Vortices as source of aerodynamic force?

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

  1. Remmlinger
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    Remmlinger engineer

    I watched Viola's video and it makes me wonder. His first claim at 1:30 is already wrong. The bound vortex is NOT the result of the velocity gradient within the boundary-layer. Instead it is the result of the entire circulation around the airfoil that is enforced by the Kutta-condition. Even a foil without a boundary-layer, but attached and parallel flow at the trailing edge can be modeled by a bound vortex and produces lift. In the next step he mixes up the bound vortex of the circulation with the shedding of vortexes in a v. Karman vortex street when the flow is separated. These two phenomenons have nothing to do with each other.
    There is the possibility, that at large angles of attack, when separation occurs, a vortex is formed at the leading edge of a delta wing. This vortex stays attached to the wing and produces additional lift. The two vortexes from the leading edges leave the wing as trailing vortexes at the rear tips. Polhamus has investigated this effect and there are several NACA-reports about the Polhamus' leading-edge-suction-analogy.
    Viola has stirred everything together into a soup that is not digestible.
     
    Last edited: Mar 30, 2021
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  2. CT249
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    CT249 Senior Member

    One way of looking at it would be to ask whether such knowledge is necessary to be successful at winning races, if that is one's goal.

    Having sailed with and/or interviewed a bunch of top sailmaker/trimmers (including America's Cup winners, Olympic medallists, qualified engineers with 8 Hobart wins, etc) and talked trim and sail setting with Olympians, one can say that few if any of them deal with the theory at that level. Since a grip of theory at this level is not needed to win Olympics and the AC, it can be said that it's not needed at lower levels.

    Arguably, the reality is that in the unsteady flow we deal with, one's time is far better spent learning how to constantly trim and adjust, with all the compromises that can entail, based on a much more basic understanding of theory. Take something like getting a Laser upwind in 7 knots or getting a Formula 18 or 36' J-boat downwind in 20 knots and it's hard to see how the layman will find the minutea (sp) worth getting into, when simply training hard on the known issues will take up far more time than most of us ever have.
     
  3. AlexanderSahlin
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    AlexanderSahlin Junior Member

    My cat, Skilla just made a precision jump with a perfect point-landing on my shoulder. So I asked her what the function for her variable moment of inertia-tensor was, and what software she used to determine her angular momentum tensor. She replied "Mjau".
    I think something similar is valid for top-athletes in most sports, also in sailing.
    But when you design boats, especially new concepts of boats, it is an advantage to understand the physics behind sailing.
     
  4. CT249
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    CT249 Senior Member

    From the comments on the video it appears that you don't believe that sails create drag. In that case, what is the force that is pulling a windsurfer rig to leeward when it's not sheeted in? When one is stationary with the rig sheeted out, as when waiting for a start, one has already normally compensated for the simple effects of gravity on the weight of the rig - but when a gust hits the force pulling the sail to leeward increases dramatically no matter whether the sail is a fully-battened one that is backed or not, or a "soft" short-batten sail. According to standard sailing theory, that force is parallel to the apparent wind and therefore would seem to be defined in conventional terms as drag. If a sail doesn't cause drag, as you claim, then what is the force that increases so dramatically as windspeed increases, all else remaining the same?
     
  5. CT249
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    CT249 Senior Member

    Yep, the top sailors certainly do spend a lot of time practising their physical skills like that. They also seem to have a very strong concept of the wind as something that fluctuates dramatically - something that slower racers just don't seem to understand. A general high-level concept of the physics, plus a knowledge of the tradeoffs between lift and drag that accompany changes in trim, seems to be enough for even the top sailors. Then they spend a lot of time on trial and error sailing, which is arguably more effective than any amount of theory most of the time, because the theory seems to concentrate on steady-state conditions and therefore may ignore far too much of what happens in the real world.

    I'll admit that I've never really discussed how much the top guys apply any high-level knowledge of physics, but I've been around when (for example) John Bertrand discussed trim with Skip Lissiman (one of the trimmer/sailmakers on Australia II), sailed and talked trim with Grant Simmer, sailed and talked trim and ideas with cat Olympic medallists/sailmakers and top Moth guys, etc.

    From what I understand from reading and that sort of personal conversation, much of the same applies to the people who designed new concepts like windsurfing and monofoiling Moths. They had a good grasp of models of physics that, even if not necessarily perfectly correct at the level of tiny vortices, allowed them to work out what could and what couldn't work.

