Vortices as source of aerodynamic force?

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Mar 26, 2021.

  1. CT249
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    CT249 Senior Member

    Al, I apologise for failing to make clear in my earlier questions that the wind wasn't changing, or believed to be about to change, in strength. The point was that in some conditions (ie flat water) it's often faster to move to a lower drag/low lift sail trim than in more normal conditions. That is something that some of us find easier to think of when we think in terms of lift and drag.

    It seems from your post above that you are inferring that Doug hasn't sailed a "real sailing boat". He's won at least one world title in very real boats, and got at least second in a worlds sailing one of his own designs. I would suggest that his experience in real boats is superior to yours.
     
  2. Sailor Al
    Joined: Feb 2021
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    Sailor Al Senior Member

    You aren't referencing the L/D ratio, but talking about reducing the FT , so why don't you just say "a less powerful sail" instead of confusing us with lift and drag?
    [EDIT] I incorrectly referenced FLAT when I meant total aero force FT
    FWIW, my experience indicates that in light conditions, <5 kt and flat water, we're searching for as much power as possible, so slackening the rig to induce sag, reducing prebend to induce depth and easing the outhaul is the name of the game.

    No, not at all. Take a close look at the sentence I'm quoting and tell me that it makes sense!
     
    Last edited: Apr 12, 2021
  3. CT249
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    CT249 Senior Member

    I don't just say "a less powerful sail" because the intention is not to depower, but to use the fact that (in the examples I gave) we have moved to flatter water and therefore, all else being equal, will be encountering less hull drag. That means that we can accept a reduction in rig lift/sail power and move to a higher-pointing mode, which is very, very different from a depowering mode.

    The high-pointing shape may actually have lower lift (edited for mistaken term), but that's not what we are looking for; it's an inherent trade-off because we are trying to get lower drag to point higher. Therefore saying we are trying to get "a less powerful sail" may be confusing, because it's very different from the situation where we are overpowered and therefore want "a less powerful sail". The low drag/low lift settings you need to point high in flat water and ideal pointing conditions are obtained by using very different settings to normal depowering settings and therefore using a different term can be very useful.

    In the example of the Farr 40 versus a J/24, if one thinks in terms of lift and drag and their relationship to the hull's drag characteristics, one can see why a Farr 40 can carry flatter (lower drag) sails than a J/24, which has a fatter, heavier and higher-drag hull in proportion. Similarly, one can understand why a Formula 18 cat carries flatter sails than a Farr 40 even when both are sailed to their optimum in the same conditions, and when neither is overpowered.

    If you tried to maximise light-wind power in an F18 by obtaining similar draft to (say) a Laser or J/24 you'd be slow and low, because rig drag appears to be so critical in the cat. And referring to heel isn't going to help, because in light airs we want to heel the cat to fly a hull - but we want to reduce aero drag too.

    You may not find this necessary, but we're all different and some of us prefer to think of things in different ways.

    Re Doug's post; no, I don't understand it, and that's not a bad thing. Doug knows his stuff. If I don't understand what he's saying then he is likely to be saying something will expand my knowledge, if he explains it in a slightly different way or if I can re-read the post and grasp the concepts and forces.

    The great thing about this forum is that we've had Doug, Tom, Mikko and Mark Drela who give us theoretical knowledge that, unlike the stuff often thrown around, actually matches up with the real world.
     
    Last edited: Apr 13, 2021
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  4. jehardiman
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    jehardiman Senior Member

    Al, this is the really important point you are missing...without the hull to work against, it doesn't matter how much force a sail generates...regardless of direction. It is the interaction between the sail force/drag and the hull force/drag that drives a sailing vessel in a particular direction....upwind or downwind. No hull force, then the only movement is in the direction of the net aerodynamic force...i.e. drift, not sailing.
     
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  5. Sailor Al
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    Sailor Al Senior Member

    Still waiting for the penny to drop! (Hint: think about "regardless of direction".)
     

  6. Sailor Al
    Joined: Feb 2021
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    Sailor Al Senior Member

    Interesting development.
    I have been in correspondence with the afore-mentioned Dr Ignazio Maria Viola at Edinburgh University over the past three weeks, in which I have detailed a number of objections to his theory relating to vortices being the source of the lift.
    His responses have been very unconvincing and he has failed to respond to my latest email altogether.
    Today, I accessed the University's website and found that neither the video presentation nor the supporting paper are mentioned.
    Maybe too soon to draw conclusions, but interesting
     
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