Is circulation real?

Fish or cut bait. Prove me wrong.

 

I assume compressible above is a typo.

From my limited reading on the subject the term incompressible in this context is shorthand for the effect of compression below Mach .3 is small enough to be ignored. I saw it described as a rule of thumb.

 

Damn! yes, will edit and fix.

 

I'm afraid that this statement is just a correct restatement of the problem.
It is "argument by assertion" and doesn't progress the argument
My argument is that at any speed, from zero to just short of light speed, air is a gas and is compressible according to Boyle's law: PV = Const.
Assuming it is incompressible below M 0.3 may simplify the maths in fluid dynamics, but doesn't explain, for example, the source of the low pressure to leeward of a sail.

 

Assuming the gas follows Boyle's Law is equivalent to assuming the flow is isothermal (constant temperature). In reality outside of the boundary layer the flow will be much closer to isentropic (constant entropy). For confirmation consult graduate level aerodynamic text books.

 

well... I feel a little sad... reading this again ... Man....we are at post #395, and none of the different explanations that you have been given in the thread do not give you the source of the low pressure to leeward of a sail ? You know what ? At this point, just pretend it's magic. It's as good as everything that has been told.

 

Yes, indeed, and I think we are approaching the same page.
Whether it's isothermal or isentropic, it's still compressible, not incompressible isn't it?

 

I hope that's just another poor attempt at a joke.

 

I think you offered Circulation Theory as the source of the speed differences.
By now, I'm pretty conversant on the subject and challenge you to provide a published reference that supports your statement.
All the ones I have read have "circulation" travelling upstream on the lower surface, but this circulation is not the air speed.

 

Just to keep this thread ticking over, I thought I'd offer this little gem.
The link to the paper that started this thread is now dead, but I have tracked down a copy here
I realised author is clearly a nutter on Page 3 when introducing us to vortices, he offers:
"Electrons are said to possess spin, contributing vorticity to atoms and molecules. These in turn form part of larger vortices such as ocean and atmospheric currents."
I then identified him as a director of a pump manufacturing (now defunct) business located in Queensland, Australia. Whilst he titles himself as Dr. Day, I suspect it's more to do with his qualifications as an osteopath and acupuncturist than it is to do with the hard world of science.

However, before proceeding with his bizarre explanation about how Circulation generates lift through a form of "pressure impulse" that is transmitted by diffusion, he does make some very prescient observations on Page 3:

I'm not the first to suggest that there's no acceleration of airflow over the wing and Bernoulli's theory has nothing to do with aerodynamic lift.

 

While (for some reason) you may never accept the validity of Bernoulli's equation, perhaps you'll have any easier time with continuity (The mass flowing between adjacent streamlines stays constant.)

If so, then you have to agree that the bunching up of streamlines on the upper surface just aft of the leading edge shows larger velocity and the spreading out of streamlines on the lower surface shows lower velocities.


I'm anxious to see if you will try to claim that compressibility of the flow invalidates my conclusions or that smoke trails don't count as experimental evidence.

 

[I engineered a way to ban myself from posting to sailing forums a few years ago as it was taking up too much time and getting in the way of work that I should have been doing, but I'm going to make an exception here which I don't want to follow up with any additions.]

Thanks Doug for that picture and comment about the mass staying constant between the lines. I've been trying to work out for years why it is that the lowest pressure over the wing migrates from the back to the front edge, but I think I can see why now. It's always been clear to me that when you move the wing forwards you will create a space (vacuum) behind the top surface with lower pressure, but what's always bothered me is that it looked as if the lowest pressure should be nearer the back than the front. Initially it must be there before the flow establishes itself, but what is it that drives it forward, and why doesn't the higher pressure air above the wing just push straight down into the space to fill it before the lower pressure air coming from the front of the wing gets a chance to do so?

Vacuums do not suck, of course; things are pushed into them, and the strongest push is coming from under the wing with the biggest pile-up under the front. That's clearly squeezing the air above and squirting it through over the wing towards the space that's been opened at the back - the place that's most free for it to be force itself into. The effective pressure of this moving stream of air is different in different directions due to its speed of movement aft, so even though it's at low pressure, it presses in that direction with greater force, and that's how it outguns the air being pushed down from over the wing in pushing into that space over the back half of the wing. Sometimes the air pushing down does actually win the fight and the wing stalls, but even when it loses, it's still pressing downwards into the stream from the front and helping to raise the pressure over the back of the wing. Circulation is not a mechanism, but a mere consequence of all this.