    The AC 75 simulations may show that a more detailed physical model allows people to predict things to a dramatically higher level than before, but it would be interesting to see how sailable such a craft would be in different situations. As windsurfers and 18 Foot Skiffs have shown, what works in an area like the America's Cup course may not work at all on the Solent or Sydney Harbour where theoretical possibilities collide with real world gusts, shifts, lulls and tactical issues. Would the simulations for the AC75s be able to accurately predict such a situation and would the boats really work in such a situation?

    The "layperson" sailor deals with the latter sort of world, rather than one where the higher levels of theory dominate IMHO. And the guys who did things like create foiling Moths, windsurfers, assy spinnakers etc are also people who have a deep and vital understanding that a theoretical knowledge "steady state" aerodynamics is less important than an understanding of the real world of gusts, shifts, chop, gybes, etc.
     
    Last edited: Mar 29, 2021
  6. Sailor Al
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    Sailor Al Senior Member

    You express it more elegantly than I could have, but that's also where I hit the wall.
    My thoughts were that the velocity gradient does not induce a vortex. A vortex has circular motion and there's no motion perpendicular to the surface, it's all parallel, so no vortex.
    Shouldn't we call him out? I have added a comment to his video... maybe you could add some comments?
     
  7. jehardiman
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    jehardiman Senior Member

    Actually, I think you may be missing the whole point of the exercise. Physical vorticity exists in the near field TBL and the far field wake. What doesn't exist is bound "circulation" and the Kutta condition...those are contrivances to make the math work. As Doug McLean alludes to in his video, people get caught up in the forces when they should be concerned with the energy. Where is it coming from, where is it going to, and Newton's Third Law. It is the vorticity in the near field that is generating the "push" to form the vorticity in the far field, they are inextricably linked.
     
  8. Sailor Al
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    Sailor Al Senior Member

    TBL: Turbulent Boundary Layer (I'm not a fluid dynamicist) ? Just how deep is that in the real world of a well trimmed jib upwind? Don't the streaming luff wools tell me there's no significant turbulence, so no vortices.
    Not sure what point he's missing.
     
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  9. jehardiman
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    jehardiman Senior Member

    You are absolutely incorrect that a streaming telltale is a sign of no turbulence. What it is a sign of is lack of separation. FWIW, the TBL is an inch or so thick by the leach.
    Here is what you are missing on the concept. Think of a fluid as a mass of shapeless rubbery blocks (because on an atomic level it is) that repel each other, but with any force you can press them into contact. They have volume, mass, friction, etc. Now imagine them in a quiescent state; they are not moving, not rotating, at constant temperature, but they are pushing back on each other. Their energy is all potential. Now we push a thin horizontal plate surface through them from right to left. The particles on top of the plate will begin to rotate CW and those below the plate will rotate CCW due to contact with the plate as well as moving moving the particle toward the left. This ratio between rotation and movement is kinematic viscosity. A "friction" force is generated by the work needed to cause the particles to spin, and if the plate had shape, a "pressure" force would be generated to push the particles out of the way. Relative to the plate, those spinning particles are still moving "aft", so like your telltale all the "streamlines" are aft even though there is "turbulence" in the fluid. Additionally, by kinematics, those friction and pressure forces are "pushed" out from the surface into the fluid. Now return to the spinning particles in contact with the plate. Based on the kinematic viscosity, the "outward" surface of the particle will have a relative speed faster or slower than it advances. If the particle in contact is spinning CW faster than it is advancing, the next particle "out" that is touched by the inner one spins in the opposite direction, in this case CCW, and so on through the fluid. In an ideal world, this spin would propagate throughout the entire fluid and all the work done by the plate would be evenly distributed throughout with just as many particles rotating CW as CCW....zero net rotation. But the real world is not ideal. By the action of <yet unwritten doctorial thesis probably involving micro topology and molecular forces, which is the whole point of this exercise> two of the particles <conceptually two next to each other on the top surface, both rotating CW> start rotating around each other, leading going over trailing still keeping the same rotation. Now not only is there internal rotation of the masses, but external rotation. This requires more more work to be done by the plate, so the mechanism of how this rotation forms is critical. Eventually, this rotating pair gathers up others until sufficient force is exerted by the other particles in the fluid not in touch with the surface that the "vortex" "separates". In this case your telltale will show this separation.
    And we haven't even addressed the issue of a non-quiescent fluid.
     
