Is circulation real?

I'm pretty sure the instantaneous reversal of a molecule's velocity vector normal to the surface that you describe is not quite what Prandtl had in mind when describing how the particles in the fluid adhere to the boundary layer.
Or what is expressed in the standard boundary layer diagram, e.g Anderson.

I think you'll find they are referring to the velocity of the fluid tangential to the boundary, not normal to the boundary.

 

Reading the Brenner paper woke me up like a hitting a cold shower on a hot day (sorry, in Sydney, currently 30°C).
It seems I am not just challenging 100 years of theoretical aerodynamics, but along with him, we are challenging the whole 250 years of fluid dynamics!
It’s embedded in his introduction:
"This experimental fact negates Euler's 250-year old generic, mass-based definition of the velocity field in fluid continua, undermining thereby the heretofore seemingly rational foundations of fluid mechanics and derivative subjects.”
And again in his closing paragraph:
"In summary, the work briefly reported upon here, if, independently substantiated by others, negates the foundations of 250 years of fluid mechanics, as well as derivative subjects,…”
Your selection of the Einstein quote reverberates with my anxiety about the meaning of flow velocity in the boundary layer. I'm not talking 5 mm close, or 1 mm close, but down in the scale required to identify the no-slip boundary - maybe 1,000 MFP's?, that's 1,000 x 0.00007 mm = 0.07 mm. That's the thickness of a hair. It's clearly impossible, an experiment can't be imagined.

Brennan has clarified what has been niggling away at the back of my brain for a while. Below a certain scale, the velocity of a gas ceases to have any meaning.

Trawling through his lugubrious penultimate paragraph:
"In any event, the fact that the relationship between the experimental measurement of fluid velocity and the symbol v appearing in fluid-mechanical equations, posited by Euler some two-and-a-half centuries ago, has, until now, never been properly subjected to rational inquiry and subsequent critical experimental test (most simply, in compressible fluids at rest), points up the sagacity of Einstein's advice. This object lesson is particularly striking when viewed in the context of the large number of students and professionals, scientists and engineers, prominent and otherwise (including, of course, myself at an earlier, more naive stage of life), who, despite their often extensive exposure to fluid mechanics, failed to question Euler's implicit assumption positing equality of the fluid's Eulerian* and Lagrangian velocities." leads to perhaps an oversimplification, but it works for me:

Plugging "v" into Euler's equations and expecting them to be useful for calculating lift is clearly a pipe dream.

What happened to the substantiation of his work?

*Eulerian velocity is the velocity of the gas as a continuum: Lagrangian velocity is the statistical mechanical velocity of the molecules.

 

Here is the link https://www.researchgate.net/scientific-contributions/Howard-Brenner-9657018 where the publications of H.Brenner ( affiliated with MIT) can be found. Brenner proposed, for our concern, is the bi-velocity concept, introducing the "volume velocity" with the aim to overcome the theoretical limitations of the use of the "mass velocity" by Euler and his followers :

"molecular dynamics (MD) simulation evidence showing, contrary to current opinion, that the NSF equations
are not, in fact, applicable to compressible gaseous continua, nor, presumably, either to compressible liquids."
[A molecular dynamics test of the Navier-Stokes-Fourier paradigm for
compressible gaseous continua, Howard Brenner, Nishanth Dongari and Jason M. Reese]

I don't actually work with rarefied compressible fluids, and even when I did, @Onera, we stayed focus on NS equations with the classical boundary layer theory, even for determining satellite re-entry into planet's atmosphere at hypersonic speed. But I'm observing that, since several years, questions about the "non-slip" boundary condition are poping out. Our current calculation's power allows particles simulations to some extends, like MD simulations, that give results very close to experiment, without the need of introducing the "non-slip" condition @wall surface. In this context, his work is cited as a reference for the determination of the "artificial viscosity", used in such particles simulations.

That's all I know, and this is quite nothing...

 

I'm no Hercule Poirot but I think you know a lot more than you're letting on!

As you know, I have serious doubts about the no-slip boundary, which is one of the 10 Commandments of fluid dynamics! Even Brenner subscribed to the theory!
This is the first hint that I may be onto something!
Do you have any references I could follow. You have provided the ResearchGate list, but all the papers are behind a pay-wall, and anyway they will probably be way above my pay-grade.
Please, throw me a line.

 

Is there any basis to you knowing what Prandtl was thinking?

 

I know I'm going to regret this, but here goes.
Yes.
I've done some reading of his work.



Take a look at this:
"In this layer-the so-called "Prandtl boundary layer"-there is a steep increase in velocity, to which the frictional force is proportional (see Chap. XV). Although ideal frictionless liquids slide over boundary surfaces, the particles of a fluid of even very small viscosity adhere to the body so that their velocity with respect to the body is zero."
It's that pretty unambiguous word "adhere" that gives it away.

 

That is an interpretation of Prandtl by a lecturer.

 

Yes, I know. But it's been done by some pretty well qualified people.
Do you have access to a translation of the lecture itself? I can't find one.

 

Leaving Prandtl's inner thoughts alone, is there something about my explanation that you find wrong?

 

As long as your erroneous interpretation of the no-slip boundary is left unresolved, there's little point in discussing your "explanations".

 

Do you have any explanation of why it is wrong? We only get unsubstantiated claims from you. Petulance is not science.

 

Even if you choose to ignore the Prandtl interpretation, I did also refer to the "standard" interpretation :

 

Yes, Sailor AI, it will be a pleasure to share some document that I have that could help you in your task. I've kept so many papers, for times where I would be able to think deeper about some physics principles that make me inconfortable. In my daily life, these principles happen to be helpfull, for my work to be done, but also for my work to be shared. Please allow me to make a rough selection, because my library is so huge..

Beyond the limit of the generally accepted knowledge, lies another knowledge that is still to be discovered. Difficult to grasp with actual words, I am quite reluctant to express, in public, the ideas and feelings that I have. You've made the same observation, that it is difficult to find the right people, who won't tell you are crazy, and whose discussions and exchange will be profitable to your research.

That said, regarding the "non slip" boundary condition, please consider this thought : If it could be observed, via appropriate experiments, that, in the vicinity of a wall, the flow velocity is null, does it obligatorily mean that a friction force, from the wall, is excerted on the fluid particles ? How can a flow stick to the wall, without the wall being sticky ?

 

I think the friction is evident in the heat generated from air flowing over a surface.

 

It's 5 in the morning so this has been penned with my brain still asleep.
It was you who alerted me to Brenner's anecdote about Einstein's advice to Weissenberg.
Is not the fact that this experiment cannot be imagined sufficient to render the concept null?
I will flesh this out more once I fully emerge from slumber.