supercavitating hydrofoil sections

Discussion in 'Hydrodynamics and Aerodynamics' started by Sherkin, Feb 18, 2014.

  1. Sherkin
    Joined: Feb 2014
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    Sherkin Junior Member

    Anyone have good resources/links for supercavitating hydrofoil sections?

    Also a stupid question but most articles mention high speed, but if you were a moderate speed and had a long enough cord then that would be comparable, if the Reynolds number is the same?
     
  2. Baltic Bandit

    Baltic Bandit Previous Member

    From what Larso reported on his Sail Rocket project, no. That there are magical things that start happening at the super-cavitation boundary. AFAiK this iis an area of research not of anything with good resources and links. few boats go fast enough to worry about supercavitating foils and most have a lot of proprietary data associated
     
  3. Sherkin
    Joined: Feb 2014
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    Sherkin Junior Member

    Cheers, good to know, I thought my google foo was having an off day.
    If you had a reynolds number of 40x10^6 at a speed of 18knots (very long cord) What would you do to reduce drag?
     
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  4. Baltic Bandit

    Baltic Bandit Previous Member

    I'm far from a foil expert. Though there are some in this forum. sorry
     
  5. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    If a foil is operating close to the surface, the pressure-side of the foil is taking more of the load. The ultimate event is when the foil is so close to the surface that the stagnation point moves down towards the pressure side. Then there will be a free jet from the leading edge, and the suction side is open to the atmosphere ("ventilated". This is called "broaching", and is followed by a sudden decrease in lift and L/D ratio.

    In this case, a cavitating, or semicavitating profile will work fine, particularly if its ventilation can be controlled. A common characteristics of these profiles is that the pressure side has some camber, and a sharp leading edge. There are some Russian profiles that work fine in "close-to-surface" operation. There may be something on "Gurevich" if you google.

    That said, I must add that cavitating and ventilated flows are a bit off common procedures, and if you are a novise to hydrofoils, there are easier ways to go.
     
  6. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Short chord, high speed and long chord, low speed with same Reynolds number would not be similar for cavitation. Cavitation depends on the shape (but not size), speed, water density and difference between water vapor pressure and hydrostatic pressure.
     
  7. BMcF
    Joined: Mar 2007
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    BMcF Senior Member

    If I understood the intended use of the "foils" on the concept being explored by the original poster, there is no lift wanted nor expected from them. They are intended to dampen vertical motions only, simply by their presence and large planform areas.
     
  8. Sherkin
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    Sherkin Junior Member

    Correct and I am looking to reduce drag.
     
  9. lohring
    Joined: Nov 2006
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    lohring Junior Member

    I did some quick research on foil sections as they apply to surface piercing propellers. It discussed supercavitating as well as ventilated flow and was designed for non technical readers. See Props - The Theory.

    Lohring Miller
     
  10. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    Supercavitating sections would not be suitable for the heave-plate application. The drag would be much higher than for a subcavitating section. You stated that high speed was not required, and the sole benefit of supercavitating sections is they produce less drag than a subcavitating section when operating at speeds beyond where the subcavitating section experiences a loss of lift and increase in drag due to cavitation.

    Using a supercavitating section on a low-speed craft is like saying, "A Ferrari has a rear engine, and it goes fast. My Volkswagen Beetle has a rear engine, so it will go fast, too." Supercavitating sections are used on craft that go fast - they are not really what makes the craft go fast.

    Any decent subcavitating section will have much the same drag for your application. The main ways in which you can reduce the drag will be to reduce the wetted area and to reduce the physical thickness.
     

  11. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Perhaps using active ventilation rather than passive caviation. This will allow the effectiveness of the foil damping to be varied accros the speed range without paying the cavitation penalty, as well as being more healthy for the structure. See some of Ceccio and Beck's work.
     
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