"A golf ball is a bluff body and most of the drag is due to airflow separation--the flow cannot close behind the ball and stay attached," says Steve Ogg, vice president of golf ball research and development for Callaway. "The dimples transition the flow from a smooth laminar state to chaotic turbulent state. The turbulent mixing increases the momentum of the air at the surface of the ball, allowing it to stay attached longer." A car is obviously not the same type of shape as a golf ball, but the idea behind MPG-Plus is that dimples on a car body could have a similar effect. In automotive aerodynamics, the surface of the car is "dealing with turbulent flow," says Bill Pien, supervisor of aerodynamics in Ford Vehicle Engineering. "In normal operation, the skin friction accounts for no more then 1 percent of the total drag since the main drag generation mechanism is the vehicle shape," he says.
[...]
"We had experimented with the sharkskin wrap in the mid 80s when we developed a low-drag Probe IV concept car which had a Cd of 0.15," Pien says. (Cd, or
coefficient of drag, is the ratio of the drag force to the force produced by the dynamic pressure times the area measured.) "We did not see improvements in drag so we moved on to work with the vehicle shape."
[...]
Where the flow is already turbulent, the dimpled skin is more likely to have a negative impact--they need to be selective in their choice of locations on the vehicle for application of the skin," Ogg says.