Pickup truck vortex used on a car?
 

I was looking at buying a volvo 850 wagon which has a nice boxy rear end. It occurred to me that that back end looks a lot like the back of a pickup cab.

There is that diagram floating around here about pickup aerodynamics, which shows the small wing on the roof and a half tonneau being the best aerodynamically. I imagine (correct me if I'm wrong) it establishes a stable vortex in the pickup bed, and the air just flows over that vortex down to the tonneau cover. I just wondered if we could move this to the back of my volvo (if I buy one).

The wing is easy, just stick it on the roof, projecting out the back. For the "pickup box", why not just get a receiver hitch arrangement and mount a short, wide box there. The vortex would reside in the box, which could also be used sometimes for carrying stuff. One could easily add a curved surface inside the box to make the vortex even more stable.

If doable, this should be a lot easier than adding a trailer with long, sloping sides and top. The box could physically touch the bodywork so there is no gap, unlike a trailer. Also the rear view mirror still works. The back door is accessed buy simply pulling the pin and removing the box from the receiver hitch.

I'm assuming the axis of the vortex is horizontal. Is that correct?

This is very interesting subject with some claiming improvements and others stopping short of calling VGs snake oil. There is an existing three on vortex generators chech it out.

Yes the axis is horizontal.

While that set-up represents an improvement to pickups, I suspect it wouldn't for the wagon. The wagon is starting out from a better place with the nice clean cut-off. If one is willing to do the work of replicating a truck box, one could just as easily- or even moreso- hang a nice partial or full tail back there.

wing/box
 

Here's a rendering of a quasi-commercial setup which combines the wing and box into an aerodynamic boattail/box-cavity/storage container.
It rolls up to the SUV/wagon receiver hitch on four castor'd pogos,which rise and stow into the belly while on the road.
3-4 mpg.
http://i1271.photobucket.com/albums/...ntitled1-7.jpg

I am seeing improvement on aero optimized hatchback (Prius C) with VGs installed along roof spoiler, so it isn't that you have to start from very bad to see improvements.

I'm saying the pickup and wagon are two different scenarios; pickup being worse.

Vortex generators are the devices for remixing and increasing speed of boundary layer. As such they need to be applied in the place where they improve aero efficiency of existing design, not to alter it as they are really not capable of doing it.

The was a study which used a combination of VG and spoiler an succeeded in completely eliminating trail wake. But as I recall they tried 26 different combinations in wind tunnel, something we can't.

terms
 

I think were getting our terms mixed up a little... A truck bed has a recirculation bubble, where a vortex is air spinning off a generator or wing longitudinally... Also horizontal axis, but towards rear.
That's what grays garage tought me anyways. Check out his vids.
Aerohead has done a nice rendering of your idea. Would shift center of gravity back a bit
Good luck

OK, recirculation bubble. I was not talking about those little vortex generators at all. Sorry for misleading people...

Aerohead has the right idea with his first picture, except that the top of the box is removed and the bubble sits in there, and also there needs to be a wing on top of the roof.

But I get Frank's point too; the devices are to compensate for the pickup being worse off in the first place, and may be pointless to apply to a wagon.

Interestingly, my Ram pickup is rated at 25mpg (highway) while the Volvo I am looking at is probably a bit worse, even though the Ram is a much larger and taller vehicle. I guess engine efficiency has gone up over the years.