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 Post subject: Supercavitation - just dreaming aloud.
PostPosted: Wed Aug 27, 2014 5:06 pm 
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Joined: Tue Apr 22, 2003 1:37 pm
Posts: 89
Location: Hong Kong
I saw the news below and my mind started wondering...

Is there any applicability of this to foiling?
My understanding is that "super-cavitation" starts at about 70km/hour.
But if somehow, we were able to pump air from inside the wing to envelope it in bubbles, would that help to start going sooner? Just a crazy idea..

Please those who understand fluid mechanics of this science, maybe you can explain why this would not work or maybe why it would?



 Post subject: Re: Supercavitation - just dreaming aloud.
PostPosted: Wed Aug 27, 2014 6:26 pm 
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Joined: Fri Jul 16, 2010 6:27 am
Posts: 3037
Location: Ford Lake, Michigan
good idea but it doesn't work in the most simple application. ventilation and cavitation both occur in ordinary hydrofoils, ventilation occurs when air is dragged down into the foil, cavitation can occur even at very low speeds. When this happens, the foil is no longer riding on water, but riding on air. The tiny foil is then no longer big enough to support the rider and it crashes down because there is no lift. You would need a foil big enough to support the weight of the rider in air (10m or so <-- the same size as your kite).

I think what might be possible is to design a partially cavitating/ventilating system where at high speeds, most of the foil is supercavitating (or ventilated) but you still have part of the foil supporting the weight of the rider.

Another, much more simple option is a bi-wing design. At low speeds, you ride it with both wings in the water, the top wing is large and supports your weight at lower speeds, as you gain more speed, you pop up until your whole weight is on a tiny foil, this can let the foil operate at both low & high speeds. Indeed the upper wing will be entirely ventilated as it will be out of the water entirely.

 Post subject: Re: Supercavitation - just dreaming aloud.
PostPosted: Wed Aug 27, 2014 7:13 pm 
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Location: rhode island
I agree with edt.

cavitation and ventillation can reduce skin friction.

generally, the drag coefficient of a foil designed to cavitate is higher that the drag coefficient of a streamlined shape (Eppler, NACA, etc) that does not cavitate while generating the same lift.
for that reason folks dont usually opt for a cavitating shape untill they have to.

however, once conditions are severe enough to induce cavitation in the streamline profile, the cavitating design can have less drag. (i.e Sail-Rocket and Hydroptere)

in the grand scheme of things
the speed of kite hydro-foils is pretty low ( approx. reynolds number= 1-2 million )
at those speeds there are a few other 'pie in the sky' drag reducing tricks worth discussion
like laminar flow profiles or polymer drag reduction (polyox)

reducing the skin friction of the strut may be worth while if you negate any need for lift there.
potentially you could add any of the above techniques to the strut
reduce surface area to the structural minimum
ride closer to the water surface (w/o ventillating the wing)

but back to edt's position....its not easy

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