Swept Volume Theory

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Aug 2, 2022 at 8:50 PM.

  1. Sailor Al
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    Sailor Al Senior Member

    This theory will trigger the re-evaluation of every textbook that uses fluid dynamics or Newton's laws to explain how sailboats sail upwind and how planes fly.

    The February 2020 edition of Scientific American published a feature article No One Can Explain Why Planes Stay in the Air which described the modern paradox that there are two competing explanations for how a wing works, neither of which fully explain the phenomenon of lift but somehow complement each other to provide an explanation that is regarded as sufficiently acceptable to train private, commercial and airforce pilots.
    This article uses the logical principle of relativity and the principles of classical mechanics to fully resolve the paradox of the competing explanations for the source of the aerodynamic force that is fundamental to modern aviation. It should prompt a reappraisal of every aerodynamic explanation of the effect of wind over a sail or a wing.
    For a racing yachtsman, it finally provides a sound basis upon which to analyse and perhaps start to understand many of the experientially correct, but totally unexplained principles of sail trim.
    The idea that the low pressure above a wing results from air expanding into the void created by the passage of the wing is not new, indeed it was ascribed to Prof Mark Drela in the above referenced SciAm 2020 article. Unfortunately he dismisses it for reasons that, as explained in Appendix 1, are themselves without merit.
     
  2. tlouth7
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    tlouth7 Senior Member

    Hi Al, I like the theory which is pleasingly intuitive. I have a couple of questions though:

    - Why do you say that current theories are incorrect? They give results that seem to match reality pretty/extremely well.
    - On what basis do you claim that CFD only applies to incompressible fluids? I work on CFD of compressible fluids everyday. In any case at the regime of a sail treating air as incompressible is a fairly good approximation - density changes are small.
    - What predictive power does your theory have? Can it help me to find sail trim settings? Or design a rig? Does it predict stall?
     
  3. Sailor Al
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    Sailor Al Senior Member

    Glad to hear.
    No, they don't. See Q3 in the article.
    They all use the theories of Bernoulli, Kelvin, who clearly state their theories relate to incompressible fluids.
    But that's where you are so wrong. Compressibility doesn't address density. It relates volume and pressure. It doesn't care about mass.
    Not yet, but neither has CFD! I hope SVT will lead to explanations. I already have one:
    The "lee bow" manoeuvre works because it's analogous to the ground effect of a wing. The windward boat's sail works to compress the air between the two, increasing the compression on my windward side and reducing the pressure on his leeward side.
    I hope it will lead to the explanation of all the principles of sail trim, but that will take time. It;'s only a very new theory.
    See Q11 for my answer to that.
    [edit]
    Sorry, my mistake. The "lee bow" manoeuvre increases the pressure on the windward side of my sail (good) and also increases the pressure on his leeward side (bad).
     
  4. tlouth7
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    tlouth7 Senior Member

    As per Sagan's principle it is not my job to persuade you that there is value in existing models. A single line stating that they are wrong is not a refutation.

    You make a statement that "CFD refers to fluids, specifically: incompressible fluids." which is simply incorrect. CFD is based on conservation of mass, momentum and energy. Note no mention of Bernoulli or any of the other theories that might be misapplied. The underlying equations of CFD are fundamentally applicable to compressible fluids, though it is true that often a simplifying assumption of incompressibility is used.

    Ultimately it doesn't matter whether a model is based on real physical underlying phenomena, it only matters whether the model is useful. Clearly lifting line theory and so on were useful when we only had limited computing power, then CFD became useful when we did, and wanted to push the envelope beyond the region for which the approximations of those earlier theories were valid.

    Your model might also be useful, but that doesn't make the preceding models invalid and so there is no need to tear them down. Providing an intuitive understanding of where lift comes from would make it useful, but at the moment no more so than any of several intuitive explanations I could give based on existing fluid mechanics.
     
    Last edited: Aug 4, 2022 at 1:06 PM
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  5. gonzo
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    gonzo Senior Member

    Density changes in direct proportion to compression. For example, air compressed to half its volume has double its density.
     
