Vortices as source of aerodynamic force?

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

  1. Sailor Al
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    Sailor Al Senior Member

  2. jehardiman
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    jehardiman Senior Member

    " All models are wrong, some models are useful". Vorticity is just the most obvious physical manifestation of work being done by the fluid. Thus, they are seized upon for all sorts of theories. Whether better or worse than others...<shrug>..."All data looks good sometimes."
     
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  3. DCockey
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    DCockey Senior Member

    At some level kinematic description of flow using the mathematical concept of vorticity becomes confused with physical cause and effect.
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Don't you just love jehardiman's distillations on so many subjects. Funny, and probably more true than not.
     
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  5. jehardiman
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    jehardiman Senior Member

    Thank you for the complement.
    Let me be clear here; often what I opine is not cynicism, it is the realities of having been an engineer working with very expensive "can't fail" projects. Engineering is messy. For high risk development projects we typically had three (or more) engineers develop loads and forces based on different individual analysis. Then we swapped them around for second party review. If all was good, all the answers generally clumped together and we took the least favorable. Sometimes though, you get an outlier and you need to figure out why. Often it is due to a limitation in the theory or method used. Not that the theoretical model was wrong in total, just that it didn't fit the particular case. I see it as unfortunate that a handful of digital products are starting to dominate engineering "analysis". Sometimes these "analysis" are just plain wrong because the coded assumptions are inappropriate. It becomes an issue when the "operator" (I wouldn't call them an engineer) doesn't even understand that the output is wrong. I saw this a fair amount my last few years as an engineering supervisor.
     
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  6. gonzo
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    gonzo Senior Member

    The first time I used FEA with Creo, it surprised me that applying a force could deform a body infinitely. The software assumes the material never reaches the yield point. Unless you enter a limit manually, the results are nonsense. Models and theories work within their limits. As an engineer I use Newtonian physics for boats, machinery, etc. However, relativity and quantum physics should be used at different scales or velocities. Most importantly, the results of a method should be compared to known measured values. I believe a good dose of cynicism is good for science and technology.
     
  7. Sailor Al
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    Sailor Al Senior Member

    But, dear forum members, these conclusions come from Dr Ignazio Maria Viola who is Reader at the School of Engineering, University of Edinburgh, and Fellow of the Royal Institution of Naval Architects, and I ask again:
    Are they relevant to the layman sailor?
     
  8. gonzo
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    gonzo Senior Member

    I think that using a square flat plate to explain airflow around a sail is not relevant. However, I can see it being useful in architecture to calculate forces in a building.
     
  9. Sailor Al
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    Sailor Al Senior Member

    But he introduces the presentation with "How do sails generate forces?" Do you think he answers that question?
     
  10. DCockey
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    DCockey Senior Member

    No. Vortices as used in the video are a mathematical convenience for describing certain fluid flow fields, not a fundamental cause of fluid flow.

    For those with an engineering/phyiscis background and a knowledge of the basics of fluid mechanics Doug McLean in Understanding Aerodynamics - arguring from the real physics provides a good discussion of the physics of aerodynamics. McLean discusses theories of lift and drag which are very similar to those in the video.
    Video of Doug McLean discussing Common Misconceptions in Aerodynamics :
     
  11. jehardiman
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    jehardiman Senior Member

    Practically? No. Because practical sailing is all about wind, water, and sail trim. Theoretically? Just another screwdriver in boxes of screwdrivers Naval Architects have.
     
  12. Sailor Al
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    Sailor Al Senior Member

    I like screwdrivers, but if they don't work, I throw them away. I can't see that this one ever worked.
     
  13. Sailor Al
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    Sailor Al Senior Member

    So he's confusing circulation and vorticity in flow fields with real vortices and real flow in fluids? Do I have that right?
    McLean's video would seem to support that theory.
    If so isn't it a pretty fundamental mistake that an associate professor in the school of engineering should not be making?
     
  14. DCockey
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    DCockey Senior Member

    The contents of the video as a useful tool might be a stretch.
     

  15. jehardiman
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    jehardiman Senior Member

    While in the grand scheme of things most of the ground he covered is well trodden...but there is that little TBL bit at the beginning where I think it offers insight to modeling turbulence development and separation vortices.
     
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