Technologie: Aerodynamik: Page 4

The Origins of Drag

Move any object through the air, and you'll get some drag. Most flying bodies (not including the occasional golf club) have a streamlined profile by design or by nature, so that they cut through the air cleanly with minimal drag. But a golf ball has to be (guess what . . . ) a ball, so it is destined to be an air punch rather than an air knife. This makes for a large drag force.

The air hits the front of the ball, creating a high pressure area, and splits around to the sides. But like a teenager in a Camaro, it's going too fast to make the turn to the back of the ball. It separates from the surface, leaving a low pressure wake like the one a boat leaves in the water. This combination of high pressure on the front of the ball with low pressure on the back is the main source of a ball's drag.

This may seem hopeless, but it's not. Maybe you can't control the teenager, but you can put better tires on his or her car. The solution? Dimples. When the surface of the ball is covered with dimples, a thin layer of air next to the ball (aerodynamicists call it the boundary layer) becomes turbulent. Rather than flowing in smooth, continuous layers (a laminar boundary layer), it has a microscopic pattern of fluctuations and randomized flow. Here's the good part: a turbulent boundary layer has better tires. It can better follow the curvature of the ball's profile. It travels farther around the ball before separating, which creates a much smaller wake, and much less drag. In fact, a dimpled golf ball has only about half the drag of a smooth one.

Putting it all Together

If we combine the spinning motion, which warps the airflow and creates the lift, with the dimples, which reduce the wake and cut the drag, we get the flow pattern around a golf ball.

Proof, you say? Professor F. N. M. Brown of the University of Notre Dame was famous for his work in visualizing flow patterns by injecting streams of smoke into a wind tunnel.

To the left is one of his pictures of an actual spinning dimpled ball. While the spin rate he used appears to have been quite low by golf ball standards, you can still see the warpage of the flow field, especially in the angle at which the wake trails away from the back of the ball.