Brick, I may be wrong here, but I believe as long as you've got attached flow at the rear of the deck lid, you can effectively ignore the turbulence at the bottom of the rear glass. It's a bubble, entrained in laminar flow.
Where you would want to deal with that low pressure is on a vehicle that didn't achieve flow reattachment by the end of the deck lid.
I'd skip that part and head straight to lip extension prototyping
Metro, I am no expert on this... but he might want to try some sort of gentle roof spoiler if the flow is detaching somewhere at the roof/window junction.
Again, I am no ultimate authority on this, but shouldn't drag be in large part the vector sum of pressures on the car? I.e. a bubble, even if enclosed in laminar flow, is still a low pressure region and hence you still have a region of high pressure at the front of the car that is directed to oppose the car's motion, and two regions of low pressure at the back of the car. One being the enclosed bubble and the other being the vacuum behind the trunk? Add them both together and they still point backwards.
Force = Pressure * Area (N/m^2 * m^2 or pounds per square inch * square inches).
This stuff is confusing, because I'm not sure which rule to ultimately go by, conservation of momentum or vector sum of pressure. Seems that (at least according to the NASA site) both theories are correct. I'm not sure what you were taught in school or reading science textbooks, but I got quite a bit of exposure to the incorrect "equal paths" theory explained in the link. I feel a little vindicated that it never made sense to me at the time and now I have something better to go with.
At least from the pressure point of view it is possible to verify via tuft testing that there is a vacuum at the back of the vehicle, and we know there will be positive pressure at the front of the vehicle.
Just thinking out loud as to how to explain what is the difference between a bubble and a boattail via the "Newton" method of conservation of momentum, with a boattail you only have skin friction slowing down the air, whereas in the other case the air might follow a similar path but with different acceleration due to accelerating towards the pressure... hence a net change in momentum towards where the car is going.
I'm going to have to ditch coroplast as a material to make a spoiler out of. Metal is looking like the best way to get a working prototype, so I'm going to stop by Home Depot to look at what they have for aluminum flashing and a better cutting wheel for the rotary tool. After looking at the lid again I think it will be alright to start with a straight spoiler as long as it can curve just enough to match the crown of the trunk lid, which is slight.
New plan: I spent my lunch break at Home Depot and found some 6" wide aluminum flashing and a hand seaming tool. The vision that I have in my head is a lip spoiler that sits on top of the trunk lid and uses the 20mm wide, 20 degree angle suggested in the aerobytes article. I'm going to have to roll the edge, maybe even double it over entirely for strength. I think I also want to use two of the existing mounting holes to restrain the ends rather than counting on mounting tape to force it to follow the curve of the trunk lid. It probably won't be as pretty as the spoiler on the new Civic coupe that I followed into the parking lot, but it should get the job done if I can wrestle it into the right form. I'm just worried that it will be short on strength, in which case I might need to go back for some heavier gauge stuff.
Yeah, that was the least expensive way to go (under $5 for 10'). It seems really thin, which is why I'm worried about rigidity. But what I have drawn up should have some inherent strength since it isn't just one straight bend, but three bends that roughly follow the contour of the lid. Kinda hard to describe without pictures, but you'll see once I get it into roughly the shape I want. The loads on the working surfaces shouldn't be too high since it's such a shallow angle, anyway. I just need to make sure it doesn't flap in the breeze!
I went back and upped the ante on both materials and tools. First, the hand seamer turns out not to be the right tool to get the nice, clean, straight bends that I'm after. So the first thing I picked up was the 12" bending tool that I passed over yesterday (which was stupid because it's the less expensive of the two tools.
The other thing I picked up was a 60" length of heavy-gauge aluminum ductwork. (I don't know what it's called but it forms 1/2 of a rectangular duct.) It seems like really durable stuff, which should make it a little easier to work with and much more robust once the product is in use.