Was Marchaj having us on?

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Apr 12, 2021.

?

Did Marchaj know he was wrong when he claimed, on P199 in my post #63, that "A arrives ...before B".

  1. Yes, and therefore he was "having us on".

    100.0%
  2. No, he didn't understand that the air flows faster over the upper surface.

    0 vote(s)
    0.0%
  3. He was right, air flows travels over the respective surfaces at equal speed.

    0 vote(s)
    0.0%
  4. He confused A with B. (The pic shows B arriving at the TE before A!)

    0 vote(s)
    0.0%
Multiple votes are allowed.
  1. DCockey
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    DCockey Senior Member

    I'm not skeptical about circulation being present when an airfoil is lifting. I was referring to your apparent skepticism.
     
  2. Mikko Brummer
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    Mikko Brummer Senior Member

    So it does... my example was not very good, actually, since it appears to be a potential flow viz.

    @Sailor Al - In my opinion, you should take a somewhat more humble attitude: In Finnish, we have a saying "Acting with the confidence of total ignorance" - not saying that you are doing so, but always good to remember. Ludwig Prandtl was one of the great minds of the last century, and possibly the greatest mind within aerodynamics ever. Read the Enigma, I suggested it because besides giving a solid base on aerodynamics, it is fascinating reading. Thank you @DCockey for recommending it years ago.
     
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  3. Remmlinger
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    Remmlinger engineer

    Dear Mikko,
    you are like a good professor who cares for all students and wants to leave no one behind. Normally this is rewarded with gratitude. But sometimes there is a teenager in the class, who has a problem with authorities. He is too lazy for serious studies. Instead, he is proud of his fearless attacks against the peers, only to disguise his ignorance. For the benefit of the whole class, the professor should kick this pupil out. This is my experience at school, it can not be transferred to a forum like this, but may be it could start some thoughts.
    Best regards
    Uli
     
  4. AlexanderSahlin
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    AlexanderSahlin Junior Member

    What you see in Prandtl's movie is the downwash generated by the lift of the airfoil and how this downwash together with the fluid behind and in front of it generates a starting vortex and also a stopping vortex when the airfoil has stopped generating lift. So it has very much to do with the generation of lift.
    Yes, in the potential flow theoretical model the starting vortex will survive forever. In real flow it can survive quite a long time, but after some time (typically seconds to hour depending on size) it will vanish because of friction.
    Concerning your poll about C.A. Marchaj I prefer not to participate. Marchaj has done important and sometimes very creative contributions to research on sailing. See e.g. his experiments on rolling sailboats on a dead run in Aero-Hydrodynamics of sailing.
     
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  5. Sailor Al
    Joined: Feb 2021
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    Sailor Al Senior Member

    BUT,But, but...According to Marchaj, it's the circulation that generates lift! What happens in your model when the circulation vanishes.... does the aeroplane drop from the sky?
     
  6. Mikko Brummer
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    Mikko Brummer Senior Member

    Yes! And the sailboat starts drifting with the wind.
     
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  7. mc_rash
    Joined: Aug 2020
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    mc_rash Junior Member

    @Sailor Al please find the real solution behind the physics of foils, lift and circulation and write a book over it.
     
    Last edited: Apr 22, 2021
  8. AlexanderSahlin
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    AlexanderSahlin Junior Member

    I shall make a last attempt to answer your question in a serious way. The potential flow theory is not "my" model, although I have used it, because it is very convenient and powerful under such conditions where it is valid. The potential flow theory was developed some 100 years ago, and have been used by probably millions of aerodynamicists. In the potential flow model the starting vortex does not vanish and the aeroplane will fly as long as you have circulation around the moving wing.
    In the real world, where you have friction, the starting vortex will vanish many wingspans behind the aeroplane, but the wing will go on generating lift and the pressure-field around the wing will go on decelerating the flow below the wing, accelerating the flow above the wing, generating upwash in front of the wing, downwash behind the wing, resulting in a bound vortex (or circulation) around the wing and tip-vorticies behind the wing.
    Sailor Al, you seem to be quite interested in understanding aerodynamics, and we have suggested some litterature, that can explain the subject in more detail than is reasonable on a forum like this. I am also sure there must be someone who gives courses in aerodynamics in the Sydney area or maybe on the internet. Although I don't teach aerodynamics, I have understood that some other members on this forum do.
    Good Luck!
     
  9. patzefran
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    patzefran patzefran

    Alexander, to be complete the starting vortex vanishes, but it has diffused in the fluid in many small vortex and his energy and Kinetic momentum is still there.
     
  10. patzefran
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    patzefran patzefran

    I mean Angulal momentum , of course !
     
  11. patzefran
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    patzefran patzefran

    Angular !!!!!!!
     
  12. AlexanderSahlin
    Joined: Oct 2014
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    AlexanderSahlin Junior Member

    Thank you patzefran, yes the angular momentum has to be conserved. When you stop generating lift this is fulfilled by the stop-vortex.
    Btw. there is an edit-button, so you can edit your own messages you have posted. Very practical.
     
  13. Sailor Al
    Joined: Feb 2021
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    Sailor Al Senior Member

    But back to my point about whether Marchaj was having us on.
    It would seem that his use of the word "circulation" in the first 31 pages of Part 2 followed Lewis Carrol's Humpty Dumpty quite nicely:
    “When I use a word… it means just what I choose it to mean – neither more nor less.”
    A word search for "circulation" is revealing (download a digital copy and do it yourself, I posted a link earlier).
    On page 186 he talks about "the velocity of circulation Vc" in relation to the speed of the fluid rotating around a vortex, explained with a photo of a tornado.

    On P189, he ventures a definition and a formula* for circulation:
    upload_2021-4-23_13-0-51.png

    On P190, he equates Vc and Gamma:
    upload_2021-4-23_13-1-35.png

    By P192 he is lost in the the belief that that circulation exists as a physical phenomenon:

    upload_2021-4-23_13-2-10.png


    His four redefinitions is most Humpty Dumpty-like!

    *In vector calculus, circulation is the line integral of a vector field around a closed curve, its units are L^2T^-1 e.g metres squared/sec.
    (And oh, yes, since we don't know the vector field of the airflow round a wing (or a sail), we have no function to integrate, but that's another story. )
     
    Last edited: Apr 22, 2021
  14. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    You do know that Lewis Carrol was a professor of logic.

    It looks like Marchaj was doing exactly what he meant, defining circulation to mean what he meant it to mean and making sure his readers understood it meant no more and no less.

    I do like the way you are talking his text apart. Brilliant, really.
     

  15. Sailor Al
    Joined: Feb 2021
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    Sailor Al Senior Member

    Yes, but I don't know about Marchaj.
    Does that mean you're coming round to my point of view?
     
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