100% Bluff Body Drag Reduction Theory
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This may be the Holy Grail of auto aerodynamics.
Unfortunately, like the Grail, it may be unrealizable for mere men. From time to time, posters ask why hasn't anyone come up with a way to reduce a vehicle's drag similar to the bulbous bow that is used to reduce/cancel the bow wave on large ocean-going ships. Well, I stumbled on this presentation which shows at least a theoretically possible way to do it. I'm skeptical. This appears to be too good to be true -- or at least too impractical to implement -- but it is also too good not to share. First the pictures, extracted from a PDF: Attachment 5218 Attachment 5219 Well, there it is in all it's impractical glory; 100% bluff body drag reduction. Things aren't perfect though, the plate-bluff body system reduction is only 62% If your vehicle is 50 inches wide, all you need do is have a vertical plate ~18 inches wide held in place 75 inches in front of the vehicle. Simple? Yes. Practical? No. Nonetheless, it would be interesting to see if it worked in real life. I suspect that even a small crosswind component would greatly diminish or wholly negate any benefit. All of this from: Bluff Body Aerodynamics, Lecture Notes by Guido Buresti. Department of Aerospace Engineering, University of Pisa, Italy. 6.12-16.2000; Pages 28, 29. |
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So you're saying that if our cars looked like an angler fish, with a protrusion in the front 75 inches ahead of the bumper at was 18 inches wide, it would split the air in front of the vehicle and allow it to pass by unmolested?
http://www.icmcreative.co.uk/uploads...0113102801.jpg I think this sounds familiar to the way that Olympic divers part the water with their palms, so they leave almost no splash... Brilliant, someone needs to make a concept out of Poly Carbonate right away! http://z.about.com/d/diving/1/0/o/B/-/-/flatHand.jpg |
He said "bluff" body, not "buff" body:D
http://z.about.com/d/diving/1/0/o/B/-/-/flatHand.jpg It's interesting that the larger square body is blocked by the smaller round body, while the larger round body is blocked by the smaller square body. Experiments with this might be easier than boattails and such. You can place something on a pole out in front of your vehicle and keep an eye on it, while you can put tufts on your windshield where you can see the effect without cameras. |
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I posted this "theory" only after a lot of thought. As an idea has a lot of attractiveness. However, for use on the road in the real world, it has a lot of practical problems. And I'm sure that the pure aerodynamicist members are rolling on the floor. If they can catch their breath, I'd like to hear what they have to say. I can only imagine how your local law enforcement would react. :eek: Conceptually, this idea would fit nicely into the generally accepted idea that more can be gained aerodynamically by looking at the front of the car than at the rear. The current thread on the Geo Metro boat tail is a masterful implementation of theories to clean up airflow at the rear, to the tune of about 15% drag reduction. http://ecomodder.com/forum/showthrea...mpg-10691.html Now, what if, just what if the same or greater gains could be made at the front end? Would it work? We'll never know until someone tries it. I find it interesting that Geo boat tail extends backwards about the same distance that the plate would extend forward. Come to think of it, this "leading plate" is sort of like the boat tail, only without the sides. |
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The diver tapers his body as much as possible so it is more like a boattail than a blunt rear of a car. |
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think it would take more than just a pole. It would probably require an A- frame at top and bottom. The low point connection wouldn't seem to be a big problem. But the upper point of connection with the body work would be around the top of the windshield. Maybe the leading plate wouldn't have to extend vertically the full height of the car. Perhaps just part way up, say to the belt line/bottom of the windshield. [Edit: There is always the possibility that turning the plate on its side, so it was horizontal, might work too. I think it would be easier to implement at any rate.] Who knows? Not me for sure. :rolleyes: Quote:
Yes, for the typical car. (But not for a car with a boat tail.) If this worked, perhaps there would be little need to work at the rear. At this time this is an "idea experiment," and I'm not capable of mounting a rigorous defense of it. |
yaw.
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More importantly, none of us are riding around in a square cylinder, either oriented longitudinally or transversely. Even describing any vehicle as a bluff object, it will be at best an irregular bluff object.
