tip vortex thrust potential?

Discussion in 'Hydrodynamics and Aerodynamics' started by lunatic, Mar 2, 2013.

  1. lunatic
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    lunatic Senior Member

    Having trouble putting a leading edge vortex on a forward surface to improve upwind performance of a vortex driven low aspect sweptback rig. Leading edge vortices readily form on the drag producing backside, but the high lift makes some great off wind sailing with surprisingly small sail area. Can a leading edge vortex (LEV) be made to form up against a forward facing surface, or do I have the wrong vortex?
    Boeing 787 sweptback wing tip seems (found only photos, no drawings) to have extreme washout to neg AOA. If, under the tip vortex, would this forward facing surface produce some thrust? The small LEV on the underside is drag, but the large tip vortex on the right planform might more than compensate, and the overall wake might be reduced by some cancellation in the intersection of the counter rotating LEV and tip vortex.
    Any examples of the energy from tip vortex being recovered for useful work on a fixed (non-
    flapping) wing, or any vortex on a forward facing surface producing thrust? Here are 2
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19860017728_1986017728.pdf
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810009463_1981009463.pdf
    In the first on page 226, fig 6 first graph, flap at 10*, suction is increased forward, decreased aft, a direction towards all thrust, no lift and perhaps better wind ward performance.
     
    Last edited: Mar 3, 2013
  2. Submarine Tom

    Submarine Tom Previous Member

    Cool project 'tic!

    Hope you get some good advice soon.

    Unfortunately, it wont be from me.

    I am not knowing.

    Good luck!
     
  3. Erwan
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    Erwan Senior Member

    Hi Lunatic,

    Try to Googlize "Split Tips + Induced drag",

    Usually, depending on the configuration, wind-tunnel tests claim better L/D by 15% to 25%,

    So it is a bit equivalent as transforming tip vortex drag into lift ?

    That is just a suggestion, I am not a CFD engineer; You'll find much more clever people than me on this forum.

    Good luck in your research

    EK
     
  4. Petros
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    Petros Senior Member

    This is a much more complex issue than you think. Vortex generators ALWAYS costs you drag. the reason they are used on large aircraft is because of inadequate design and modeling tools during the design phase, after the first flight, when all the tooling has already been built and aircraft are in production, they find large separation bubbles near the ailerons (very dangerous, risk of flutter and loss of control). Too late and too costly to redesign the wing, so they install the vortex generators to keep the flow attached in these areas, to maintain aileron effectiveness. Occasionally the drag of the vortex generators is less than the drag of the separation bubble, and overall drag is reduced. If the design tools were effective it is always better to design not to have to use vortex generators, only in a very few examples is the use of vortex generators done as part of the original design. I think their appearance means a fix needed to be done after they realized their design methodology was inadequate.

    This is not converting a vortex into thrust, it is reducing drag by keeping the airflow attached. Reduce drag and the L/D improves, which is the primary driver in an aircraft design.

    Vortex generators are also used to improve control effectiveness at low speeds, when not enough air is flowing over the control surface (ailerons, rudder, etc), by energizing the flow over the surface with vortex generators you improve low speed control. the cost of this extra control is increased drag (at all speeds), that energy to create the vortex comes out of the thrust of the engines. So there is no such thing as a free lunch, and there is no such thing as free lift. Drag always goes up with the use of vortex generators.

    Vortex generators are almost always used to correct a design mistake, or to compensate for other more important design considerations. The "vortilons" used on the Rutan Long EZ is an example, it greatly improves low speed control and stall behavior on the swept wing. The swept wing was a necessary design element to get the wing tip rudders further aft, (as it is in a transonic aircraft to avoid sonic speeds on a high speed wing). These vortex generating devices have minimal effect at normal cruise speed since they are mostly aligned with the airflow in cruise condition, but at high lift conditions (low speed) there is a lot of span wise flow (undesirable but it comes with swept wings), so large vortexes are generated to improve elvon/aileron control and prevent main wing stall. But they too cost a few knots of cruise and top speed, their presence harms high speed performance, but a cost well worth it to improve low speed safety.

    As far as you idea for a sail, I see no advantage. If you know the flow that well over the sail, why not just redesign the sail for better performance without the vortex generators?

    I am not sure I am understanding your use of "backside" or front-side of sails, the low pressure (lee) surface generates most of your drive (or lift). Separation seldom occurs on the high pressure side (windward side), so to keep the flow attached you would want vortexes to form on the low pressure side. This can be done with vortex generators on the sides of the mast, and will improve the ability to point higher into the wind, and prevent stall, but at a lot of increased drag at all points of sail. Most conventional sails however have poor shape where the mast meets the sail and at the trailing edge (roach) of the sail, so much of the sail has large areas of separation, as much as 30 percent or more. Optimized racing sailboats are another story of course, but I am referring to conventional sails on your typical recreational sailboat, large and small.

