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.

A pdf papaer on the study :

http://gtresearchnews.gatech.edu/res...w05/trucks.pdf

OK, but how much energy is used to compress the air? My natural pessimism says it's going to take more than it saves.

I think it would be much easier to kamm or boat tail the trailer.

But lets make things more complicated!

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.

Are you guys even reading the article ? ;)


( http://gtresearchnews.gatech.edu/res...w05/trucks.pdf )

I wonder if that's why my tailpipe is located kind of high:

http://image.caraudiomag.com/f/85940..._side_view.jpg

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.

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.

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.

That would be proportional to the volumetric flow requirement.

On a race car you'd only need it on the straightaways at higher speeds. Under braking, the extra drag would be useful. The car would also use a sort of regenerative braking...simply pump up the air tank with a wheel driven air pump.

On a car driven at highway speeds for long periods of time, you'd have to see if it was better to burn fuel to make X amount of horsepower to go a target speed or better to reduce the amount of horsepower required by reducing drag and then burn fuel to (somehow) make your compressed air contraption work.

Still...I don't think it would be that difficult to duct the inside of the rear wheel wells to the back of the trunk. That should be a straightforward HIGH pressure to LOW pressure job.

Renault are working on something similar: The Synthetic Jet.
http://pdf.aiaa.org/preview/CDReadyM...V2006_3337.pdf
Green Car Congress: Renault Altica: 44MPG Diesel Concept with Active Airflow Management
http://www.people.vcu.edu/~kmmossi/d...ddox_mossi.pdf
http://depts.washington.edu/cims/pub...actuators1.pdf

It's not strictly compressed air but it is doing the same job. It's also been talked about here without much resolution on the matter of compressor efficiency.
Didn't the P-51 mustang have this technology? It enabled it to go further on a tank of gas. I think there would be a smaller percentage improvement on a car using this but it would be an improvement all the same.

ollie

Perhaps this is already answered somewhere, and I missed it, but why go to the trouble of compressing air when you already have a large volume of hot gas under pressure, flowing out the exhaust? Just duct it to the appropriate places, making sure that it doesn't get into the cabin...

moving engine or at least the radiator the the back, could accomplish two tings at once... it would delete a grill at the front allowing for better aero and allow for better controlled cooling if air would be ducted (in part) trought the radiator that would exit in the wake of the car

Thanks for the brainstorming guys.

It's too bad you can't just connect a Shop-Vac to your cars AC outlet. :D

( It wouldn't produce nearly the amount of power needed though !!! ( right ? )

How do you create .25 PSI in an open area?

I have no clue.
I'm wondering the same thing.

Rising Fuel Prices Renew Interest in Fuel-Saving Technologies for Heavy Trucks | Georgia Tech Research Institute

This may have been posted already.

It also looks like it is sucking air in from the side.

http://www.gtri.gatech.edu/files/media/truckfuel1.jpg

If half of the HP required to drive 55 is to push the car through the air, it stands to reason that there is a lot of HP available to redirect to the problem of form drag. Specifically, reduction in the form of pressure redistribution.

For example, If 50% of your drag at 55 is pressure drag, then a car that takes 20hp to go 55 would have 5hp to burn to break even. Using any less would be an improvement. 5hp to run a high flow turbine is a lot. I'll bet in the above story, they are using about a centrifugal compressor, not a piston compressor. If you run it off the exhaust, then you even use some waste heat and have even more hp to play with.

I was toying with the idea of a hole thru my van a while back and this makes that idea seem less crazy that it did at the time. Pressurize the air in that duct and you have something.

From a DIY standpoint, the biggest stumbling block of this approach is figuring out how best to apply it to a particular vehicle.

With boat tails, Kammbacks, wheel skirts etc., we can fairly easily see whether the air likes them or not by tuft testing.

But there's really no easy way for the backyard aerodynamicist to visualize the best position/angle/flow rate of a compressed air device without a wind tunnel. We can't tape tufts onto air behind the vehicle!

