Rear window fairing (roof extension)
 

Many of the automakers today employ a fairing that protrudes from the rear of a car's trunk or a van's top. I assume this helps aero. I have also seen where people put vortex generators on the roof of their car in front of the rear window to help aero.
So, why hasn't anyone tried putting a fairing over their rear window in place of the vortex generators? I guess I need to understand why the fairing works on the trunk. Can anyone clear this up for me?

I'll take a stab as I really don't fully understand those little lip spoilers enough (enough to my satisfaction :p)

http://www.primitiveengineering.com/...ex_gen_cfd.jpg

This image shows CFD of a Mitsubishi Lancer IIRC. The right side doesn't have vortex generators (so ignore the left side). Notice how the blue area (lower velocity air) pretty much misses the trunk lid on a downward angle? By adding a lip there, flow is straightened out before leaving the vehicle and I would think would make flow interaction from swirling air below the car and above the car less of a problem...

If someone has a better understanding, please chime in :)

My understanding is that one would want to minimize the blue area to prevent vacuum (drag). The vortex generators seem to help the air "turn" around the rear window. I still don't understand the tail fairing as that would seem to increase the blue area behind the trunk, unless the fairing allows the pressure behind the trunk to be higher (green).

We're talking about pressure drag here, which is the result of the pressure differential between the front and back of the vehicule. By adding a properly designed lip spoiler, the pressure over the surface between the spoiler all the way to the roof of the vehicule increases, hence reducing the pressure differential. So the net result is less drag.

Anything that extends the trunk long enough for the flow to reattach will have the effect of increasing the pressure over the rear window and trunk of the vehicule.

So a roof tail would be bad, VG's are good, and the longer the tail fairing, the better, right?

On a notchback yes as it would potentially prevent the flow to reattach at the trunk. On a squareback they can be desirable if they taper the roofline down at a 12 degrees or so angle.

On a notchback yes, they can help reattach the flow on a shorter distance. On a squareback, which is a lot like a trailer, there are some sources saying that they can be used to create what looks like a boat tail made out of vortices (SAE paper 2003-01-3377 as an example).

VG's have a minimal impact on FE however you look at it though.

Yes. Past a certain lenght the drag reduction is marginal though.

I've seen a lot of roof extensions that I think actually help, on squarish backed vehicles like hatchbacks, wagons, and SUV's. If the roof slopes gently downward towards the rear then an extension or roof spoiler can continue that line past the rear window. (A steep-sloped roof or hatch doesn't help; the air flow detaches rather than sliding along the surface.) The lengthened slope reduces the final cross sectional area at the rear, making for a smaller wake or turbulence zone. This is especially true if the sides are angled inwards a bit also.

Take a look at the writeup on Kammback in wikipedia:
http://en.wikipedia.org/wiki/Kammback

The roof extension type "spoilers" are really an attempt at a very short Kamm back. I'm sure one can argue a case that the overall benefit of this is very slight but that's the case for most of the features that contribute to better aerodynamics or fuel economy. And I don't think it's such a small gain. A roof that slopes down and sides that taper inwards towards the rear will have less drag than the same design with a roof and sides that continue level and straight towards a vertical back end - all other things being equal of course.

I don't know about this. The more I look at this, the more I am starting to believe that it is a way to add downforce to the car without adding drag the way an angled spoiler would do. I suppose that a true Kammback or teardrop design would make the car very light in the rear, just like air flowing over a wing. A Prius is a perfect example for someone to test FE with and without it's rear fairing.

Not so much adding down force... Just eliminating lift :D But it's to the same end result :D

I want to say this cautiously. I am no expert by any means but I am 99.99% certain that the vortex generators are not used to reduce drag on the Lancer. Rather they are used to clean up the airflow into a more laminar flow over the rear spoiler to increase down force.

The IIRC is a small 4 door, 4 cylinder sedan with a huge turbo and the car is designed to go like stink and handle. Look carefully and you can clearly see the vortex generators several times in this movie. http://www.metacafe.com/watch/342989...tion_mrfq_360/

Trust me, they are for downforce only, you can not even mention fuel economy and the Lancer IIRC in the same sentence with out being laughed at. There are only a very few cars that are faster or handle better than this car and they all cost 3 times as much.

They definately don't make the flow over the rear more laminar as vortex -generators encourage the opposite of smooth flow, but you may have an argument.

Also, I'm pretty sure that if they did aid the spoiler they would not increase downforce but rather decrease lift. It may be semantics, but they are two different things in my mind. A true spoiler "spoils" the lift of an object, it doesn't generate lift in the opposite direction. Decreasing lift decreases induced drag, which is good for the FE minded, and it also returns the weight of the car to the rear wheels. For example, an airplane spoiler isn't generating down force as much as its returning the aircraft's weight to the wheels for braking.

Race cars definately generate downforce, but they use airfoils rather than spoilers. When it wasn't outlawed, Lotus used a skirt and a specially shaped underside to generate a venturi effect to create downforce. On the other hand, drag racers and cars like the Porsche Carrera use spoilers.

Lastly, while those vortex-generator may not exist for FE, air drag is still an enemy of someone racing their car. While it may not help in slow maneuvers, it should help anywhere high speed runs exist.

- LostCause

rear window fairing
 

The late-model Corvette uses a glass" fairing" which tapers both in plan and elevation to clean up the wake behind the cabin so air will hit the top of the "trunklid".Since these cars are driven flat-out on the Autobahn they need the downforce and directional stability of the high-drag ,enormous tail,especially in inclement weather.The "boat-tailed" rear windscreen has the shape of the cap on my truck and where Chevy uses it for speed,it cheats the gas pump a bit for me.On a notchback car,such a fairing would block access to the trunk,so provision would have to be made to tackle that animal.

OK, so it looks like any fairing that auto makers put on a car, whether is it a notchback, Kammback, or squareback, is used for downforce and not for lowering CofD. So then, any theories on how to reduce CofD by changing the fairing? Why couldn't someone extend the bumper out towards the rear and make a Kammback off the rear of the trunk?

apgrok1 - There are lots of examples of roof extensions (on hatchbacks/wagons) and trunklid lip extensions on sedans which reduce drag. Some of them also happen to reduce lift at the same time. Yet others reduce lift at the expense of higher drag.

adbso - I think you're right in that the goal of the Mitsu engineers was to get more clean airflow to the decklid spoiler on that car. That said, if the spoiler wasn't there, the net effect of the VG's would have been to very slightly improve Cd and reduce fuel consumption.