If the source of the pressure distribution around a lifting airfoil is known, why is not explained in textbooks?

[Squizzie]

TL;DR Summary

Why, if the source of the pressure distribution around a lifting airfoil is known, is it not explained in any textbook?
This question presupposes that textbooks do not explain the source of the pressure differences around a lifting airfoil, so the first challenge would be to demonstrate that it is in fact the case that an explanation is not to be found. I think I can present convincing evidence that it isn't.

Philosophically, it is impossible to prove a negative, so I will turn to one of the most recognised authorities, Anderson, J. D. (2011) "Fundamentals of Aerodynamics" Fifth Edition. An on-line version can be borrowed from archiv.org at Fundamentals of aerodynamics : Anderson, John D., Jr. (John David),

On p. 282 he states:
"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. "
And in the references section 1.5, on p. 18:
" However, in these and all other cases, the aerodynamic forces and moments on the body are due to only two basic sources:
1. Pressure distribution over the body surface
2. Shear stress distribution over the body surface
No matter how complex the body shape may be, the aerodynamic forces and moments on the body are due entirely to the above two basic sources. The only mechanisms nature has for communicating a force to a body moving through a fluid are pressure and shear stress distributions on the body surface. "

However, there no explanation is provided in Section 1.5 for the pressure differences.

An exhaustive search of the aerodynamics literature, including Lanchester (1907), Prandtl (1922), Pope (1947), Clancy (1975), Bertin (2009), Mclean (2013), Drela (2014) has similarly revealed no explanation for the pressure differences.

So my question is, why isn't the pressure distribution around a lifting airfoil explained in the literature?

 

[Baluncore]

So my question is, why isn't the pressure distribution around a lifting airfoil explained in the literature?

It is, but you ignore it because you do not understand the mathematical explanation.

The lift and drag on a cylinder with circulation can be solved.
https://en.wikipedia.org/wiki/Magnus_effect#Pressure_gradient_force

Airfoil analysis started with the Joukowski theorem, that was later improved with the Kutta-Joukowski theorem.
https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem

That is a complex transform that maps a circle into a representative airfoil, and the flow around a cylinder, into the flow around a wing. When the circulation of the Magnus effect is added to the model, the lift and drag on the airfoil can be explained and computed, with a little help from conservation of energy.

The only true explanation of lift and drag is through the mathematical transformation of circulation. Because there is no magic potion that will teach you the mathematics, you may well be unhappy. Don't get angry with the texts, just think harder, and study more mathematics.

 

[A.T.]

However, there no explanation is provided in Section 1.5 for the pressure differences.

Why does the existence of a pressure difference need to be explained, when an object is not symmetrical along the flow? It's rather unlikely and a special case that no lift is produced in that case.

If you don't mean the mere existence of lift, but its quantitative amount, then there is no way around the complex math, which often can be solved only numerically.

 

[Dale]

TL;DR Summary: Why, if the source of the pressure distribution around a lifting airfoil is known, is it not explained in any textbook?
This question presupposes that textbooks do not explain the source of the pressure differences around a lifting airfoil, so the first challenge would be to demonstrate that it is in fact the case that an explanation is not to be found. I think I can present convincing evidence that it isn't.

I can present conclusive evidence that it is:

Airplanes fly

Engineers go to school, they read the literature, and are then able to apply the information that you claim is missing in order to design aircraft.

 

[Lnewqban]

TL;DR Summary: Why, if the source of the pressure distribution around a lifting airfoil is known, is it not explained in any textbook?
...
So my question is, why isn't the pressure distribution around a lifting airfoil explained in the literature?

What is that known source that no book explains?

 

[Squizzie]

What is that known source that no book explains?

I intentionally employed a conditional phrasing, "if the source of the pressure distribution around a lifting airfoil is known..." to highlight a potential deficiency in our current understanding.
Anderson's assertion, that "the true physical sources of aerodynamic force on a body are the pressure and shear stress distributions", is essentially a restatement of the fundamental principle that gases exert force on surfaces through pressure and shear stress.
Without further explanation for the pressure distributions in Section 1.5, the reader is left to conclude that the source of the pressure distribution was known.