    Not sure it would be worth it to have vortex generators, you can point higher, but you forward speed is diminished. With a good sail/mast design you will have a better L/D meaning that you will move faster but not be able to point as high. If you were in a race, and it was all up wind, it might save you one or two tacks. If that if faster overall than a faster boat that has to make an extra tack is hard to tell, possible but not assured. You will be slower at all other points of sail, so the boat without the vortex generators wins, unless perhaps the whole race was done up wind.

    There is no way to "convert" drag into thrust, drag is always bad. In high lift/slow speed conditions, the drag is not as harmful, so when trying to out point another sailboat, vortex generators and other means of generating more thrust (or lift), might get you an advantage, but that is the only condition that will. Reducing drag and reducing areas of separated flow by refining the shape of the sail and mast are usually much more productive, and the direction most racing sailboat designers go.

    It is important to understand the forces involved and the mechanics of the flow before you put things on your sail by copying what you see in commercial aircraft. I hope this helps. good luck.
     
  5. DCockey
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    DCockey Senior Member

    Winglets, raked wing tips and similar configurations at wing tips can reduce drag epending on the constraints on the wing design if properly designed and implemented. Probably the most common explaination is they do so by increasing the effective aspect ratio of the wing.
     
  6. lunatic
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    lunatic Senior Member

    Not using small vortex generators (horns of ignorance) attached to surface but mainforce vortices such as large LEV formed by low AR sweptback planforms for their high lift but drag due to flat lee ( backside) aft facing surface ruins windward performance. If the strong LEV suction could be removed from the lee side to a forward facing surface, there would be, ideally, forward thrust with no heel,see links. Low pressure from a resultant tip vortex, if on a forward facing surface, could also produce thrust, but would it outweigh the drag penalty? Sail a LEV driven rig off wind and you would be delightfully surprised, but not up wind. There may not be a solution for this , but the process has been instructive through responces like yours thanks.

    It is important to understand the forces involved and the mechanics of the flow before you put things on your sail by copying what you see in commercial aircraft. I hope this helps. good luck.[/QUOTE]
     
  7. Petros
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    Petros Senior Member

    I am sorry, I just went back and reread your starting post and looked at those old NACA papers, I did not quite understand what you are asking. I am not sure you are reading the reports properly, the LE flaps they use actually kill the lift by reducing the size of the vortex on the upper surface, and thereby reducing drag. In the early days of delta wing aircraft vortex lift was not that well understood, highly swept wings would form large vortexes on the low pressure side (upper surface) of the wings when at very large angles of attack (like 40 deg, way over the normally stall angle of 15 deg). These large vortexes would keep the flow attached on the wing and generate large amounts of lift. This high angle of attack made it hard for the pilot to see forward, but allowed the high wing load fighters to land a relatively low speed. I think this flap was an attempt to bring the angle of attack down so the pilot could see, and it appears there was even an idea to use them for roll, pitch and yaw control (the TE flaps or elvons typically loose effectiveness that far back on the wing at low speeds). The trapped LE vortex would created an effective large radius LE so the flow would create more potential flow lift and less vortex lift, this is how the L/D is improved.

    I do not see this idea as beneficial at all to a sail plan. The vortex lift is effective for small sails, but it comes with a large drag penalty, if you kill that lift with a LE flap, you also will kill your forward drive. You might achieve a better L/D, but your total drive force will be reduced. Although sails work the same way wings do, their beneficial effect is somewhat different. On an aircraft L/D is the main driver, they had to use the delta wing so it would fly at supersonic speeds, but in the subsonic cruise mode it was not a very efficient flyer. On a sailboat at low speeds and pointing high into the wind, the best lift you can generate drives you forward, drag is not quite as critical because of the lower speed. But at higher hull and winds speeds in a reach, L/D becomes more important on a sailboat. In a reach the drag create more heeling moment, which limits the total amount of drive (or lift off the sail) you can use.

    If you want to improve your sails performance you need to improve the flow over it with an improved shape, to take advantage of all of the sail area you have available. Older designs tend to have large area of the sails with separated flow (doing nothing but creating drag), with as little as one third actually creating useable drive. Higher aspect ratio, better shaped mast at the mast/sail junction, full battens with a cut to maintain a better shape, elimination of the sharp peak, etc. all improve sail performance. this is the direction all performance sailboats have gone. Traditional rigs evolved from square riggers, and were discovered by trial and error, with little understanding of the forces that generate drive.

    Many outdated ideas still linger in many sailors vocabularies from the older inadequate understanding of fluid mechanics. It always drives me crazy when I hear some old salt say, "you want more curve in your sails in light winds so you catch more wind". Arggh! the sails "catch" the same amount of "wind", you want more camber to accelerate the low amount of mass crossing over the surface. the thrust of the sails come from accelerating the mass of the air over the surface. F=ma, the more curve or camber in the sail, the higher the acceleration, the more thrust.

    To be fair, fluid mechanics is not intuitively obvious, that is why it took so long to develop controllable aircraft, and efficient propellers. Yet sail boats have been around since the dawn of recorded history. they only worked well downwind and the earliest sailboat had to have rows of oars so they can move the boat around. I just wish the sailing schools would teach it correctly and this obsolete terminology will eventually go away.