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  10. Remmlinger
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    Remmlinger engineer

    I do not think that a comment would have an impact. If a person ignores 100 years of research......
    Any textbook on aerodynamics will explain the lift-force. I recommend "Low-Speed Aerodynamics" by Katz & Plotkin.
    The method to place vortex-lines (singularities) on the surface of the foil is a mathematical model to simulate the potential (inviscid) flow around the foil. The integral around the foil is used to calculate the lift-force.
    The shedding of votices into a v. Karman vortex street behind a bluff body or a foil at large angles of attack is a physical phenomenon, that can be observed in the wind-tunnel (smoke-streaks). It is not a mathematical model. A flat plat at 90 degrees to the flow will create a massive vortex street but absolutely no lift.
    The vorticity in a turbulent boundary layer is on an extreme micro-scale. The microscopic eddys that are much smaller than the thickness of the boundary layer develop on any fixed wall, that contains a moving fluid. This has nothing to do with lift.
    Viola mixes all these phenomena that have nothing to do with each other into an obscure theory. He should not be a lecturer, he confuses his students.
     
  11. DCockey
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    DCockey Senior Member

    You are confusing a "vortex" as in a rotational flow structure similar to a tornado, and "vorticity" which is vector measure of the rotational nature of the flow. Think of vorticity at a point as if you could look at small volume of fluid going past a point and observe its rotation, if any, as it passes that point. The rotation described as a vector is the vorticity at that point. By their nature boundary layer have distributed vorticity.

    Now to add to the confusion a classic vortex has zero vorticity outside of the vortex core. An neutrally buoyant small object moving with the fluid in the vortex outside of the core will remain facing the same direction as it goes around the vortex core. An analogy would be sitting on a swivel seat on a turntable (not in the middle), and as the turntable revolved you remained facing in the same direction. Your travel around the turntable would be the circulation but your vorticity would be zero.
     
  12. Sailor Al
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    Sailor Al Senior Member

    Where am I doing that? I haven't used vorticity. The video at 2:00 introduces a vortex, which I interpret as fluid rotating in the conventional sense, and proceeds to circulation, it doesn't mention vorticity.
    Remember, I'm trying to determine to what extent if the material in the video relates to the real world of airflow around a sail.

    Oh yes, as well, he has the vortices rotating clockwise over the upper surface and anti-clockwise over the lower surface and then bundles them together as a single one rotating clockwise. I don't follow!
     
    Last edited: Mar 30, 2021
  13. AlexanderSahlin
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    AlexanderSahlin Junior Member

    If you want to get a better understanding of aerodynamic lift I recommend that you forget Violas's theory. I have watched it twice now, without understanding it and it seems to me that he tries to use potential flow theory -a mathematical model, that is ONLY valid at zero vorticity and also requires that you can neglect viscous forces- for the separated flow behind a stalled sail, that has lots of vorticity and viscous dissipation. His theory gives the same circulation around an airfoil as the potential-flow model yes, but if you model the wake behind a bluff body with such a pair of thin vortex-sheets, as he displayed, you should get zero pressure-drag according to the potential flow model. This is what you try to obtain for a base-ventilated hydrofoil strut e.g.
    To get pressure-drag for such flow you need viscosity or turbulent mixing (or the combined effect of them) to get the exchange of momentum between the wake and outer flow, that gives the pressure-drag.
    So look in some textbook on aerodynamics, as Remmlinger suggested, or you can also read Aero-Hydrodynamics of sailing by C.A. Marchaj (quite long with some anecdotes, but good) or look at the first chapters of Fluid-Dynamic Lift by S. F. Hoerner. Both these books are written, so you shall understand them without the highest level of mathematics.
     
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  14. Remmlinger
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    Remmlinger engineer

    Good point! Here it is:
     

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  15. DCockey
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    DCockey Senior Member

    The flow outside of the core of a point vortex is irrotational. The flow is only rotational at the core of a point vortex. However there is circulation around a point vortex.

    A fundamental mistake made by many folks new to aerodynamics is to assume the flow around an isolated vortex is rotational, with "particles" of fluid rotating. It is not rotational. The particles of fluid around an isolated vortex are not rotating. They are only translating.
     
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