 

[Squizzie]

Airfoil analysis started with the Joukowski theorem, that was later improved with the Kutta-Joukowski theorem.
https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem

But there's the paradox. On p. 282 Anderson states:
"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. The Kutta-Joukowski theorem is simply an alternative way of expressing the consequences of the surface pressure distribution" (Anderson's emphasis of consequences)

Anderson does not claim that the Kutta-Joukowski theorem explains the pressure distribution.

He merely states that it expresses the consequences of that pressure distribution, and again refers us back to Section 1.5 for the true physical sources.

That Section 1.5 does not explain the source of the pressure distributions is at the heart of my question.

 

[Baluncore]

Anderson does not claim that the Kutta-Joukowski theorem explains the pressure distribution.

I agree.
You asked specifically about the "source of the pressure distribution around a lifting airfoil".
The fundamental is the Magnus effect, and that must be explained and understood first.
The Kutta-Joukowski theorem then maps that circular distribution onto the surface of the representative airfoil.

 

[Lnewqban]

I intentionally employed a conditional phrasing, "if the source of the pressure distribution around a lifting airfoil is known..." to highlight a potential deficiency in our current understanding.
...
Without further explanation for the pressure distributions in Section 1.5, the reader is left to conclude that the source of the pressure distribution was known.

Perhaps all the authors have considered the use of words related to airflow sufficient to infer that a constant supply of kinetic energy reaching the wings is indispensable for lift and flight to happen and to be sustained.

I would say that in this specific case, "the pressure and shear stress distributions" are the measurable ways in which that energy is transferred from the airflow onto the wings.

 

[russ_watters]

I intentionally employed a conditional phrasing, "if the source of the pressure distribution around a lifting airfoil is known..." to highlight a potential deficiency in our current understanding.

What deficiency? What does 'known but not explained' even mean in this context? It sounds like a self-contradiction. We have the math to calculate it(do you agree we have the math?) and airplanes that fly based on that math(do you agree they are designed based on that math and that the do, in fact, fly?). Is that not what you are looking for? What, exactly, are you looking for that we don't have? Or what, exactly don't you agree with?:

...an explanation is not to be found. I think I can present convincing evidence that it isn't.

Get to your point/misunderstanding. We don't like these games.

 

[Squizzie]

The fundamental is the Magnus effect, and that must be explained and understood first.

Does this imply that Anderson’s description of the Magnus effect explains the origins of the pressure distribution around a lifting airfoil?

 

[Dale]

to highlight a potential deficiency in our current understanding.

And yet, airplanes fly as designed according to our current understanding.

 

[Squizzie]

What deficiency? What does 'known but not explained' even mean in this context? It sounds like a self-contradiction. We have the math to calculate it(do you agree we have the math?) and airplanes that fly based on that math(do you agree they are designed based on that math and that the do, in fact, fly?). Is that not what you are looking for? What, exactly, are you looking for that we don't have? Or what, exactly don't you agree with?:

Get to your point/misunderstanding. We don't like these games.

What is not explained in the aerodynamics literature is the source of the pressure distributions.
That the aerodynamic force (resolved into lift and drag) is due to pressure distributions is basic physics.
The Kutta-Joukowski theorem explains consequences of that pressure distribution.
Anderson, on p. 282, asserts the physical basis is explained in Section 1.5, but I am unable to find the explanation.
From his responses, @Baluncore understands what's missing.
I assure you this is not a game.

 

[russ_watters]

What is not explained in the aerodynamics literature is the source of the pressure distributions.
That the aerodynamic force (resolved into lift and drag) is due to pressure distributions is basic physics.
The Kutta-Joukowski theorem explains consequences of that pressure distribution.
Anderson, on p. 282, asserts the physical basis is explained in Section 1.5, but I am unable to find the explanation.

