This would also explain the shape at the top of my car's hatchback, and around the upper half of the hatch glass, which I've always wondered about. It's a built-in spoiler, meant to force flow separation early to avoid the negative effects of having the flow wrapping partway onto the glass before going turbulent.
I'll get a pic and post it tomorrow.
Moral: everyone, go outside with your duct tape and cardboard and square off all your cars' trailing edge radii.
Looks like the small spoiler keeps the TOP air flow going straight back and then the low pressure area at back is filled by air from the SIDES?
If you look at the Airtabs applications...that might be how they work...causing a straight sheet of air...a clean break?
I was assuming that they caused the boundary layer to follow around the curve into the lower pressure area behind the car.
If you look at how the Airtabs are made...look like they lift a stream of air in the center and force it out away from the surface plane to some extent? Between the Airtabs seems like there would be a lower pressure area that might supply air to the center??
Whereas the vgs I made induce trailing vortexes. By placing them 4" apart...the 2 vortexes from ajoining vgs counter-rotate and shouldn't interfere with each other.
My ZX has a small spoiler at back...so maybe the top vgs help to keep the air more down on it...while the side vgs help air getting around the back...or maybe hinder it?
Leading the perpetually ignorant and uninformed into the light of scientific knowledge. Did I really say that?
a new policy....I intend to ignore the nescient...a waste of time and energy.
You know, I'm starting to think that it wouldn't be that difficult to make an effective lip spoiler. I had a look at my girlfriend's mother's Mercedes E300 Turbodiesel (I think it's a '99) and that has one built right into the trunk lid. It's literally just a vertical lip formed into the top of the trunk, right at the rear edge. As those sources point out it seems like it's just a matter of optimizing size and angle. I think that the tougher part would be making it look good, and then reliably measuring its effects without a wind tunnel.
I agree: a lip spoiler is an easy item to make, and the theory behind its purpose is solid (& logical, which makes it even nicer). As for angle, I think the safest bet would be to simply extend the plane of the trunk lid. As for size, I'm less certain, but the article mentioned 20 mm.
Originally Posted by brick
think that the tougher part would be making it look good, and then reliably measuring its effects without a wind tunnel.
You can probably guess where I place looks on the scale of things
But you hit the nail on the head about reliably measuring effects.
Logically, a lip extension should improve aero. But the change will likely be small enough that on-road testing - whether coastdown based, or speed-controlled & averaged bi-dir. runs - may show nothing conclusive; the effect will likely be lost in the normal static of variability.
It's something I've been thinking about lately, since there are a number of other known aero tweaks I'd like to try, but whose individual effects (if properly executed) are known to be small. How to quantify...
As for angle, I think the safest bet would be to simply extend the plane of the trunk lid. As for size, I'm less certain, but the article mentioned 20 mm.
Sounds fair. The other car I looked at this weekend was my girlfriend's sister's '98 BWM 328 coupe. (What can I say, their father is German-born.) Anyway, that also appears to have a built-in extension that I can only assume is designed to do the same thing. The horizontal part of the trunk lid is simply extended ~20-30mm beyond the vertical part of the trunk lid below, tapering backward (err...forward?) to meet it. It makes a lot of sense now that I know what they were trying to accomplish.
Spent four hours traveling for work today. It's amazing the number of newer cars with an aero device at the rear of the roofline! Magnum, Pacifica, Caravan, Matrix, Trailblazer, Beemer and Lexus Suvs etc. Must be there for a reason....
"If the engine ain't running it ain't using gas!"