Is circulation real?

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

  1. Paul Scott
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    Paul Scott Senior Member

  2. Sailor Al
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    Sailor Al Senior Member

    Not sure why you are concentrating on the liquid boundary layer. The current subject is the zero velocity component of the aerodynamic boundary layer.
     
  3. Paul Scott
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    Paul Scott Senior Member

    Fluid flow is fluid flow? But now I realize it’s not? :rolleyes:

    Oops.
     
  4. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Science history has shown that some true remarkable achievements have been made by "outsiders", stepping on toes to reach out prominent figures in their field, desperately stretching their arm to deliver, to the "highness", the key that would allow them to go further. Keeping the enlightment of your comment, also sharing your point of view regarding the difficulty of proposing new theories, I would quote Max Planck, nobel prize in 1918 for his work on the Quantas :

    " Eine neue wissenschaftliche Wahrheit pflegt sich nicht in der Weise durchzusetzen, daß ihre Gegner überzeugt werden und sich als belehrt erklären,
    sondern vielmehr dadurch, daß ihre Gegner allmählich aussterben und daß die heranwachsende Generation von vornherein mit der Wahrheit vertraut gemacht ist
    "

    (A scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die and a new generation grows up that is familiar with it.)

    I'm not an academic researcher, but science has always been such delicious treat for my mind that I firmly kept every bit of knowledge in my brain, like a child who makes the candy last as long as possible in his mouth. Having struggled with flows around rudders, keel, daggerboard, sails, hulls, beams, superstructures, struts, with some very different budget sizes, I encountered the limit of the theory so many times, that I feel that something new is needed in fluid dynamics for us to go further. If only we are able to listen to others.
     
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  5. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Here is a short article that explicitely give the limit of the "non-slip" condition in fluid flows:

    "The no-slip condition has been found to be inapplicable in special circumstances such as at a
    moving contact line when a drop spreads over a solid surface, or in flow of a rarefied gas through
    a pore of diameter of the same order of magnitude as the mean free path of the gas molecules"
    https://web2.clarkson.edu/projects/...es/Boundary Conditions in Fluid Mechanics.pdf

    Also, this article recalls that this "zero speed at wall" should, indeed, be splitted into two parts : The normal speed at surface, and the tangencial speed at surface. Bringing nuance in our discussion. It also recalls that the boundary conditions at wall differ from Euler to Navier-Stokes. Finally, it also gives some historical hints about the Newton's Slip, "emphasize how difficult it was (?and still is?) to understand the motion of a fluid in contact with a solid body.
    https://www.ias.ac.in/article/fulltext/reso/009/04/0050-0060
     
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  6. gonzo
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    gonzo Senior Member

    Not at all. You still don't understand what a theory is. A theory provides the framework to explain the physical world. It is useful because it generates useful designs or techniques. Just because you claim it is an impossible physical condition, it doesn't make it so. Statements of opinions are not facts. A fact would be you providing some kind of evidence that the basis of the theory is wrong. However, designs that use the theory work; that is a fact.
     
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  7. David Cooper
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    David Cooper Senior Member

    I don't want to get dragged into this so I won't bother to respond even to non-valid objections, but the thread's stuck in a rut and I'd like to see if I can help it move on.

    (1) Air molecules move faster than Mach 1 - they typically move at 1100 mph. The speed of sound is lower than that because most of the transmission is not straight line, but involves a lot of zigzagging (and this also leads to sounds becoming more muffled over distance). When we talk about the air in the boundary layer being almost stationary, that's referring to the average speed of the molecules: they're rattling about at 1100 mph, but making very little progress along the surface.

    (2) Everyone who sails should learn early on that the wind is faster higher up, so we need twist in sails. As you get closer to the ground, the speed of the air gets ever closer to zero, and this theoretically hits zero at the boundary, but the boundary layer is infinitely thin - clearly less than the size of a molecule. The molecules closest to the surface are moving a considerable distance in and out from the surface as they bounce around - far greater than their size. They will typically move 1.38 × 10^−5 cm away from the surface before hitting another molecule, so that distance might be a good way to define the boundary layer, and its speed relative to the surface will not be zero, but it will be so close to zero that it could be considered to be zero as the error will be infinitesimal - indeed, such an error is no different from the wing/sail being 2.76 × 10^−5 cm thicker than measured. So, you're dealing with a simplification which works because any errors resulting from it are too insignificant to be worth considering, unless your focus is on understanding every aspect of mechanism.

