[Kevin Cooper]

If you take a brick and round the front edges just enough to achieve attached floe, then the drag coefficient will drop from 0.80 to about 0.30. At this level, the forebody drag is close to zero abd the drag comes mainly from the base pressure and to a lesser extent from skin friction. As the edge radius is further increased, the drag coefficient changes very little more. The explanation is that the curved edges have suction pressures that pull forward while the central, flat region has positive pressures adding drag. As the edge radius becomes smaller, the edge suctions become larger, offsetting the drag increase from the larger flat region. This process of balancing positive and and negative contributions continues until the flow separates and the edge suction diminishes. See: "The Effect of Front-Edge Rounding and Rear-Edge Shaping on the Aerodynamic Drag of Bluff Vehicles in Ground Proximity" SAE 850288 for a detailed discussion of this behaviour.