Swept Volume Theory

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Aug 2, 2022.

  1. gonzo
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    gonzo Senior Member

    I did not refer to any rig type not using all the sail area available. I made a comparison between a sloop and a cat.
    You claim to be a racing sailor, this should not need explanation, unless you need a primer on trimming sails.
    Apparently you have never trimmed sails to balance the steering of a boat.
    Apparently, you have never raced any handicap classes that measure fore triangle.
     
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  2. Sailor Al
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    Sailor Al Senior Member

    I suspect you meant "between a sloop and a catboat", i.e one designed to sail upwind with two sails or one designed to sail upwind with one sail (Finn, Laser etc.), in which case you're talking apples and oranges.
     
  3. gonzo
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    gonzo Senior Member

    Not at all. I don't care what is driven by the rig. The same boat with different rigs is the only way to compare rigs of same area.
     
  4. CT249
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    CT249 Senior Member

    I know it's off topic, but that very old claim that we'd still be land-bound if the Wright Brothers paid attention to the scientists of their time is completely wrong. The very first concrete step the Wrights took when Wilbur decided to experiment with flight was to write to the Smithsonian and ask it to send him all the current writings on flight. They used Smeaton's theories on air pressure (IIRC) and Lilienthal's studies of airfoils to determine their foil sections, and when the second or third glider failed to perform as predicted it caused them major heartburn because they DID pay attention to earlier scientists and experimenters like Smeaton. It turned out that Smeaton was wrong, as the Wrights found by building their wing tunnel.

    The cliche that the Wright Brothers ignored accepted science is utterly wrong, and it may have increased the tendency to downgrade the actual expertise of scientists and experts. While I'm not interested in analysing the model of flow like Sailor Al is (there is too much other stuff that we can learn AND APPLY imho) one thing I noticed long ago was that the information that people like Tom Speer (RIP) and Mark Drela provided here conformed exactly to what we see in reality on the water. That cannot be said of the stuff that Marchaj, for example, wrote.

    From a very quick look, it seems that following the Swept Volume theory would lead one to over-sheet the sail, but that may well be missing something since I'm just scanning it while waiting in a phone queue.
     
  5. Sailor Al
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    Sailor Al Senior Member

    Rather than showing it is completely wrong, doesn't your example actually support the "very old claim"?
    You say that they did indeed pay attention to the scientists by using the scientific material from the Smithsonian, and the glider failed.

    So in this example, wouldn't it have been better if the Wright Brothers had ignored the accepted science and gone straight to their wind tunnel?
     
    Last edited: Dec 19, 2022
  6. gonzo
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    gonzo Senior Member

    Not at all. They conducted a scientific study and experimental process. Ignoring all previous process may have led the to jump off a cliff and die.
     
  7. Sailor Al
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    Sailor Al Senior Member

    Reading just what @CT249 reported, I think they a) undertook the scientific study, b) implemented the theory and found it failed, c) abandoned the theory and then d) undertook their own independent scientific experiments in a wind tunnel and e) implemented (successfully) what they learned from d).
    I'm pretty sure they would have saved themselves a lot of time and effort if they had skipped steps a) through c) and had gone straight to step d).
    And they still would not have jumped off a cliff and died.
     
  8. CT249
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    CT249 Senior Member

    No, because they learned an enormous amount from the accepted science; got good results using it; tried another aerofoil and found that it failed and then worked out what had gone wrong.

    Why the hell would they ignore all the huge amount of information others had already learned? They didn't know what they needed to know until they had already stood on the shoulders of others. Who the hell would be so arrogant as to assume that no one else had any information worth learning?

    As Wilbur wrote after their work with the second wind tunnel, the useful information from Lilienthal's book so outweighed its few errors that it was "an extraordinary one" to be created by one man.

    For you to basically sit back and not only claim to know better than the aerodynamicists but also better than the Wright Brothers is just weird. FFS, is there anything you reckon you don't know better than anyone else?
     
    Last edited: Dec 20, 2022
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  9. skaraborgcraft
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    skaraborgcraft Senior Member

    Try the Woodenboat Forum. Place is full of know-it-alls.
     
  10. Sailor Al
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    Sailor Al Senior Member

    You wrote "They used Smeaton's theories on air pressure (IIRC) and Lilienthal's studies" and "..second or third glider failed to perform as predicted.."
    I was just pointing out that on the evidence you provided in the post they would have been better off going straight to their wind tunnel.
    I'm not sure why you didn't include this additional information in your earlier post, as it does indeed support your conclusion.
    Have you had a chance to take a longer look? Do you still think it would lead to over-sheeting?
     
  11. gonzo
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    gonzo Senior Member

    Many people jumped off cliff with devices that didn't work and died. What makes you so sure that if they had been so arrogant as to completely ignore all previous knowledge they wouldn't have jumped off a cliff? The people that jumped and died shared your opinion that their square wheel was better than the usual round ones.
     
  12. Sailor Al
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    Sailor Al Senior Member

    I'm suggesting that anyone with the nous to build a wind tunnel in order to learn about lift, like the Wright Brothers, would also have the sense not to jump off a cliff.
    I really don't understand your vitriol.
    It's not arrogance that prompted me to think outside the box, it is from a deep concern that the present state of knowledge is based on such obviously flawed premises, and that it doesn't answer the underlying question of the source of the pressure differences around a sail that generate the thrust and leeway/heel forces on a yacht.
     
  13. messabout
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    messabout Senior Member

    I suspect that Sailor Al may be just messing with us for his own amusement. Alternatively, he may be one of the Flat Earthers. In any case this has been a fun thread.

    Merry Christmas to all. Here's wishing you health, wealth, and happiness......That includes you too, Sailor Al.
     
  14. gonzo
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    gonzo Senior Member

    I wonder what the boundary layer on a reindeer looks like.
     

  15. Sailor Al
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    Sailor Al Senior Member

    Here's a demonstration of how the pressure regions are generated using a "bubble raft" trapped below a glass sheet over water and a wiper blade to represent the sail moving through the apparent wind.
    The motion of the bubbles show how the low pressure to leeward and the high pressure to windward are generated simply by the passage of the sail through the air.
    To leeward, the pressure is low, as the surrounding air bubbles are sucked into the swept area behind the sail.
    To windward, the pressure is high, as the sail sweeps the existing bubbles into previously occupied space.
    This is the basis of Swept Volume Theory, which accounts for the pressure differences without recourse to Bernoulli, fluid dynamics or Kutter-Joukowski circulation.
    A new approach involving the gas laws, molecular-kinetic theory, thermodynamics and statistical mechanics will have to be developed to get some useful numbers.
    It will take someone out there with the knowledge, skills and ambition to crack it.
     
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