[aerohead]

Originally Posted by winkosmosis

Conventional wisdom is apparently wrong. Making a roof curved does reduce drag, even though frontal area increases. Obviously the best thing would be to keep roof height the same but curve the windshield but that's not usually practical. <warning! scribd link which slows down some browsers!> In This Chapter the Modifications That Were Carried Out To

Roof: Roofs are designed with a convex shape to ensure sufficient rigidity. For stylistic reasons an attempt is made to keep the convexity as small as possible. Fig 5.9 shows this for a medium sized notchback car. If the convex shape is designed so that the frontal area A of the vehicle increases, the aerodynamic drag of the vehicle decreases.



On the other hand, if the original roof height is kept constant the front and rear windows must be curved into the roof contour to eliminate obstruction of the view. This leads to expensive windows but results in lower drag.

The measurements plotted in Fig 5.10 (after Buchheim et al) show the same tendency for a car with a fastback. Here the chord length of the roof arch was used as the reference variable for the curvature.



Aerodynamic drag reduces with increased convexity for two reasons. First, the higher convexity allows for a larger radius at the transition from the windshield to the roof. This results in a less pronounced suction peak at this location. The momentum loss in the boundary layer during the following less steep adverse pressure gradient is therefore smaller and the boundary layer itself is less endangered by separation. Second, the convexity provides for gentle deflection of flow at the rear and the pressure rise at the rear end is therefore enhanced. The convexity of the roof and the rear end shape must be carefully matched.

"If the convex shape is designed so that the frontal area A of the vehicle increases,the aerodynamic drag increases." Hucho