lift without downwash?

Discussion in 'Hydrodynamics and Aerodynamics' started by lunatic, Oct 4, 2012.

  1. markdrela
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    markdrela Senior Member

    Holy smokes. That web page is incredibly misleading.

    Their formula gives the overall flow circulation in terms of the rotational surface speed Vr at the cylinder surface: G = 2 pi R Vr
    This is the "rotating cylinder drags the flowfield along" model. Unfortunately it is completely bogus. The cylinder rotation cannot transmit a torque to the fluid past the boundary layer thickness, which is tiny on the upwind side.

    The correct model is that the cylinder rotation generates circulation by delaying separation on the backward-moving side, and promoting separation on the forward-moving side. It is the asymmetry in the boundary layer separation points -- in effect an asymmetric stall -- which creates a net circulation and a corresponding lift.

    I'd expect better from NASA. If I was the webmaster there I'd take down that page, or at least fix it up.
     
  2. PI Design
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    PI Design Senior Member

    Good to know NASA know what they're doing!
     
  3. Remmlinger
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    Remmlinger engineer

    Leo, I could not resist to compare the test data from your report with my "work-horse" which is K├╝chemanns modified lifting-line method. The result is attached and I think for engineering purposes it is absolutely sufficient. On top of it the method requires only milliseconds computertime.
    Uli
     

    Attached Files:

  4. daiquiri
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    daiquiri Engineering and Design

  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks, Uli.
    I have a lot of time for Kuchemann, Gersten and some other German work on
    wing theory from that era and the 1960's.

    There are several very good approximations like Kuchemann's that I have
    included in that draft compendium. I will expand it when I get time.

    The approximate formula for elliptical planforms by Hauptman and Miloh is
    also very good and, with the approximate formula for the elliptical integral of
    the 2nd kind, it can all be done in Excel. That also only takes a fraction of a
    second.

    I haven't had time to see how those engineering approximations translate to
    other, more complex, calculations, such as estimating the leading edge
    singularity strength etc, which are important for the planing splash problem.

    Even Vortex Lattice Methods can have some difficulties when you examine
    them with a numerical microscope. There are some funny kinks that appear in
    VLM calculations of the behaviour near the leading-edge which require some
    tricks to eliminate them.

    I couldn't get AVL running on my computers here. It will be interesting to see
    if Drela et al have eliminated those kinks, or whether it is unimportant
    because they are only really interested in integrated quantities like the lift.

    I was having such a gentle, relaxing time programming a new version of
    Flotilla to handle Surface Effect Ships, and now I have this distraction to
    wrestle with. I should thank you, I guess. :)

    All the best,
    Leo.
     
  6. Will Fraser
    Joined: Feb 2014
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    Will Fraser Senior Member

    Apologies for dragging this thread out of its peaceful slumber in the archives.
    I was just reading an Arvel Gentry article (The Origins of Lift, 2006) that mentions that "deflecting the air downward" is a three-dimensional effect. It did not sound right, so I ran a quick cfd on a foil with 1m chord and stretched the computational domain out to 150m behind with 25m above and below. I still end up with a net uniform vertical velocity component in the wake far aft- see pic attached.

    Is this different from the previously discussed definition of "downwash"?
    Or does the key to "no downwash" lie in the fact that an infinite wing that produces a finite amount of lift require the lift per unit span to approach zero?
     

    Attached Files:

  7. philSweet
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    philSweet Senior Member

    What were the boundary conditions? If you let everything escaping out the bottom reappear at the top, you are modelling an infinite stack of airfoils, and that has a different sort of 'farfield' law than a single airfoil does.
     
  8. markdrela
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    markdrela Senior Member

    What he said.

    Will Fraser:
    If you are simply setting freestream flow at the inflow, then the right way to do this study is to increase the flowfield grid equally in all directions, so that it remains a square with the airfoil in the center. You should find that as you make the box bigger, the downstream downwash magnitude gets smaller in inverse proportion: Doubling the box size halves the downwash. In the limit of an infinite box the far-downstream downwash is zero.
     
  9. Will Fraser
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    Will Fraser Senior Member

    Thank you, my results check out.
    It is really nice to have the Clever League on speed dial.

    How does Tom Speer's comment in post #18 tie in with all of this? It reads as if finite lift is a condition for no downwash. If I am measuring a finite amount of lift on my 2D unit of span, does that not translate to infinite lift over an infinite span?
     

  10. markdrela
    Joined: Jun 2004
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    markdrela Senior Member

    The 2D wing is "less infinite" than the unbounded flow around it. Or another way to look at it: A 1-meter span section of a 2D wing is finite and has a finite lift, but it imparts this finite lift force to an infinite amount of fluid below and above it. So the resulting velocity of this infinite amount of fluid is zero. This is partly arm-waving, but should get the point across.
     
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