sail aerodynamics

http://www.boatdesign.net/forums/sailboats/newport-27-rudder-2886-2.html#post31559 #27

 

Ah, the "horns of ignorance" as Jimmy Doolittle III (test pilot & grandson of the same) calls vortex generators.

I have two reservations with regard to the papers on humpback tubercles. The first is the wind tunnel tests were done at low-ish Reynolds numbers, and the benefits may not be the same at high Reynolds numbers. Britt Chance could probably identify with that.

The second is the tubercles may not have a hydrodynamic function at all! The researchers assume that because the turbercles are on the fin leading edges, they must be hydrodynamic. In the photos I've seen many of them are encrusted with barnacles, which are hardly clean hydrodynamic configuration. What if humpbacks evolved tubercles because other humpbacks found them sexy?

 

Here's a reference to a very recent contribution from the Uni. of Adelaide researchers.

"Performance Variations of Leading-Edge Tubercles for Distinct Airfoil Profiles"
Kristy L. Hansen, Richard M. Kelso, and Bassam B. Dally,
AIAA JOURNAL, Vol. 49, No. 1, January 2011

ABSTRACT
An experimental investigation has been undertaken to determine the influence of sinusoidal leading-edge
protrusions on the performance of two NACA airfoils with different aerodynamic characteristics. Force
measurements on full-span airfoils with various combinations of tubercle amplitude and wavelength reveal that
when compared to the unmodified equivalent, tubercles are more beneficial for the NACA 65-021 airfoil than the
NACA 0021 airfoil. It was also found that for both airfoil profiles, reducing the tubercle amplitude leads to a higher
maximum lift coefficient and larger stall angle. In the poststall regime, however, the performance with largeramplitude
tubercles is more favorable. Reducing the wavelength leads to improvements in all aspects of lift
performance, including maximum lift coefficient, stall angle, and poststall characteristics. Nevertheless, there is a
certain point at which further reduction in wavelength has a negative impact on performance. The results also
suggest that tubercles act in a manner similar to conventional vortex generators.

 

"Hey mate, she's got nice tubercles."...sounds plausible.

Still, it seems the tubercles have to offer a pragmatic benefit, as someone suggested, to become part of the evolutionary trend, at least in the sense the bumps do not detract from survival, the mating/propagation issue notwithstanding.

And, from the literature, looks to be the case.

Big boobs may be in now, but seem to have little to do with our survival, and most gals I know, prefer the lighter variety.

 

Fortunately, the ignorance is a built-in feature of the mankind. It tempts us with an infinite possibility of discoveries.
Fortunately, the mankind cannot resist temptations.

The term "Horns of ignorance" reminds me of the so-called "Coefficient of Safety" or "Factor of Safety" (FoS).
This coefficient is also called by some (and I can't agree more) "Coefficient of Ignorance", because it is directly proportional to:
- our ignorance of variables which might screw up our most carefully performed calculations.
- our ignorance of the underlying physics.
- our general ignorance (and this one is big).

Could be, but I vote for hydrodynamic reasons. I have read that they have a very particular feeding technique (http://www.enchantedlearning.com/subjects/whales/species/Humpbackwhale.shtml) which requires a good low-speed maneuverability.

And besides that, how can anyone find pimples sexy?

 

Tubercles

I guess there is only one fun way to find out, we gotta grab the humps...by the horns :]

My dinghy is small, so it is also slow...I might just see some improved slow action...

 

been reading along with this one for a while now, T has a good point about those tubercles being just some odd ball form of sexual dimorphism, happens all the time and just about every species has an example or two. Speech development is thought by some to be a form of phylogeny as well.

The barnacle whale relationship is an interesting aside from the conversation but does have some pertinent aspects. For instance most barnacle species are specific to a particular species of whale but more on topic is the fact that the larvae, once they find a whale, walk around on the whale till they find the preferred spot. then dig in. So if there tubercles are a common resting place for these barnacles then they might be a form of bio-symbiotic hosting not previously recognized. From what I can remember the relationship between barnacle and whale is considered obligate comunalism or something like that, which is about one step short of parasitism but the host neither gains nor looses anything in the deal. Point being that if it does in some way benefit the whale to host the barnacles then there's another reasonable explanation for the tubercles that doesn't involve hydrodynamics.

lots of various reasons for physical structures that don't have much to do with efficiency in the animal world.

good point T
cheers
B

 

Tubercles

The tubercles could be simply a means to attract, hug or hold on to a mate and control krill and/or maneuver...but I have to defer to those with more research and science.

