Foil Cavitation at Lower Speeds Than Expected

Discussion in 'Hydrodynamics and Aerodynamics' started by Doug Halsey, Aug 11, 2015.

  1. schakel
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    schakel environmental project Msc

    At 0:06 until 0:07 you see the fast backwards propagation. It last 1 second so you must look very carefully.
    Next frame (even more spectacular) is at second 11 until 17, (slowmotion)
    Next at 21 seconds. 33 until 41.
    So four takes where you can see the bubbles going backwards because of remaining underpressure.
    Still a strange phenomena to me. How can the underpressure remain in water while it is surrounded by water?
    Pascal law on fluids is applicable so why do the bubbles occur?
    https://en.wikipedia.org/wiki/Pascal's_law
    My guess it's because of whirling that creates the underpressure.
    Must whirl very fast to obtain something like that, but it happens.
     
    Last edited: Sep 15, 2018
  2. Doug Halsey
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    Doug Halsey Senior Member

    I might have misinterpreted what you meant by backwards propagation. I'm thinking of the phenomenon where ventilation can originate downstream of the foil's trailing edge, and then propagate forward toward the foil. I'm not seeing that in this video, but it's hard to tell from the underwater shots where it's not clear where the foil is.

    Why the bubbles to persist for so long is a different question that I can't answer easily, but I don't think Pascal's law is involved. That seems to apply to fluids in closed containers (with walls). In an open body of water, where you could be sailing, I think the pressure disturbances propagate at the speed of sound, rather than instantaneously.
     
  3. P Flados
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    P Flados Senior Member

    The trail of bubbles is clearly not "cavitation". Cavitation occurs when the pressure of a fluid drops below the vapor pressure (less than 1 psia for water at room temperature). When real cavitation occurs, the bubbles collapse near instantaneously as soon as the pressure exceeds the vapor pressure. The collapse generates shock waves which can cause "cavitation damage" to nearby items (foils on fast boats, impellers/diffusers on pumps, etc.).

    Ventilation is where air is entrained below the surface. The video shows what looks to be an amazingly long ventilation trail with air bubbles slowly rising to the surface long after the foil is some distance away.
     
  4. baeckmo
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    baeckmo Hydrodynamics

    This is a nice example of how a foil tip vortex presents itself long after the foil has passed. Schakel is completely right, the bubble is actually propagating backwards. In the last passage, you can see the foil passing the camera at some depth, and without visible gas content. Later, the foil seems to approach the surface, and then suddenly you can see a gas-filled (not vapour) column propagating backwards.

    The vortex strength of the rotating fluid "left behind" by the lift generating foil is not strong enough to cause vapour cavitation, but it is persistent enough (and not interrupted by cross-flows here), and strong enough to suck surface air down along its track. The "gas-pipe" has a definite length, defined by the balance between dynamic pressure in the vortex core and the water pressure at the specific depth.

    The phenomenon mimics the condensed water seen in aircraft wing tip vortices, only without the clear backwards propagation. Thanx for sharing the example S!
     
  5. Doug Halsey
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    Doug Halsey Senior Member

    As I pointed out in Post #122, my confusion was with the ambiguity of the phrase "propagating backwards."

    I think you (baeckmo & Schakel) are using it to mean "going aft or downstream," whereas I was thinking you meant "moving in the opposite direction from usual (i.e. forward towards the foil)."
     
  6. revintage
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    revintage Senior Member

    Quite a while ago Doug Halsey and Tom Speer discussed cavitation of Doug´s Broomstick.

    Starting at post #33, Tom Speer recommended to either try a "coke bottle fairing" at the joint or "Perhaps the simplest option at this point would be to cut off the bottom of the V and add a flat piece connecting them together, changing the V to a trapezoidal U."