You could guess, and then do coastdown testing, but it's very time consuming and getting good data in on-road testing is very hard.

Until one of us comes up with a way to smoke test our cars...

On the race cars, it's not really to reduce drag down straights. It's for cornering. If you look at the Chaparral 2J or the Brabham BT46B cars, they have skirts that basically seal off the bottom of the car. If you take the small vaccum generated by the fans & multiply that by the surface area under a car, you get huge amounts of "downforce". It may have reduced drag a little bit, but you could go around a corner like you wouldn't believe.

Backyard aerodynamicist here.

Volumetric requirements. take surface area of rear of your car in ft^2. Multiply that by your desired speed in ft/min. This would roughly give you the cfm required to fill that hole. 55 mph is 4840 ft/min. Turns out to be a whole bunch of air.

You could try to use an electric cooling fan ducted down to the underside of the car.

Don

Is that a cubic foot per minute? That is a ton of air.

But if the air came out of a slot going around the rear edge of the car and the slot was angled such that the air formed a virtual boattail (or kammback), it would need much less air and therefore energy.
I think it's a bit excessive to try fill the void behind the car with air flowing the same speed as the car.
What's needed here is a Blown Kammback. Much like the blown flaps we hear about on planes. I haven't even got around to making a regular kammback but some of you guys here seem to have the time/abilities etc.!!

ollie

I just played around with the calculator and then accidentally deleted my post. :(
I tried using drag area (because it is easier to find), speed and distance. I'm not sure what it means but I think I found a fun way to compare a Hummer and an Insight. I call it DRAG VOLUME. :)

The drag area for a Honda Insight is 5.10 sq ft.
The drag area for a Hummer H2 is 26.5 sq ft.
60mph is 5280 ft/min.
5.10 x 5280 = 26,928 (cubic feet/min?)
26.5 x 5280 = 139,920

The Hummer moves 26,928 (drag volume?) of air in a mere 1016 feet. If the Hummer goes 1016 feet per minute that's just 11.5 mph.

Sorry. I'm too dense to figure out what you are saying here.
Are you saying that at a mere 11.5 MPH a Dummer moves 26,928 cu.ft of air ?

( Imagine you are explaining yourself over again to a six year old. )

Thanks

Staying on topic here but veering off course a little -
I have a Civic hatch with the little mini tailgate.
Other posters here have found that opening the windows in their cars actually does not hurt the FE as much as they thought.
If I were to open my tailgate, as well as my windows, air would swoop into the cabin and find its' way out the back of the car through the opening - hopefully filling in my wake some.
What do you guys think will happen ?
A. I will die from all the fumes coming into the cabin from the stupid hole I made
B. The drag will actually be worse.
C. I'll be fine because the air ( and fumes ) will exit the cabin out the back. Possibly a very slight increase in FE.
D. Nothing. ( A wash )

Here is an example of the tailgate down ( Note that in the picture the hatch is open. For the test, only the tailgate would be open ): http://farm4.static.flickr.com/3500/...f5be95ee21.jpg

Old Suburbans that had roll down glass in the tailgate exibited the same properties. Kept you cool too. As far as asphyxiation goes, Most trucks & SUV's have the exhaust turned to the side for this reason. A simple clamp on extension with a 90 degree bend should keep you consious.

Don

Without making any guesses on the drag, I think your handling will be pretty squirrelly. I'm visualizing it without any real aero-ducation, but I think the air flow off the bottom of the car will be quite significantly altered relative to the air flow over the car. Induced lift since the low pressure zone will be close to the top of the vehicle and farther from the bottom?

But then again, I'm pretty clueless.

I'll try this idea out next time I go visit my relatives ( for X-mas)
Extremely easy test to set up, but I'll have to relocate my plate.
I don't have cruise control, so I'll just try and keep it a steady as possible.

