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

    OK, to recap:
    1. Anderson, J.D. submits an argument based on the isentropic acceleration of gas to prove air can be considered incompressible below M 0.3. (Which I believe is madness)
    2. I asked for an example of isentropic acceleration of gas. (OK, because I believe it's not theoretically possible)
    3. You submitted the NASA paper claiming it is an example which claims "the density of the gas remains constant" with no explanation - as a fact. (again I believe that's false.)
    4. I asked for an explanation of that fact along the lines of why the sun will rise tomorrow, or gravity is a force of attraction, or the speed of light is absolute. All of which can be justified in a couple of lines*
    You responded with a dozen paragraphs of unattributed, obscure dogma that I am just not going to grace with analysis.

    Don't you see, you are digging yourself a deeper hole?


    *
    1. The sun will rise tomorrow because the earth is a rotating spheroid in an orbit around the sun.
    2. Gravity is a force of attraction because gravity is the name of the force that scientists have adopted for the attraction force between massive objects
    3. Einstein demonstrated the speed of light is relative with a simple thought experiment.
    [EDIT] OK, I'll relent with one analysis:
    On your first point: That is NOT the definition of the compressibility of a gas. The compressibility of a gas is encapsulated in Boyle's Law: PV=Constant (later enhanced to Charles' Law PV = nRT). [/EDIT]
     
    Last edited: Oct 2, 2022
  2. gonzo
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    gonzo Senior Member

    Do you claim the NASA paper is false or that they falsified the experimental data? We are getting deep into the realm of conspiracy theories.
     
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  3. Sailor Al
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    Sailor Al Senior Member

    I don't think the paper is quoting experimental results, so I don't claim they falsified experimental data.
    I am not a "conspiracy theorist", they make claims without credible evidence.
    But it is a conundrum. Why did they make the claim? It seems to me to be incorrect and I am testing that interpretation in this thread. So far M. Cattelliot is the only responder and his explanation is patently absurd.
    Can you offer an explanation?
     
    Last edited: Oct 2, 2022
  4. Alan Cattelliot
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    Alan Cattelliot Senior Member

    I re-checked the calculations I gave. I see no mistakes.

    I think that we just find where you are stucked. The definition of the compressibility is explicitely given in : " mesure of the relative change of the volume when a pressure is applied". You cannot derive the compressibility from the gaz law itself. You must add another equation, requiring the mass conservation, before using the Charle's Law. Also, we don't speak the same language, here. In your definition, a compression of a material is an increase of its pressure. In the definition I use, a compression is an increase of its density. When seeing the pressure distribution around the Cark-Y airfoil, you may probably believe that, wherever the pressure is greater than the absolute pressure, here 101325 Pa, the density of the air increase. Which is not true, in this example.

    The sun will rise tomorrow if it doesn't explode during the night. ;)
     
  5. Sailor Al
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    Sailor Al Senior Member

    What is the source of that definition?
    What are the units of "compressibility"?
    It completely ignores temperature.
    [EDIT]
    Don't bother answering those. It doesn't matter, you are still presenting a dozen unattributed paragraphs of dogma to explain a simple fact.

    We know the sun won't explode tomorrow because of science. Don't be silly. [/EDIT]
     
    Last edited: Oct 3, 2022
  6. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Here is one, amoung plenty.
    Compressibility - Wikipedia https://en.wikipedia.org/wiki/Compressibility
    Here is a nice representation of the difference between compressible and incompressible, found in the attached paper.
    upload_2022-10-3_7-44-51.png

    it's in [Pa]^-1

    Absolutely not. In the iso-thermal case I developped, I substitute the temperature according to the Charle's Law :
    upload_2022-10-3_8-3-35.png
    With the underscripted T that stands for "Temperature", the term on the left should be read "the derivative of the density with regards to the pressure at constant temperature". Constant temperature = iso-thermal, as illustrated in the example of a bicycle pump.

    I presented rigourous mathematic calculations in relation with the definition of the compressibility and the Charle's Law. Every professor in thermodynamics will do the exact same demonstration, so I don't really know if it can be attributed to someone in particular. And I don't know if the fact is so simple. If that would be the case, we wouldn't have this discussion.

    Sorry...bad joke....
     

    Attached Files:

  7. Sailor Al
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    Sailor Al Senior Member

    Maybe it is, but it is not an answer to my question:"why is it so?" (the density of the gas remains constant)

    Your explanation doesn't refer to the NASA paper's process ("As a gas is forced through a tube, the gas molecules are deflected by the walls of the tube...." and doesn't end with " and therefore the pressure remains constant".
    That is how an explanation is structured in science (and I think that's our common language).



    P.S. your attached paper "A compressible plasticity model for pulp fibers under transverse load" has absolutely nothing to do with the thermodynamics of gas !
     
  8. Alan Cattelliot
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    Alan Cattelliot Senior Member

    No, the paper is the source where I find the diagram, that could help you to distinguish between compressible and incompressible. It seems that source is very important to you.

