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  #46  
Old 11-23-2010, 04:35 PM
Paul Scott Paul Scott is offline
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Quote:
Originally Posted by markdrela View Post
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
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  #47  
Old 11-23-2010, 04:41 PM
Paul Scott Paul Scott is offline
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"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
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  #48  
Old 11-23-2010, 04:46 PM
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Doug Lord Doug Lord is offline
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Quote:
Originally Posted by Paul Scott View Post
"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
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  #49  
Old 11-23-2010, 04:47 PM
Paul Scott Paul Scott is offline
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Thanks Doug
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  #50  
Old 02-07-2017, 08:17 PM
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brian eiland brian eiland is offline
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Bridgedeck Centerboards

Quote:
Originally Posted by grob View Post
Tom,

My original question was meant to be about the section design of surface piercing rudders. You seem to be talking about foils as in foils for generating lift in the vertical direction. Do your comments still apply?

Also why does a sharp leading edge minimise wave making and spray generation?

Gareth
I was looking back at this subject thread for info on ventilation problems that one might incur with vertical foils such as centerboards and dagger boards on level sailing multihulls.

Gareth, I was also surprised at the direction the subject thread took in discussing 'vessel lifting foils' as opposed to leeway reducing foils or rudder foils as you had asked about.

Later in the discussion I found this:
Quote:
Originally Posted by Paul Scott View Post
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
I'll have to see if I can find & wade thru this research, but i have a feeling it may go more deeply into the theory of this ventilation problem than I care to go at this time. I am looking for the best manners to avoid as much ventilation of a vertical foil centerboard(s) as discussed in this recent subject thread,
Bridgedeck centreboard why don't they work???
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  #51  
Old 02-13-2017, 04:49 AM
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brian eiland brian eiland is offline
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Surfboard Fins

Quote:
Originally Posted by boogie View Post
hi guys,

no more football or kangachook fins on windsurfers...
and lexan as a material for making them is long gone too....

outlines and sections have become a lot cleaner/simpler in recent years, but straight leading and trailing edges are not that common either. windsurfer fins vary a lot in shape and size depending on the gear they are used on. for light wind course racing [no centerboard] they are around 700mm long and for speed sailing they can be as small as 220mm with the same scaled outline. wave and freestyle fins are probably more what most people imagine when they hear windsurfer fin. like half dolphin tail or a dorsal fin.

i'm involved in windsurfing now for about 8 years and i have not seen a single fin with a fence. maybe it's time to spin the wheel again and try some out.

ventilation is more of a problem for the large lightwind race fins rather than the speed record ones. i personally have so far only managed 36.2kn on my GPS, so i think i'm still quite a long way from running into cavitation problems.
it is hard to tell when the top guys are wiping out at +40kn what exactly it was, but they all use fine entry sections on the foils of around the 8-9% thickness and quite a bit of rake in them [20-30deg]. the CL values are pretty small at those speeds despite the fins being very small in area.

would a cut out at the waterline or a reduction of chord length [with the same absolute thickness or greater %-age for structural reasons] actually be the best to prevent the ventilation?
if the suction peak close to the leading edge is the main contributor wouldn't an increase of the chord at the waterline drop the local CL and thus reduce the peak?
cheers
boogie
Everytime I come back to this subject thread I find another interesting posting.

So how did these whale tubercle fins work out?
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  #52  
Old 03-08-2017, 07:54 AM
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frers33 frers33 is offline
 
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i am trying to figure out the chord and length of a straight daggerboard to reduce the leeway on 30-45ft displacement monohulls. i have already picked an asymmetrical foil based on Cl, Cd, Cm etc...

my question is how much of a horizontal lift force must a daggerboard generate to work against the leeway when going upwind? typical speeds of 6-8 knots.

100 lbs ?
200 lbs ?
300 lbs ?

thank you.
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