sail aerodynamics

Ivor, please take part. You are thinking about this stuff, and no one is going to jump on you for it. No one has asked me to keep it down, and Lord knows, I have deserved it at times.

As far as devices, start Googling! You never know where they lurk, and a lot of them are not even on the Web. I found Elkaim's piece while Googling 'reynolds numbers of hulls', so there you go. As far as Elkaim's sort of thing, I don't know of a lot out there that are modern, outside of speed record stuff, and some of the open canoe, and older IC guys. You might try wing sails, etc. Sites come and go...... Print them out, as they do tend to dissappear forever. There are academic pages too.

The list I have tends to revolve around flow re numbers from 30,000 to 3 or 4 million, which usually gets me into the UAV, MAV, RC, and wind turbine sites.

My feeling on this forum is that the only sin is not trying.

Paul

 

Paul,
Please read attachment,
Ivor Bittle

 

Ivor, being a retired professor of music, I have some idea of theses tomfoolery. The thesis has holes you could run a truck through, but given the paucity of this sort of stuff for sailboats outside of this site, it is somewhat instructive. The section he uses does have a lot in common with some other mid low re sections (?? ~ 250,000) I've run across, and I was particularly interested in the discussion of flow separation towards the nose, and the use of the reverse curve towards the trailing edge, which in certain circles is kind of controversial. It would have been interesting to see some closeup pics of flow around the section while sailing. They may have been reinventing the wheel, but they did build and sail it, and it interested me that the wing really only started working (if I remember right) at 8K or so, which would be about the right windspeed for a 3d foil to start performing better than a curved plate (according to other authors), i.e. sails on a catamaran. I thought this might stimulate some responses concerning modern wing mast theory vs sails vs 3d sections for different re.

If you really want to get into the politics of higher learning, I suppose you could try Sailing Anarchy, but I'm really more interested in the aero part of the design, as the control system seemed a bit dodgy to me. (This all reminds me of the old joke- why are the politics in academia so vicious? because the stakes are so low........)

To sum up, the thesis is interesting to me because of re regime explored. (And the poetics involved.)

Paul

 

Paul
I am not sure if it has come up on this thread before but it could be worthwhile playing with JavaFoil if you want to get into low speed foil design. It is nice software to use and gives good results. I have checked its output against test data from Selig et al and it is favourable.

Martin Herpperle also has some of his own foils that are used for wings and he gives their design working range.

I have used JavaFoil quite a lot to develop shapes for low Re# water propellers and the results have been good.

Rick W.

 

Ivor,
I wanted to give you rep points because of your answer, but it happens I had given you already. So, from here my applause.

Cheers.

 

JavaFoil Presentations

Here are a few JavaFoil presentations on this subject thread addressing my 'sail interaction' (slot effect) contentions:

http://boatdesign.net/forums/showpost.php?p=89766&postcount=148

http://www.boatdesign.net/forums/showpost.php?p=89921&postcount=151
_______________________________________________________________

Here's another posting referencing some of these images...from a mast-aft subject thread

Hello again Tom. Once again I must beg off on a ‘quantitative’ reply to the advantages of the headsail arrangement on my mast aft rig concept. I believe just as with the rigging load questions, a straight-forward quantitative calculation as to the superiority of the efficiency of the headsail over the mainsail may well be an incalculable quantity from a purely mathematical standpoint. So I have to rely on a variety of observations gathered from history, from pass designer’s applications, from current theorist, from real time sailor’s experiences, etc.

So I will present a few of those observations:
1) Lets begin with a ‘pictorial’ example. Rick Loheed recently posted these images here, “I found a post that had a reference to JavaFoil, Martin Hepperle's 'relatively simple' inviscid foil analysis program that will do multi-element airfoils. It is great for illustration purposes here. Clearly it shows the affect of the whole system- actually, as a cascade of foils. Further Aft foils must have more incidence- but when incidence is added, they help increase circulation around the whole system, increasing the forward foils effectiveness by inducing more incidence and accelerating more mass about the whole mess".
Which sail looks most effective at driving you forward.

2) Another JavaFoil analysis by Rick here, “Here is a comparison using Javafoil of a simple 15% camber (pretty high lift) 40% max camber location Jib/main combination arbitrarily loaded to near Max CL, and a 20% Clark 'Y' wingmast shape based on methods from Tom Speer's wingmast paper. For an input Aspect ratio of 10 for each case, this simulation shows a Max Cl of 2.11 for the main jib combo readily achieved for the combination, whereas the wingmast gets to a fairly typical Cl of 1.2 max”

