A "new" theory of lift ?

[rb1957]

i'd like some more informed aerodynamists' opinion on Mr (Dr?) Johnson's opinion that lift is not caused by circulation. From

http://knol.google.com/k/the-spell-of-kutta-zhukovsky-s-circulation-theory#

...

"it is shown that the lift you experience when you fly, comes without circulation, as displayed in the following figure showing the lift and circulation of a Naca0012 wing as function of the angle of attack, computed by solving the Navier-Stokes equations for the flow around the wing:

pic doesn't show, sigh

We see that the lift increases linearly with the angle of attack up to 16 degrees, while the circulation stays
basically zero up to 10 degrees: Lift and circulation are not equivalent as in Kutta-Zhukovsky's formula"

there is an impressive looking pic showing lift increasing with AoA, as expected, but "circulation" remaining constant, and close to zero. this sort of breaks the linkage between circulation and lift, but i'm smart enough not to take things I can't derive myself at face value.

As far as I've read Johnson doesn't propose a consistent new theory, but tries to explain lift and drag at near separation AoA.

opinions ?

 

[MikeHalloran]

In my limited understanding, circulation is a mathematical artifice, introduced in order to allow use of a closed system model to approximate the real system comprising an airfoil traveling in a free field, and the resulting upper and lower surface 'flows'. Neglecting induced vortices, the only actual flows are of nominally stationary air molecules flowing 'vertically', away from and then back toward their original locations as the airfoil passes by.

The operative word here is the verb 'approximate', so I'm not surprised when different math models don't agree with each other or with reality.




Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

http://knol.google.com/k/why-it-is-possible-to-fly#view

Is where they try to explain it. Except they don't actually explain it. At least not in any clear concise way. Every time you think they're about to summarize they give a link to a different page and go on rambling.

Essentially they're saying it's 3D turbulent flow separating and causing a down wash if I understand correctly.

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[KENAT]

Oh, and unless things have changed a lot in the last 11 years, which is possible, I thought all computer CFD were fairly unstable approximations. They seem to be relying on CFD a lot to justify what they say.

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[IRstuff]

The circulation approach was a mathematical artifice to get the solutions to the equations to come out "correct."

I think the modern CFD programs can now run the full-blown Navier-Stokes equations. Prior to that, I think they were running the Euler equations.

TTFN

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[IRstuff]

Oh, and the problem is supposedly that the Euler equations did not handle unsteady flow, which is supposedly what the actual mechanism boils down to, i.e., the steady-state solution is unstable, and unobservable in real life.

TTFN

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[KENAT]

OK, So anyone know of a website with a coherent explanation of the current theory. Equivalent to the 2 sheet (including diagrams) explanation we had to do in my first year fluids class for circulation theory?

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[IRstuff]

I'm a bit dubious about the stuff on Knol, since it's not unlike Wikipedia, and worse, since it's written be specific people who may, or may not, have agendas of their own, and there appears to be less peer review than even Wikipedia.

To wit, they claim that NASA is confused and offers 3 bad explanations "but offering no theory claimed to be correct." And yet, the NASA site has, on each of the "wrong" theory pages, a link to the "correct" theory NASA page:

http://www.grc.nasa.gov/

http://WWW/K-12/airplane/eulereqs.html

which then links to the unsteady NS equation page:

http://www.grc.nasa.gov/

http://WWW/K-12/airplane/nseqs.html

TTFN

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[rb1957]

yeah, that was were i was before the other thread was RF'd (apparently the OP was a student). just as IR posts, yes NASA refutes 3 incorrect theories of lift, but then (not reported by Johnson) go on to say the Newton and Bernoulli are both correct.

Also i was going to say the Johnson doesn't prove circulation is wrong he just says it is (the way i read his paper).

 

[IRstuff]

I was lazy earlier, the NASA "right" page is here:

http://www.grc.nasa.gov/

http://WWW/K-12/airplane/right2.html

The problem with circulation is that there is no physical mechanism by which it can occur, nor by which it can be observed.

