Is circulation real?

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

  1. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Can we safely say the original question has been answered?
     
  2. Sailor Al
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    Sailor Al Senior Member

    I claim it is dead and buried since the link to the original referenced paper is now dead!
     
  3. Mikko Brummer
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    Mikko Brummer Senior Member

    High Lift Prediction Workshop https://hiliftpw.larc.nasa.gov/index-workshop1.html
     
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  4. latestarter
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    latestarter Senior Member

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  5. Alan Cattelliot
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    Alan Cattelliot Senior Member

    A long time ago, In my opinion ....

     
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  6. Earl Boebert
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    Earl Boebert Senior Member

  7. Alan Cattelliot
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    Alan Cattelliot Senior Member

    @SailorAI

    I've read with a carefull attention your Swept Volume Theory. As already said, I would find very nice to have a more simple theory to explain the forces acting on, let's say, a sail shape.

    Let's consider the case of a flat sail going upwind (from right to left). According to your principle, the forces acting on this sail should be proportionnal to the swept volume, which is proportionnal itself to the speed of the sail. Should we say, that, according to the swept volume theory, the force acting on the sail should be proportionnal to the speed ? In that case, the swept volume theory would not be in accordance with the basic formulas obtained by measurement, where the forces are measured in proportion with the square of the velocity. What do I miss, here ?
    upload_2022-8-29_19-7-50.png
     
  8. Sailor Al
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    Sailor Al Senior Member

    No, there is no claim in the theory that the force is proportional to the swept volume.
    You are talking about the size of the force, which is much more complex than that, indeed, in "Unanswered Questions" I specifically say:
    "..this theory doesn't lead to a simple mathematical solution..."
    and:
    "none of the current explanations have come up with the answers either."

    What I am trying to do is to get greater minds than mine to use thermodynamics (instead of fluid dynamics) to work out how quickly the temperature differences of the air in the bubbles dissipates into the surrounding air.
     
  9. Sailor Al
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    Sailor Al Senior Member

    I have found the reference for the assumption that, for aerodynamic calculations, air can be considered incompressible when below about Mach 0.3.
    in Anderson's Fundamentals of Aerodynamics.
    In the introduction paragraphs of Section 8.5 WHEN IS A FLOW COMPRESSIBLE? he asks the reader to:
    "Consider a fluid element initially at rest, say, an element of the air around you. ... Let us now accelerate this fluid element isentropically to some velocity V and Mach number M, say, by expanding the air through a nozzle.
    He doesn't present a procedure for how the air is to be accelerated through a nozzle, so I wonder if someone could please sketch an apparatus for this experiment.
     
  10. mc_rash
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    mc_rash Junior Member

    It does not matter how the air is accelerated since it's a theoretical physics question. Don't ask WHY and HOW but think about WHAT will happen.

    In Germany we call it "Gedankenexperiment".
     
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  11. Sailor Al
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    Sailor Al Senior Member

    Yes, and one of the greatest thought experiments was Einstein's one involving a train, a mirror and two observers, in which by changing the one parameter, speed, of an otherwise replicable "experimental apparatus", he could demonstrate that the speed of light was absolute.
    The point is that the apparatus of his Gedankenexperiment could be rendered in a diagram. The "thought" component, which could not be replicated, was increasing the speed of the train to a speed approaching the speed of light.

    I am asking for a diagram of Anderson's Gedankenexperiment.
     
  12. mc_rash
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    mc_rash Junior Member

  13. Sailor Al
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    Sailor Al Senior Member

    Thank you for finding that but you have to agree that it doesn't contain a sketch of the apparatus, only a single component (the nozzle).
    upload_2022-9-27_6-25-57.png
    Here is an example of sketch of an apparatus. It shows the whole system, not just the components.
    upload_2022-9-27_6-33-22.png
    [EDIT] The reason I'm looking for this is that I'm not sure that the laws of thermodynamics allow for even an ideal gas to be accelerated isentropically.[/EDIT]
     
    Last edited: Sep 26, 2022
  14. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Laws of thermodynamics are generally applied under several assumptions, in order to derive practical formulas. By itself, the notion of "isentropy" is one of these assumptions.
    Isentropic process - Wikipedia https://en.wikipedia.org/wiki/Isentropic_process#Isentropic_flow

    As a consequence, losses are also defined as the difference between an ideal process -ie adiabatic and reversible - and a real process. Das Gedankenexperiment gives us a simplification, as well as a reference for calculations. Laws of thermodynamics do allow isentropy to be possible. You can figure it by considering their mathematical formulations, all based on partial derivatives. The existence of these derivative, in the expression of the thermodynamic laws, is the mathematical reflection, so to say, of our current undestanding of how real physical systems evolve, spontaneously, or under certain external conditions. Independent systems, like ideal gases, are the subjects of the thermodynamic laws. Under the Boltzmann conditions, we care not to take into consideration systems that would not behave according to these thermodynamic laws.
    Entropy (statistical thermodynamics) - Wikipedia https://en.wikipedia.org/wiki/Entropy_(statistical_thermodynamics)

    So can we calculate all the processes that are listed in the first link, and use their results in our everyday life.
    Ideal Rankine cycle : Isentropic compression in a pump
    Ideal Rankine cycle : Isentropic expansion in a turbine
    Ideal Carnot cycle : Isentropic expansion
    Ideal Carnot cycle : Isentropic compression
    Ideal Otto cycle : Isentropic compression
    Ideal Otto cycle : Isentropic expansion
    Ideal Diesel cycle : Isentropic compression
    Ideal Diesel cycle :Isentropic expansion
    Ideal Brayton cycle : Isentropic compression in a compressor
    Ideal Brayton cycle : Isentropic expansion in a turbine
    Ideal vapor-compression refrigeration cycle : Isentropic compression in a compressor
    Ideal Lenoir cycle : Isentropic expansion

    That said, can we realize any isentropical acceleration of particules, or systems of particules (gases) ? The answer is no, because it is only a theorical state. Approaching this theoretical state is really difficult and not very usefull for any pratical applications. By scientist, who shall make precise energy measurements, are bound to make all the effort they can to be as close as possible to isentropical systems. If you want an example of really hardcore apparatus made for this, take for instance the Large Hadron Collider. 50 kilometers of tubes and connectors (including not only the main loop, but also the experiment loops), cooled near absolute zero, keeped at a pressure between 10e-10 an 10e-11 mbar.
    Grand collisionneur de hadrons https://home.cern/fr/science/accelerators/large-hadron-collider

    If you wish to have a great moment in discovering this apparatus, I recommend this film :
     

  15. Sailor Al
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    Sailor Al Senior Member

    But is it a theoretical state? Could it exist in an ideal (inviscid) world?
     
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