3) Excerpted from aerodynamicist and North Sails consultant Paul Bogataj’s paper, “How Sails Work” http://www.northsailsod.com/articles/article6-1.html
‘Sails in Combination’, “Each sail by itself is much simpler than the combination of a foresail and mainsail as in the sloop rig. The sails are operating so close to each other that they both have significant interaction with the other. The most interesting feature of this is that the two sails together produce more force to pull the boat than the sum of their forces if they were each alone.
The foresail of a sloop rig operates in the upwash of the mainsail. The wind as far upstream as the luff of a genoa is influenced by the upwash created by the mainsail. Hence, a jib or genoa in front of a mainsail has a higher flow angle than it otherwise would have by itself, causing an increase in the amount of force that the forward sail produces. So, while the mainsail is experiencing detrimental interference from the foresail, the foresail benefits from the interference of the mainsail. Notice that more air is directed around the curved leeward side of the foresail. This causes higher velocity (lower pressure) and more force. The net result is that the total force of the two-sail system is increased, with the foresail gaining more than the mainsail loses”

4) Its been some 40 years ago that I did extensive study of some of the classic sailing boat design books, but I can distinctly remember how there was applied a ‘performance factor’ to the headsail of 1,3 to 1.5 more effective than its actual sail area when computing the CE of the sail plan. So even though the theory was not thoroughly understood at the time, real time observations came into the equation.

5) Redcooprs was talking of the subject under the ‘Fraction Rig’ discussion (www), “However, the design of sailforms is very much a practical nature. In terms of sailing, the feel is that our jib supplies the driving power - and wind tunnel tests also show that it has a very large Cl compared to the main. The main on the other hand, is very responsible for the righting moment and general tuning of the boat.”
...item #24
http://www.boatdesign.net/forums/showpost.php?p=110983&postcount=24

 

Brian
There is some nice work there - thank you for the references. The comment of Rick Loheed about aspect ratio correction in JavaFoil is of interest. I use my own empirical method for induced drag for my props but I often just use the JavaFoil information for rudders and sails. It looks OK but I should check to see how it compares. I expect it assumes semi-elliptical shape.

Rick W.

 

All these discussions don't address the main function of the slot: It's primarily a boundary layer control device, which allows the overall slotted section to tolerate a larger AoA and hence a larger CL before stall, compared to a single sail of the same area. If the AoA isn't increased, then the slot has essentially no effect on the lift.

The attached inviscid panel calculation plots show this convincingly. The first case is a typical AC jib+mainsail airfoil, at 18 degrees AoA. The second case is the same, except the jib has been translated so as to almost close off the slot. The CL is nearly identical at the same 18 degree AoA. The jib translation has been done perpendicular to the freestream, so the projected chord is the same, for a fair comparison.

 

What happens to the curve if the same jib area is used and the overlap is varied? In other words, how much overlap is needed to produce the high CL? Any?

What does the L/D curve look like for same area and AoA with varied overlap?

If the performance is related to projected chord, I would guess that the overlapped area is next to useless?

 

Thanks, Rick, that's a nice vote of confidence for Java foil. My last computer hated it sooo much that it didn't really work, but I have a new toy now, so I'll give it another try.

I really need to give the Hepperle site the time it deserves.

Do you run different re's for indivual propellers?

Paul

 

But if jib overlap (leach) ends at the point on the lee of the mainsail where the mainsail's lee flow is at it's max velocity, doesn't that help influence the Kutta condition at the leach of the jib, decreasing the deceleration of flow to the leach on the lee side of the jib? That would be good, wouldn't it?

Paul

 

Yes, it does, and yes, it is. This is what A.M.O. Smith calls the "dumping effect". There doesn't have to be as much deceleration toward the leech of the jib, so the boundary layer isn't as stressed and is less prone to separation.

For the Kutta condition itself, it's the normal velocity at the leech that requires circulation on the jib to balance it out.

 

Actually, not so much. The "normal-velocity at slat TE" effect described by AMO Smith actually has little effect on the lift. See the attached panel solution. Whether the jib's leech is just under the mast or just above the mast doesn't affect the lift very much, despite a very large change in the normal velocity seen by the leech.

In light of classical thin-airfoil theory, none of this is very surprising. This theory indicates that for a given AoA, lift is almost entirely determined by the camber at the rear of the airfoil. Modifying the shape near front like in this case has almost no effect. In the context of this theory, a small slot between the jib and mainsail is just a tiny interval where the modeling vortex sheet has zero strength. This small interval will only affect the flow in its immediate vicinity, but it cannot influence the overall circulation and lift.

 

Paul
Yes - Also I adjust Re# over the length of the blade using a segmented approach. I have a neat way to take it into account for a blade I can explain if you are interested.

I believe my prop optimising model gives better results, and is more flexible, than JavaProp but I use JavaFoil to generate the polar curves. I did buy Volume 2 of Selig's low speed foil data but have found JavaFoil is so close it does not matter.

It would be really nice to have a 3D version of JavaFoil but have to do with 2D for now and use empirical additions to get something that relates to 3D.

Have put this question to Leo Lazauskas but he said it is way too complex for him to fit into his current work program.

Rick W.

 

Scarburgh (1709 AD) "To which therefore t'is said to be a Normal Line."

Drela (2008 AD) "a small slot between the jib and mainsail is just a tiny interval where the modeling vortex sheet has zero strength."

It seems to me this argues for overlap? But what would it argue if the main were a symmetrical foil, and the jib a curved plate? Low (80,000) re? 3,000,000 re? The same?

Paul