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)
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  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)
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Multiple votes are allowed.
  1. jehardiman
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    jehardiman Senior Member

    Damn, dipping back into the energy tar pit again...
    Both the Flettner rotor and the pitched baseball are items that generate a force (i.e. "lift") based on external energy...just like sails...except they generate lift from the rotation of the body, vice the rotation of the flow. In all cases, it is the in fluid rotation caused by the body movement that changes the energy allocation in the fluid which makes the differential pressure that causes the net force. A baseball travels and moves in particular directions based on the energy imparted to it by the pitchers hand.
    Physics can explain the fastball’s unexpected twist, new study finds https://arstechnica.com/science/2018/11/physics-can-explain-the-fastballs-unexpected-twist-new-study-finds/
    A sail boat moves due to the flow and subsequent rotation of the fluid over the sail tempered by the hull forces. If you have no hull, then you just drift like a feather in the wind.
     
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  2. Sailor Al
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    Sailor Al Senior Member

    Simply repeating the silliness in an increasingly haughty tone of voice doesn't make it less silly
    upload_2021-4-16_17-47-34.png
    There's no way, in either frame of reference (earth or boat) where there is any air flowing from right to left (lower Vc <--- in the diagram)!
    And anyway, a wing with a zero degree (0°) AoA like in the diagram generates no lift at all, just a little frictional drag! There's no aerodynamic force being generated at all!
     
    Last edited: Apr 16, 2021
  3. patzefran
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    patzefran patzefran

    Sailor Al,
    Seems the origin of your trouble is in the wrong reading of this figure. Marchaj don't say there is a net flow adverse on the extrados of the foil. He says the presence of the foil results in a clock wise contribution to the velocity along the foil ( referred as a circulation bound vortex ) which add to the maistream flow. As the incrompressible irrotational flow equations are linear these flow solutions can be added and gives a net velocityV0+Vc on the intrados and V0 -Vc on the extrados, according to Bernouillis law this gives a differential pressure on the foil which is at the origin of the Lift. When a step incoming flow is applied to the foil, to keep the whole flow irrotationnal, an adverse vortex is created which drift thereafter in the flow to keep the total Kinetic momentum of the flow equal to zero. This vortex can be seen on the very interesting computer code solution given by Miko Brunner.
     
  4. Howlandwoodworks
    Joined: Sep 2018
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    Howlandwoodworks Member

    Sailor AI,
    In the lustration Fig. 2.13 above the small gear at the back that says starting vortex that is the power sources/motor that starts and keeps the cylinder turning.
    A page of Popular Science Jan. 1984

    upload_2021-4-16_4-35-27.jpeg
     
    Last edited: Apr 16, 2021
  5. Mikko Brummer
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    Mikko Brummer Senior Member

    If you look at the first few frames of the simulation, you can distinguish the starting vortices at the leech of the jib and the main - Marchaj's smaller cogwheel represents these, only in 3D these are vortex tubes. The cogwheel is not a bad analogy, in my opinion, of the start of the "circulation". This old sim is low resolution - attached a frame of a more recent sim, with starting vortex tubes showing both from the main & the jib.
     

    Attached Files:

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  6. Mikko Brummer
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    Mikko Brummer Senior Member

    The profile is asymmetrical - it would certainly produce lift at zero angle of attack.
     
  7. Mikko Brummer
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    Mikko Brummer Senior Member

    @Sailor Al, on a positive note, I totally agree with you about the discussion of lift-drag vs. drive-heel, when it comes to sailboats & sails. Rather than saying "a sail is not a wing", I would say a sailboat is not an aeroplane. I will try to find the time to get back to that subject/post.
     
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  8. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    Must repeat Jehardiman et al here. If you imagined Marchaj's diagram meant there is right to left flow under the illustrated foil with the starting vortex, than your understanding is so feeble there's no basis for further discussion. It surprises me that you have so much detailed knowledge of the text of Marchaj, and no comprehension at all. Start again. It is admirable on your part to want to climb higher than shoulders of the old giants we stand on, but first you must reach the shoulder. At the moment you're lost in the giant's trouser cuff.
     
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  9. Sailor Al
    Joined: Feb 2021
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    Sailor Al Senior Member

    Let me share a page that I photographed from Gustave Eiffel's fabulous book.
    Look at the results from Wing 8b. which closely resembles Marchaj's image, except it has a very slight concavity on the lower surface. Kx and Ky are forces parallel and perpendicular (Drag and Lift resp.) to the incoming air flow.
    At zero AoA, there does indeed seem to be a small lift component, so maybe you're right. (But only just!).
    upload_2021-4-17_10-38-0.png
     
  10. DCockey
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    DCockey Senior Member

    The definition of angle of attack is an arbitrary choice. Probably the most common reference is the line from the trailing edge to the furtherest point on the nose. Another for airfoils with one side flat is a line on the flat side. Then there are the references derived from a particular mathematical formula used for defining the shape of the airfoil. And sometimes the line is defined by position of the airfoil relative to the free stream which results in zero lift.
     
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  11. patzefran
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    patzefran patzefran

    Good Summary, should end this useless debate !
     
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  12. Sailor Al
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    Sailor Al Senior Member

    What's your point? AoA is normally defined as the angle between the chord and the airstream and the chord is the line from the leading edge to the trailing edge. There's not much debate about that.
     
  13. Sailor Al
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    Sailor Al Senior Member

    Far too soon, we aren't close to a consensus on my proposition.
     
  14. patzefran
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    patzefran patzefran

    Physical truth is not like democracy, it dont rely on likes, vote or blog consensus !
     
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  15. Howlandwoodworks
    Joined: Sep 2018
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    Howlandwoodworks Member

    Last edited: Apr 17, 2021
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