Thursday, April 24, 2008

Ventilation Whoas

Little thread on Sailing Anarchy got me thinking about ventilation. I'm sure this is probably obvious to anyone who has watched the videos, but this is my take on the phenomenon.

Main foil sails through a small trough (wand wake, or just bad luck) and gets air onto the low part of the strut, which is rapidly immersed again, Whereupon ambient pressure goes WAY down from atmospheric, and then that little bit of air next to the foil just progressively expands to fill the entire low pressure region - PV-nRT? Air expands to fill the region of the foil where ambient pressure is lower than atmospheric - if it were higher than atmospheric then air bubble should not expand at all or "move". Amazing how much space air takes up when the pressure goes down.

That's my theory and I'm sticking with it. You need a way to get air onto a part of the foil that is normally continuously immersed - whether AOA sucking it down a vortex from the surface, or some sort of local temporary thing just putting it down there (e.g. hand of God). From there it just spreads like a turbocharged fungus.

I think putting fences on the leading edge is wrong. Bubble is always on the aft part of foil on the laminar flow sections and propagating down the concave part aft of max thickness. Rudder might be different story if you steer too hard at the wrong speed, but once established these foil farts all propagate the same way - down the trailing part of the foil, and onto the lifting foil if it gets that far, where the pressure is even lower...

Maybe (colder) more viscous water makes the pressure lower on aft part of foil. Seems like it might. Will have to try it out with a scale model and some maple syrup. And pancakes - no tow test is complete without pancakes and a wormhole or two.


Anonymous said...

Good theory.
Why is cold water worse?
Air bubble gets into the laminar separation bubble and expands there?

Karl said...

I don't know is the short answer. I am not an expert in this stuff and as Gui says I pretty much make it up as I go along.

My hunch is that it is all about pressure gradients and that there is more negative pressure on the usual negative pressure locations of the foil when the water is more viscous. Not sure how to model that but I suppose I can jam it into XFOIL at different Re and see what the pressure numbers look like.

Ventilation happens at the leading edge with surprisingly little AOA (air sucked down from surface) or with wand wake interference (temporary surface height discontinuity). The relative contributions of the two mechanisms on rudder and DB are likely different.

From what I know about LSB (not much) it would make sense that air expansion would occur on the same part of the foil section. Strut is generally laminar flow to 60% and you do not see shaggy air bubbles hanging around until the pressure recovery concavity which should be the same place the LSB occurs. But you only have an LSB if you have water on the foil. All this Re talk would seem to go out the window when you put some air down there. From what I understand critical Re for an airfoil is 70k and it's hard for me to see any part of the foil getting that low at the speeds in Becks' video or the one from JPZ - at least until the air hits the foil.

Unknown said...

You're overcomplicating it all! Use a better rudder section and it doesn't happen at all....

Karl said...

Cookie -

Yes you're probably correct, but it's good fun to have people far smarter than me tell me how full of crap I am.

I can see leading edge sharpness being an issue with the rudder section. Is that what you are referring to?

Anonymous said...

fence does not have to be a "fence" look at f4 phantom wing le.