Peeping Toms need to get to work.

 

I hope it's okay, but since the revival of this thread lead me to note Perm Stress' post I thought I'd reply.

PS, I understand the point but I think there's a fair bit of evidence to say that a 30% increase in basic performance potential will lead to instantly noticeable results.

A few examples;

The modern assymetric spinnaker has nothing like a 30% speed increase (all round) over a symmetrical kite (I think it was around 3% from memory IF the spinnaker size was increased), but from the very first time the assy was used it was extremely competitive in both 18s and 14s.

The first Windsurfers were a vast step into unknown territory, but there's some evidence that they were very quickly sailing to about 90% of their performance potential.

The early trapezes, which were quite difficult to use, lead to an instant noticeable performance increase although they only add something in the region of around 2% to overall speed.

The first solid wing sails and wing masts would surely have required at least as much re-thinking as a crab-claw sail would require, yet their performance potential (which was slow to prove itself) lead to many experiments from "mainstream" sailors.

Many early multis proved their performance potential very quickly despite their novelty.

So while conventional rigs are better optimised, with respect, if there is any rig that can create a 30% improvement it has never turned up in races, and much smaller improvements are picked up avidly by racing sailors. The advent of the totally novel form of sailing (and rig) that was windsurfing is surely proof of the fact that good sailors can get to grips with very novel rigs very, very quickly.

Finally, there are claims that pacific sailors actually switched to sprit rigs after seeing Europeans sailing them, which would indicate that the crab claw may not have been superior.

 

CT 249

Ilan replied to you at #23

http://www.boatdesign.net/forums/multihulls/wing-sails-36306-2.html#post451440


P

 

Fat-head and Square-top Mainsails

There are some interesting observations over here on triangular, vs fat-head, vs square-top mainsails.


http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/aspect-ratio-headsail-vs-shroud-angle-39379-4.html#post483474

 

Racing Rules Designing our Boats ??

I was just going back thru this subject thread and ran across this series of postings #370 to #374

I've often expressed this same siniment about handicap racing rules designing our vessels rather than mother ocean.

I want to revisit these discussions when I have a bit more time.

 

It's a fact and has been been so ever since boat races have been organized. See the link in this old post of mine: http://www.boatdesign.net/forums/boat-design/buttock-angle-37434.html#post453579

And when it wasn't handicap rules, then the hull shape (of both sail or motor boats) was historically often determined by the need to (pick your choices): maximize cargo or fish storage volume, allow access to shoal waters, make the boat beachable or groundable during low tides, ease the lifting of fishing nets, increase maneuverability, increase the initial stability, make the construction easier and cheaper, keep the CoG low etc. etc. Many things in the list besides (or before?) hydrodynamics and seakeeping.

Today we often tend to see boats as objects designed for leisure, relax, fun, speed, competition, exposition etc. But throughout centuries boats have been, above all, working tools. The concept of yachting has appeared no more than 300 years ago, so the work-related issues were historically much more important than powering, comfort or aesthetics.

Resuming, all this is to say that hullforms have always been dominated by different types of constraints, many of which have nothing to do with sailing or hydrodynamic qualities in a strict sense.

Cheers

P.S.
Looks like the above-mentioned link no more contains the article about history o handicap rules. That's a pitty, it was a nice reading.

 

Not to corrupt a thread, but this is close enough: whatisit with the new Boeing 787 and those pointy wingtips. I've just gotten my two sailboats into the flathead main config, to minimize tip vortex stuff, etc etc, and now Boeing does this to me. OK, so we're only sub-sub sonic: I'll assume the practice is still worth pursuing: flathead mains.

 

Raked wingtips are found on a variety of fast birds and fish, e.g. swallows and tuna. (Their wing shapes are usually known as "lunate" or crescent-shaped).
The rake increases the effective aspect ratio, as do endplates and winglets, but without increasing the bending moment as much at the wing/fuselage intersection.

Leo.