Foil Cavitation at Lower Speeds Than Expected

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

  1. schakel
    Joined: Jul 2008
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    Location: the netherlands

    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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    Location: California, USA

    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
    Joined: Oct 2010
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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
    Joined: Jun 2009
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    Location: Sweden

    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
    Joined: Feb 2007
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    Location: California, USA

    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)."
     
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