    Why one can forced a gas through a tube without changing its density ? It's true that I gave no direct answer. Indeed, I secretly expected that you could get your answer by this demonstration, because there is no better accepted answer that is not formulated by one self. I demonstrate that the compressibility is the inverse of the pressure. I recall : If the pressure is high, the compressibility is low. Thus, my conclusion and my answer to your question is :

    The density of the gas remains constant when forced trough a tube because the pressure is maintained constant.

    How one can forced a gas through a tube without changing its density ? By equilibrating the pressure in the tube. In the diagram below, found here Pressure-fed engine - Wikipedia https://en.wikipedia.org/wiki/Pressure-fed_engine, the tubes we are talking about are in red and in green. The pressurized gas chamber, in blue, serves this purpose.

    upload_2022-10-3_9-39-17.png

    In the example of the Clark-Y airfoil at waterlevel, the pressurized gas chamber is the earth atmosphere.
     
  9. Barry
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    Barry Senior Member

    Gas Laws: Overview https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/States_of_Matter/Properties_of_Gases/Gas_Laws/Gas_Laws%3A_Overview#:~:text=The%20gas%20laws%20consist%20of,Equation%20and%20Ideal%20Gas%20Law).

    You refer to Charles Law as PV=nRT. This equation is called the Ideal Gas Law and is the consequence of the amalgamation of Charles, Boyle and Avogadro's laws.

    It would be appropriate to use the proper nomenclature
    ________________________________________________________________________________________________________________________________

    P1V1 = P2V2
    n1T 1 n2T2

    This is the General Gas Law which is just shows that for a contained, but variable volume, Temperature, Pressure and (edit) Volume can all be changing to satisfy the equation . ie right side = left side
    Assuming that n1 and n2 are the same, then the equation becomes:

    P1V1 = P2V2 Obviously T2 should be under P2V2.
    T1 T2

    P1V1/T1 = P2V2/T2
    T1 is ambient


    So when (and I hope that I am not misunderstanding Alan C comments) he mentions Isothermal, I am taking it to mean that T1 and T2 do not change. Then you say that he is ignoring the temperature component.
    So assuming that P1 increases to P2, ie compressing the gas, then V2 and T2 will change. T2 due to the compression BUT this occurs during the transition to P2, and over time, T2 will come down to ambient T1.

    So can compression occur without an increase in Temperature?

    Theoretically no, but under the right conditions, the temperature could remain the same within extremely small values.
    That can occur with two sets of condition:
    1)External cooling of the envelope to maintain T1, or
    2) an very slow compression to allow for natural temperature adjustment to ambient.

    I believe that in a recent post or posts that you were floating the idea of "bubbles" of hot air adding to the force developed on a sail. I had said that I thought that a temperature increase would be insignificant. True to form,
    you critiqued this comment. As a "mathematical scientist" you would have been able to determine the temperature increase of based on the static pressure at the stagnation point say with a wind speed of
    15 mph BUT this temperature increased area would be mixed with so much surrounding air to make temperature irrelevant
     
    Last edited: Oct 4, 2022
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  10. Sailor Al
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    Sailor Al Senior Member

    Thank you, I stand corrected and will mend my ways.
    Whilst the pressure changes over a wing or sail that are required to generate the force are small relative to atmospheric pressure, they are nevertheless sufficiently large to drive a 6-tonne keelboat through the water at 7 knots and support a 500-ton A380 in level flight at nearly Mach 0.8.
    It is the fundamental nature of Boyles' law (PV=C for const T) that causes me to challenge the "air can be compressible [EDIT] of course, the clam is that it is incompressible! My typo.[/EDIT] below Mach 0.3" statement in the derivation of the aerodynamic force over a wing or sail.
    I have to accept the claims that the assumption of incompressibility below Mach 0.3 yields acceptable correlations with experimental results in CFD because they are widely claimed and I have no experience with which to challenge them. The fact that I can't follow Anderson's justification of the claim is personally annoying but not a show stopper.
    The fact remains that Boyles' Law does describe the practical behaviour of air. Sails and wings generate aerodynamic force and that force arises from pressure differences. The source of those pressure differences isn't explained in Anderson's book or any other aerodynamics textbook that I can find.
    Circulation theory (getting back to the subject of this thread) doesn't yield a solution to the source of the pressure differences.
     
    Last edited: Oct 5, 2022
  11. Sailor Al
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    Sailor Al Senior Member

    I have just re-read your post an realise there's a misunderstanding. They are not "bubbles of hot air", they are the soap bubbles filled with helium that I use in my explanation of the source of the pressure differences in Swept Volume Theory
     
  12. Alan Cattelliot
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    Alan Cattelliot Senior Member

    upload_2022-10-4_8-36-13.png
     
  13. Sailor Al
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    Sailor Al Senior Member

  14. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Yes, I suck in presentation... Also believe me, I try my best. It is also very good that we use computer's characters to exchange, because my writing style is far far worst .... Still, does this crappy presentation help, at least a little bit ?
    The concept of circulation together with the Kutta-Joukowski condition contains no expression of the pressure, you are right, SailorAI. You have to put Bernouilli in the game to do so.
     

  15. Doug Halsey
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    Doug Halsey Senior Member

    Still howling at the moon, I see.

    Oh well, it won't hear you.
     
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