possesses bulk, because the fluid mass is displaced and this remains true even at very low RE numbers in purely laminar flow conditions.
5. But when it doesn't have bulk, vortices are still produced. The flag could have absolutely no thickness whatsoever, yet it would still flutter. If its leading edge did NOT oscillate, it would still flutter.
6. Eliminate every large scale cause from the equation and the flag would still flutter. Eliminate the mast; eliminate the leading edge thickness; eliminate whatever you like. It hardly matters.
7. When you have an entity in a fluid mass flow, the fluid's viscosity plays a role. There is adhesion to the body. At the skin of the body, the velocity of the fluid is virtually that of the body and progressively approaches that of the fluid mass flow with increasing distance from the body's surface until, at a given distance, it matches that of the greater volume of the fluid mass. Between these two "altitudes", the velocity of the fluid mass is somewhere in-between that of the body and that of the greater fluid mass. This zone is the Boundary Layer and within it, the relative velocity of the fluid mass obeys the inverse square law as a function of the distance from the body's skin.
8. The very presence of the body in a fluid mass flow MUST change the velocity of the fluid close to itself and it DOES. Two mechanisms operate here.
9. The first mechanism is Bernoulli Effect. Change in velocity causes change in pressure perpendicular to the flow. That causes curvature of the flow and cannot fail to set up oscillations and vortices.
10. The second mechanism is on the micro scale. Tolmien-Schlichting Waves are generated in the boundary layer and are responsible for boundary layer growth. In an expanded boundary layer, vortices WILL occur and increase in magnitude downstream - which they DO. Although there is no officially recognised cause known for these TS Waves, I can tell you all with authority that THE CAUSE IS MOLECULAR SHEAR, according to my own choice of terminology. I claim the right to that terminological liberty, because I claim its discovery and as a result, the body of knowledge in this field has been EXPANDED.
11. Vortex streets are small potatoes. They're the product of other causalities.
Cheers, everyone!

RE: Why do flags flutter?

Madprof.
I´m talking about the flag fluttering, not the nature of turbulences (I thought this was the question).
On the other side.
If you want to demonstrate that viscosity is the responsible for turbulences I think that you are late (unless you are Prandtl or Glauert by night).
Congrats for the discovery. Please, tell us when you get the Nobel. We´ll all be there with flags and no masts fluttering in the wind. :DDD

Cheers.

RE: Why do flags flutter?

Hi Saskimoto,
The natures of turbulences are underpinned by causes. Turbulences cause the flag to flutter, obviously enough. Ipso facto, whatever causes turbulence also causes the flag to flutter. Merely a different depth of analysis applied to the phenomenon.
Prandtl's and Glauert's work should hardly need to be re-demonstrated, though it surprised me a little that something so critical to the whole question's body of answers would be in any way refuted in favour of the vortex street phenomenon: a causality factor, certainly - but itself a secondary cause-and-effect-combined at best, which itself has more basic and fundamental causalities, that actually take place at molecular level within the boundary layer.
Collectively, have we not all been aiming after the nitty-gritty causalities of flag flutter?
The discovery of Molecular Shear might hardly be enough to win a PhD - far less a Nobel Prize, but thanks for the vote of confidence. In a limited range of applications which unfortunately cannot include flags, perhaps I may cut-and-paste your confidence vote to my work on attenuating Tolmien-Schlichting Waves in the boundary layer? Realising that TS Waves cannot be eliminated because they're intrinsic to the boundary layer phenomenon, I nevertheless have something that will reduce their effect quite considerably - more so, I believe, than 3M's Micro Riblet contact sheet. Micro Riblet is known to reduce drag by about 7% through direct passive containment of vortex growth. My concept achieves something very similar, but through the active elimination of progressive wave amplification, instead. It's a different approach, tackling the phenomenon at a different level by different means. The two are not incompatible, either. They can be used in conjunction with each other, possibly yielding overall drag reductions of up to 20%, optimistically.
In the meantime, I've been studying an altogether different set of interrelated phenomena for almost four years now - those of Gravity, Unified Field, Over-Unity, Scalar Energy, the Zero Point Field and Spacetime. This links with the work of Nikola Tesla, James Clerk Maxwell, J.J. Searle, Thomas Townsend-Brown, Werner Von Braun, Faraday, Beifeld, Poynting and Einstein.
As was the case with TS Waves, in-depth study led to invention/design and also more recently to development of my own theory concerning these things - Gravity and its direct causality in particular. I believe I have the answer to the cause of Gravity at the level of the relationship between Baryons and Leptons, though this is presently unproven.
The theory is in its early stages of evolution presently, but looks good and is now progressing more rapidly than I can test experimentally. If the theory proves correct and the prototype I will commence constructing early next year tests successfully, both the PhD and the Nobel will be in the bag, plus a lot more. There should be far-reaching implications in the Quantum and Cosmic realms, plus much more. And one should not ignore the possibility that the speed of gravity may actually be the speed of light squared.
Furthermore, IF I am correct in any reasonable measure and the prototype works as expected, Mankind will have its "Next Wheel". A primitive one, but a Gravitron, nevertheless.
Now, this is entirely outside the scope of aerodynamic engineering.
So is the question, "What causes Gravity?" though I'd welcome attempts to answer it other than, "Oh, come on! Everybody KNOWS that Mass causes Gravity - right?"
Because IF I am right - then "everybody" is very wrong.
Cheers, all!