    Foil Cavitation at Lower Speeds Than Expected https://www.boatdesign.net/threads/foil-cavitation-at-lower-speeds-than-expected.53927/page-3#post-746625

    Tried to interpret two of Speer´s suggestions. Also added a third one that has been discussed. As only straight foils are available, question is which way to go?

    flatbottomvfoil.png
     
    Last edited: Feb 18, 2020
  7. tspeer
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    tspeer Senior Member

    The first option is what I had in mind. I can't tell what you are doing with the second option, because you haven't shown the longitudinal profile. But I think a small fairing like you've shown may be ineffective.

    The third option is probably going to be much like the second. Extending the foil will lessen the loading and reduce the velocities near the joint a little, but I don't think you'd see much difference.
     
  8. tspeer
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    tspeer Senior Member

    You had it right at first. Air is sucked into the trailing vortex when it gets to the surface, and that air goes upstream inside the vortex core until it reaches the foil. The vortex may already be filled with water vapor, because the pressure in the vortex core is quite low. Atmospheric pressure is greater than the water vapor pressure in the core (otherwise, the oceans would boil away), so if the core gets close to the surface it's easy for air to push into the vortex and run right up to the foil.
     
  9. revintage
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    revintage Senior Member

    About the second and third option, let’s say we hit it off and made an ideal ”coke bottle fairing”? From an earlier duscussion, I am aware of that it is no easy task to do the ”ideal” fairing.
     
  10. Doug Halsey
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    Doug Halsey Senior Member

    revintage: If the V-foils for your boat have the outer legs of their V's extended past the apex, you will probably have the apexes out of the water whenever cavitation might possibly occur. In that case, the extra fairings wouldn't matter. They might still be helpful in other conditions, but they seem like an unnecessary complication for any but the most determined amateur builders.
     
  11. tspeer
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    tspeer Senior Member

    You need to be able to estimate the surface pressures in order to properly design the fairing. A panel code is about the simplest method that will do the job. The biggest challenge is creating the surface grid for the panel code. Most panel codes originated in the aerospace industry, and our hydrofoils don't much like airplanes. But you can do it with CAD scripts.
     
  12. Doug Halsey
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    Doug Halsey Senior Member

    A paper I happened to be reading recently has some interesting comments about this.

    "The Mechanism Of Ventilation Inception On Surface Piercing Foils"
    SAGE Journals: Your gateway to world-class research journals https://journals.sagepub.com/doi/abs/10.1243/JMES_JOUR_1974_016_005_02?journalCode=jmsa

    It says that, for surface piercing foils, this mechanism is only likely to occur for aspect ratios smaller than about 1.5, so I would assume it could be avoided by not flying too high.
     

  13. tspeer
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    tspeer Senior Member

    For a more modern look at ventilation inception, see On the Road to Establishing Ventilation Probability for Moth Sailing Dinghies. They discuss a couple of different paths to ventilation, including tip vortex ventilation. They also talk about a filament vortex that extends down from the free surface near the strut, and is another route around the surface seal.

    Another interesting paper is Experimental and Numerical Investigation of Ventilation Inception and Washout Mechanisms of a Surface-Piercing Hydrofoil. They present this map of ventilation vs angle of attack and Froude number based on depth of immersion:
    [​IMG]
    Here's yet another paper that compares CFD and model tests: Numerical Simulations of a Surface Piercing A-Class Catamaran Hydrofoil and Comparison against Model Tests. They were not able to reproduce in CFD the ventilation observed on the model. I think this is a good indication that we're along way from being able to predict ventilation. One complicating factor is ventilation depends on environmental disturbances as well as the design of the foil. So we may never be able to completely nail it down.

    In conversations with the C-Fly designers, they have observed waves breaking over their main foil, trapping a bubble of air, and leading to ventilation that way. They had to add fences to counter this behavior.

    Flying lower helps in several ways. Besides putting more of the foil farther from the free surface, it also results in a lighter load on the strut and the pressures will not be so negative. But the wetted surface is greater, so there's a performance penalty to be paid for being cautious with regard to ventilation. I guess that's what makes racing sporty!
     
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