I noticed that the truck that has the airflow setup in the image above has a raised section that creates sort of an air pocket. This makes me think of the air pocket that was on the back of the UCDavis Taurus.

Drive "fast" when you do the test, it will help show the benefits, if any, better.

What do you guys think will happen ?
B. The drag will actually be worse.

Your interior will now be part of the wake bubble. Air will enter through the tailgate area and get sucked out your front windows at the low pressure area by the A posts. It will also be loud. You'll also enjoy road spray on the inside of your windshield if the road is wet. :)

It sucks to be part of the 0.3% of the world that thinks an SUV is an ugly contraption. :(

Why does it suck? Remember: you're in the top 0.3% !! You should be proud :)

Would the air entering the tailgate have more force than the air trying to escape through it ?

Also, I assumed that the wake was already pretty messy back there, and that the messy air coming out the back would be pretty much the same .

Thanks for your advice.

If I know from the start that that the results of the testing will be more drag, I won't even waste my time with it.
Will the drag still be worse if I create a smooth channel for the air to run out the back ?
I plan to create a channel running from the passenger side window out to the back of the car. ( I'd remove the passenger seat and fold down the back seats flush.
Since this would be asymmetrical, would this lead to any problems with the test ?

In the real world, this would never be practical on a car like mine, but as I look at cars such as a VW Jetta, it seems that you could create a channel for the air that would be weather tight and would not cause extra wind noise. You could exhaust the air out slots on the back of the trunk.

Here is a really sloppy example :

http://farm3.static.flickr.com/2655/...4fd51e47_o.jpg

most people don't think about these things... if all cars look a certain way they think it looks good especially if comertials cofirm that.

if all carmakers would suddenly bring out boattails or if the next james bond movie would have a production car with a boattail than people would want it (of course a true solid boattail is just very unpractical for most people wich is why cars don't have them)

Smoke testing JUST might be doable. I hear (and saw for myself) that seafoam through the throttle body creates an ass ton of smoke. I am thinking seafoam (or other similar) injection maybe directly into the exhaust? in the lead car the subject car follows close in and a chase car alongside off to the side with a video camera.

get the smoke going video tape the results. not perfect but MIGHT garner usable results.

I am thinking kamback and air injection under the kamback to virtually "extend" the kamback.

as for costing more energy than saved this is an oxymoron. entrophy says this is 100% what will happen 100% of the time. Period so even saying it kind of sounds silly.

what matters is NOT energy in versus energy out (which is ALWAYS negative) but DOLLARS in versus dollars out.

even if it takes 3 times the energy you save is the alternative CHEAPER.

ie compressing the air at home uses your household current and as we know this is virtually FREE compared to the cost of gasoline. so unless your using hundreds of kilowatts each drive your never going to spend as many dollars on E as you do on GAS. This is why EV's are so damned desirable.

you run into another problem though. your going to expend you compressed air VERY VERY quickly. as a short TEST with say a scan gauge this might garner results but you only going to be able to carry at best a few minutes of air power with you without going to very very expensive tanks and pumps. ie you need a giant tank at THOUSANDS of PSI not the measily 120-150psi your can do with conventional affordable tech and the 20-40 gallons of air you can carry at that pressure.

you need more along the lines of 50-80 gallons of air at 4000-5000psi to get anything usable. the COMPRESSOR to do that costs more than most of our cars cost and it was not designed for such a duty cycle ie even more expensive.

real time generation is likely going to be whats needed. I am not sure how to do this. it does make the thought juices flow though :-)

correction
 

I found the reference for earlier work:
"Trapped Vortex Flow Control For Automobiles," by J.J.Cornish,III,Chief Engineer,Lockheed Aircraft Corp.,Marrieta,Georgia division.
Presented,Los Angeles,California meeting,Aerospace Industries Association of America( 100 Wilson Blvd.,Suite 1700,Arlington,VA 22209 USA Aerospace Industries Association ).