Swept Volume Theory

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

  1. mc_rash
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    mc_rash Junior Member

    I see some discontinuity, be carefull with this kind of graphs!!
     
  2. Sailor Al
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    Sailor Al Senior Member

    All good and true Newtonian mechanics
    I am appealing for experimental evidence for "accelerating the fluid" when the fluid is a gas, like air.
     
  3. Doug Halsey
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    Doug Halsey Senior Member

    The theoretical curves computed the velocity values directly, with no assumptions about the pressure.

    If the experimental points were computed from measured pressure values, doesn't that give you some confidence in both the data and the theory?
    NACA66(215)-216.jpg
     
  4. Sailor Al
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    Sailor Al Senior Member

    You presented the fig in response to my appeal for experimental evidence of velocity change.
    The "experimental" data in the figure is computed from experimental pressure, not experimental velocity.
    I'm appealing for experimental values of velocity, not computed values of velocity.
    There is a fundamental difference.
     
    Last edited: Sep 11, 2022
  5. Sailor Al
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    Sailor Al Senior Member

    Remember, I'm appealing for experimental data, and so this is what I'm seeing from your submission:
    experimental.jpg
     
  6. Doug Halsey
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    Doug Halsey Senior Member

    But you're ignoring the axis label.

    NACA66(215)-216_Partial2.jpg

    Since the theoretical data came from inviscid, incompressible potential flow, Bernoulli's equation applies exactly to it and since the theoretical and experimental results are in such good agreement, there is no reason to doubt that the velocities shown are an excellent approximation to the experimental velocities just outside the boundary layer (if it had been possible to measure them). The scientists at NACA apparently had no doubts - hence the label on the axis.

    Of course, a radical skeptic such as yourself, will never accept an appeal to authority like that, but then I wonder what measurements of flow velocity you would ever accept. Certainly not those from handheld anemometers with a ball rising inside a tube or spinning cups. Their calibrations must rely on theories involving total pressure, drag, and other relations of unknown accuracy to you. And of course, they're limited as to how close they can get to wings, sails, etc.

    IMG_0841.JPG

    Laser Doppler Velocimeters might do better, but do you really understand what goes on in the conversion of the received signals into a velocity measurement?

    Radical skepticism is such a sterile philosophy! If you can't accept the opinions of experts in your field, would you accept those of people who know even less about it? Or would you just end up paralyzed by indecision?

    Speaking for myself, I would trust the results shown in that graph more than the readings from any possible gadgets that could have been placed in the flow.:)
     

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  7. latestarter
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    latestarter Senior Member

    Sailor Al wrote “Going back to first principles the reason for the confusion was obvious. They were trying to explain it from the point of view of wind flowing over the sail.

    When considered from the perspective of the sail moving through still air, the air to windward becomes compressed and creates a region of higher pressure, while the air to leeward moves into the void resulting in a region of lower pressure. A pressure difference is the only way a fluid, like air, can impart a force to an object.”

    That is a good description of a yacht on a dead run, there is no air flow across the sail, so the force is generated as you suggest.

    It is truism that the pressure on the windward side of an object is raised and on the leeward side is lowered but where does that get us.

    Your change of frame of reference does not eliminate airflow over the sail, when close hauled as you agreed previously, tell tales would still stream in your scenario.

    The problem I have is solely applying that principle to when there is airflow over the sail.

    The assumption is that because that is the mechanism by which a sail works on a dead run then it is the only method by which a sail generates force.

    I attach an amended sketch for Fig 11.
    Mine shows A and B as vertical straight lines representing a dead run.
    Just drawing a curved surface at an angle on fig 11 is not giving a true impression.


    fig 11A.jpg
     
  8. Sailor Al
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    Sailor Al Senior Member

    Don't you see that is a circular argument?
    The experimental data - reported as (v/V)^2 is the velocity that has been calculated from the measured pressure, presumably using Bernoulli's incompressible flow theorem. So of course it is going to match the "calculated value". Both the Experimental and Calculated values are calculated using the same theory!!!!

    At this point I would accept almost any experimental at data at all.
    It would be better if it were a) from a peer reviewed paper in a reputable journal, or b) by an established academic in the field.
    It was the Babinsky video, from a source of category b) above, that early on in my evolution provided fuel to my conviction that Bernoulli doesn't apply to air.
     
