Is circulation real?

Discussion in 'Hydrodynamics and Aerodynamics' started by Mikko Brummer, Jan 25, 2013.

  1. Sailor Al
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    Sailor Al Senior Member

    If it is just dust as soon as I drive off it is blown away by the air that is moving over the surface.
    Otherwise it takes a sponge and water to shift because it has adhered to the paintwork and there's not enough friction in the air that's blowing over it to shift it.
    Seems to me you are adding to my doubts about air being stationary at the base of the boundary layer.
     
  2. Sailor Al
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    Sailor Al Senior Member

    Thank you for the video, it hammers yet another nail into this particular coffin.
    upload_2022-11-24_8-25-10.png
    "Hoist by his own petard"?
     
  3. Sailor Al
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    Sailor Al Senior Member

    Actually, no, that's not the the case. Here's the quote from O.G Tietjens translations of lectures by Prandtl in Fundamentals of Hydro- and Aeromechanics:
    upload_2022-11-24_8-59-0.png
     

    Attached Files:

  4. Alan Cattelliot
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    Alan Cattelliot Senior Member

    You would have to take a microscope in polarized light to see the bunch of water molecule glued to the wall.The water molecules near the wall do not move, so there is no accumulation at the interface. It may be more evident @3.00, in this other video.


    Inside the boundary layer, the flow has a "U*"(traditionnal notation) velocity component, that drives the white bubbles and prevent them to accumulate. The viscous forces elongate their traces inside the boundary layer. What is to be observed here is the shape of this elongation. Prantl's postulated that the "non-slip" condition correctly reproduces mathematically this shape.
     
  5. Alan Cattelliot
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    Alan Cattelliot Senior Member

    You can also check this one (shorter)

     
  6. latestarter
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    latestarter Senior Member

    Just curious as to what difference it makes whether it is no slip or very slow, or are we in an angels on the end of the pin discussion.
     
  7. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Not at all, Latestarter, not at all. This time, I think that SailorAI is really putting his finger on something. Since several years, some airplane designers have come accross this https://www.csc.kth.se/~cgjoh/newtheoryflight.pdf . A punch in Prantl's boundary layer theory.

    As the authors say :

    "The new theory of flight is the result of a new capability of computing tur-
    bulent solutions of the incompressible Navier-Stokes equations at affordable cost
    for large Reynolds number using slip or small friction force boundary condition
    documented in detail in [18, 48, 48] allowing in particular accurate computation
    of lift and drag of arbitrary bodies. "

    Also this (special dedicace to SailorAI ), without any respect to elders... :

    "• Downwash generates lift: trivial without explanation of reason for down-wash from suction on upper wing surface.
    • Low pressure on upper surface: trivial without explanation why.
    • Low pressure on curved upper surface because of higher velocity (by Bernouilli’s law), because of longer distance: incorrect.
    • Coanda effect: The flow sticks to the upper surface by viscosity: incorrect.
    • Kutta-Zhukovsky: Lift comes from circulation: incorrect.
    • Prandtl: Drag comes mainly from viscous boundary layer: incorrect."


    When designing an aircraft, you don't, in general, use heavy hi res fully turbulent-viscous coupled [...LES, RANS....] Navier stokes solver. By removing the "non-slip" conditions to the calculations, they have shown that results can be produced, in sufficient accordance with the measurements, at a low cost of computation. In my post Is circulation real? https://www.boatdesign.net/threads/is-circulation-real.46025/page-25#post-935836, I shared an interesting paper about superfluids flows. Because of their properties, superfluids behave as if their viscosity would be very small. Although they conclude that the momentum conservation was responsible by itself of lift, they do not really developped the numerical approach they used. Interestingly, several domains of integration have been used, in an attempt to see the influence of the number of tip vortices, inside that domain, I therefore made my own conclusion, that goes in the direction of this "New flight Theory". The surface vorticity creating turbulent separation could indeed be the true mechanism of lift generation, explaining also birds and insects flights, that cannot be explained totally, for now.

    Still, the conventional approaches give excellent results. But it is clear to me that the mechanism behind the wall to fluid interaction is not very well understood yet.
     
  8. Sailor Al
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    Sailor Al Senior Member

    Glad to have you on board.
    I enjoyed the first half of the paper.
     
    Last edited: Nov 24, 2022
  9. Sailor Al
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    Sailor Al Senior Member

    The suggestion that the difference between no slip and very slow slip is a matter of degree is as absurd as it a discussion about whether no angels can stand on the end of a pin or one angel can stand on the end of a pin is a matter of degree.
    It's not a question of whether you can stand 10 angels or a hundred! It's about the existence of angels at all.

