The reason why golf balls have dimples is a story of natural selection. Dimples maximize the distance golf balls travel. Dimpled balls travel up to four times farther than smooth-surfaced golf balls.
In the early days of golf, smooth-surfaced balls were used until golfers discovered that old, bumpy balls traveled longer distances. The science of aerodynamics
helps explain the dimpled phenomenon. The dimples reduce the drag on a golf ball by redirecting more air pressure behind the golf ball rather than in front of it. The higher levels of pressure behind the golf balls force them to go far distances.The dimples change the levels of pressure by bringing the main air stream very close to the surface of the golf ball. The dimples, or “turbulators,” increase the turbulence in the layer of air located next to the surface of the ball. This high-speed air stream near the ball increases the amount of pressure behind the ball-thereby forcing the ball to travel farther.
History of the Golf Ball
The early golf ball, known as a featherie, was simply a leather pouch filled with goose feathers. In order to obtain a hard ball, the pouch was filled while wet with wet goose feathers. Since people believed a smooth sphere would result in less drag (and thus fly farther), the pouch was stitched inside out. Once the pouch was filled, it was stitched shut. Therefore there were a few stitches on the outside of the ball. The ball was then dried, oiled, and painted white. The typical drive with this type of ball was about 150 to 175 yards. Once this ball became wet, it was totally useless.
In 1845, the gutta-percha ball was introduced. This ball was made from the gum of the Malaysian Sapodilla tree. This gum was heated and molded into a sphere. This resulted in a very smooth surface. The typical drive with the gutta-percha ball was shorter than that obtained with the featherie. However, according to golf legend a professor at Saint Andrews University in Scotland soon discovered that the ball flew farther if the surface was scored or marked.
This lead to a variety of surface designs which were chosen more or less by intuition. By 1930, the current golf ball with dimples was accepted as the standard design. The modern golf ball consists of rubber thread wound around a rubber core and coated with dimpled enamel. The dimples are arranged in rows. The number of dimples is either 336 for an American ball or 330 for a British ball. The typical drive with a modern golf ball is about 180 to 250 yards.
The Dimples
Why, then, does a golf ball have dimples? The answer to this question can be found by looking at the aerodynamic drag on a sphere. There are two types of drag experienced by a sphere. The first is the obvious drag due to friction. This only accounts for a small part of the drag experienced by a ball. The majority of the drag comes from the separation of the flow behind the ball and is known as pressure drag due to separation. For laminar flow past a sphere, the flow separates very early as shown in Figure 1. However, for a turbulent flow, separation is delayed as can be seen in Figure 2. Notice the difference in the size of the separation region behind the spheres. The separation region in the turbulent case is much smaller than in the laminar case. The larger separation region of the laminar case implies a larger pressure drag on the sphere. This is why the professor experienced a longer drive with the marked ball. The surface roughness caused the flow to transition from laminar to turbulent. The turbulent flow has more energy than the laminar flow and thus, the flow stays attached longer.
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| Figure 1: Laminar Flow Over a Sphere. | Figure 2: Turbulent Flow Over a Sphere. |
So, why dimples? Why not use another method to achieve the same affect? The critical Reynolds number, Recr, holds the answer to this question. As you recall, Recr is the Reynolds number at which the flow transitions from a laminar to a turbulent state. For a smooth sphere, Recr is much larger than the average Reynolds number experienced by a golf ball. For a sand roughened golf ball, the reduction in drag at Recr is greater than that of the dimpled golf ball. However, as the Reyn olds number continues to increase, the drag increases. The dimpled ball, on the other hand, has a lower Recr, and the drag is fairly constant for Reynolds numbers greater than Recr.
Therefore, the dimples cause Recr to decrease which implies that the flow becomes turbulent at a lower velocity than on a smooth sphere. This in turn causes the flow to remain attached longer on a dimpled golf ball which implies a reduction in drag. As the speed of the dimpled golf ball is increased, the drag doesn’t change much. This is a good property in a sport like golf.
Although round dimples were accepted as the standard, a variety of other shapes were experimented with as well. Among these were squares, rectangles, and hexagons. The hexagons actually result in a lower drag than the round dimples. Perhaps in the future we will see golf balls with hexagonal dimples.
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| Figure 3: Golf Ball with Round Dimples. | Figure 4: Turbulent Flow with Hexagonal Dimples. |
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