Vortex lift on fractional rigs

Discussion in 'Hydrodynamics and Aerodynamics' started by BMP, Jul 23, 2022.

  1. BMP
    Joined: Jun 2022
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    BMP Junior Member

    BLUF: Q does/could a fractional rig use vortex lift over the mainsail?

    I have been reading about delta airfoils (sails and wings) and how the vortex lift (or increased flow attachment if you prefer) offers greater lift and more forgiving angle of attack particularly in the compound or ogive configureation.
    It got me thinking, for aircraft the compound delta wing has a lot of advantages having to do with the vortex flowing over the main wing increasing the range of angles the wing can provide lift at without stalling.
    F18
    [​IMG]
    MiG 29
    [​IMG]
    I saw a picture of a square top fractional rig and thought how the combined airfoil was similar:

    [​IMG]
    Also, I was sailing on a Dana 24 (I think) rigged as a cutter with a 100% genoa and the owner said that is was faster with the staysail, which I would have though just added drag.

    But then I thought about vortex lift and how having a vortex flow over a foil increases the possible angle of attack and therefor performance (at the cost of some drag) like the tubercles on a humpback whale's fins.
    I watched some excellent simulations of the airflow through a sloop rig and saw that the highest lift portion of the sail was where the vortex flowed over it.
    Could vortex lift be a key to a more powerful and forgiving (particularly off wind) sailing rig if the head sail(s) were designed to generate one or several vortexes over the main?
    Or am I all wet?
     
  2. patzefran
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    patzefran patzefran

    Delta wings are used on Supersonic planes fighters (or Concorde) as they have less drag in supersonic flow. At low speed regime they develop vortex flow wich deliver lift with high drag but need
    higher angle of attack, which is not important as this occure during take of and landing and they have plenty of power. Crab claw rigs use vortex lift which is adequate, except for upwind
    petormance.
     
  3. Skyak
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    Skyak Senior Member

    Very roughly, a vortex is a column of fluid with rotational energy but zero net momentum direction. Lift is a net change in momentum of the fluid so efficient lift generation generally seeks to avoid vortex generation.
    Going down wind is most often a wind limited condition where you can use all the drag and lift you can generate. Generating vortices is not a bad thing if you can hold on to them thus avoiding the loss of energy to creating new ones. Downwind sails cycle of building and shedding vortices is well known but if you can figure a way to beneficially hold on to the ones you have already made and avoid making new ones you might have something.
     
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  4. BMP
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    BMP Junior Member

    Thanks.
    My thinking was if you could make use of vortex lift off the wind, and therefore more forgiving sheeting like a crab claw, but use the normal high lift of the Bermuda rig going to windward you might have the best of both worlds.
    Kind of wishing I had gotten an engineering degree...folly of youth and all that.
     
  5. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Hi BMP,

    Your question is very interesting. Indeed, pure downwind sailing is an example of how to use vorticities to create drive force.

    In racing cars, for instance in Formula 1, the vortices are indeed used, also not directly to produce an additional lift, but to drive the flow around the car.
    Variations of vortices: vicious or virtuous? - F1technical.net https://www.f1technical.net/features/21854

    For our purpose, I came across a study ( that I am trying to find in an old hard drive ), about bamboo sails, made without any tissues, only plank linked with bamboo fibers, with space in between. This study clearly shows that the lift coefficient calculated using the planform and the classic prantl's formulation cannot explains the lift generated by the whole sail. A student of mine was working with North Sails on new sail shapes for multihulls, and he made, at this time, the idea that we could allow regularly spaced elongated holes in the sail, to improve its lift to drag ratio, and make the sail more tolerant to stall. His idea could have been used also for every onboard or outboard control surface, like rudders, daggerboard, or foils.

    Unfortunately, we did not push his idea to real CFD testing, nor windtunnel.

    Reconsidering this fact in the light of your question, it might be possible that, this extremely "fractionned" rig could indeed take advantage of the vortices created on every plank of the sail...

    Just an idea....
     
    Last edited: Jul 31, 2022
  6. Skyak
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    Skyak Senior Member

    Tricks using vortices to combat stall have been proven. When you say you can use them to improve the lift/drag you only lift one of my eyebrows in suspicion. Any vortex is sapping energy so the virtual structure you create with it generally has a real equivalent that is more efficient by eliminating that loss. I know this categorically shuts down more possibility than I have the right to but that is all I have learned from the smartest people I know. Your F1 tech is an example of using a vortex to make virtual structures the rules wouldn't allow if they were real.

    My best ideas for work with vortices is to use shark tooth structures to locate them so that the boundry layer stays thin and stable.
     
  7. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Ah ah ah !!! I very like you being suspicious ! It's ok, also prudent, and it's the true behaviour of a good scientist or engineer.