    The fact that I you have chosen to suggest a very slow slip to support your position shows wilful distain for the fact that I have not made any claim about size of the slip.
    Stay out of the kitchen if you can't stand the heat.
     
  10. latestarter
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    latestarter Senior Member

    Sorry if I hit a raw nerve, “angels” is just a figure speech in hindsight I would replace it with “how important is it”?

    I do not have a position, the longer the thread goes on the more I realise how complex the subject is.

    My question resulted in a very informative response from Alan Cattelliot and praise from him to you.

    I am not in the kitchen and given rising energy prices and the coming winter any heat would be welcome. :)
     
  11. Sailor Al
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    Sailor Al Senior Member

    It wasn't the Angels reference that hit a raw nerve. The angels analogy is a very useful one when arguing reductio ad absurdum.
    No, the issue was your apparent disrespect for the difference between "none" and "some" and its implications for fluid dynamics.
    Maybe you are not familiar with the seismic influence of Prandtl's announcement of the no-slip boundary layer in 1904. For the ivory-towered mathematical fluid dynamics community, it was a gift from the gods. For most of the 19th century they had been struggling to make their mathematical model of fluid flow conform to experimental results. It appears that by applying the no-slip boundary to their model, the "D' Alembert's Paradox" was miraculously resolved. Then along came Joukowski, Kutter, Navier Stokes, all the way to Gentry, Marchaj, Anderson and Drela, who swallowed this myth to develop the nonsense of modern aerodynamics, with its Circulation Theory, Starting Vortex, Lifting Line, Vortex sheets and surface friction to explain lift and drag.
    It seems that there was little appetite to invest time and effort in performing experiments to actually discover whether or not Prandtl's discovery was valid, or to actually measure the depth of this boundary layer.
    So no, not a problem with angels.
    Sorry for the tirade.
    @gonzo, @Doug Halsey, @DCockey, @Barry, any comment?
     
    Last edited: Nov 25, 2022
  12. Sailor Al
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    Sailor Al Senior Member

  13. DCockey
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    DCockey Senior Member

    Your assumption is completely wrong. Boundary layers have been the subject of very extensive research since the early century, and that research continues. The structure of boundary layers and the interaction between solid surfaces and fluid flow including the validity of the "no-slip" conditions have been probed innumerable times using many methods including hot wires and laser anemometry.

    The no-slip conditions is valid until you look at length scales comparable to the mean free path of fluid molecules between collisions with other fluid molecules. At that level the general treatment of a fluid as a continuum also becomes invalid. The behavior of discrete molecules need to be analyzed. By the way the mean free path of molecules in air at sea level atmospheric pressure and density is around 60 nan0meters or 0.00006 mm, which is around 1 / 1000 of the diameter of a human hair. So in other words the no-slip condition is a valid approximation for virtually any purpose other than in very rarified conditions such as on the edge of space.

    D'Albert's paradox is that the inviscid and incompressible flow around a solid object does not produce drag unless vorticity is somehow generated by the object. The no-slip boundary condition does not enter into it. What Prandtl's division of the flow around airfoils, etc into an outer potential flow away from solid surfaces and an inner boundary layers near solid surfaces did was to provide a framework for understanding that how vorticity was generated by solid surfaces and shed while the flow away from the surfaces and outside the wake was inviscid and irrotational, and to then make calculations.

    I assume you do not fly on airplanes because the design and engineering of aircraft depends extensively on the use of boundary layers and the no-slip conditions.

    Do you think you are the first person in over 100 years to ask these questions?
     
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  14. Sailor Al
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    Sailor Al Senior Member

    So how do you account for post #497?

    I only have 1,000 hrs private, but I'm pretty sure no-slip boundary layers had nothing to do with the development of C172 wings!

    I can't find many others, can you assist please?
     

  15. DCockey
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    DCockey Senior Member

    The "no-slip" condition applies at the surface, not away from the surface. The velocity is non-zero any small (or large distance) away from the surface. The marker fluid is being stretched in the boundary layer so that it is no longer visible all the way back to the stagnation point. (Too much precision is being attributed to low resolution demonstrations and video.)

    Why would you think the developers of C172 wings did not know and use standard aerodynamics theory and methodology? Prandtl's concepts were accepted for several decades before the C172 was developed.

    Two alternatives:

    Sailor Al's claim that "modern aerodynamics" then the theory and formulas used to develop virtually all aircraft flying today is wrong. Many very smart people have not been able to see the flaws in the theory.

    Sailor Al does not have sufficient understanding of the theory.​

    I'll leave it to the reader to decide which they consider to be the more likely and how improbable the other is.

    Time to go back to sidelines of this thread.
     
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