    Anyway, could you please try and articulate what you hope to achieve by putting a LE flap on your sailing rig? did I understand it correctly, you think it will improve your drive?
     
  8. yipster
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    yipster designer

    By now i'm curious enough to open the doc's above later today as i'm still confused its about wingtips, leading edge profiles, delta wing, vortexgenerators or leading edge extensions. Once there was the concorde and many fighterplanes use vortex lift, did read up on that few years ago but still puzzled by Marchai on the crabclaw sail. Vortex lift gave exellent figgers and wasnt a windtunnel error he mentioned before taking up para gliding. Scale model crabclaw proas go like rockets so whats the mistery? Cant belive its only convention we hear so little on vortexlift. Than again still have to open the docs, they might be on another subject alltogether ;+)
     
  9. Erwan
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    Erwan Senior Member

    Lunatic,

    Re-reading your post, I realized, I didn't catch the LEV;
    So here is a question: is your concept similar to what Marchaj observed a long time ago with the crab-claw rig?
    Regards

    EK
     
  10. Submarine Tom

    Submarine Tom Previous Member

    As I recall from reading the downloads days ago, they regard slats at the leading edge which are proven effective.

    Wing tiplettes save ~2.7% fuel on commercial airlines.
     
  11. lunatic
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    lunatic Senior Member

    Yes,but Marchaj does not describe the large drag of lift on the flat lee side which makes these rigs so poor upwind and hinders interest in there development. In ideal upwind sailing all force vectors would point dead upwind along the line of travel. Very little of a "lift" vector does this in laminar (potential?) flow. Trying to place the intense suction of a vortex on a more forward facing surface to produce more thrust, less heel and leeway without too much drag penalty. Just begun model tests but skeptical of success without much theory to back it up.
     
  12. yipster
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    yipster designer

    yup, those docs are about vortex lift as Marchaj tested and i've been asking about in the exellent sail airodynamics thread
    have a delta kite hanging above my head here ive been testing with as well as in falcon (good till the march 15) both fun but no results
    from what i checked; vortex lift likes a given degree of sweptback delta, a sharp leading edge bend like a trapezium to start the vortex
    LEX or leading edge extensions, slats or canards give high lift for less drag than flaps if i stil got that rite
    did drew slats on masts but must be reversible like the desired leading edge on a delta vortex on a sail, not as easy as inflatable battens or sails
    very complex but interesting stuff and above papers discus a slat / vortex combination i belive while i simply wonder..
    if Marchaj might have missed something and if a well set crabclaw is indeed superior, interesting enough me thinks :p
     
  13. DCockey
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    DCockey Senior Member

    Having the aerodynamic force from the sails point upwind would be very interesting.
     
  14. tspeer
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    tspeer Senior Member

    A vortex flap can reduce the drag by tilting the skin under the vortex forward, but drag will never be as low as when the leading edge has attached flow and you get leading edge suction.
    I don't think you will get any drag reduction by producing a vortex on the pressure side and trying to get it to cancel the tip vortex. The combined vortex would have to be the same size as before because that is dictated by the velocity across the wake, and that velocity is due to the lift. So any reduction in tip vortex strength would have to come at the expense of lift.

    Given that a vortex can't just end in the fluid, the leading edge vortex would trail back and merge with the trailing vortex in any event. Unless it was bounded by some structure, such as fence or winglet.
     

  15. Petros
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    Petros Senior Member

    you are indeed confused, not unusual because fluid mechanics is not intuitively obvious so do not feel bad, it just goes with trying to understand a complex issue. Potential flow lift is what I call the lift that comes from air flowing attached over a curved surface, straight clean flow is the most desirable. It can be turbulent or laminar, and still be attached and still generate lift. Laminar is clean flow where the layers are not mixing, turbulent is where the layers are mixing, but it is still attached to the surface. If it becomes detached, you loose lift and drag goes up. However if you create a trapped vortex, you can also generate lift. It creates lift by curving or accelerating the far field flow, and the vortex sort of creates the curved surface for the flow to curve around.

    It is curving, or accelerating, the mass of the air that creates lift. you can accelerate air with a curved surface, or with a vortex. The problem with vortex lift is it take a lot of energy to create, so drag is always higher with vortex lift. In vortex lift you also have very large separated areas over the surface.

    Your idea appears to be to create a vortex on the upwind side of a surface (the sail) to create up wind drive. The energy it takes to make the vortex has to come out of the wind stream working against the sails, which means in order for that to work you have to get more energy (in the form of upwind drive) out of the wind than the wind is putting into creating the vortex.

    So, how is it again you propose to get more drive out of the sail than the wind is putting into it?

    Those technical papers propose to reduce the drag of the delta wings by weakening the upper surface vortex by created a smaller leading edge vorex with the LE flap, this alters the flow over the upper surface to weaken the large vortex. I do not think this is what you had in mind.
     
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