    In the amplification theory - branch of the theory of optimal control - the energy loss in vortices strongly depends on the turbulence rate of the flow. 9 is the extreme value i often use, for instance when using Xfoil, when I want to simulate numerically the polar curve of an elongated lifting surface. Because, in real applications - not in wind tunnels - the flow is always turbulent, and the Reynolds number is always in the order of 10 to the power of 6. It's a good approximation if you want a rough estimate of the boundary layer thickness. But we must not forget, that, on elongated lifting surfaces, the tip vortice is induced by the crossflow (Z+) of the boundary layer. Fences act as to block this crossflow, and thus reduce the tip vortex. Of course, each fence put on the lifting surface do increase the whole wetted surface area. As a consequence, these static control surfaces (fences) are to be extremely well balanced, not to degrade too much this lift to drag ratio. We then talk about a robust control. A better solution is to directly manage the tip vortex by putting the device directly at the tip, by the use of a cylinder or a winglet. On commercial airplanes, this robust control is now very often used.

    Your idea of directly controlling the boundary layer is indeed a very good idea, that is practically used in every wind tunnel facility. On the most simple version, rubber bands with calibrated sand glued on are put at some places, in order to reproduce the boundary layer obtained by numerical simulation. 3M also sold some adhesive surfaces that can be put on real airplanes. Unfortunately, these material were costfull, and were to be replaced very often. As a result, they have not much been used. Some racing sailing teams also used special paints developped to produce the same effect, with acceptable results, but, as for the airplanes, the low durability to weathering of such paints as discouraged them to go further, not to mention that these prefabricated surfaces are very often banned by class rules. It is also the case in many sports, like swimming or cycling, although, in these applications, their use would have been far more profitable. I remember the case of Olympic Games, in Australia, were a swimmer had trouble using a special suit with a squale like skin Question of equity, I think.

    I would not mention but quickly, the "infinite horizon control", which can be obtained by the use of dynamical piezoelectric actuators to control the boundary layer. The results obtained are very good, but I see not very often lifting surface equipped with these. Probably that too much of a knowledge from the flow around the lifting surface is required, making inevitable very expensive studies and testing, to make a good use of them.

    In plane view, as you said, shark tooth structure do produce good results, that are easier to calculate. This idea has been used on the rudders of the SWAN 50, and maybe be inspired from the turtle's fin ( although they have put these indents at the tip, rather than at maximum lift position... ).
    IMG_20220731_233647.jpg
    IMG_20220731_234041.jpg


    If I have no real clues concerning the true effect of using a grid instead of a plain surface to increase the lift to drag ratio, but I have very good demonstrations that vortices can indeed be used for tweaking the flow around a symmetric profile, making it more elongated when convenient, as in the F1 application. Such profiles has been created in the scope of racing boats -like ACC- and exploit very finely the inflexions on the extrado and the intrado, "filling" them with vortices at low angle of attack. When the angle of attack increase, these vortices disappear. Several regimes are thus used, with the same profile. The polar curves of these symetrical profiles tends to exhibits "buckets", exactly like those you can find on the polar curves of non-symetrical profiles, resulting in greater lift to drag ratios at low attack angle.
     
    Last edited: Jul 31, 2022
  8. Skyak
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    Skyak Senior Member

    Well, I still hold tight to my view that the path to high lift/drag in unconstrained design is to minimize vortices, but I have already learned new things just discussing this. I thought those leading edge tubercles were designed in for greater flow attachment at high lift and turbulent conditions.
    If possible I would like to turn the conversation to conditions where I don't have such a strong opinion and confidence -maneuvering and control. I am sure that vortices are key to these properties and I have no intuition about how they are working. The two important products I am trying to understand are kites and monofoils.

    (443) Triton Foils T1 Explainer - YouTube

    I know all the basic mechanics of what makes a foil more difficult to turn but I don't know how they are able to avoid a tail wing and keep pitch stability. Obviously the ducktail foil profile and the sawed off looking tips must be the trick or at least I can't see another reason for them. I also wonder if the highly swept V is crucial for stability or is it just trying to recapture performance in such a low aspect wing? Because it is low aspect it must have huge tip vortices but it seems they have a way of using them to enhance stability so they are saving the entire tail structure and its drag and downforce. That is a significant tangible savings.

    Some power kites (LEI) also have the swept planform and the "S" suction side profile. For moment coefficient?
     
  9. BMP
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    BMP Junior Member

    From what I'm reading it seems that laminar flow or at least attached flow is best for lift but harder to maintain in real life, vortices can improve the attachment but at the cost of some additional drag. So, if you wanted a boat that would sail closer to the wind and be more forgiving of sheeting angles, but not as fast, using vortex generators (delta/crab claw sails, thick line luff lacing, multiple head sails) would be the way to go?
     
