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Old 12-28-2009, 06:57 AM   #1 (permalink)
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100% Bluff Body Drag Reduction Theory

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:

Click image for larger version

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Click image for larger version

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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.


Last edited by Rokeby; 01-04-2010 at 09:58 AM..
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Old 12-28-2009, 09:25 AM   #2 (permalink)
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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?

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!
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Old 12-28-2009, 10:06 AM   #3 (permalink)
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He said "bluff" body, not "buff" body



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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Old 12-28-2009, 10:14 AM   #4 (permalink)
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Quote:
Originally Posted by Funny View Post
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!
That would seem to be a good analogy.

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.

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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Old 12-28-2009, 10:27 AM   #5 (permalink)
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Quote:
Originally Posted by Rokeby View Post

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.
Whose generally accepted idea is this??

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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Old 12-28-2009, 10:27 AM   #6 (permalink)
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Quote:
Originally Posted by thatguitarguy View Post
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.
A large part of implementing a "leading plate" would be holding it in place. I
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.

Quote:
Originally Posted by thatguitarguy View Post
Whose generally accepted idea is this??

The diver tapers his body as much as possible so it is more like a boattail
than a blunt rear of a car.
I think I meant to say that aero mods should first be made at the front.

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.

Last edited by Rokeby; 12-28-2009 at 12:50 PM..
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Old 12-28-2009, 03:32 PM   #7 (permalink)
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Old 12-28-2009, 05:21 PM   #8 (permalink)
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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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Old 12-28-2009, 05:34 PM   #9 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
yaw.
Frank Lee,

Are you providing the correct name for a non-centerline wind component
mentioned in my first post,

Quote:
Originally Posted by Rokeby View Post
... I suspect that even a small crosswind component would greatly
diminish or wholly negate any benefit...
Or have you mispelled yawn?

I'm hoping the former.
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Old 12-28-2009, 05:45 PM   #10 (permalink)
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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.

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