Not all aerodynamic cars have to draw on the same set of forms. The 2010 Kia Ray (or PHEW Ray) manages to look slippery without resembling a blend of Tatra and Citroen shapes.
The most distinctive element is the Kamm tail, a feature Alfa Romeo and Zagato used in the 60s. The very sharp rim that defines the cut-off tail is there to improve the airflow break-away. A rounded edge would cause more turbulence (that’s why the tail of the first Audi TT has a small lip attached on the bootlid).
As air moves down the top and sides and then encounters the end it will tend to become turbulent. Up until the point the car’s surfaces begin to taper inward, the flow is laminar (which is good). Then, at the corners or ends, the flow will encounter perturbations due to variations in pressure. These complex vortices arise as inertial forces in the medium (air) begin to dominate over viscous ones. The sharp edge seen here on the Ray’s tail reduces that interaction of forces and so reduces the drag that would otherwise act on the car (were it ever to make production).
The mathematics of fluid dynamics are horribly complex. Einstein said the first thing he’d ask God about when he went to heaven was fluid dynamics. Rather than model the behaviour of fluids mathematically (which you can do anyway), it is often better to suck it and see by setting a scale or full-size model in a tank and running air and smoke plumes over the form to observe how it behaves.
Taking advantage of water’s higher viscosity (and thus lower speeds) Mercedes famously honed the ugly hulk of the 1995 W210 E-class in a vat of water and let streams of gas show where any turbulence was being generated.