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Those diagrams are entirely fascinating, though somewhat counter-intuitive. Ive been researching aerodynamics for years, though mostly for car/racing purposes (gaining downforce while minimizing drag). Its great to know theres always more to learn on topics like these.
I find it a bit interesting we dont see any iterations of that bus that has the bottom taper up? On race cars that tapering from the bottom is a diffuser, it creates downforce, but also minimizes drag. Is the increased downforce, and thus increased tire friction, the reason these "tails" do not taper from the bottom as well? From my understanding, tapering on the top and the bottom would be the best way to minimize drag, at the cost of slightly increased downforce. Hypothetically you could taper it to the size of a pencil, then with an air jet shot through this convergence point, you effectively get an "infinitely long" tail, without the need for additional panels. This is because the air jet, which needs to be faster than the air travelling around it, acts as a place where the slower moving air can grab onto (instead of the side of your car). This same phenomena is seen in modern jet craft pulse detonation engines, it allows for drastically increased efficiency on these pulse engines.
I do know they do similar things on some semi trucks, or at least toyed with the idea; in which they would have fast moving air "curtains" expelled the rear of the truck. This would limit the turbulence behind the truck and minimize its drag. As I recall this system drastically helped decrease drag and increase mpg, but the cost was substantial.
edit: The main reason I guess we dont see that tapering from the bottom partly relates to why it works as a diffuser. On a flat bottomed car such as a dedicated race car, the rear tapers up before the end of the car. This causes the air before the taper to speed up to maintain constant flow, this affects the pressure under the car, essentially pulling the car down (downforce). However it does increase drag as stated prior, due to the air velocity being faster after the taper. The only thing I can think is that this increased air velocity would cause aggressive turbulence past the end of the car, potentially nullifying the beneficial effects? Can anyone shed some light onto this?
It looks like the second diagram does touch this topic, and Cd is decreased, but its not a common theme I have seen on this site. Is it something that is being overlooked, or something that is purposely being ignored because it's benefit is negligable (and really only applies to cars with flat undertrays) or because it is simply not achievable in most applications?
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