RE: Why do flags flutter?

I think one of the the basic questions here is whether or not the oscillation of the flag is fundamentally a forced oscillation or self-excited instability. If it is the former than the flag should be fluttering with a frequency near one of the fluid frequencies. For example if it is due to vortex streets generated from the flag pole then the frequency of oscillation should be near that of the shedding frequency of the vortex street and would be a function of the geometry of the pole. If it is due to a self-excited instability, as in a true flutter problem, then the oscillation of the flag would be near that of the first couple modes of the flag, ie quite low.

These things could probably be checked fairly simply by going out and measuring the frequency of oscillation, approximating the wind speed and measuring the geometry of the pole.

I have a paper due on Thurs though so I don't have time now... : )

RE: Why do flags flutter?

Oh, just one other thing regarding vortices shed by a mast. Yes, they'll determine periodicity of the dominant flutterings in the flag, but conditional upon the Reynolds Number (flow condition) operating about the mast. The biggest amplitudes are produced at relatively low RE numbers in the rough order of 500, where the flow is "Turbulent Laminar". The vortices are big, rhythmical and geometric, while the point of flow stagnation on the mast's leading surface correspondingly cycles from left to right.
When the RE < 50 and the flow condition is "Laminar", the flag must generate most of its own vortices and flutter, because the mast will not shed any significant turbulence at all.
When the flow condition is in the other extreme - say RE > 1,000,000 and "Turbulent Separation" is occurring about the mast, the amplitude and periodicity of ripples in the flag are very different: being very much less and very much shorter. They're also far more complex, reflecting the flag's presence in the mast's totally chaotic turbulent wake.
It's interesting that the flag operates as a very good visual downstream indicator of the Reynolds flow condition about the mast.
But there is another consideration. The flag is a second entity in the same stream, but may well behave as though it were very much a contiguous part of the mast in some respects. In cases of "Laminar Separation" and "Turbulent Separation", the flag may be either partially or fully in the shredded wake of the mast. It is possible that at some wind velocities, "Turbulent Reattachment" of the mast's shed boundary layer may occur at the flag's surface. It would again show such a thing visually with very small ripples of short periodicity upstream, but with larger ripples of longer periodicity in the trailing part of its length.

RE: Why do flags flutter?

Thanks PJA!
Good observation!

RE: Why do flags flutter?

Three points, in decreasing order of banality
   
   1) The flag starts off drooped, ie in a somewhat chaotic geometry. It seems to me that expecting something that starts off with an ill defined geometry is unlikely to attain a nice rectangular shape under the influence of an unstable set of forces.
   
   2) Real world winds vary in direction and strength, contnuously. Around here I'd guess that the variation in strength is around 30% over the course of 5 minutes, and the wind direction changes by 20 or 30 degrees in the same timeframe.
   
   3) If the flag is limp then the only way that forces can be transmitted are in the local plane of the material, ie membrane forces with no bending. A flag that is streaming perfectly is not in stable equilibrium, as soon as the free upper corner moves out of plane it will start to descend. As it does so a pleat forms, which will create lift. This tends to move the material back up into plane, but will inevitably do so a bit too much, in an underdamped system. So as the upper free corner returns to position it will have an upward velocity. This will then create tension pleats in the material, which create a downward lift.  And so it goes.
   
   If we put the flagpole horizontally it is possible for the flag to assume a somewhat stable shape. Do you think the free corners will point up - because they locally develop more lift than local weight, or down, because they don't generate enough lift, or horizontal? Will they flutter around a stable position, or just sit there, or does this stable shape never form in practice? Obviously it can if the material is sufficiently stiff.
   
   So, my prediction is that the flutter pattern will be dependent on g, and Et^4, as well as all the usual suspects.

