Best shape for surface piercing foil

[Paul Scott]

Originally Posted by markdrela

You sure about this? I don't think you NEED separation to get ventilation.

During the Decavitator project, I had a couple of students do some towing tank experiments with an angled surface-piercing lifting wing for their SB thesis. The goal was to characterize the onset of ventilation. The airfoil was some generic low-camber section, maybe NACA 2410 if I recall correctly.

The most interesting discovery was that the wing had two stable states at most speeds and alphas:
1) normal operation
2) fully ventilated operation

At the start of the tow run the wing would always be in normal operation. It then might or might not switch to the ventilated state. The higher the speed and the higher the alpha, the more likely the switch to the ventilated statem but stalled operation was not a requirement to get ventilation. At low speeds ventllation was unlikely to occur naturally, but it could still be forced to occurby tossing a cup of water at the wing as it ran by in the tank. In all cases, once ventilation started it would persist for the rest of the tow run. The load cell indicated that there was a dramatic reduction in lift from the ventilation. Total drag didn't change much, but this just meant that CDp increased significantly while CDi decreased (from the reduced CL). All as expected, I guess.

We ha a camera on the tow carriage and watched the ventilation-start process. It would always begin near the LE-surface intersection, possibly at the Cp minimum, and then propagate down and back, covering most of the wing surface.

Since I am apparently obsessed by leeboards (and windwardboards for that matter), I have remembered this post, and once in a while I have come across a report/thesis etc. that seems to address the problem in some way. I realize that this thread is old, and the mechanism involved may have been understood completely by now, but I haven't seen anything here on BDF, so I've run across a report in the Journal of Fluids and Structures that seems to address the subject, and It is available on the web- Google- "Unsteady free-surface wave-induced boundary-layer separation for a surface piercing NACA 0024 foil: Towing tank experiments" by Metcalf, Longo, Ghosh, Stern-

Especially interesting is a mention of Zhang and Stern 1996 (in the Reference section), in which contains the following- "Additionally, a necklace vortex can be observed wrapping around the leading edge of the foil for higher Fr. Nodes and saddles of separation and attachment are identified and used to describe the flow patterns." Which seems tantalizing, since it seems to locate a low pressure(?) discontinuity that exists at a leading edge. Could this sort of condition be what is tripped into ventilation by the cup of water that Mark was talking about above?

I don't know if anyone will take the time to wade through the piece, but if you do, is it germaine? The main reason I ask is there is an observation in a thesis on a page I'll have to look up and post immediately that points out a problem with NACA 0015 at certain re is a microscopic flow detachment that kind of reattaches before the 'officially recognised' flow detachment and subsequent turbulent reattachment that happens very near the leading edge, which is in the general area where ventilation is observed to begin in the water, at any rate, on ventilating foils. It seems there are all sorts of flow discontinuities happening in the laminar flow zone? Or is the idea of laminar flow only a kind of large scale concept with many small scale complexities existing below a certain scale of observation?

Paul

[Paul Scott]

"System identification for precision control of a wingsailed GPS-guided catamaran" Elkaim
Stanford University. Look at page 168. Also has a symmetrical airfoil section the author generated for the project that's kind of cool looking.

Couldn't get the address to paste. Sorry.

Paul

[Doug Lord]

Originally Posted by Paul Scott

"System identification for precision control of a wingsailed GPS-guided catamaran" Elkaim
Stanford University. Look at page 168. Also has a symmetrical airfoil section the author generated for the project that's kind of cool looking.

Couldn't get the address to paste. Sorry.

Paul

=================
Here it is: (be patient this takes time to load-couldn't post the pdf)
http://users.soe.ucsc.edu/~elkaim/Do...imThesis01.pdf

[Paul Scott]

Thanks Doug