 

It's true that you can describe the flow over an airfoil without mentioning circulation. But if your description is capable of defining the velocity distribution, then you can go on and compute both the lift and the circulation, and you will find that they are proportional and that you get no lift unless you have circulation. You might not be convinced that circulation is the cause of lift, but at least you would see that they are correlated.

Some of the various theoretical methods of computing flows over airfoils start by computing flows without circulation and they invariably get zero lift. (I'm referring to the total lift. If multiple bodies are present in the flow, the individual ones can be lifting upwards or downwards, but their sum is always zero.) When a vortex (or vortex distribution) is added to the flow model, the overall lift then appears. In this sense, at least, you can say that including the circulation caused the lift.

A similar thing occurs in the physical situation. The original flow has no circulation (and no lift), but in passing over the airfoil, it acquires both. What you have been describing are your ideas of how that happens. Personally, I'm happy with the conventional story found in all the textbooks, involving the starting vortex.

 

Hi Doug, Since we have been locking horns over the past months and years, I feel that I have come to know you as an individual with whom I would like to be able to engage in a productive exchange of ideas.
I understand that from your education and professional career you have far more experience and learning than me in the science and engineering of fluid dynamics.

From following your posts on other topics, I learn that you are not only a keen sailor, but have the enthusiasm, curiosity and ability to push the boundaries, to explore the practical application of foils to achieve higher sailing performance.

From your exchanges with me, I am aware that you have an extensive technical library and are willing and able to engage in a technical discussion. I am also aware that in your opinion my views verge on heresy , and you are frustrated (and grumpy) that I continue to make claims that in your opinion are baseless and nonsense.

The fact that I posted this recent post indicates that I am very keen to continue the conversation, and I am delighted that you have responded to it.

So, let me please clear up a few misunderstandings.
It is not true that I don't accept the validity of Bernoulli's equation. Of course I do, it has been scrutinised by scientists, mathematicians and engineers for hundreds of years.
What I don't accept is that it applies to the air flow over a sail of a yacht or the wing of an aircraft.
And whilst I believe there are multiple reasons for that, the one that I think is most revealing is its requirement for an increase in the speed of the fluid to generate a low pressure.
As I said, this is one of the reasons, but from a purely logical point of view, if a theory fails on a single reason, then the theory fails.
It is for this reason that I have pursued this line of argument and why the issue of whether the air speeds up over the top of the foil is so important to me.
And yes, I am not going to forget that despite considerable effort, you have been unable to provide experimental evidence of an increase in air velocity over a lifting foil.

In this latest post #401 you argue that the bunching up of the streamlines shows a larger velocity.
Given that this is a still, black&white photo, with no indication of speed at all, I just don't know how you can make that claim. There is nothing in the photo to support any assertion about speed or velocity.
And no, I don't claim that smoke trails don't count as evidence.
Indeed, I believe that not only do smoke trails count as evidence, but, in Babinsky's video, they provide indisputable evidence that not only is there no velocity increase, but in fact the air over the top of the foil actually slows down.

I have been fortunate to be able to engage Prof. Babinsky in a spirited debate on this issue and he has been unable or unwilling to resolve the apparent paradox.
[EDIT] I am thinking of publishing that conversation but am hesitant about professional privacy. Do you think I should? At no stage in the email exchange did he or I make any statements about privacy or confidentiality. It makes fascinating reading! [/EDIT]

I know that fluid dynamics is a powerful tool for hydrodynamics, and that in many arenas it provides a powerful tool for studying airflow, but it has failed to provide us as racing sailors with a framework for understanding the how's and why's of sail trim, rig trim and even some manoeuvres (the lee-bow for example).

What I am trying to do is to free us from the shackles of fluid dynamics and aerodynamics in order to re-think the way we analyse and talk about the way we operate racing sailboats.

I honestly hope that I can encourage you to remove your cloak of fluid dynamics and look at my argument with a fresh set of eyes.

 

And if I can quote Anderson J.D on Page 284 of "Fundamentals of Aerodynamics" 6th Edition, having spent 200-odd pages discussing circulation, he says:
"The circulation theory of lift is an alternative way of thinking about the generation of lift on an aero- dynamic body. Keep in mind that the true physical sources of aerodynamic force on a body are the pressure and shear stress distributions exerted on the surface of the body, as explained in Section 1.5."
Section 1.5 contains no explanation for the source of the pressure differences that give rise to the aerodynamic force.
And this is the current required reading for the undergraduate study of aerodynamics at Sydney University. No wonder they are all confused!
[EDIT] see my annotated extract from Anderson . Just read the call-outs and highlighted text to get the message, but the whole 5 pages is quite revealing.
[/EDIT]