sail aerodynamics

Discussion in 'Hydrodynamics and Aerodynamics' started by Guest, Mar 21, 2002.

  1. CT249
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    CT249 Senior Member

    I'll be interested to see results when they line up against a fleet of comparable boats. Why they haven't done so (as far as I can see from results searches) when there is a sportsboat fleet in their home town is beyond me.

    Double surface soft sails are not new, after all. The sector of sailing which partial-length double surfaces are most commonly used (windsurfing) often rejects them due to handling and complexity issues, even for high performance sailing. Of course that doesn't mean they are not great sails (personally I love big double-surface luff windsurfer sails) but the downside should be noted.
     
  2. sandhammaren05
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    sandhammaren05 Senior Member

    It's an airfoil. The lift is due to circulation just like a wing or propeller blade. The net lift acts at the center of pressure, so you have to use the rudder to aim the boat where you want. Study wing or hydrofoil theory and you have it.
     
  3. MikeGBR
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    MikeGBR Junior Member

    I was very interested to stumble across this thread and have spent the last couple of days reading it from start to finish. Thanks to all who contributed to the discussion and shared their expertise and knowledge.

    One item that particularly caught my attention was Tom Speer’s reference (post #60) to A.M.O. Smith’s observation, in his paper on high lift aero dynamics: 'if you place a bluff body like a circular cylinder behind and below the trailing edge, you get an increase in lift on the surface due to the deflection of the flow at the trailing edge'.

    I wasn’t aware of A.H.O. Smith’s paper but had come across studies of the flow around headlands on the east coast of Australia, which found that off-lying islands eliminated back eddies.

    I have been wondering for some time whether it would be of any significant benefit to put rods (possibly split back-stays) behind a mainsail to allow the main to be sheeted harder, without stalling, to create more lift? Equally, whether the type and positioning of shrouds could have significant influence on close sheeted, non-overlapping headsails? If so, what would be the smallest diameter of the rod to be effective and how far should it be positioned from the trailing edge for optimum performance?
     
  4. sandhammaren05
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    sandhammaren05 Senior Member

    You're talking about the same effect as the flap on a wing, or the cup on a prop. Consider two flat plates at the same angle of attack, one with the flap the other without. The flap can increase the lift by about 30% depending on flap angle The flap or cup must be right at the trailing edge unless you want to mess up the Kutta condition.
     
  5. tspeer
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    tspeer Senior Member

    Getting more lift in a sail rig is easy - just add more sail area. Reducing the drag, while maintaining the same lift and height of the center of effort, is hard.

    Smith's illustration of a very large cylinder behind and to windward of the trailing edge was in no way intended to be used as an actual device. He was showing that the cross-flow produced by the cylinder changed the boundary conditions at the trailing edge and this altered the lift on the entire wing. The cylinder took the place of a slotted flap, but it was the most inefficient slotted flap possible, and was a shape that would produce no lift by itself. His point was that a real slotted flap produces a similar cross-flow, and this cross-flow accounts for the effect of the flap on the wing.

    Placing stays near the trailing edge of mainsail will not increase the lift on the mainsail. Many boats already have such an arrangement with running backstays.
     
  6. MikeGBR
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    MikeGBR Junior Member

    Thank you for your immediate and helpful replies. Back to the drawing board!
     
  7. brian eiland
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    brian eiland Senior Member

    Flaps on Airfoils, the MastFoil concept

    I see the subject of 'flaps' has come up again.

    In terms of sailing rigs (afterall we are are a boating forum, not an aircraft one,...very different Reynolds numbers), I would like to reintroduce this very interesting experimentation and implementation being carried out by Chris White

    Page 31 of this Sail Aerodynamics discussion...
    Another subject thread on MastFoil:
    http://www.boatdesign.net/forums/multihulls/chris-white-atlantic-47-mastfoil-40670.html

    ...short video:
    https://chriswhitedesigns.smugmug.com/ATLANTIC-CATAMARANS/Atlantic-47/Atlantic-47/i-KCxP879
     

    Attached Files:

  8. brian eiland
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    brian eiland Senior Member

    V-flap Foil

    https://chriswhitedesigns.smugmug.com/ATLANTIC-CATAMARANS/Atlantic-47/Atlantic-47/i-RnZMM5k



    ...a reply posted by another gentleman very interested in this technology...
     
  9. brian eiland
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    brian eiland Senior Member

  10. Spiv
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    Spiv Ancient Mariner

  11. brian eiland
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    brian eiland Senior Member

    Thanks for that link Spiv. I posted this comment on that link just now...
     
