Coanda effect drag reduction (and propulsion?)
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...