  10. Sailor Al
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    Sailor Al Senior Member

    There is no such thing as "vortex lift".
    The "lift" on an aircraft is just the mathematical resolution of the vertical component of the aerodynamic force generated by the wing. As explained in Aerodynamic force on a sail, the aerodynamic force arises because the sail is compressing the air to windward and decompressing it to leeward. Forget "Vortex lift", it's a furphy.
     
  11. Skyak
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    Skyak Senior Member

    I am as open to innovation as anyone you will find, and I have no idea how creating more vortices would be more effective up-wind than a foil that is designed to minimize vortices.

    Upwind is case of diminishing angle of attack, so a single high aspect foil with elliptical loading to minimize vortex generation is most efficient.

    Flow over a foil generally starts out laminar and can stay laminar for a while if you accelerate it very smoothly. This is a very important to lift/drag because laminar is lower friction. As the fluid continues over the foil it gets harder to hold laminar flow because of less favorable pressure gradient so turbulence begins and grows. Turbulence disturbs the flow making more turbulence. The good aspect of turbulent flow is that it is stickier, so the foil shape can bend the flow more. Bending the flow is what makes lift so the end of the foil is a game of bending the flow as much as possible without growing the boundry layer as much.
     
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  12. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Well... I guess we're talking here from more practical applications. Theoretically, you're right, in a sense ( plz, see my remark about pressure in Is circulation real? https://www.boatdesign.net/threads/is-circulation-real.46025/page-19#post-932452). Practically, your remark does not add very much here, to my opinion of course
     
    Last edited: Aug 3, 2022
  13. Alan Cattelliot
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    Alan Cattelliot Senior Member

    I see the video for quite a time, but not entirly. One essential advantage with swept wings is to be able have to equilibrate your pitching moment as you wish. As a consequence, efforts on surface control are also minimised. Watch out on your reflexion on the tip vortices, because a swept wing should not be confound with a delta wing. The aspect ratio of a swept wing being greater than the one of a delta wing.

    The argument of the monowing generating less drag than the traditional configuration have been verified in aeronautics. Fuselage to wing & tail interference and Görtler vortices generation are important loss of energy. But like St Thomas, I would prefer checking myself, by making the calculations, if this advantage for monowing is also relevant for their usage in a hydrodynamic foil. Because no one, no motor, no tank, no control lines is in there, the volume of the "fuselage" of the submerged part of the can be extremely minimised, and placed into the already stall flow region, to minimize the interference drag.

    Another comment, if you have ever played with paper planes, you will know for sure that the duck tip that you see on the monowing of the video makes your paper plane fly for a greater distance. The idea is to force circulation (or "recompress" the flow, in other words) at the tip to avoid local instabilities in the flow due the acute angle at the tip of such monowing. It is a tricky part, because if this duck tip is too large, you will have an inverse effect...

    Also, from what I know, it is more complicated to manage the roll and yaw moment of a monowing. Concerning their yaw moment on the horizontal of an hydrofoil, it shouldn't be such a problem, since you've got the vertical part of the hydrofoil to manage that. Considering the roll moment, it can probably also be a point in the bucket of a monowing, with it's rotating capabilities. However, watch out that there is also a counterpart also, because bank fly is more difficult to recover also.

    Finally, and again, advantages and drawbacks are to be evaluated very precisely, and taking account into all the parameters of the problem. A kite monowing should not be compared to an hydrofoil monowing. In doing that, you neglect :
    - their flying domain : very different from one another, so the flow regime may vary, and so do polar curves also.
    - their configuration : a monowing of an hydrofoil does not come alone. The entire hydrofoil should be considered. For instance, the drag of the monowing of the hydrofoil is only a part of the drag of the whole hydrofoil. As a consequence of what has been already said, the difference of total drag between monowing and wing+control may be small. At the boatyard, where I used to work before having my own business, there was a clever windsurfer who wanted to test foils. What he did was gluing crude fiberglass rods in place of the wing at the end of his fin. That's worked very well. So to say that there may a little bit of commercial proselitism in the video.

    As St Thomas, I rather do like small calculations on real geomtries, just to be sure. Have you test yourself these monowings on hydrofoils ?

    As for the kite's V-shape, please wait my next comment. Thx
     
    Last edited: Aug 3, 2022
  14. Sailor Al
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    Sailor Al Senior Member

    Qualified confirmation is a good start.
    I think the practicality of the remark lies in its answer to the OP's question "Q does/could a fractional rig use vortex lift over the mainsail?"is that Swept Volume Theory explains why the answer "NO".
    BMP asked a direct question, and I provided the answer. I think that's helpful.
     

  15. Alan Cattelliot
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    Alan Cattelliot Senior Member

    A direct question with an inadequate wording has just one answer, and yours is absolutely right. Although it closes too quick the conversation. That's my point.

    How can you pose the right question when you want to get extra intell you cannot name ? (Which is always the case for certain questions). Inevitably, you make mistake in your question.
    How can you get an answer to a question hill posed ?
    Ask it to someone sufficiently open to understand the question behind the words.
     
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