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
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.
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.
Phenomenon is called Karman vortex. In this particular case the spinning air effect was caused by pressure difference (vacuum) created by rapidly spinning water, which "sucked in" air. The vortex continued to grow till the water had momentum, or till it reached the ground. Low pressure area continued to grow with the vortex. After the vortex stopped growing, it continued to spin, until its centripetal force was too weak to compensate the buoyancy of air bubble. This phenomenon is created by hydrofoil all the time. More speed = more momentum to the spinning water. In this case, first vortex was created very close to the water surface, so the vacuum was able to suck in air. In this example
you can see creation of 2 vortexes. Case is very simillar, only here two vortexes were strong enough to suck in air. If some sort of dye were in the water, you could see those vortexes being made all around the trajectory of hydrofil´s neck.
And NOT only, this exact phenomenon is, what causes kite lines to drag, and flags to flap.
Conclusion: more round is the neck of hydrofoil, more drag in the form of Karman vortex is present
Edge19 wrote:Thanks Krucifix, when you say more round is the neck more drag... What are you referring to? Wing tips?
I am sorry, I dont know the terminology. By neck I ment the long and narrow part from board to that ufo-shaped-wing thing on the bottom (I believe from some technical-ish picture that that part is called the strut) However, pretty much any object which moves with rapid speed in liquid object will suffer with this kind of drag. But more aerodynamically is the object shaped, bigger is the critical speed at which vortexes are being made. This si why some local Cessna has narrow wings, but supersonic F22 has wings quite wide.