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 ?

 

[aeroafrix2]

@vortexman,
I understand how "passonate" you are about 'convincing' others of your understanding of 'circulation theory', however, I think it is not out of place for anybody to raise their own view point on the matter especially in a forum such as this. And just to clarify that I actally studied aerodynamics from undergraduate to post-graduate level and I have studied to a large extent "circulation theory" as it relates to lift, hence there is no need for the comments in your previous post.While I agree with most of your post on circulation theory as it relates to lift generation, your reply post to IRstuff, I thought was out of place. All I only requested was your own insight into how you think both momentum theory and circulation theory "inter play" to produce lift in the light of your response to IRstuff- maybe you should read your post again.

And by the way,I wouldn't want us to derail the topic, however I will like tosay categorically that Bernoulli Equation can and has been be faulted. Mind doing a little research on that in your spare time? Remember no man is an island when it comes to knowledge!

 

[btrueblood]

"categorically that Bernoulli Equation can and has been be faulted. "

Um, grammatical errors make interpreting that statement a bit difficult, but if you are saying the Bernoulli equation is wrong...then you need to supply quite a bit of proof yourself before many people will take you seriously. Certainly there are limitations to Bernoulli's equation, but it's basis is simply a simplification of the same Navier Stokes equations used for CFD, and is adequate to model a rather enormous variety of flows. Similarly, potential flow with circulation is adequate to model the lift characteristics for wing sections, at far lower computational cost and better accuracy than fully viscid RANS CFD modelling, all things being equal.

 

[vortexman]

aeroafrix2,

The original post sought an aerodynamicist's opinion about the "new theory" that lift is independent of circulation. That issue, and the related issues that arose in the discussion are very straightforward to deal with the use of very well-established aerodynamic theory. By well-established, I mean that there is no reasonable doubt about the elements of theory I'm referring to.

I have no comment about your educational background, but you have made a claim that I would like to ask you to support. Specifically how has the Bernoulli Equation been faulted? Please explain this very clearly so that I can understand it.

Thanks,

vortexman

 

[gerritgroot]

I don't remember in which one, but in one of those classics
either:
Prandtl & Tietjens, "Applied Hydro- and Aeromechanics"
or:
Prandtl & Tietjens, "Fundamentals of Hydro- and Aeromechanics"

Not only the existence of circulation in case of lift, but also the whole proces of how it comes actually into being, is demonstrated by experiments in those books.

They usually come in a pack and are not expensive. The fact that flying without the proper circulation, i.e. without viscosity, is not possible, becomes also clear in those texts.

 

[rb1957]

i've dredged up this site for further research ...

http://www.desktop.aero/appliedaero/potential3d/liftingline.html

... it discusses a how bunch of fluid dynamic theories. i found it interesting that it says most CFD programs solve linearised approximations of Navier-Stokes ... hence different programs could give different results due to different assumptions.

i too would love to know how/why bernoulli is wrong ...

 

[Dan320]

Vortexman,
I believe "Faulting the Bernoulli Equation" refers to not accepting it is being used "backwards" i.e. airspeed causes a drop in pressure but saying: Pressure differential causes airspeed.

Much of the new theories of lift center on this.

 

[vortexman]

Dan320,

I'm not positive that I understand what you mean, but I'll offer the following comments: 1) The Bernoulli Equation "works", in the sense the it always gives the right answer, when used correctly. There is nothing about causality in the Bernoulli Equation or in its application. 2) This notion of causality makes no sense; you can't say what causes what, and you don't need to. 3) There are no "new theories of lift". There are perhaps some "new" methods, such as numerical ones, but none of them replace or disprove the "old theories", which all remain valid.

vortexman

 

[rb1957]

how does an airfoil create lift ?

lift is created by the low pressure on the upper surface and the high pressure on the lower surface.

why ? how are these pressures created ?

the airflow above the airfoil is faster and below it is slower, as required by bernoulli.

why ?

it just is. experiment shows this. CFD, for what it's worth, shows this.

or maybe, it is a result of circulation around the airfoil.

why ?

it just is.

 

[gerritgroot]

how does an airfoil create lift ?
lift is created by the low pressure on the upper surface and the high pressure on the lower surface.
why ? how are these pressures created ?

By means of viscosity. The air follows the shape of the airfoil, because it sticks to it. The only way to do so is by means of a centrifugal force, which in turn can only be caused by pressure gradients in the radial direction of the curve

the airflow above the airfoil is faster and below it is slower, as required by bernoulli.
why ?

Because the air does not disappear nor gets created around the airfoil and because of the statement above. If there is less place, and the amount of air going in (between infinity and your airfoil) is equal to the amount going out, it must go faster, because the cross section is smaller. (Compare internal flow)

it just is.

See above, it is not "just"

experiment shows this. CFD, for what it's worth, shows this.
or maybe, it is a result of circulation around the airfoil.
why ?


CFD just solves your equations, either complete or not, on such a small scale that they can be linearised, just like any finite something method on any problem in physics or engineering.

Not only the experiment shows this, the theory also shows this, the experiment of Prandtl only confirmed it.


it just is.


No it is not "just"
The circulation and the lift ar both a result of the air following the shape of the airfoil, for being sticky

 

[Dan320]

Whatever you say, Vortexman.
Old lift explanations state that pressure drops when airspeed increases. If you think about this for a while you will find it makes no sense, even if it is seemingly in accordance with Bernoullis conservation of energy equation.
When you realise that differences in air pressure is what causes the air to move, and not the other way around, everything becomes more understandable.
That is what I think people mean when they "disprove Bernoulli".

 

[gerritgroot]

@Dan320.

It makes perfect sense, see my previous post.

What you say is not true. If I throw a stone through air at rest, there's no pressure drop moving any air, it's the stone that is moving. Any further existing pressure gradient is a local distortion caused by the presence of the stone itself not by any pressure drop in the farfield (remember that the stone flies through air at rest).

The fact that one may consider an equivalency by holding the stone and moving the air doesn't change anything to the situation.

Vortexman is right when he states that you can't say what causes what. The only thing you can say is that if one of the variabls changes, the other ones will change in accordance with Bernoulli as long as you fulfill the conditions for the law's validity.

 

[rb1957]

dan,

what causes the pressure difference in the first place ?

you can create it by say opening a can of pop/soda/soft drink ... as we know the pressure difference between the contents of the can and the outside atmosphere causes the can contents to rush out untill the pressure is equalised. the can had a higher total pressure than the surrounding atmoshpere.

i guess we could create a similar set-up with a wind tunnel, such that the total pressure is increased due to airspeed (ie static pressure remains the same) and then open the tunnel to the outside ... the tunnel contents will rush out the same (because they have a higher total pressure than the surrounding atmosphere.

i don't think you can say pressure gradients create velocity ... i think bernoulli says static pressure and dynamic pressure are exchangeable, in the same way that potential and kinetic can be exchanged. i don't think you can say potential energy causes kinetic energy ...

 

[KENAT]

Can I check what people mean by 'momentum theory'?

Are we talking about conservation of momentum?

Or the idea of particles bouncing off the angled 'aerofoil'?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Dan320]

This discussion has gone off in so many directions the forum seems insufficient. Why can't we all meet in say, Rio for a conference?
Sign me up.

 

[rb1957]

great idea, but i think we'd only be able to afford Rio, North Dakota