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Why do flags flutter?

Yes, in the real system where there are so many variables and different mechanisms operating, the influence of gravity as Greg indicates cannot be ignored and neither can the vortex street per RB1957 & Saskimoto be dismissed.
And streaming the flag in the horizontal plane should produce interesting behaviour to compare with the vertical. In either plane, transverse and diagonal ripples in the flag are certainly another consideration.
Something has just occurred to me here, where the corners of the flag at the trailing edge are concerned. I haven't examined it in detail yet, but I'll resort to geometry for a moment in pointing this out.
1. Mark a midway point between top and bottom of the flag on its leading edge.
2. Use that point as the origin of a series of concentric circular arcs described across the fabric of the flag.
3. Make the radial increments small enough to isolate each of the corners in its own arc radius.
4. Now do the same, using the top and bottom corners of the flag's leading edge as the origins of two similar sets of arcs, increasing by the same increments.

Do they tell us anything about the role of the trailing edge corners? I think they might.

RE: Why do flags flutter?

Oh, and apologies to all for throwing in a change-of-subject and non-related question.
Greg's pointing out 731-376 is noted.
It wasn't intended to corrupt the thread, but purely in response to the Nobel Prize reference. Whether I'll put the question to the forum in a different category remains undecided, yet.

RE: Why do flags flutter?

If I can take us back to the vortex street phenomenon one more time, there is something to remark concerning the alternating nature of the vortices that would shed from a mast at an RE of about 500.
Assuming dead-steady wind of a velocity that would create the particular flow condition called "Turbulent Laminar", what actually causes the asymmetry in flow that sets up the vortical pattern?
Obviously, some condition within the boundary layer must generate that - otherwise the flow either side of the mast would simply meet downstream of it with equal pressure and cancel, exactly per the "Laminar" flow condition. Presto! No vortices shed by the mast!
But this is Turbulent Laminar flow, brought on by a higher wind velocity than would cause the purely Laminar condition, so what is the mechanism operating that cycles the downstream flow into a vortex street in this particular way?
I think that it only takes a very small asynchronous perturbation in the flow to trigger it and the flow volume/velocity/density equation in turn supplies the amplification and determines the periodicity (in its relationship to the thickness of the mast and consequent measure of flow displacement) in a naturally elastic system.
I strongly suspect that Molecular Shear around the mast may be responsible for that asymmetry at grass roots level - in other words, the TS Wave phenomenon.
Also, there would be comparatively little tension in the fabric of the flag, since the vortices impress a very slowly moving boundary layer upon the fabric, due to "upstream" direction of helical flow at the periphery of each vortex where it is in contact with the flag, as opposed to the downstream flow direction farthest from the flag on the opposite side of each vortex.
I think of it as a kind of "false boundary layer condition" imposed upon the flag by the mast, transmitted by the vortices it sheds.
Therefore, in the presence of the mast, the flag's behaviour would be rather languid compared with its behaviour in an air mass flow of the same velocity without the mast being there to disturb the flow from upstream.
And molecular shear at the flag's surface itself would play almost no significant part, since it is highly dependent upon flow velocity to produce the laminar tearing effect in sufficient strength. About the mast itself, this tearing effect would be significantly stronger.
Another thing of interest is the manner in which the vortices dissipate themselves in the downstream wake.

RE: Why do flags flutter?

I overlooked one other thing to mention where the vortex street is concerned.
There is a flow condition of the Turbulent Laminar kind where the vortices do not alternate, but occur in pairs downstream from the mast. The RE number applicable is higher than that of a purely Laminar flow, but lower than that of the Turbulent Laminar with alternating vortices.
There is enough low pressure behind the mast to curve the flow sufficiently to set up the vortices by drawing flow from either side into an upstream direction to fill the void created by the mast, but not enough to force the vortices to alternate. Several factors appear to contribute to this condition: velocity change and consequent pressure variation is sufficient to curve the flow enough, but velocity is insufficient to generate vortices of a larger diameter - ones big enough to interfere directly with one another in the mast's wake. These vortices can be seen to be of a diameter of less than half the mast's thickness, where the alternating vortices are about twice that diameter, approaching the full thickness of the mast in diameter.
Asymmetry, on the assumption of a TS Wave origin is not forced, presumably due to lack of molecular shear in that relatively low airflow velocity range. So there is insufficient disturbing energy to vary the pressure differential from one side to the other and so set the vortices to alternating with each other.
And the flag's behaviour in this stream?
Very, very languid, I would say; and the huge vortex street ripples of the alternating street would have to be altogether absent.

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