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Are you providing the correct name for a non-centerline wind component mentioned in my first post, :thumbup: Quote:
I'm hoping the former. |
I find the first thumbnail in the first post more interesting, partly because it shows a relatively small rod parting the wind for a boxy building shape, but more so because it shows actual smoketrails to support the theory. Also, that the rod worked better when closer to the box.
This seems to work similarly to the way a canard wing affects and enhances the airflow hitting a main wing behind it on an airplane. The void behind a car is large, and to fill it with a boattail, the tail has to be large enough to fill that void. In contrast, if a small shape before a car can act to part the air before it gets to the car, then it's kind of a canard fairing/vortex generator. What's nice about this is that a smaller shape is going to be easier to experiment with, it doesn't need as much support structure as a boattail, and with a minimalist triangulated frame would present less surface to a sidewind, hence (hopefully) less of a negative yaw impact. Fun to think about.:thumbup: |
OK.
yaw. and speed. Maybe if that thing is aimable- veer it off towards the xwind. |
Funny. I was picturing a car with a "nerf bar" sticking out about 6' in front of it.
#1. I know that if something sticks out, what? 12" behind your taillights, you need a reflector. But how far in front of your headlights can it be before, I don't know, the cop isn't gonna buy, "Hey, buddy! It's just a nerf bar! Doncha 'member seeing them on '50's hotrods?" :eek: And, then, as Frank, with such minimalism one could not accuse him of pedantry, pointed out, the optimal location would necessarily vary with air speed and direction relative to the vehicle. You would conceivably need a system that was able to instantaneously adjust the location of your plate/rod via computer and electric or hydraulic actuators. Likely in all three axes. Oh, the magnitude of the technology needed! That would almost be as complicated as, what? micromanaging each cylinder's spark event and injector duty cycle based on instantaneous readings from knock sensors? Yer right, Frank. Couldn't be done. Nothin' to see here. Move along. Just razzin' ya, Frank. ;) |
This reminds me of a "spike" appendage tested on rockets about 30 years ago, as shown in Aviation Week and Space Technology. I think it was a SLBM rocket, fairly short and squatty to fit in the submarine launch tubes. Upon deployment, the spike (which looked like a big spike or nail, with the disk head at front) moved forward on a telescoping tube or rod. The wind blast hit that, shed, and formed a sort of aerodynamic bubble in which the main rocket body traveled. Overall, it reduced drag at high speeds as the missile accelerated.
I've wondered about using such an idea on a motorcycle, which being narrower, would presumably mean a smaller disk deployed closer to the headlight. As it is, there is quite a lot of turbulence and buffeting behind a standard windscreen, due to Von Karmann vortex street effect. Trick is to shed such oscillations far enough behind the rider's helmet to keep the helmet in a protected bubble, where it is much quieter. With this device, perhaps the whole upper bike could run in the bubble wake. Thoughts? |
And, I wonder what difference if the body in front had a convex (leading edge of an elipse) shape with a serrated edge.
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I thought this was already well known... isn't this why conventional cab trucks are more aerodynamic than cab-over ones?
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Similarly, you'd expect a teardrop made from Lego blocks to be more aerodynamic than a box made of Lego blocks, even though both present the same surface area perpendicular to flow. |
apples n oranges boys
review the concept again |
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It's literally better for the air to form it's own bubble in front of the flat nose of the CoE truck, because there is more skin friction and shape induced drag for the same surface area on the extended nose trucks. When the airflow "hits" the cab nose at highway speed, there becomes a bubble effect on the CoE trucks that displaces air further ahead of the cab due to air spoiling at the nose, creating it's own "teardrop-ish" shape. The extended nose design could have something in it's favor if it were actually shaped to divert airflow from the smaller nose up over the cab and around the widest points without having to make contact with them, though. Watching the teardrop shape, we understand that nose extensions are only necessary to an extent ahead of the primary body, and the angles at which they direct flow can be very steep compared to tail angles. I'm trying to find a virtual image of the bubble that forms in front of a CoE in a wind tunnel, but I'm not having any luck. Also, I found this post here: Quote:
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The main point of my post was that neither truck had had any treatment. In OE form, the "conventional" truck has more skin friction and shape associated drag, because the flow has to change direction so many more times.
