Flow pattern video of experimental sail needs analysis.

Discussion in 'Hydrodynamics and Aerodynamics' started by lunatic, May 18, 2011.

  1. lunatic
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    lunatic Senior Member

    The flow pattern in drawing 1 and at 20* -30* angle of attack (AOA) in videos http://www.youtube.com/watch?v=z91al4uBm9g consistently appears during best performance in sailing telltaled prototypes (drawing 2). These flat sails have no forward facing surface except 1 1/2" leading edge (LE) round spar, yet seem unexpectantly handy to windward despite drag of backside lift. Sharp LE rigid wings were poor performers on all points of sail. I have no velocity or pressure measurements for these flows but a 27 sq ft sail would surf a Sunfish in 15 -20 mph wind. Seems to have some potential, but what is going on here?
    Much of lee surface flow is spanwise (accelerating? attached?) up into leading edge vortex (LEV) and some chordwise on aft expanding area above foot with tip vortex below. Upper flow exiting trailing edge (TE) is pulled up into trailing LEV (drawing 1). Would there be any advantage to pulling all disturbed air into a single trailing vortex, with all surface flow and all TE flow up into LEV? What happens to downwash, induced drag, circulation, closed vortex system? What would be going on here?
    These sweptback rigs have a pronounced delay in response to wind gusts, I assume this is time for LEV and subsequent surface flow to take effect, and may be related to some unique handling characteristics. Model 2 of video has LE sweepback similar to sailing prototype B in drawing 2 which had exceptional tacking ability. Initial intense vortex lift, starting at apex, has large turning moment resulting in tacking speeds that could catapult an inattentive sailor from the boat. Other prototypes were self-trimming, some sailed without a sheet. The aerodynamic center seems quite lively in these rigs and might be exploited for some smart handling.
    All models in video have a triangular section LE spar, base facing forward, for upwash on more forward facing surface and a vortex inducing sharp edge (drawing 3). Model 1 and 2 are rigid flat plates, model 1 has tear drop section stiffening foot. LE spar of model 3 is straight for ease of construction and extends above output of grated fans producing ambient flow but shows LEV forming over cambered sail for additional forward facing surface. I assume spanwise flow is accelerated into LEV without the constraint of pressure recovery and attachment might be maintained by rotating LEV (drawing 3). Looks to have potential for high lift, but for what drag? All this forward facing surface depends on minimum initial sweepback yet enough for good LEV formation.
    Unfortunately, this seems to conflict with single trailing vortex idea as sailing prototypes E and F (drawing 2) with greater sweepback had more flow up into vortex, though heeling in actual sailing may be a factor. No prototypes had adjustable sweepback while sailing since a graduated sweepback would be fairly constant for good initiation and maintenance of LEV? and a rising apex would eventually create a foot LEV, competition for surface flow and another trailing vortex in the wake.
    Reducing lower aft area by shortening foot and increasing TE rake produced more upward flow in model 2 with hollow leech and model 1 with fence, less so with only straight leech. Same aft cut on model 3 with quickly improvised very baggy sail resulted in only thin line of chordwise flow on aft half of foot. Cupped leech looked very draggy but, with telltales streaming parallel to TE, is this really a TE? seems more of a tip in need of a fence. Last video images show fence on cut away model 3 with all flow up into LEV and only a weak tip vortex, almost a single vortex system.
    These flow patterns look somewhat structured but, turned so far from ambient flow, is any advantage gained. Future direction hard to determine with so many assumptions and questions, and as I barely have a grip on classical theory, I could use some feedback before building another prototype.
     

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  2. gonzo
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    gonzo Senior Member

    The video seems to show a rigid sail. That would make its behaviour very different from a soft sail. Also, you only show one side of it. In which ways do you claim it is different from a lateen or crabclaw sail? Have you tried putting telltales on any other sail for comparison?
     
