lift without downwash?

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

  1. gonzo
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    gonzo Senior Member

    Lunatic should define downwash so we don't argue about different things.
     
  2. daiquiri
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    daiquiri Engineering and Design

    This link indicates correctly the downwash as a phenomena associated with a 3D wing, and generated by the wing-induced trailing-edge vorticity.
    It is presented in the simplistic form, where only wing-tip vortices are shown, which is incorrect. A correct illustration and description would be the one in this page: http://en.wikipedia.org/wiki/Horseshoe_vortex
    where the vorticity is considered along the whole wing span, being the strongest near wing tips.


    The illustration in that page is wrong because it shows a 2D flow around an airfoil, and indicates the outflow from the trailing edge as downwash. That is not a downwash as defined in aerodynamics. Once again, a downwash is a net vortex-induced downwards velocity component and is ultimately what makes the difference between lift forces produced by a 2D airfoil and a 3D wing at a same AoA. A 3D wing will always give less lift than a 2D airfoil at a same geometric angle of attack. The reason for this lower lift is the decrease of the AoA seen by a 3D wing, due to a net downwash. So it is IMO wrong, even from the educational point of view, to call downwash something shown on an illustration depicting a flow around a 2D airfoil. Downwash is a flow feature of finite 3D wings.

    I am enclosing two pdf files which highlight my points. They are not rigorous from the formal point of view, but are intended to help better comprehension of these concepts.

    Cheers
     

    Attached Files:

  3. TeddyDiver
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    TeddyDiver Gollywobbler

    No math, just a bit of causality.. Angle of attack produces pressure gradient, pressure gradient produces lift, lift produces downwash, downwash vorticity, vorticity drag...
    But of course it's only a point of view :D
    BR Teddy
     
  4. quequen
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    quequen Senior Member

    Just trying to clarify what I understand by "downwash" in practice (as maths and aerodinamics are far away from my knowledge):
    - Sails are vertical wings, downwash (in a sail) is the bending of back flowfield towards windward. That is associated to Lift, as I understand it.
    - I used 2D figures just for simplification, so I'll try with 3D figures now: the pit at the clouds on the cesna image is caused by downwash, as the airfow under the chopter. Lifting Line graph shows vorticities causing downwash (and lift).
    I can't understand yet why this phenomena isn't related to local flow around the wing :confused:.
    If I understand Lunatic's question well, he asks for some kind of sail (wing) that leaves the back flowfield unchanged while generating Lift at the same time.
    Any guidance is welcome...:)
     

    Attached Files:

  5. daiquiri
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    daiquiri Engineering and Design

    Quequen, your understanding is correct, and Lunatic's question regarding downwash and lift has been answered by Tspeer and by myself. It could be possible to have lift without downwash, but the sail would need to have an infinite span. Since an infinite span is not practically possible, it means that it is not practically possible to have a sail which produces lift without downwash. :)
    Your question about local airflow around an airfoil was answered by Mark Drela in his post #16. What doubts are still bothering you?
     
  6. baeckmo
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    baeckmo Hydrodynamics

    The definition of downwash in the field of aerodynamics is, as daiquiri has pointed out, the effect of the 3-dimensional flow. It is the source of tip vortices ONLY, and this is what you see in the very instructive picture of the Cessna. But that is NOT what you see in the chopper picture, that is the integrated downward velocity vectors from the rotor blade foils plus the effect of the tip vortex downwash.

    As the downwash to some extent represent a loss of lift (paid for by increased propulsion cost), a variety of methods are used to recover some of the energy in those vortices; the near-vertical tip winglets on modern aircraft just one type. The delta wing at high angle of attack is in effect making use of a series of downwash vortices that form over the upper wing surface a.s.o.

    The term "downwash", as I understand it came into use when it was discovered that those whirls have a profound and dangerous influence to an aircraft that is crossing those trailing vortices at a substantial distance after the craft that has generated them.

    So: Is lift possible without downwash? Yes, certainly, but under special circumstances. Just do not confuse it with the vertical velocity vector that is the result of the vertical momentum change that is necessary for a foil to create lift!
     
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  7. daiquiri
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    daiquiri Engineering and Design

    To fuly understand the difference between a near-field and far-field flow, and how the "downwash" in the far field of a 2-D lifting body vanishes, a simple example of a lifting circular cylinder can be used.

    [​IMG]

    A lifting cylinder is a special case of an airfoil - an ideal airfoil with no sharp trailing edge. It is a rotating cylinder immersed in a uniform fluid flow, for example a horizontal airflow. The lift thus generated is called the Magnus Effect, after the german scientist Heinrich Magnus who was the first one to describe it.

