Air bubble lubrication successful trials

Discussion in 'Boat Design' started by jakeeeef, Oct 22, 2021.

  1. BlueBell
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    BlueBell "Whatever..."

     
  2. Barry
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    Barry Senior Member

    an interesting video

    So the boat when the complete footprint is under bubble attack, the boat does not sink. His reasoning is that the water coming up vertically is counteracting the loss of buoyancy created by the bubbles.
    The water is not travelling upward, the bubbles are. The bubbles are creating a very slight turbulent surface. His conclusion is not correct

    At 1.13 into the video, when completely in the bubble field, you can see right down to the plimsol/bootstripe/antifouling paint line. When the boat was not sinking as ordained.
    At 1.55 to around 2.16 PRIOR TO THE BUBBLE FIELD BEGINNING only part of the hull is in the field, the boat is pre-immersed almost to the gunwale. mmm???
    The missing piece is a picture when partially in the field before the boat filled with water. It is easy to see the water on the rear deck before the boat actually sinks.
     
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  3. DogCavalry
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    DogCavalry I aim to misbehave.

    Most excellent. At 20% there's obviously free stream paths to the surface where literal holes are made. Although I wonder where they got the figure 20% from. Out of their hats, I'd bet. And 20% at what depth. On the bottom, at the piping?
     
  4. DogCavalry
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    DogCavalry I aim to misbehave.

    More observant than me @Barry .

    As a thought exercise to demonstrate why bubbles don't reduce bouyancy, imagine instead an equivalent amount of styrofoam pellets being released. If you had a huge pile of styrofoam trapped under your boat you wouldn't fear sinking. Instead you'd be worried about being lifted up and tipped over.
     
  5. Heimfried
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    Heimfried Senior Member

    A partly disagree. There is some friction between water and rising air bubbles so an upward climbing bubble stream will also drag some water with it. Otherwise the water surface would not become a kind of "bubbling/boiling fountains" - fast moving and (I suppose) often more than a feet high - at the area of the injected air. I don't think only the busting of bubbles at a virtual calm water surface could create such fountains.

    Thus I think there will be a upstream of water also, much slower than the upstream of the air, but not to be ignored.
     
  6. DogCavalry
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    DogCavalry I aim to misbehave.

    Quantitative analysis vs qualitative analysis.

    How much upward velocity? How much air percentage? The 6% 20% quoted in the video are meaningless.

    It's not a video about research, it seems to be to win a bet, or assuage an ego. There's no instrumentation, just statements of faith, and simple falsehoods.
     
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  7. Blueknarr
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    Blueknarr Senior Member


    The tube of bubbles in a small aquarium are a pneumatic pump. They pull water from beneath the gravel.
    The O2 and CO2 gas exchange is at the surface not in the bubbles
     
  8. portacruise
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    portacruise Senior Member

    Will the stream of bubbles emitted from deep below act the same way as bubbles emitted at the hull surface?
     
  9. Blueknarr
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    Blueknarr Senior Member

    Bubbles don't care where they are created.
     
  10. DogCavalry
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    DogCavalry I aim to misbehave.

    The only difference would be the generated upward current, and the evolved state of bubbles. The first is hardly relevant for a vessel underway. The second is just that rising in water the bubbles are shearing apart and forming smaller bubbles all the time. Stable small spheres at depth, as they rise they expand with falling pressure until they are too big, and shear apart again. A process that continues to the surface. Bubbles formed at extreme depth will form a more uniform distribution of sizes, having gone through this process many times. Although the median size may be bigger than the size of bubble generated by something like Silverstream. Hmm, there's a research paper in there.
     
  11. Doug Halsey
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    Doug Halsey Senior Member

    If this were true, then daggerboards or keels would be unable to provide any lateral resistance.
     
  12. DogCavalry
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    DogCavalry I aim to misbehave.

    Sorry, Static pressure. I thought that was obvious to anyone following the conversation to that point.
     
  13. Doug Halsey
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    Doug Halsey Senior Member

    Static pressure? I think you mean stagnation pressure (aka total pressure).
     
  14. Barry
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    Barry Senior Member

    I see where you are going with this.
    With this in mind, in a planing hull, the pressure acting on the hull changes ahead and behind the stagnation point, for reference, the highest pressure line about 1/3 -1/4 way back from the leading wetted edge.
    So forward of the stagnation line, the pressure goes from the high at the stagnation line to atmospheric and sternward of the stagnation line, the pressure goes from this high to the transom where the pressure
    is almost atmospheric.
    To any bubble that is injected just behind the stagnation line (highest) will have the sum of the dynamic and static pressure at that point and as it move rearward the pressure will change in the bubble.

    For simplicity I am following a bubble moveing longitudinally in a straight line from a point on the stagnation line to the transom, which it does not as the bubble will move away from the keel line depending upon deadrise and
    other parameters

    So then the pressure to get a bubble into a point on the hull must be slightly over the sum of the dynamic pressure and static pressure. ( again a small point, I would expect that a bubble in the injector "nozzle" would become entrained
    onto the hull surface due to viscous drag. Baekmo perhaps a comment here.)
     
    Last edited: Oct 29, 2021

  15. alan craig
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    alan craig Senior Member

    I sailed on a reservoir which was being aerated from a giant compressor; you could sail right over the bubbles.
    Gas bubbles moving up through a tube will drag water with them, like the aquarium aerator mentioned above. Also used in airlift bioreactors and other industrial devices to both add a suitable gas and to circulate the liquid. From my experience at work it was really difficult to control the size of bubbles, higher or lower flow rates and different grades of frit (porous glass or ceramic bubbler) didn't make much difference, maybe surface tension has a big influence.
    I know this doesn't help much with the original concept....
     
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