    (A large part of the problem we have with this subject is that people tend to use high level rules in their attempts to explain mechanism instead of going all the way down to the fundamentals, and the more trained they are to think using high level rules, the more room there is for magical thinking to slip into their imagined mechanisms without them realising it, as with where people assert that circulation is the cause of lift rather than a consequence of it.)

    If you scatter dust across a surface and blow over it strongly but smoothly, the dust may not move at all because the air flow is so slow that close to the surface. To get it to move, you need turbulent flow with little gusts hitting the surface downwards to lift the dust up into faster flowing layers. The fact that the dust can be removed by blowing doesn't mean that the boundary layer moves fast - it's turbulent air that sets the dust loose.
     
  8. Sailor Al
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    Sailor Al Senior Member

    Thank you, but I don't think you have.

    Here are the sticking points I raised in #531:.
    1. How thick is the stationary part of the boundary layer?
    2. To whose version of aerodynamic theory do you subscribe?
    3. Please provide a reference to one or two experiments validating no-slip boundary.
    4. Where's a) the theory or b) evidence that the air comes to rest at the interface?
    We have had some suggestions for Q1:
    @DCockey suggests it's like a bungee, if it could stretch indefinitely it would eventually become too thin to see. [ corrected]
    @gonzo suggests it's one molecule thick, but that's only for the purpose of calculus (?)
    @David Cooper suggests it is infinitely thin - clearly less than the size of a molecule.
    @Barry suggests it's the first molecular layer in contact with the interface
    A few, including @David Cooper have suggested that maybe the concept of zero velocity can be watered down from an absolute value to a matter of degree.

    I am suggesting that in the face of these irreconcilable differences, that the answer is much simpler such as maybe there is no "no-slip" boundary layer at all.
     
    Last edited: Dec 2, 2022
  9. Paul Scott
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    Paul Scott Senior Member

    has anyone studied dust on a surface during flo
    I Googled ‘ dust in airflow’ & got many hits on why dust sticks, which is kind of logical, if you think about it: sticky dirt, to electrostatic attraction, and beyond, mainly around ducts & fans. How thin does a boundary layer need to be before atomic structure matters? Is there a sliding scale between boundary layer, what, roughness, thickness, stickiness, and Reynolds number (viscosity?)?
     
    Last edited: Dec 1, 2022
  10. Paul Scott
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    Paul Scott Senior Member

  11. Sailor Al
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    Sailor Al Senior Member

    Last edited: Dec 2, 2022
  12. Sailor Al
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    Sailor Al Senior Member

    Not sure where you are going with this. Would you like to elaborate a little?
     
  13. Paul Scott
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    Paul Scott Senior Member

    The dust analogy :))which I like fwtw) has been around a long time, so I Googled it (sic ^), and was kind of astonished about the number of hits from various points of view, as well as the kinds of dust are out there & how each (re)acts. (Is there official wind tunnel dust?)

    The depth of the boundary layer is being kicked around here- non existent to thick. If it were thin to the point of atoms, some really spooky stuff might be going on? How do you measure (observe) the BL without changing it?

    Then there’s really thick boundary layers (If that’s what you want to call it), so the sub Re 100,000 stuff: different aspects of the boundary layer. The works cited in the Swift article are cool. Sub 50,000 re gets even more interesting. (And there’s been a lot of work in the last decade on it) That, and I really really like low wind sailing, 1-4 knots. So much so that our custom 40’ sled has a static Prismatic in the .53s, and a pretty high aspect rig. Apparent wind gets very sailor specific. Why Einstein liked to sail? (Do telltales change the wind speed? Or location?) Smoke helps see what might be going on, but it gets strange, esp if the air is very smooth, like when the only airflow is cold, moist, thick, slow & sliding down a smooth hill to the water.
     
  14. Sailor Al
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    Sailor Al Senior Member

    Just for clarification, it's not the thickness of the boundary layer itself being kicked around here. It's the thickness of that part of the layer, as Prandtl describes, whose velocity is zero, or equal to the velocity of the boundary. Necessary for the "no-slip" condition so essential to aerodynamic theory.
    Sounds like conditions get light and cold in your neck of the woods.
     
    Last edited: Dec 2, 2022

  15. DCockey
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    DCockey Senior Member

    I did not say or suggest the "stationary part of the boundary layer" is "much thicker than the mean free path of air (N2, O2, etc)".
     
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