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 Post subject: vortex bubbles propagating backwards on hydrofoil
PostPosted: Mon Aug 25, 2014 12:58 pm 
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Tip vortex bubbles propagating backwards under the water during a upwind tack on a Liquid Force kite hydrofoil. It seems that the trigger is ventilation when the tip of the wing strikes the surface but hereafter it is an open question whether the fast reverse propagating line of bubbles is created due to the ventilation or actually cavitation



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 Post subject: Re: vortex bubbles propagating backwards on hydrofoil
PostPosted: Mon Aug 25, 2014 1:21 pm 
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This is a ventilation issue. Not going fast enough for cavitation. Most foils I have seen have the same issue if the wingtip gets close to the surface. Usually the ventilation bubble sheds from the wingtip pretty quickly.


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 Post subject: Re: vortex bubbles propagating backwards on hydrofoil
PostPosted: Mon Aug 25, 2014 1:58 pm 
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thanks for the vid.
fun to watch.

i believe your first assumption is correct.

surface air is entrained by the wing tip once it pierces the surface.
low pressure due to high AOA and speed during the maneuver both contribute to entraining the air at the wing tip.

possibly a combination of recirculating flow caused by boundary layer separation on the upper face of the wing tip and the tip vortex are able to carry the entrained air for significant distance while the foil returns to more benign design conditions.
its curious that the strut did not appear to suffer from similar type of ventilition.

if cavitation were present, ithink you would first see it along the upper surface of the wing where pressure is the lowest (near the leading edge but AWAY from the tips) . (i admit , the pics are not detailed enough for me to really tell one way or another).
designers typically try to minimise the amount of lift at the wing tips to reduce spanwise flow and the intensity of the tip vortex. The benefits being improved lift/drag efficiency and handling

cavitation is not really an 'entrainment' type of phenomenon.
low pressure sites on the wing make bubbles (boiling water) which immediately collapse once the pressure returns to normal. bubbles are generally smaller in size at inception. if some are due to air coming out of solution, as opposed to water vapor, they may remain and be visible downstream after the foil has departed.

its hard for me to rationalize ventilation leading to cavitation because sucking in air would limit the wings ability to generate lift which is proportional to fluid density. No lift means no low pressure regions and no mechanism to boil water.


If you knew the board speed ,AOA, and foil profile, one could estimate the local cavitation number along the blades surface. My guess is that the board was not going fast enough.

no matter, still fun to watch
regards,
-bill


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