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