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neither truck has a pre splitter on a jousting probe in front of it
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Back on Topic, the original post reminds me of a techology that was researched that is referred to as "Super Cavitation". Please google that term for more information. |
I believe we've "probed" this topic before here?
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theory
I think the images are in 2-dimensional flow,so they'd be a little desceptive for 3-D.
A fellow at Bonneville in 1990 was attempting such a thing by attaching a sphere-on-a-stick ahead of a streamliner,and then attempting to draft behind it's wake. I was given to understand that the fellow had just received a graduate degree in aeronautical engineering from Cal-Tech,or somewhere like it,and that this "technology" had been the basis for his Master's Thesis. I was running my own car and didn't have time to visit with the fellow.It would have been interesting to hear about it all. As far as I know,he did not set a LSR with the setup. |
Supercavitation
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associated with super high speed underwater objects; submarines, torpedoes, and even bullets. It has to do with creating a gas bubble around the object. It would appear that in water at least, speeds ~180kph, ~110 mph, are required. SUBMARINE HEROES I have found indications that the phenomena can also be created in air. I haven't yet found what speeds are necessary to create/sustain supercavitation in air. But, from the cavitators pictured below on aircraft/missles, it would be well above typical highway speeds: :p "Different nose geometries can be used to create supercavities flat discs, cones, 'gear shaped' plates and cones (top and middle), faceted concavities and cavitators with inscribed cones that move in and out like the tips of ballpoint pens (bottom)." Attachment 5243 It is noteworthy that the pictured cavitators are much closer to the main body that the 1.5 x the main body's width given in the original architecture derived arrangement. It may also be significant that the main bodies are long, thin, and aerodynamically clean, not a "bluff body" like a car. This from a very short preview article in Scientific American. Access to the whole article requires a subscription or on-line purchase: How to Build a Supercavitating Weapon: Scientific American Wouldn't it be a surprise to be able to create the supercavitation phenomena with an simple plexiglas plate, sort like an external grill block cum bug deflector, located 6 or 8 inches in front of the grill, mounted to the front bumper? I suppose the effect wold be to place a single whole-vehicle vortex generator in front of the car to pre-energize the boundary layer. :confused: Help! Somebody stop me! I'm saying things I know next to nothing about! :eek: This quote isn't particularly encouraging: "But super-cavitation technology might also be applied to aircraft, giving them brief stints of hypersonic speed when required, either for orbital insertion or escape from nearby threats. Note that SC transport requires a shape much like that of a rocket. " And there is always this: :o Quote:
More research needed... |
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The pictures in the first posting don't show what the speed factor is. I don't think that the speeds we travel is anything close to what is needed for true cavitation, but a shape that would create some kind of vortex generation is more feasible, using the same principle.
How about this. Instead of thinking about a whole vehicle generator, start small and create a probe that would stick out in front of your mirror to generate a vortex to lessen the drag on the mirror. This would be easier to tuft test, and wouldn't get as much potentially undesirable attention as a clown nose on a pole in front of your car. And the smaller size would allow you to experiment more with the effects of different shapes shapes and sizes. |
cavitation
Correct me if I'm mistaken,but while cavitation and supercavitation are certainly phenomena which operate within the study of fluid dynamics,for our purposes,they are limited to "liquids" not vapor or gases.
The low pressure around the structure allows momentary phase change to a vapor ( cavitation ) which,after passage of the structure attains a pressure regain at which ambient surrounding temperature allows for dew point condensation back to a liquid form,collapsing the bubble. In the air,as an aircraft accelerates through stages of Mach you will observe condensation clouds of water vapor created as the local compressive heat of compression is released and visible droplets form in the colder low pressure of the rarefaction behind the supersonic shockwave. For cavitation of the air to occur,the gases which form the atmosphere would have to be destroyed into free radicals and free atoms.Yes? |
Beam me up Scotty...
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Phil -
In short, you're right. Everything I've read and seen about this particular function of aerodynamics places it so far outside the realm of "normal" operation that it's particularly useless to discuss in our forum. It's still an interesting read, though, if you're into that sort of thing. Rokeby - Thanks for finding my thread. I forgot it was here. :thumbup: |
interesting
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I ain't dun yet.
