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. Sailor Al
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    Sailor Al Senior Member

    That is not an example of having us on, he's just playing a word game, there's no intent to have us believe his statement.
    A person is "having us on" when he/she aims to expose our gullibility by saying something he/she knows is wrong, to see if we are silly enough to believe him/her.
    It's a British phrase that maybe has not travelled well to the US. Although Marchaj was Polish, he lived in the UK and became quite a Brit.
     
  2. DCockey
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    DCockey Senior Member

    Sometimes posts say more about the person posting than about the subject of the posts.
     
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  3. Sailor Al
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    Sailor Al Senior Member

    Can we at least agree then that he was wrong?
    upload_2021-4-19_11-36-53.png
    Check the Cambridge video.
    upload_2021-4-19_11-38-45.png
    In fact the air over the top of the wing reaches the trailing edge long before the air flowing under the bottom of the wing reaches the trailing edge.
    Thus:
    No stagnation point at S2.
    No vortex generated between S2 and the trailing edge
    No counter-rotating cog to start the "circulation".
    Whether he knew this or not is open to speculation.
     
    Last edited: Apr 18, 2021
  4. philSweet
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    philSweet Senior Member

    No. He wasn't wrong there and none of your thus's follow from that fact anyway. You don't understand the time evolution of the start-up. In an accelerating system, the pressures aren't as developed as they are in steady state. So at the start-up, circulation is zero, and S2 is exactly as drawn until sufficient speed is gained. And then the stagnation point shifts with the evolving nearfield flow and developing pressure gradients.

    On startup, the trailing edge cusp triggers flow separation from the lower surface flow. Any flow separation creates a volume of entrained fluid that travels with the body (in this case, an entrained wake), and the stagnation point is either on the body where the flow is attached, or on the boundary of the entrained fluid if the flow is separated. The entrained wake evolves quickly during startup. Thus, the geometry of the surface where the stagnation point can be changes very quickly.

    If the foil was to magically go from zero to speed V instantly, the pressure gradients would develop over time. Initially, the flow at the bottom would arrive at the trailing edge ahead of the top flow. Shifting to the model's perspective, as the starting vortex and it's velocity potential disappear in the distance, leaving only the bound circulation, the top flow begins to arrive at the trailing edge ahead of the bottom flow. This only takes a distance of a chord or two, so the development is quick. The model correctly predicts both the steady state pressure gradients on the body, and the time evolution of pressures due to unsteady flow. Marchaj correctly depicts the flow features during startup.
     
    Last edited: Apr 19, 2021
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  5. philSweet
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    philSweet Senior Member

    Regarding the survey, none of the above. He was correct, but you erred in describing it. The startup condition is different from the steady state condition.
     
  6. Sailor Al
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    Sailor Al Senior Member

    So your interpretation is that the starting vortex is created at "start up" due to the "A" particle arriving at S2 when the "B" particle reaches the trailing edge, but in the steady state the "B" particle arrives at the TE after the "A" particle as shown in the Cambridge video.
    Is the startup condition instantaneous?
    Is the circulation instigated in this instantaneous "starting vortex"?
    And does this circulation survive during the steady state of a multi-hour plane trip?
    Why is there no evidence of circulation in the video?

    I assume you are deducing this from Marchaj's book, not from your own research, in which case, I think you have drawn a very strange interpretation from "the flow has just started".
     
  7. Will Gilmore
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    Will Gilmore Senior Member

     
  8. Sailor Al
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    Sailor Al Senior Member

    I suspect some serious post-production work!
    upload_2021-4-20_14-56-53.png
    Without a commentary or attribution, I'm not sure how it helps.
     
  9. Will Gilmore
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    Will Gilmore Senior Member

    I'm afraid I have nothing helpful to add of my own. I just thought this showed the phenomenon in question at the start of movement and how it transformed once lift was established.

    If you didn't see what I saw, take another look. I couldn't speak to the actual causes, but the counter circular movement is there at the startup.
     
  10. Mikko Brummer
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    Mikko Brummer Senior Member

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  11. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Nice visualization, but I think this classical visualization by Ludwig Prandtl shows the downwash and development of start- and stop-vorticies at 2.00 even more clearly:
     
  12. Sailor Al
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    Sailor Al Senior Member

    @AlexanderSahlinA great video, but what has it to do with the creation of lift on an aerofoil? There are lots of weird and wonderful phenomena when you start to drag objects through fluids: Check out the great videos of vortex rings but they add nothing to the current discussion.

    I think you are supporting philSweet in his belief that circulation is instigated in this instantaneous "starting vortex", and this circulation generates lift and somehow survives hours-long flight.
    Where's the evidence?
    I guess you would tick the third box?
     
  13. Sailor Al
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    Sailor Al Senior Member

    One book at a time, please!
    The reviews indicate it's about the scientific conflict between the Brits and the Germans in the early part of last century about the source of lift, and finding that in the '30s circulation theory prevailed. An interesting read, I'm sure for historians, but I'm more interested in the present, and what's being taught now.
    [EDIT]
    I suspect I would agree with the reviewer who opened with:"what a suavely packaged piece of tomfoolery this "enigma of the airfoil" is."[/EDIT]
     
    Last edited: Apr 20, 2021
  14. DCockey
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    DCockey Senior Member

    Circulation skepticism ...... interesting.
     
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  15. Sailor Al
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    Sailor Al Senior Member

    It's taken this long for you to get there?
     
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