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  6. Sailor Al
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    Sailor Al Senior Member

    I suspect that was a typo which should have read "The underlying equations of CFD are fundamentally applicable to incompressible fluids" not "compressible fluids".
    That makes a big difference to the argument. Can you confirm please?
     
  7. Sailor Al
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    Sailor Al Senior Member

    Yes, thank you.
    I should have been clearer. When talking about compressibility, it was in the context of the gas laws, specifically Charles' law: PV=RT.
    No involvement of mass, no involvement of density.
     
  8. tlouth7
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    tlouth7 Senior Member

    I can confirm that I made no typo. CFD is fundamentally applicable to compressible (sic) fluids.

    Careful now: the correct equation is Pv = RT, or PV = nRT

    Both v and n relate to a mole of the relevant gas, which is where the mass term appears.

    In any case I think this discussion of density is a distraction, I only introduced it as a shorthand for changes in the volume occupied by a fixed quantity of gas which is exactly what you expect to see for a compressible fluid at changing pressure as per the equation above.

    Do you expect to model the changes in pressure as adiabatic? If so you need to consider the T term as well.

    I guess the obvious question is, what shape does the pressure distribution take from the high and low regions adjacent to the sail to the free stream? Without recourse to the classical fluid mechanics that you disparage how can you find that?
     
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  9. Sailor Al
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    Sailor Al Senior Member

    I think you are wrong, but I'm not a CFD expert. I understood that CFD was based on fluid dynamics, which does relate to incompressible fluids. But anyway, it's not germane to the development of SVT.
    A great question, thank you.
    As the sail moves through the air, the process will compress the bubbles to windward and they will heat up, and decompress the bubbles to leeward and they will cool down. After its passage, they will cool and heat respectively to the surrounding air temp. So no, the process is not adiabatic.
    Another great question, thanks, and the answer is I don't know. I have spent many hours trying to model the movement of the warm small bubbles to windward and the big cool bubbles to leeward, but haven't yet cracked that particular puzzle. Fortunately it doesn't impact the theory but may yield some very useful explanations of the fundamental principles of sail trim.
    I hope that this new theory will spur greater minds than mine to some of these answers.
     
  10. tlouth7
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    tlouth7 Senior Member

    I wouldn't call myself an expert, but I am a professional user of CFD, and I can assure you that I am correct. It is nonsense to say that fluid dynamics relates only to incompressible fluids. It is an entire field of study and a big proportion of that is related to work on compressible fluids (indeed almost all these days, because incompressible fluids are effectively solved).

    Quite, I will try to say no more on the subject.
     
  11. rnlock
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    rnlock Senior Member

    The February 2020 edition of Scientific American published a feature article No One Can Explain Why Planes Stay in the Air...
    I was very disappointed in that article. A theory isn't wrong because you can't explain it in words in such a way that everyone can understand it. If my former housemate, who's now an astrophysicist, can't explain some arcane phenomenon to me, I don't assume it's not true. My comprehension or lack thereof doesn't affect physics, nor does anyone else's. The universe is allowed to be as complicated as it needs to be, despite what we might like. All those allegedly incompatible explanations are like the 5 blind men with the elephant who each think the other 4 are wrong.
     
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  12. Sailor Al
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    Sailor Al Senior Member

    The Scientific American Article points out that even amongst the experts (my italics) no one can explain how planes fly. It's not just dummies like us who struggle! Take another look. That's the paradox that it's explaining.
     
  13. rnlock
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    rnlock Senior Member

    Not explaining and not understanding are not the same thing. If you throw in the Kutta condition, lift is inevitable.
     
  14. Doug Halsey
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    Doug Halsey Senior Member

    I mentioned in another thread that that article was the last straw and I cancelled my subscription to Scientific American after having subscribed for almost 50 years.

    It should be obvious, but apparently some people still need to learn that if they don't understand something, it doesn't mean it's wrong!
     
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  15. rnlock
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    rnlock Senior Member

    I still read it, but I feel like it's a hollow shell of its former self. For that matter, the New Yorker is getting sloppy about correct usage. It's murdering words.
     
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