  3. yipster
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    yipster designer

    i went kiting delta wings, try'd telltales without getting much insight
    havent taken the fan out yet, but find this interesting stuf, busy with other things tho but keep me posted!
    have you seen the testrig Marchaj used and illustrated in his book seaworthines the forgotten factor?
     
  4. lunatic
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    lunatic Senior Member

    Model 1 and 2 are rigid, model 3 has a soft sail though, in last part,it has a rigid TE fence. Windward flow was chordwise parallel to ambient. No claims here, flow is unique to these various planforms and likelly exist elsewhere.
     
  5. lunatic
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    lunatic Senior Member

    In Marchaj's "Sail Performance", chapter 11, The Sail Power of Various Rigs, he warns that "commonly used tell-tales are of no help in tuning the crab claw sail..." but gives little warning of the backside drag of LEV rigs, and favors axisymmetrical flow.Chapter ends with "Such research, although interesting, was beyond the scope of our investigations". Seems still work to do.
     
  6. brian eiland
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    brian eiland Senior Member

    Hi Lunatic,
    Am I to assume that the video posted, and the text are yours?

    Interesting little test rig set-up you had there. I'm playing around with some model test ideas myself
    http://www.boatdesign.net/forums/sailboats/scale-model-testing-sailing-rigs-outdoors-52977.html

    If that was your test rig, can you tell me more about it,...fan source, wind deflector, etc. Are you, or have you done more testing beyond this video?

    I have to look at the video again, but it appears as though your foil has a squared off leading edge as would a block of foam would have,...or is that some sort of optical illusion on my part.
     
  7. Petros
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    Petros Senior Member

    the "sail" is acting like a vortex generator. vortexes generate a lot of lift, but they are "costly" in terms of drag. Not an efficient way to generate lift, but it is very stall proof that is why they use leading edge extensions on many fighter aircraft to generate lots of lift for high g maneuvers, the flow stays attached and the aircraft in control.

    There is no new discovery here, you have to put some means of measuring forces on it to know if it will actually work better than a conventional sail. I would expect you to find it has very high drag in relation to the lift it generates (low L/D).

    Some of the statements in the video seem to indicate the person is not familiar with fluid mechanics. He said something like "down wash is eliminated", if you have no down wash you have no lift. So that can not be true if he expects to drive a boat with that rig.
     
  8. lunatic
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    lunatic Senior Member

    These tests were just to establish flow patterns, flow was from a 36" fan through 2X2X2" grate. No more model testing, but have had fun sailing these rigs, backside drag though problem upwind. Squared off LE is attempt to put vortex on forward facing surface, failed to do this despite many configurations.
     
  9. lunatic
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    lunatic Senior Member

    Wet behind the ears in fluid mechanics! Hard for me to see the down wash when most of the surface flow is pulled up into the LEV? Some delta LE extensions present a more forward facing surface to reduce the backside drag. These rigs are a delight to sail off wind, have surfed a Sunfish hull with a 27 SF sail but I can't solve the backside drag problem or even find the down wash.
     
  10. daiquiri
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    daiquiri Engineering and Design

    The downwash is the velocity component perpendicular to the free stream inflow, observed far behind the finite-span sail or wing, after all the local effects (like the LEV-induced transverse flow components over the sail surface) have disappeared. That's why you can't notice it in your experimental setup.

    You might want to check this thread for more info: http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/lift-without-downwash-44937-2.html

    Cheers
     
  11. brian eiland
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    brian eiland Senior Member

    Lunatic, you ought to find this link interesting,...recently posted over on the aft-mast discussion since it also has some Delta sails cotributions.

     
  12. lunatic
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    lunatic Senior Member

    No experience with tankers but best performance sailing a delta on a Sunfish hull was with no camber and only offwind, high lift vortex forms on back side of sail and I have failed to move it onto a forward facing surface. Seagate claims upwind performance with cambered low aspect rig?
     

  13. brian eiland
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    brian eiland Senior Member

    Confused me also..
     
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