Is circulation real?

Discussion in 'Hydrodynamics and Aerodynamics' started by Mikko Brummer, Jan 25, 2013.

  1. Sailor Al
    Joined: Feb 2021
    Posts: 586
    Likes: 22, Points: 18
    Location: Sydney

    Sailor Al Senior Member

    Thanks for the reference, I wouldn't have found it. Seems your source is Charlie Crummer "Lab manager for Lower Division Physics for the University of California at Santa Cruz."
    I rest my case. Plywood junk.
     
    Last edited: Nov 30, 2022
  2. Sailor Al
    Joined: Feb 2021
    Posts: 586
    Likes: 22, Points: 18
    Location: Sydney

    Sailor Al Senior Member

    I have no idea what you were saying earlier, but I am encouraged by your final sentence.
     
  3. Alan Cattelliot
    Joined: Jul 2021
    Posts: 293
    Likes: 75, Points: 28
    Location: La Rochelle (Fr)

    Alan Cattelliot Senior Member

    :) Thank you.

    Earlier... I just said that the QM analogy shouldn't be pushed too far in this example. Collision-based algorithms for flow calculations do exist. But they require mathematical assumptions. TDT is another assumption that fits the experimental results provided quite well. But adding an underlayer does not help explaining the underlying physics.

     
  4. gonzo
    Joined: Aug 2002
    Posts: 16,359
    Likes: 1,409, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    You are using a theory to prove or disprove another theory but providing no experimental data. Experimental data is you litmus test, so this is a contradiction in your argument, which makes it flawed.
     
  5. baeckmo
    Joined: Jun 2009
    Posts: 1,524
    Likes: 506, Points: 113, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    It all boils down to the statements in posting no 567. The molecules perform a random dance with an average velocity determined by the temperature and molecular mass. In a stationary volume of gas, the velocity is the same, but since the vector sum of all those random movements is zero, no velocity is imparted to the mass of gas, neither into the bulk mass, nor into the part(s) adjacent to a solid wall.

    Now, when there is a movement of the gas mass in one direction or another, it means that the random movements of a specific molecule is a vee bit "less random", which we will observe as a movement of the gas mass.

    I'm not clear over Sailor Al's reasoning here (is anyone but SA?), but my interpretation is that he associates the random molecular movements with a de facto gas movement of the same velocity in the gas layers close to the boundary. And, unfortunately, as long as he clings to that misunderstanding, there is no progress possible.
     
    DCockey likes this.
  6. gonzo
    Joined: Aug 2002
    Posts: 16,359
    Likes: 1,409, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Fluid dynamics uses statistical methods and calculations that give real world results for engineering. As you say, the Brownian motion of the molecules statistically cancel each other out. However, there would be no pressure without that motion.
     
  7. Sailor Al
    Joined: Feb 2021
    Posts: 586
    Likes: 22, Points: 18
    Location: Sydney

    Sailor Al Senior Member

    Yes, that's a reasonable restatement of Kinetic Theory. I would suggest that the velocity of the molecules is determined by the temperature and pressure, rather than temperature and mass, but that's only a quibble.
    I don't know why you are attributing that incorrect assumption to me.
    Indeed, I am arguing the exact opposite!
    It is the NASA paper @gonzo submitted Boundary Layer https://www.grc.nasa.gov/www/k-12/BGP/boundlay.html that contains the statement "As the fluid moves past the object, the molecules right next to the surface stick to the surface." that is contradicted by Kinetic Theory.
    As you say, it is the bulk movement of the gas molecules that imparts the velocity to the gas.
    Prandtl's no-slip boundary layer states that the gas is stationary with respect to the boundary.
    My question is, if the velocity of the gas is the velocity of the "bulk mass", then the gas that is stationary at the boundary has to have "bulk", i.e. thickness. So how thick is the stationary "bulk" of gas in the boundary layer?
    My claim is that there is no answer to this question, so Prandtl's theory must be false.
    (Wipes sweat from brow!)
     
  8. Paul Scott
    Joined: Sep 2004
    Posts: 376
    Likes: 24, Points: 18, Legacy Rep: 84
    Location: San Juan Island, Washington

    Paul Scott Senior Member

    Thoughtful reply! Thanks!

    If we do live in a Quantum Universe, I think it can be argued that our brains have adapted to it (or it to us?o_O), which is why, for example, I think Tom Speer and many others start with an observation, move to an abstraction based on that observation, and then to models based on different languages of scientific inquiry. Both particles and waves were (are?) useful concepts in Quantum Mechanics, as are particles and flow in fluids. My aim with the two articles was to find work attempting to bind (?Wrong word?) particle and flow together, and up pops statistical analysis. (For example, electrons only tend to be where they might be :rolleyes:). My use after that of the search term Quantum Theory was more a Hail Mary on line rhetorical search device to bring focus on, and any benefits of, a field that is obviously grappling with dualities in a number of ways, and having some success, since no other questions brought any results about a theory that might be bringing together two of the intellectual dualities of lift, besides a statistical approach, and I did get results. (As an aside, part of my graduate work was in mathematical model making in the non empirical sciences- but while that might:eek: be enlightening, seemed a bit edgy, and turned out, specifically, futile, for a discussion of fluid flow. What if 1 doesn’t equal 1 sort of thing…Papaliolios let me play around with his Quantum Dice some too, which probably is a good story more that an excuse for my frame of mind:).). Anyway, movement towards more inclusive theories is a difficult business.

    I’m not a fluid flow scientist, but an interested amateur at best, so apologies if I’m stepping on toes.
     
    Alan Cattelliot likes this.
  9. gonzo
    Joined: Aug 2002
    Posts: 16,359
    Likes: 1,409, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    That is more than a quibble. They are two completely different definitions. Pressure if Force per Surface Area. Mass is Density times Volume. Pressure is an intrinsic property whereas Mass is and extrinsic property (dependent on volume).
     
  10. Sailor Al
    Joined: Feb 2021
    Posts: 586
    Likes: 22, Points: 18
    Location: Sydney

    Sailor Al Senior Member

    In the context of the discussion, it is a quibble.
    Please focus on the discussion.
     
  11. gonzo
    Joined: Aug 2002
    Posts: 16,359
    Likes: 1,409, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    The discussion is based on some common agreement of what basic principles are. This is a complete different definition of the basis for Kinetic theory.
     
  12. Sailor Al
    Joined: Feb 2021
    Posts: 586
    Likes: 22, Points: 18
    Location: Sydney

    Sailor Al Senior Member

    Are you channelling C Crummer?
     
  13. Sailor Al
    Joined: Feb 2021
    Posts: 586
    Likes: 22, Points: 18
    Location: Sydney

    Sailor Al Senior Member

    Whether the velocity of the molecules depends on the pressure or the density is irrelevant to the discussion. The point is that the molecules are whizzing around randomly at about Mach 1.
    The discussion is about whether Circulation is real. And since Circulation depends on Prandtl's no-slip boundary, with which I have a problem, the discussion has recently been about the thickness of the stationary part of that boundary.
    I'm still waiting for someone to come up with an answer to that question.
     
  14. gonzo
    Joined: Aug 2002
    Posts: 16,359
    Likes: 1,409, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    The thickness, as used in the theory, is one molecule.
     
    Paul Scott and DCockey like this.

  15. Sailor Al
    Joined: Feb 2021
    Posts: 586
    Likes: 22, Points: 18
    Location: Sydney

    Sailor Al Senior Member

    One molecule of air, with a velocity of zero?
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.