    The equations of the flow field around a lifting cylinder can be found in any textbook about aerodynamics and in many websites. For example: http://www-mdp.eng.cam.ac.uk/web/library/enginfo/aerothermal_dvd_only/aero/fprops/poten/node38.html

    The cylinder has a diameter "a", is rotating and is immersed in the uniform horizontal fluid flow with speed Vinf. The components Vx and Vy of the freestream velocity are:
    Ux = Uinf
    Uy = 0

    In the polar system of coordinates, defined by distance r from the center of the cylinder and angle theta from the x-axis, the freestream velocity decomposes into a following components:
    Ur = Uinf cos(theta) (radial velocity component)
    Ut = - Uinf sin(theta) (tangential velocity component)

    [​IMG]

    Now consider the equations 4.127 and 4.128 in the above webpage (again: http://www-mdp.eng.cam.ac.uk/web/library/enginfo/aerothermal_dvd_only/aero/fprops/poten/node38.html). They give the radial and tangential component of the velocity field in any point around a rotating lifting cylinder, defined by coordinates (r, theta).

    What happens to the flow field far-away from the cylinder? It's easy to find out - you just have to impose r = infinite. The equations 4.127 and 4.128 then become:
    Ur = Vinf cos(theta)
    Ut = - Vinf sin(theta)

    But hey, hold on! Aren't these the same speed components of the uniform horizontal freestream velocity? Yes they are! :) In other words, the far field of a 2-D lifting cylinder is made of just horizontal uniform flow, though the cylinder is producing lift. No downwards-directed flow components, no downwash!

    The same result would be obtained in case of a 2-D airfoil. So the downwash, as noted several times by now, is a far-field characteristics which doesn't exist in case of 2-D lifting bodies. Once again, it is a vortex-induced flow feature of 3-D wings which produce lift. It is what makes a difference between 2-D and 3-D lifting bodies.

    Cheers
     
  8. baeckmo
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    baeckmo Hydrodynamics

    Beautiful, Slavi. Not allowed to give you any reps, but here they come.......!
     
  9. Stephen Ditmore
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    Stephen Ditmore Senior Member

    You're technically over my head here, but I remember reading a paper about hull/appendage combined drag where it was related to the angular momentum in a 2D plane through the wake of the boat, just behind the boat. It was some sort of CFD shortcut. I think the plane was normal to the direction of the boat. Does that bear any relationship to the question at hand?

    As for what you actually get if you put tell-tales on a sail, doesn't heel come into it, not to mention twist and its relationship to wind gradient?
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I think you might be referring to the Trefftz Plane.
    http://www.desktop.aero/appliedaero/potential3d/induceddrag.html

    Forget anything on or near near the sail.
    Downwash is a far-field concept, i.e. a long way downstream of the sail.
     
  11. BMcF
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    BMcF Senior Member

    All I know about downwash is...it has typically been so important to account for, and so difficult to accurately predict, that we've always engineered in a means of changing the total angle of incidence on the aft foils of the hydrofoil craft we've designed and built.

    Although the tools to predict downwas sure have improved by leaps and bounds in recent years.
     
  12. philSweet
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    philSweet Senior Member

    daiquiri, At the risk of really buggering things up, I have to point out an important difference between the Magnus rotor and a wing.

    The Magnus rotor is receiving an external torque, which it transfers to the fluid at the surface. How much do you want to bet that the energy it receives is precisely the amount needed to counter the net vertical momentum vector that an ideal fixed wing would produce.

    At the risk of being lambasted, I really like the way wiki handled the issue-

    That is from the horseshoe vortex article linked to earlier.

    Down wash here is understood to be the total sensible deflection of air opposite lift, which can be portioned out to suit the problem at hand.
     
  13. tspeer
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    tspeer Senior Member

    That's not right. In the sailing context, we'd be talking about "sidewash" instead . For the sailor, it's probably more descriptive to say, "the boat sails in a header of its own making."

    The Park Avenue boom and other wide boom shapes were an attempt to make the boom act as an end plate to reduce the formation of trailing vortices. The presence of the boom means the bottom vortex would roll up from the bottom of the boom instead of the foot of the mainsail, thus extending the effective span of the rig.

    The air does flow downward at the foot due to the displacement of the air sideways by the sail. But that's not the flow that the term "downwash" was coined to describe.
     
  14. tspeer
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    tspeer Senior Member

    The trouble is the Trefftz plane is a theoretical construct that depends on an approximation in which the vortices trail straight back from the lifting surface. But in a real flow the wake rolls up in the far field.

    Let's say you were to make measurements within, say 1 m of the trailing edge. How might you determine what the sideways velocity would be in the Trefftz plane?
     

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I know the difficulties, but I don't use Trefftz plane methods myself.
    I just mentioned it as a possibility for what another poster was trying to express.
     
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