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

    Here it is: An Explanation and Understanding of Aerodynamic Lift by ...https://www.preprints.org › manuscript › download
    At least it has full attribution, but, as noted in footnote on P1 "∗This manuscript was submitted to mdpi fluids but rejected for publication. An updated version includes changes due to critique and recommendations by the reviewers."

    I was unable to identify an answer to my question:
    1. How thick is the stationary part of the boundary layer?
     
  2. Paul Scott
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    Paul Scott Senior Member

    It’s a theory, not a cult of personality.
     
  3. gonzo
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    gonzo Senior Member

  4. Sailor Al
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    Sailor Al Senior Member

    It's called provenance, nothing to do with cults.
    I may I found something in the Appendix of your Schaffarczyk paper that might have made you think there was an answer to my my question.
    upload_2022-11-30_8-30-41.png
    Looking at line 508, I couldn't evaluate RN^(1/8) as a distance.
     
  5. Sailor Al
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    Sailor Al Senior Member

    "As the fluid moves past the object, the molecules right next to the surface stick to the surface."
    Kinetic Theory requires air molecules to move at about Mach 1. That only becomes zero at 0°K.
    I know this is a bit controversial, but on this point I think Kinetic Theory of Gases trumps NASA.
    This a product spec sheet for a device to "measure and study boundary layers in flows".
    Interestingly it features the infamous Prandtl no-slip boundary layer image under its logo:
    upload_2022-11-30_8-44-18.png
    My question relates to the thickness of the stationary portion of the boundary layer, not the overall thickness of the boundary layer.
     
  6. gonzo
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    gonzo Senior Member

    How does it disprove Prandtl theory (NASA didn't exist at the time).
     
  7. Sailor Al
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    Sailor Al Senior Member

    Prandtl's theory states "the velocity is either zero or equal to the velocity of the body." (op cit)
    Air is a fluid.
    If you want to work at the molecular level as NASA does ("the molecules right next to the surface stick to the surface."), Kinetic theory (predating Prandtl, NACA and NASA) requires air molecules to travel at about Mach 1.
    For air molecules to be stationary the temperature has to be 0°K.
    The air over my sail is about 300°K.
    I think that disproves Prandtl's no-slip boundary at the molecular level.
     
  8. gonzo
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    gonzo Senior Member

    What disproves it? I don't read any proof, experimental or theoretical. I accept that is your opinion, but it doesn't have any scientific backing. Air molecules, or any other type of molecule, can be stationary with respect to a surface if there is some kind of attraction.
     
  9. Sailor Al
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    Sailor Al Senior Member

    Air molecules can't be stationary with respect to the surface of a sail. I have provided an argument based on the Kinetic Theory of Gases.
    Somebody, please help explain the process of logical argument to @gonzo!
    [EDIT] Maybe I should have added that at 300°K in air at sea level pressure, Mach 1 is about 340 metres/second. That means the air molecules at the surface of my sail are moving at about 340 m/s, not at zero as required by Prandtl (and NASA).
     
  10. Paul Scott
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    Paul Scott Senior Member

    Provenance? :) That is a dark forest, prowling with wolves and villains. It’s like saying a Guarneri cello is a great cello if a great cellist played on one, even if he hated it, and traded it in as soon as he could. Of course, that emotional part of the story will be, ahem, left out. Goes for Strads too. The ‘approved’ (and enforced, Al) story, er, provenance, of the instrument becomes more important than the actual music.
     
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  11. Sailor Al
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    Sailor Al Senior Member

    To my moderately trained ear, Crummer's paper sounds more plywood junk than Guarneri.
     
  12. DCockey
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    DCockey Senior Member

    Velocity is a vector quantity with three components (in a 3D world). It has both a magnitude and a direction. Speed is the term commonly used for the magnitude of a velocity.

    The velocity of fluid is defined as the average velocity of the molecules of the fluid in a volume larger than the mean spacing of the molecules, but smaller than the lengths of interest.
    The velocity of a fluid does not equal the velocity of individual air molecules.
    The velocity of an individual molecule of a fluid does not equal the velocity of air as defined for a fluid.
    The speed of a fluid does not equal the speed of the individual molecules.

    When the velocity of a fluid is zero the speed of the fluid is zero, but the speeds of the individual molecules of the fluid are not zero.
    The air in a room is not moving at the speed of sound, even though the speeds of the individual molecules are close to the speed of sound.
     
    Last edited: Nov 30, 2022
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  13. Paul Scott
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    Paul Scott Senior Member

    you could have looked this up yourself, but no. Pathetic power trip. But that’s become part of your provenance

    http://physics.ucsc.edu/~ccrummer/
     
  14. Alan Cattelliot
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    Alan Cattelliot Senior Member

    In my opinion, this is a bit going too far... A wave to wave interaction (gravitationnal wave @ ocean surface, ripples patterns) do produce interferences, without being qualified as "Quantum Waves". The diffraction phenomenon can even be observed when you observe people going out of a Supermarket, through the exit door. Why ? Because the concept of flow is a statistic one, involving a great number of elements in interaction, as it is said many times in the article you provide. So, I wouldn't mind if we can just let the Quantum Double Slit experiment aside, in this discussion. Quantum is...something else... The double slit experiment, used in the quantum field, has only been made in order to demonstrate the duality wave-particule, that is assumed according to this theory. Further authorizing the superposition principle. Nothing more. I hope we won't do a remake of the Coppenhagen discussions here, when trying to get a better understanding of fluid flows and boundary layers.

    Nevertheless, the statistical approach of your article makes sense, undoubtly. It is a corner stone of the Smooth Particle Hydrodynamics (SPH) formulation, that I use for my simulations, in complement of traditionnal FE codes, since a decade now. DualSPHysics – DualSPHysics: A combined CUDA and OpenMP implementation of the Smoothed Particle Hydrodynamics method based on the advanced SPHysics code. https://dual.sphysics.org/. I appreciate being free of the concept of mesh, because, as it is said in your article, many calculations can be done, only considering the elastic collisions between "idealized" particules. The counterpart is it requires a very high number of particules for the calculations to give results in accordance with experimental results. To give you an idea, a flow around a hull, computed with OpenFoam, 1.5M points, can, with great care - and iterations...-, gives results with a precision of 5%, compared with measurements. The same accuracy requires, from the SPH formulation, at least 15M points... GPU are faster thant CPU, but considering the time passed, during the execution of the GPU, for this important amount of variables to be read and written, it becomes evident that a hardware fit to GPU computations is to be more "powerfull" than an hardware fit for FE calculations.

    Anyway, even SPH formulations, based on particules collisions to simulate fluid flows, struggle to reproduce experimental results, in the vicinity of surfaces. Mathematical tricks are used in order to re-create boundary layers. So If the article, indeed, provides a simpler mechanism for fluid calculations, many difficulties arise from this simplicity when transposed at greater scale. Difficulties that are, for now, resolved with mathematical tricks. A very basic response, comparable with what Prandtl' has done when proposing the concept of boundary layer. But again, the article fails to provide the true mechanism responsible for the wall to fluid - or fluid to fluid - interaction. However, i also want to thank you for this article. I have many questions about this interaction, and I can see that I'm not alone. Thank you also, SailorAI, for your hard work in pushing us again and again. Keep questionning the rightfull of established theories is, indeed, part of the nature of a true scientist.
     
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  15. Sailor Al
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    Sailor Al Senior Member

    Absolutely!
    I think you may be agreeing that NASA may have it wrong: "As the fluid moves past the object, the molecules right next to the surface stick to the surface."
     
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