This is a section from the
book I recently acquired:
An important parameter in researching the aerodynamics of sedan-style vehicles is the
effective angle of slope, measured from the rear of the roof to the rear of the trunk.
The above drawing (according to Nouzawa et al.[2]) shows how Cx increases depending on the effective angle of slope. Notice the similarity to the angle of slope in a hatchback, which has a minimum around 12 degrees and a maximum around 28 deg. Two ways of improving Cx is to either extend or to raise the trunk lid.
Fukada et al. ([1]) noticed that Cx could be reduced by adding a deflector to the central section of the rear edge of the trunk lid. A spoiler on the whole width of the rear edge produces a large downforce, increasing drag, but a small (1/3 width of trunk) deflector gave half the downforce and a reduction of drag (compared to no spoiler at all). The reason for this is as follows: the body of a sedan produces a pair of trailing vortices and the small deflector produces vortices of the opposite direction of rotation, thus partially cancelling the first pair.
This lead to the idea of building a triangular-shaped deflector on the trunk lid to use as a vortex generator. The drawing below shows how drag Cx and lift Cz depend on the width of the deflector.
[1] Fukada H., Yanagimoto K., China H., Nakagawa K.,
Improvement of vehicle aerodynamics by wake control, JSAE Review 16 (1995), p.151-155.
[2] Nouzawa T., Hiasa K., Nakamura T., Kawamoto K., Sato H.,
Unsteady-wake analysis of the aerodynamic drag on a notchback model with crotical afterbody geometry, SAE SP-908, paper 920909, February 1992.