I agree that supercavitation, while interesting in its own right, is outside the
envelope for application to land-bound vehicles used on public roadways. That said, I'm still poking around the 'net for more info on the original idea, as it might be inelegantly put; "a plate on a pole." :rolleyes: Edit: One little factoid from my reading to date: drag in water is ~1000 times that in air. (I suspect you guys already know this, but it's news to me) This would confirm that putting model cars in water flow tanks is good for generalized flow visualization... and only that. |
poking
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No stone should remain unturned,and discoveries in any branch of study could potentially be of benefit to us all. We never know where the voyage may ultimately lead.Thanks! |
I prefer "clown nose on a pole."---:p
But it is frustrating when people lose sight of an idea because of improper terminology or semantics. It's about the big picture. Or in this case the little picture - the one in example C in post #1. I think this is more applicable than the flat surface "blocking" for the round shape. Our cars are closer to "boxy" as in the first plate, than "round" or "spherical" as in the 2nd plate. I am going to see if I can figure out some attachment scheme to try my mirror "blocking" idea. |
drag
drag reduction requires a "single pass".No "circulation" is allowed.
The turbulence behind the lead structure is followed by pure turbulence.The circular motion of these turbulent eddies prevents their kinetic energy from EVER being converted to static pressure behind the trailing structure.It will simply be lost to viscous attrition at some point downstream converting all it's energy to useless heat. Simply softening the edges of the leading edges of the second structure will accomplish everything the lead structure is accomplishing and WITHOUT any circulation and loss of kinetic energy,making this conserved energy available to increase the base pressure behind the second structure,reducing its profile drag to a value lower than that possible when using the lead structure. So far,and for sub-transonic velocity,three-dimensional flow,the 2.5:1 teardrop body-of-revolution remains the lowest drag aerodynamic form to be discovered and published about. |
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But, since we cannot drive around in fish-like bodies of revolution, and have comparatively crappy vehicle shapes to deal with, how about somebody trying a pie-plate-on-a-pole mod for a motorcycle? After all, a motorcycle is about the worst shape on the road aerodynamically, so may have the most to gain with the bike running in the wake of the pie plate. Might be handy breaking up the wake turbulence and buffeting behind trucks, too, leaving the bike rider in a relatively calm pocket of air, saving fuel into the bargain. |
Is there any info on the speeds at which the "pie plate on a stick" idea was successful?
I think, for all intents and purposes, this will be an exercise in futility at 60 MPH. I'm saying this without experience in the field, because it's something I've never really gotten into too much, but I suspect that at the speeds that we're traveling, and even at 100 MPH, the object would still have to be ~60% of the vehicle's frontal area to have any notable effect. The lower the speed, the more of the vehicle's frontal area, and the closer to the vehicle, the object has to be to open a "pocket" of airflow. What we're talking about here basically is cavitation, except without supersonic speeds being necessary. If I had to chance a guess, I'd suggest that what's going to occur with this at our speeds, the airflow is going to begin to close behind the object, making it have to change direction more drastically to begin attached flow on the vehicle. I guess the image that keeps flowing through my head right now is a truck "blocking" for a trailer... the cab's aero structure has to be larger than the trailer to maintain attached flow at highway speeds, so the flow tripping off the edges of the cab fairing doesn't begin to roll into the space between the structures. |
I've found another paper dealing with the phenomena presented in my OP,
that is the reduction of drag on a square cylindrical prism (3D solid) "bluff body" by placement of a separate cylinder in the up-stream airflow. This new find has the drawing showing the hypothetical best placement of the main body and cylinder/plate in common with the first reference. It has different graphics though. It's biggest drawback is that other than a brief introductory paragraph it is in Japanese. Here it is, you can get to the document by using the PDF button in the: Read/Search Full Text section: CiNii- Drag Reduction of a Square Prism OK, does anybody know which of the many web-based translators will produce a readable Japanese to English version? (Note: I'm not 'net/web savvy enough to extract Japanese characters out of a PDF to input to a translator.) :confused: :( :mad: Edit: Here is a list that includes other studies which may shed light on this phenomena, most of them are also in Japenese: CiNii Square Prism |
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