Most new cars that gain low drag have sharp separation edges at the back.
If edges are round than wake is smaller but curvature produce low pressure, if edges are sharp wake is larger but dont have low pressure.
So what solution is better and why?
You can see at rear bumper one sharp line that will stop air to follow curvature of bumper.
Best Answer
On a car you want a controlled separation. The sharp edge produces a shallow pressure profile up to the edge, at which the flow separates cleanly. If the contour were round, separation would happen somewhere along the contour and will be asymmetrical between left and right sides in a crosswind. With the high pressures at high speed, such asymmetrical pressure has the potential to produce unwanted side forces which negatively affect handling.