What does the word "explain" mean to you? What sort of answer would satisfy you, if not the math describing it?

From his responses, @Baluncore understands what's missing.

I doubt it.

I assure you this is not a game.

I doubt it.

You said there is a "potential deficiency in our current understanding". You haven't said what it is. Please do so.

 

[Baluncore]

From his responses, @Baluncore understands what's missing.

It is certainly not missing from the literature, it is missing from your understanding.

 

[Dale]

What is not explained in the aerodynamics literature is the source of the pressure distributions.

And yet, airplanes fly.

I am unable to find the explanation.

Maybe that would be a more productive avenue. It is clear from the success of aerospace engineering that these concepts are explained somewhere in the literature.

 

[Vanadium 50]

"I do not understand" and "there is no acceptable explanation" are not synonyms. If we don't agree on that, this thread is doomed.

 

[Squizzie]

"I do not understand" and "there is no acceptable explanation" are not synonyms. If we don't agree on that, this thread is doomed.

I agree 100% .
We are not doomed.
I haven't said: "I do not understand".
I said: "Anderson, on p. 282, asserts the physical basis is explained in Section 1.5, but I am unable to find the explanation". It's different, very different.
Please, either a) shut me down by showing where, in Section 1.5, Anderson explains the source of the pressure differences, or
b) join my search for an explanation of why the source of the pressure differences around a lifting airfoil is not explained in textbooks .

 

[russ_watters]

Please, either a) shut me down by showing where, in Section 1.5, Anderson explains the source of the pressure differences, or
b) join my search for an explanation of why the source of the pressure differences around a lifting airfoil is not explained in textbooks .

c) You tell us what "explanation" means to you/what could be provided that would satisfy you.
d) You tell us what "potential deficiency" you see.

You say this isn't a game. To me a "game" is what happens when people people purposely hide their definitions of terms and points. It looks to me like you are doing that.

 

[Squizzie]

c) You tell us what "explanation" means to you/what could be provided that would satisfy you.
d) You tell us what "potential deficiency" you see.

You say this isn't a game. To me a "game" is what happens when people people purposely hide their definitions of terms and points. It looks to me like you are doing that.

My point is not about the quality of the explanations that have been provided.
It's not the case that I don't understand them, or that I disagree with them, or that I have a different explanation.
The point is that Anderson claims that the physical source of the pressure distribution is explained in Section 1.5, and I can't find it.
Please, take the time to read Section 1.5. It's only 13 pages, but you only really need to look at the first four pages as by then he has established the existence of the resultant aerodynamic force. If you don't have a copy, it is downloadable from the archive.org site.
Do a word search on "pressure distribution" and see whether there is an explanation.

 

[Frabjous]

The physical source of the pressure distribution is the airflow. Why isn’t this enough for you?

 

[Baluncore]

The point is that Anderson claims that the physical source of the pressure distribution is explained in Section 1.5, and I can't find it.

On page 284, of the Sixth edition;
Section 3.16 The Kutta-Joukowski Theorem and The Generation of Lift;
"... 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 ..."
That does not say it is explaining the fundamental origin of those forces, just that they are being applied to the body as explained in section 1.5

Those forces are caused by the variation of airflow over a cylinder with circulation. That is the subject of section 3.15 Lifting Flow Over A Cylinder; (Page 268 of the Sixth edition).
You need to start reading at section 3.2 Bernoulli’s Equation; (Page 209 of the Sixth edition).

 

[weirdoguy]

I think I can present convincing evidence that it isn't.

I think, I can present convincing evidence that this thread is going to end up the same way as all your previous ones. Yet again you claim that something is not in the literature, something is hidden from everyone. It's clearly not true, of course. But why on earth are you so obsessed with this paranoia?

In this thread:
https://www.physicsforums.com/threa...ainstream-physics.1058491/page-2#post-6980369

you've been proven wrong by multiple people, and you still said "no, I'm right". Why should anyone waste their time this time?

 

[russ_watters]

The thread has become moot, and is locked.