  12. brian eiland
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    brian eiland Senior Member

    Flow off that Flap

    I wonder what the flow looks like behind that flap,...would love to see a smoke tunnel image/video.

    Would it make any sense to have a small v-shaped flap like that behind a more traditional shaped, bare mast,...for drag reduction verses extra lift created??
     
  13. Spiv
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    Spiv Ancient Mariner

  14. brian eiland
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    brian eiland Senior Member

    "Think One" and Flaps

    Think one
    Flaps

    So now I am 'Thinking One' and looking at this new alternative aftmast concept I posted over here:
    Alternative Aft-mast Rig
    vizible105 catamaran rig, profile.jpg
    Looks like it might be fair to say that it presents two nice headsails to drive forward with, and a controllable flap by way of that aft mainsail/wingmast arrangement? (I might even be tempted to term that mainsail a mizzen, it is mounted so far aft).

    I even wonder if that aft mainsail/wingmast might be replaced with a more brief MastFoil arrangement operating as a flap device. :idea:

    In either case I think I would prefer this more brief arrangement on the after portion of my sailing rig than an over-tightly sheeted traditional mainsail trying to act as the flap. I've experienced over sheeted mains, and mains pulled up pass the centerline, and sails with leech control lines, and in most cases I did not see big improvements with my windward sailing,...in fact most cases less performance.


    PS: I forgot to add a link to the website (blog) of the rig's creator
    http://www.vizible.co/p/p.html
     
    Last edited: Feb 27, 2016

  15. David Cooper
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    David Cooper Senior Member

    I've been reading through this thread over the last few weeks and I'm now wondering what the current state of computer simulations is. It sounds as if the one used to create the video on page 33 takes hours or days to run and needs a network of machines to share the workload. It also sounds as if it isn't cheap to buy. I'm planning to have a go at writing my own software of this kind, but I've approached the task by working everything out from scratch, starting by thinking about collisions between air molecules rather than simply applying existing formulae. I've come up with a number of tricks for simplifying the task so that there's no need to work with individual collisions, so it's really a matter of working with numbers of molecules and vectors for average movement in boxes, and the size of the boxes can be made smaller for higher resolution results (though only near to the rig).

    There are a number of simple things I'd like to know about existing simulations first though. How long do the best 2D simulations typically take to generate end results on a single machine with different resolutions (meaning, how long does it take for the air flow to settle down: it'll take time for the stream of accelerating air to be generated round the outside of the sail, for example - I'm guessing the point of lowest pressure will work its way forward from near the leech towards the luff), and how long would the 3D ones take to do the same thing on a single machine at a useful resolution? How small do the boxes need to be before the 3D programs get results that provide a reasonable representation of the real world? My plan is to start with boxes of 1.28m sides, then subdivide to 64cm, then 32cm, 16, 8, 4, 2, 1cm, 5mm, 2.5mm, and 1.25mm around the most tightly curved components of the rig. The virtual test arena will initially be 12.8 x 12.8 x 12.8 metres in size. I'll be able to set different wind speeds and directions at different heights. One of the things I want to do is work out how the forces are transferred from the sails to the mast, sheets and other rigging and on to the hull (in addition to providing more simple results).

    I suspect the biggest problem will be simulating boundary layers correctly. It's easy to introduce drag, but the distance which the effects of it spreads over may depend heavily on the resolution used (the box sizes) next to the sails, so I'll likely have to cheat in some way. I also don't know how much resolution is needed before turbulance appears in the model, but it should be fun just to write the program and see what happens. If it ends up being useful to anyone, that'll be a bonus. If it's too processor intensive for it to be practical to do the whole 3D thing, I might restrict it to 3D slices through the rig, feeding downforce back in at the top at the same distance out from the sail so as to be able to generate figures for lift in directions other than the horizontal.

    It's already been a worthwhile exercise just thinking through the way air molecules travel about and collide with each other, because I've finally managed to work out why air accelerates round the outside of a sail. Moving a sail through the air creates empty space behind the back of the sail towards the leech. Air is then going to flow into that space, and it does so purely because air molecules can travel in that direction more easily without suffering so many collisions: they simply move in that direction for longer while travelling at the same speed as before, but the average speed of the air goes up as a result, even though the molecules themselves have not accelerated (although their speed will increase over time due to the collective effect of many collisions). What I have yet to work out is why the place of minimum pressure migrates forward towards the luff, but it must be something to do with a stream of air round the back of the sail being established over time with the acceleration leading to the area near the leech being filled up again and restoring higher pressure there as the air slows back down. That is one of the things I want to explore through simulation.
     
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