In the last picture, the vortexes (vortices?) are due primarily to the sharp corner between the vertical sides and the flat top. Round the corners, and the vortexes get smaller. Round them more, they get even smaller. Make the front profile a half-circle, and the only ones left are down at the bottom of the car--and they are much smaller.
There are lots of interactions when it comes to aero. My aero engineering friends tell me that fluid dynamics is the most empirical (e.g., "fiddle with equations until the numbers seem to come close to what we measure") and least intuitive of all of the engineering disciplines.
For example: In the aircraft world, significant reductions in drag have been found simply by adding a fillet between the wing and fuselage of the aircraft. (That's a transitional radius between the two shapes, not part of a fish.
) The interactions between air flowing over the wing and air flowing along the fuselage created significant drag, and adding a piece to make a small curve instead of a (nearly) right angle reduced that to very little.
-soD