The Science of Golf Ball Dimples

When a golf enthusiast drives his or her golf ball off the tee, it takes to flight in a roughly parabolic fashion. Although dimples can impart additional lift to a ball's flight, the more important benefit of dimples is related to drag. There are two factors that impart drag to the ball. One is friction. Surprisingly, this is not the predominant force that lessens the distance the ball travels. The primary force is related to the way air flows around the ball.

Golf Balls without Dimples

A golf ball without dimples-that is a completely smooth golf ball-experiences essentially no effect from spinning. The air stream coming off the top of the ball runs nearly parallel to the air stream coming off the bottom of the ball. Thus immediately behind the ball, the pressure drops off strongly for essentially the whole diameter of the ball. This pressure gradient (difference in pressures) between the front and back of the ball, seriously impedes its travel.

Modifying Pressure Gradient

If the pressure behind the ball more nearly matches that at the front of the ball, the ball's flight would be impeded less. Dimples accomplish this to a degree. They produce a turbulence that "throws" some of the air passing over and under the ball (and, in fact, completely around the ball) in the direction of the center of the ball. Thus there is some of the rear of the ball that is receiving additional air pressure, reducing the gradient or difference between front and back pressure, lessening drag.

Theory and the Reynolds Number

Golf ball design theory as well as all other objects traveling through a fluid medium must take into account a parameter known as the Reynolds number. The Reynolds number, is the ratio between inertial force and viscous force, and is closely associated, not only with specific numbers, but with "types" of flow. The K-8 Aeronautics Internet Textbook site informs us that the Critical Reynolds number, Re_cr, is the point at which laminar flow (essentially smooth and parallel) changes into turbulent flow (includes eddies and vortices).

Translating Reynolds Number into Golf Ball Design

The job of the golf ball designer, then, is to design a dimple pattern that decreases the critical Reynolds number. Interestingly, not only the number and positioning of the dimples is important, but their depth and shape, as well. Hexagonal dimples are superior to circular ones, producing a lower critical Reynolds number. In effect, the lower the Critical Reynolds number, the lower the velocity of the ball required to envelop more than 50 percent of the golf ball. Eventually the ball slows, laminar flow returns, and the drag increases. The longer the ball experience turbulent flow, the further it travels.

References:

The K-8 Aeronautics Internet Textbook - "Why does a golf ball have dimples?"

Knetgolf - Golf Ball Dimples