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As air drags alongside a missile (for example) it directly
slows it down. However, if the missle has weird spikey
pockets which trap air, or move it out - it may create a
'puffer jacket' of turbulent air around it - reducing its
drag. Question mark.
Perhaps other counter-intuative shaping
(Such as a
concave - ie bowl shape) nose-cone. Might be
advantageous.
@maxwell, yay! my first portion of croissant!
@bad-jim, no the other concave. so the bowl on the jet
... would hold milk in it ... if the jet ... were standing
upright. like i said, counter intuative...
@david_scothern - thanks for info, no i have no idea
what i'm talking about.
@freefall - well, this is the half-bakery...
Like this?
http://en.wikipedia...resistant_aerospike
[ShawnBob, Apr 12 2010]
see page 9
http://facta.junis....02/macar2002-06.pdf
[ShawnBob, Apr 12 2010]
[link]
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The air will still need to get out of the way. There "cone" of stable air that you imagine sitting at the front of the airfoil still needs to push the other air out of they way, by so doing pushes on the surface of the airfoil (pressure differential) and thus slows the airfoil. It matters very little if the shape of the airfoil is formed exclusively with conventional solid surfaces or by means of laminar/turbulent effects, neither is inherently more or less efficient. |
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You mean a bit like the spike on the front of supersonic jet fighters? |
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nicholaswhitworth, welcome to the HalfBakery (I speak on
behalf of..... and also with a slight slur). |
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As regards the idea, yes. As I understand it, the rough
texture of sharkskin, and the dimples on golf-balls, are all
related to this idea. So, even though this is sort of baked,
you get a [+] from me for an eminently good idea. |
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On Mythbusters they proved that a car with golf-ball style dimples gets better gas mileage. That works on the same principle. |
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The idea is not to induce turbulence, but to thicken the
layer of still air between the surface and the moving air.
This allows the latter to slip more readily over the still air,
producing a smoother travel. |
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look up "supercavitating". It's concept is mostly referred to with water/submarines but at extreme speed could apply to atmosphere as well. |
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It pretty much makes no odds what you do with the nose-cone section, as long as it's convex and smooth. In other words, what [WcW] said. The rear end is much more critical, because you're looking to get the flow you've disturbed back together again without generating monster vortices. Granted, for a rocket, the rear end aerodynamics are utterly dominated by the rocket's gas plume and you don't have such a problem. |
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I'd suggest that aerodynamics are pretty well understood though - do you have any reason, other than intuition, to believe that trying something very unusual like this will bring benefits? |
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The effect of the dimples is to increase the energy in the boundary layer. When the boundary layer is energetic, it is less likely to separate from a surface which curves away from the flowstream. In the case of a car, the effect is to allow the air to close back in around the back end of the car, thereby producing a smaller wake. |
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While this does reduce the drag on a car, it also (depending on the car) increases the lift, making the car less stable at speed. |
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If the flow is already smooth, with a gradual pressure gradient (such as around a wing in cruise flight), this is not beneficial. However, in other configurations, such as high angle-of-attack operation during landing, a high-energy boundary layer is very beneficial. The benefit is not in reducing drag, but in keeping the flow attached and delaying stall. |
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This idea is an example of "Concept X does this in situation A, maybe it will work in situation B." That is a great start and an important step, but in this case, this particular field of aerodynamics is fairly well understood, and the concept is already being utilized where it shows a net benefit. |
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Interesting article I read somewhere not about
fluid dynamics but actually civil engineering: some
researchers discovered that they could facilitate
evacuation from a building by sticking a big pole a
few feet in front of the exit. It created
"turbulence" in the "flow" of people preventing big
mobs from forming and blocking the doorway
allowing more people to leave faster. Sadly I can't
seem to find it anywhere.
Either way, [+] for all the funny missile shapes this
inspired in my head. |
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[Postscript], yes, I recall that too. It might have been reported in the UK in either "The Engineer" or "Eureka", both of which are advert-supported free magazines from the same publisher. Alternatively, it might not. You're not making it up, either way. |
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Thirded. I read that too. |
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By the way, welcome aboard [nicholas]. Yes it's true as others have stated that this effect is known and being studied and used; the fact that you arrived at the same point by intuition - //spikey pockets/puffer jacket// earns you a crusty bread treat from me [+] |
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Bye for now, I'm off to load ravers into my trebuchet and seeing how far they fly with and without spikey rubber backpacks and puffer jackets. |
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Nic, you comments would make more sense in the thread of the comments overall. |
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It's called an aerospike [links] |
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blobs on whales supposedly make them more efficient, so there are a variety of fluid dynamic realms where this works |
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//blobs on whales supposedly make them more
efficient// I find that hard to believe. Whales have
had several tens of millions of years with nothing
much to do apart from eat krill. If blobby skin were
such a great thing, they'd have evolved it by now.
Instead, they seem to rely on barnacles to create
blobs, which seems a less than ideal solution. |
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