tip vortex thrust potential?

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

  1. lunatic
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    lunatic Senior Member

    I do a lot of dinghy rowing and should try a spoon blade but vortex tips seem a stretch, nice counter-intuitive diagrams of blade travel. Most vortex research has little direct application to sailing and theory is not optimistic, especially for up wind sailing so any potential development maybe in the wind of actual sailing. One oddity of these rigs is self sheeting, perhaps due to a lively aerodynamic center moving across a long low rig, amused myself by sailing without a sheet and thinking of applications. Go to Proafile.com for more thinking but even there, many new designs have abandon the traditional crab claw, or any sweptback vortex driven rig. So there is doubt even among the believers, but it is a fascinating subject.
     
  2. Petros
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    Petros Senior Member

    It might be possible to take advantage of the vortex lift on a crab claw type of rig to beat into the wind at a higher angle than with a conventional sail. You would need a large and very carefully made foil dagger board with a foil shape that has good stall resistance.

    This could be an advantage for going through narrow channels or trying to beat out of a harbor directly into the wind. but it will not be fast, the drag penalty is high. A conventional rig that can not point as high would likely win in an up wind race, but would take several more tacks to do it.

    Not a race boat, but for fishing or sightseeing, the ability to point real high could make sailing in tight harbors and around irregular land forms much more pleasant.

    One simple way you can generate some small vortexes on the low pressure side of a lateen or crab claw rig is to wrap a heavy cord in a spiral around the leading spar. Each coil will shed a vortex across the low pressure side, and this will prevent it from stalling at high angles of attack. It will generate a lot of lift (or drive) so be prepared to counter the heel, and that is also why you will need a large efficient dagger board.
     
  3. yipster
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    yipster designer

    that vortex generator rope -seen from behind- shifts the suction to the edge
    and on the other side now also shows bigger suction areas
    Marchaj tested all his models at 10 deg heel and fasted crabclaws i saw were proas
    must be really a lot of drag that brings almost twice as much lift down to comparable figgers
     

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  4. yipster
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    yipster designer

    was playing with picture above how pressure area moves
    got people in and put windspeed way up for quick result and pc out
    sorry for that and attached pic at 10m/s wind is more like it
     

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  5. Petros
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    Petros Senior Member

    I imagine a wing (or sail) like that would be darn near stall proof. I wonder what it does for drag, can not be good.
     
  6. yipster
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    yipster designer

    for falcon cfd: sorry i made this mesh but [​IMG] want it back, what now?

    the concorde had a hinging nose and very high stall angle landing witch was still at high speed
    yes drag but cant check drag anymore without doing calculations as falcon cfd trial expired :(
    here two more pics on the basicly poor delta planform from Marchaj's exelent sail performce book
    only beeing a interested amatuer i'm now trying to comprehend rotational vs irrotational vortexis
     

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    Last edited: Mar 16, 2013
  7. Petros
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    Petros Senior Member

    the concord has the tilt down nose only so the crew can better see the runway when landing, subsonic aircraft have a blunt nose and do not have this problem. the concord low speed configuration (for landing) depends on vortex lift formed by the leading edge extensions (or strakes) running forward from the wing leading edge alone side the fuselage, in high speed flight they do little. The large vortex allow angles of attack up to 45 degrees, which generates a lot of lift for low speed landings, but also has a lot of drag. Not a problem when you have very large engines for super sonic cruising flight.

    rotational vs. irrotational flow is not really related to a vortex. Vortex flow can be irrotional, that is not what it means when they make the statement that the CFD assumes irrotational flow. The idea of doing calculations with irrotational flow is to simplify the calculations, when you have the fluid elements rotating than you have to account for their rotational energy in the calculations, way too complex even with modern super computers. Most flow in reality is irrotational anyway, the rotation only occurs in the boundary layers along a fixed surface (right at the skin) where the fluid particles can actually rotate and tumble along the surface.

    One of my fluid mechanics professors put it this way: if you took a tiny stick and floated it on the surface of your bath tub and observed it as it approaches the vortex at the drain, it will keep its orientation (it will stay pointing in the same direction) as it rotates around and around as it goes down the vortex. vortex flow is irrotational for the most part (until it makes contact with the drain), and can be accurately modeled that way. Where you get rotation is if you put the same stick along the very edge of the bath tube where the fluid is moving along it, the stick will tumble end over end. With rotational flow you have loses that have to be accounted for, and usually shows up as drag. Most CFD approximates the drag from experimental studies, it adds fudge factors based on boundary layer thickness, rather than attempt to calculate the losses due to rotational flow. best you just ignore it, it is one of those statements used to qualify the limits of the fancy numerology used to try and determine what real fluids will do before you actually go out and test a new shape in real conditions.
     
  8. yipster
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    yipster designer

    Yup, that second half and epilog is clear, great explanation I didnt checked up on yet.
    Still fascinated tho. First half I wondered if one could be more confusing than me
    Look f.e. at the high lift blown wing; takes power! Will read your post again tomorrow
    We better be sailing but for now thanks for that swirl, think I got that now
     
    Last edited: Mar 15, 2013
  9. DCockey
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    DCockey Senior Member

    Numerous Navier Stokes codes are used everyday to calculate flows with rotation.
     
  10. Mikko Brummer
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    Mikko Brummer Senior Member

    ... on desk top.
     
  11. yipster
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    yipster designer

    as if those codes are a normal thing :eek: enough to make any man shiffer

    boundry layers yes, reading up on vortex rotational flow and playing with match sticks

    dont know how falcon calculates but we can ask and try again
     

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  12. lunatic
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    lunatic Senior Member

    Reread C.A. Marchaj's Sail Performance chapter "The Sail Performance of Various Rigs" which inspired much of the interest in vortex driven sails.http://www.proafile.com/archive/article/rig_options_crab_claw Skeptical that any of the models shown created a vortex. Page 167 "...commonly used telltales are of no use in tuning the crab claw sail...". They do help in actual sailing and clearly show the vortex. Best offwind drive increased with decreasing camber, created by high purchase downhaul and compression strut between aft extending spars to prevent any camber from flex. With all lift on flat lee side at 15* to 20* AOA, I doubt any hull configuration would help solve the high drag problem. Sailed rigs were half deltas without the power of a second LE yet were still a thrill to sail offwind. Might there be enough offwind power to justify the complication of a morphing rig from potential to vortex flow. More non-sailing vortices http://www.acsol.net/~nmasters/vortex-lift/
     
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