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Old 11-13-2009, 07:50 PM   #1 (permalink)
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Thumbs up Compressed air for drag reduction ( Coanda slots )

The latest issue of Motor Trend ( December 2009 ) has a ( one page ) article on page 39 entitled 'Aero Blows' by Frank Markus .
The article states that in a series of tests using compressed air on the back of vehicles, drag was reduced by 36 % on an automobile, and an incredible 84 % on a semi truck.
The testing was done by Robert Englar at the Georgia Tech Research Institute.
Using compressed air not only reduces drag, but can also be used to better control a vehicle in a crosswind.

Another benefit is that there is no need for a boat tail at the rear of the car, or other add-ons such as spoilers.

I posted in the hopes that one of you guys might get some use out of the idea. I'd love to see a homebuilt version of this idea.


Last edited by Cd; 11-14-2009 at 12:41 PM..
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Old 11-13-2009, 07:56 PM   #2 (permalink)
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A pdf papaer on the study :

http://gtresearchnews.gatech.edu/res...w05/trucks.pdf
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Old 11-13-2009, 08:44 PM   #3 (permalink)
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OK, but how much energy is used to compress the air? My natural pessimism says it's going to take more than it saves.
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Old 11-14-2009, 01:06 AM   #4 (permalink)
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I think it would be much easier to kamm or boat tail the trailer.

But lets make things more complicated!
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Old 11-14-2009, 01:41 AM   #5 (permalink)
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Originally Posted by jamesqf View Post
OK, but how much energy is used to compress the air? My natural pessimism says it's going to take more than it saves.
Exactly. I've thought about making a duct that would transport air from the high-pressure zone in front of the car to the low-pressure zone behind it, but then decided that there would be plenty of drag in the duct, and it wouldn't be doing anything that's not already happening under/around the car. Generating compressed air will have an even lower efficiency.
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Old 11-14-2009, 12:39 PM   #6 (permalink)
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Are you guys even reading the article ?


( http://gtresearchnews.gatech.edu/res...w05/trucks.pdf )
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Old 11-14-2009, 12:57 PM   #7 (permalink)
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I wonder if that's why my tailpipe is located kind of high:



I read the article, and it does not disagree with James and Piwoslaw.

I really can't imagine a method of compressing air that's more efficient than collecting it from aerodynamic high-pressure zones of the vehicle, and Piwoslaw spells out in post five why that's almost certain to fail.

However, if they can reduce the air requirements with pulsed blowing as they mention, it might work out.
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Old 11-14-2009, 04:29 PM   #8 (permalink)
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blowing and sucking doesn't suck

Alex Tremulis believed cars would have Cd 0.12 by now,using suction slots and strategic blowing over the car body.
He cited a paper by,I'm going to say J.J.Cornish,from 1978,presented at an AIAA congress in California.
Cornish proposed using a turbocharger to run a pump which would provide both vacuum and pressure to enhance airflow over the blunt car bodies.
If any member has Paul von Valkenberg's book on race car engineering,at the back of the book I believe there are smoke flow photos of a Corvette with and without suction.The difference is remarkable!
I've seen some of Georgia Tech's stuff.
The sticking point was always the energy to run the pump.
Alex is one of my heroes.I was fortunate to meet and visit with him at his home after Bonneville before my return trip from the west coast.He was a smart man,his home had many of his design models which reflected his knowledge of and passion for aerodynamics and I would trust his judgment without reservation.
I hope those Georgia Tech people pull this off.It would be great for everybody.
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Old 11-14-2009, 06:56 PM   #9 (permalink)
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The article states that a psi of only .25 is needed to get these results.
How much power would be needed to produce .25 psi ?

Probably 99.7% of the world thinks that cars look 'ugly' or 'odd' with a boat tail. People don't want a big thing sticking out the back side of their SUV. I see this invention as a way of catering to the rest of the world.
Besides, as pointed out in another thread, one of the reason big rigs don't have boat tails is that they have length restrictions.
This invention solves that problem.
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Old 11-14-2009, 07:14 PM   #10 (permalink)
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Quote:
Originally Posted by Cd View Post
The article states that a psi of only .25 is needed to get these results.
How much power would be needed to produce .25 psi ?
That would be proportional to the volumetric flow requirement.

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