Coanda effect drag reduction (and propulsion?)

RoadWarrior's picture

Hi folks,

Been thinking about the applications of Coanda Jets and Thrusters to vehicles. One of the problem areas on a vehicle is getting air out from underneath to smoothly help "fill in" the bluff body drag right behind the lower half of the vehicle. Underbody ducting is used on supercars, such as the Ferrari Enzo Ferrari.

I have been thinking for a while of using such techniques under the back end of my van, but now I think I've got something better. It's active aerodynamics, it uses wasted energy, and it may also provide some degree of impulse as well as killing drag.

Here's a page with some of the conceptual basics...
http://www.meridian-int-res.com/Aeronautics/Coanda.htm

Now looking at that Coanda thruster, which is reputed to entrain 20 times more air than is blown out of the plenum, I'm thinking, what if I stuck something like that under the back bumper and used the exhaust to drive it? That would it seem do several things, i) it would entrain underbody air sucking it out, ii) it would fill vacuum behind the vehicle iii) it would improve exhaust extraction iv) it might provide a small amount of thrust.

So, I'm thinking hard now about making such a device. Probably will have to clean up the forward underbody a bit first though. So I'm envisaging a large radius quarter round coming from under the car, with a narrow blower slot near it's leading edge fed from the exhaust pipe. The top back edge of the quarter round will have to reflex slightly to direct flow away from the back of the vehicle, to split it away to join in the same after flow as air from over the top and sides of the vehicle. So it might look a bit like a gentle S curve.

Further enhancements might be to inject water into the exhaust to gain the most fluid velocity possible from waste heat in it. This could be temperature regulated, when it's cool still, no water, when it's hot enough, spray the water. Might need an expansion cone to do that in. Hard to figure how to do that "just so" to not kill velocity further back.

If notionally, at highway cruise, each cylinder is putting out about 50 cfm of exhaust gases, ~300cfm for a 6 cylinder motor, then a correctly designed Coanda system has the potential to move 6000cfm from under the vehicle. If our vehicle is 8 inches off the ground and 4ft wide it will have an underbody CSA of 2.6 sqft, and approx 13,700 cfm will pass under it at 60mph. Now, if all of that represents work that the vehicle has to do in pushing it under there, then by sucking out 6000cfm we reduce that load by nearly a half. However, I'm thinking that not necessarily all of that is a drag force. We might be doing way better than half the work. If we had a front air dam only 4 inches off the road, and side skirts ditto, the coanda jet/suction should be starting to suck the vehicle to the road @ 60mph.

I dunno if my math is right on this but it also seems that while moving that 6000 cuft from under the vehicle, the Coanda device may also be making 8lbs of thrust, which at 60mpg equates to losing about 1.5 square feet of frontal area off a car with a CD of 0.50, or 2.4 sqft off a vehicle with a 0.3 CD, this seems to be in addition to the parasitic drag reduction.

Dayum, this looks good enough that on a circuit racer you'd dominate the race then get banned by emergency rule changes for the next :D

If one could triple the CFM out of the exhaust by leveraging the heat to expand steam one would have a lossless underbody at 60, with the vehicle stuck to the road, have 24lb of thrust and equalise the drag on a further 4.5 square feet of frontal area @ a CD of .50

This document might have the details needed to start figuring the right shape of the Coanda device...
http://fluid.ippt.gov.pl/ictam04/text/sessions/docs/FM2/12062/FM2_12062.pdf

Anyhoo, much figgerin' needed still, looks real promising though...

Road Warrior

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Coanda effect drag reduction (and propulsion?)

RoadWarrior's picture

Got an idea for a cheap test rig. Duct tape 3 or 4 coffee cans together, (With that aluminum duct sealing tape, nut utility "duct tape" would be better) knocking holes in the bottoms of the ones in the middle. You prolly only want to cover the area between your wheels really. Make a fitting you can connect the exhaust to. Now, slice a line of slots in them up one side with a dremel, probably want about 3 slots (longitudionally) per can, with a bit of metal in between. Now tweak the slots open a bit so the leading edge is high and the trailing edge is inside the can, but don't make 'em too wide. Aim for about the same slot area as your tailpipe is. Fit under the back of the car such that the slots are at about 5 o'clock towards the front of the vehicle and the back of the coffee cans mate with the bottom vertical edge of the bumper at 9 o'clock (where the can is vertical) now use flexpipe or whatever to hook up the exhaust. Not sure how hot it will get so maybe you want to isolate it from a plastic bumper with a strip of draught sealer or something that's made from a more temperature resistant plastic (Cut strips off a dollar store silicone cookware piece, use same stuff with hose clamps to seal joints)

Now, I'd love to have a vehicle with a scangauge to do some highway runs with this on, figure it could pick up 5 mpg or better. When I get time to hack this up, I'll need to run a few tanks through to tell anything.

There's probably ways to distribute flow better if you were making a "good" version of this, and you'd probably want to use something like 12" duct or flue pipe instead of coffee cans, and blend the form with your back bumper better. Anyhoo, thought this setup might be good enough for some basic concept testing.

Road Warrior

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Coanda effect drag reduction (and propulsion?)

iburnh2o's picture

How bout something like this? applied to your car/truck/suv.

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Coanda effect drag reduction (and propulsion?)

RoadWarrior's picture

Yep, similar principles. Only we want to be using the air amplification effect to "fill in" drag vacuums, or provide small amounts of thrust rather than produce lift, which would be troublesome. Most of the aircraft applications we need to think about turning upside down or backwards, kind of.

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Coanda effect drag reduction (and propulsion?)

BMac's picture

One could build a rear spoiler like this but what about lifting the rear of the vehicle... or would you just have to angle it correctly?

another idea is that they said "...the increase in velocity produces thrust." and "The bottom line is the mass of the flow operating on the wing is doubled and the velocity of the flow through the system is dramatically increased."

Does this mean it could be used as a supercharger? I'm thinking so :D

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Coanda effect drag reduction (and propulsion?)

RoadWarrior's picture

I ran some numbers for using it for induction charging, you do get a higher pressure....... but it's typically spread out over a large area and is only slightly above atmospheric, so while you could have several square feet of coanda surface and parlay that into a few pounds of thrust, the actual PSI over atmosphere is around 1/2 to 1 max. I think a big ram intake gets you about that. (Which when you think about it is the same principle anyway)

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