  9. Sailor Al
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    Sailor Al Senior Member

    Thank you for presenting an argument based on the paper itself, not on the implications. You have correctly quoted from the introduction of the theory (although not the theory itself).
    But it isn't using a boat on a dead run.
    The whole theory is developed using the model of a yacht with the apparent wind from well ahead, not a dead run.
    All the figures are developed from Fig 3 which shows an AWA of around 45°
    upload_2022-9-13_21-59-41.png
    But that is not the assumption. The theory has been developed with an AWA of ~45° That's a close reach, not a dead run.
    I don't think you have yet got your head around the change of Frame of Reference from the boat to the undisturbed air. I'm not saying there is no airflow over the sail. I'm saying that in the FoR of the undisturbed air, the sail is moving through still air. That changes the subject from a fluid mechanics problem of air moving around the sail, to a gas theory/thermodynamics problem of a moving sail interacting with stationary air.
     
  10. latestarter
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    latestarter Senior Member

    You picked a frame of reference, I chose a different frame of reference to make my point. If your theory is valid it should work on all points
    of sailing.
    By your theory tacking down wind will not work. The swept volume predicted will be maximum on a dead run, and reduce if you deviate.

    You also wrote:-
    "In either frame of reference, the laws of physics apply. Strictly speaking, as long as the frames of reference are not accelerating, any one is as good as another."
    Then why the jump to gas theory/thermodynamics?

    BTW Bernoulli can be adapted for compressible gases Bernoulli's principle - Wikipedia https://en.wikipedia.org/wiki/Bernoulli%27s_principle#Compressible_flow_in_fluid_dynamics
     
  11. mc_rash
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    mc_rash Junior Member

    Re the compressibility (I'm talking about sails on a sailboat, not foils of highspeed plane, Concorde, ...).

    Compressibility of a fluid flow is determined by the Mach number, i.e. the ratio of flow speed and speed of sound in that fluid at that temperature at that altitude etc. (In this thread there was a discussion about more or less Mach 0.3, in the following you see the reason why it is related to compressibility)

    Speed of sound at 20° C at sealevel is 343 m/s. Taking a sailboat in account with a speed of 70 kn, and wind speed of 100 kn (just theoretical) you'll get a resulting fluid flow of 170 kn = 87 m/s (assuming the boat can sail directly into the wind which normally never would happen..)

    Mach = 87/343 = 0.25

    With Mach 0.25 the fluid might get compressed, but I guess my speed estimates (both, boat and wind) are very unrealistic and most boats operate under less speeds.

    As you can see there is no reason to take compressibility into account - for the flow arround sails of a sailboat you can trust scientists, engineers, and others, that you can assume incompressible flow.

    If you want to - Sailor Al - I will figure out why compressibility depends on the speed of sound, but it has much to do with thermodynamics I can say for sure.
     
  12. Sailor Al
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    Sailor Al Senior Member

    But what is your point?
    We agree that a sail works by having a low pressure to leeward and high pressure to windward. Are you saying this is only true dead downwind?
    I don't know what your objection is.
     
  13. Sailor Al
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    Sailor Al Senior Member

    Please, make your point.
    I'm claiming that Bernoulli's Principle does not apply to air over a sail. If it did, the low pressure to leeward would have to be caused by the air accelerating over the leeward side of the sail (or for a wing over the top of the wing). I don't think that happens. My graphic from Babinsky's video certainly doesn't show it happening, my simple experiments don't show it and Bernoulli was working with water, not air.
    If it does, then someone must have published some wind-tunnel or full-scale experiments to show it.
    I can't find evidence, Doug Halsey hasn't found it, Babinsky didn't show it.
    I am appealing for evidence that it does.
     
  14. Doug Halsey
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    Doug Halsey Senior Member

    Air on the leeward side has regions of both acceleration and deceleration. On average, the speed is higher than on the windward side. That's all that matters.
     

  15. Sailor Al
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    Sailor Al Senior Member

    If it does apply to sails, then it probably works elsewhere.
    My appeal for evidence would be satisfied with wings , windmills or wind turbines. What do you have in mind?
     
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