TTFN

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[Dan320]

I have problems with this page:

http://www.onemetre.net/Design/Downwash/Circul/Circul.htm

Why is circulation = 0 ahead of the airfoil when we can see the streamlines deflecting upwards? Why is Vk equal but opposite on top and bottom?
Seems very idealised, close to the point of being incorrect...

 

[IRstuff]

As discussed elsewhere, the circulation theory is a mathematical construct used to solve the problem of having no downturning of air to generate the lift, so circulation was added to impart a downward momentum to the airflow at the trailing edge:

http://knol.google.com/k/why-it-is-...ing_large_scale_circulation_to_potential_flow

However, since this is a totally ficticious construct, the circulation can't even be constant around the airfoil, otherwise, it wouldn't correlate with other observable effects.


TTFN

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[Dan320]

Which is pretty much what I was getting at.

 

[rb1957]

ok, circulation is an analytical construct that bridges analysis and the real world. As a theory it matches observed behaviour, and makes predictions that have been validated (no?). It is a theory with limitations eg does it predict stall behaviour ? (I don't think so).

CFD is possibly a better tool, depending on the mesh, etc (just like FEM).

To my limited understanding, the "problem" is related to potential flow models (eg predicting zero drag). but this is for an ideal fluid, no? How does potential flow theory stack up against Navier-Stokes (ie CFD) ? I'd've thought it was reasonable to make a CFD model of an ideal potential flow "problem" to clarify the idealisation and the realities.

no ?

 

[IRstuff]

KZ theory is a zero-drag approach, and requires significant tweaking to make it work at different conditions and AoAs.

The issue is simply that there is no ideality, and CFD is the only way to get simulations to match reality consistently.

TTFN

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[Sparweb]

The google knol that started this thread has a few references that I checked into. I have JD Anderson's book Introduction to Flight, and you can read a lot about the theory of how lift is produced there. The basics are in an early chapter and much more detail in chapter 5. Circulation is in the "alternative explanations" section, along with other trivia like the Magnus effect.

Abbott and von Doenhoff are more to the point in Theory of Wing Sections: they don't waste a lot of words on circulation, merely demonstrating how to use the mathematics.
Circulation is used in airfoil design - you can find it in section 3.7 of TOWS. But even so, the theory needs modification to match experiments.

To the layman and the engineer, the momentum theory is likely the most intuitive and useful. And valid.

Back to the google page: I've never met a pilot who knows how a wing generates lift, or cares, except the university edjikated ones. So I don't know why the author refers to pilot's concerns. The text fails to explain streamlines, yet relies on the concept of the streamline for its argument. No layman reading that will make heads or tails of it. No engineer reading it will glean any useful information from it either.

Steven Fahey, CET

 

[rb1957]

so the "new" theory is navier-stokes equations (as solved by CFD) ?

"KZ theory is a zero-drag approach" ... i thought the whole point to circulation was that it fixed the zero drag prediction from potential flow solutions ?

 

[IRstuff]

My reading of KZ is that it's supposed to give you a downward momentum at the trailing edge only, but has nothing to do for the drag.

I think NS, and even the Euler equations with drag, allow you simply crank the basic momentum and stress relationships without regard to "what is lift", i.e., lift simply arises from the turning motion of the air around the airfoil. It's more a description than a theory.

TTFN

FAQ731-376

 

[rb1957]

that's sort of what i meant ... we don't need any theory beyond NS to explain lift. mind you "lift simply arises from the turning motion of the air around the airfoil" sure sounds like circulation to me !?

 

[IRstuff]

Not exactly, but I get your point. The circulation theory does not attempt to explain the phenomenon; it merely makes the math come out, without regard to any physical explanation, nor can it do with without tweaking by hand to "correlate" with reality. Presumably, this is most readily apparent when comparing flow around a cylinder vs. around a normal airfoil vs. around a flat plate. Each condition, and each location on an airfoil requires adjustment to make the results match reality.

NS simply cranks the math, based on recognizable and accepted physical behavior, and does not specifically require tweaking of the math itself, i.e., running cylinders, airfoils, and plates require no special processing.

TTFN

FAQ731-376