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Why Does A Golf Ball Have Dimples? | |
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A golf ball can be driven great distances down the fairway. How is this possible? Is it only because of the strength of the golfer? Or, are there other reasons? As we will see, aerodynamic forces play an important role in the flight of the golf ball. We will start by looking at the history of the golf ball, show why it has dimples and how lift is created by the spin of the ball. We will also look at how experimental tests can be performed using a spinning ball in a wind tunnel. 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 get a hard ball, the pouch was filled, while wet, with wet goose feathers. It was believed that a smooth ball (sphere) would travel, farther, with less drag. To reduce drag the pouch was stitched (sewn) inside out. When the pouch was filled it was stitched shut with most of the stitches facing inward. The ball was then dried, oiled, and painted white. The typical (usual) drive with this type of ball was about 150 to 175 yards. If the ball became wet it was useless and thrown away. 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. But, the drive was even shorter than the earlier balls (the featherie). However, the surface of this ball was sometimes chipped or scored when hit with a golf club. A professor (teacher) at Saint Andrews University in Scotland discovered that a scored or chipped ball flew farther. This lead to a variety of surface designs. By 1930, the current golf ball, with dimples, was accepted as the standard design. The modern golf ball has rubber thread wound around a rubber core and coated with dimpled enamel (bumpy paint). The dimples are 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 yeards. 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 without dimples (while it's flying through the air!). The first kind of drag is the obvious drag due to friction. But, this is only a small part of the drag experienced by a ball. Most of the drag comes from the "separation of the flow" as the ball sails through the air. For laminar (smooth) flow past a sphere, the flow separates very early as shown in the left picture above (again, with a smooth ball). Compare this with a "turbulent flow", caused by a marked or dimpled surface. Flow separation is delayed, as can be seen in the picture on the right. Notice the difference in the size of the separation region behind the spheres. The larger (or early) flow separation causes a larger pressure drag on the sphere (golf ball). The rough or dimpled surface causes "turbulence" which delays or narrows the flow separation. This lowers the pressure drag. On a smooth sphere (golf ball) the faster the ball moves more drag is produced. On a rough sphere speed does not change the drag very much.
Although round dimples are accepted as the standard, many other shapes were tried. Hexagons (six sided) resulted in lower drag than round dimples, so maybe in the future we will see golf balls with hexagonal dimples. How a Golf Ball produces Lift Lift is another aerodynamic force which affects the flight of a golf ball. This idea might sound a little odd, but given the proper spin a golf ball can produce lift. At first, golfers thought all spin was detrimental (not good). However, in 1877, British scientist P.G. Tait learned that a ball, driven with a "backspin" (the top of the ball turning back toward the golfer) actually produces lift.
The dimples also increase lift. Remember, dimples help keep the flow attached to the sphere. The dimples also cause the flow to be "focused" into the flow of the wake. In this figure, the smoke shows the flow pattern around a spinning golf ball. The flow is moving from left to right and the ball is spinning in a counter-clockwise direction. The wake is being forced downwards. This downward movement of the wake means that a lifting force is being applied to the golf ball. Hook and Slice (golf ball moving to the left or right)i This can also be explained by the spin of the ball. If, after being struck, the ball spins to the right (clockwise) on its veritcal axis it will spin off to the right. If it spins counterclockwise (veritcal axis) then it will spin off to the left. This is called the Magnus effect. This effect is important in most ball games.
Of course, someone invented a ball that would not slice or hook. But, the United States Golfing Association felt that this would reduce the skill required to play in tournaments, so they passed a rule that made the new ball illegal. Effect of Gravity During the last part of a golf ball's flight, gravity begins to take over. As the ball slows due to drag, the lift decreases. At some point, the lift will no longer be greater than the weight and the ball will begin falling to the ground. Experiments
All these conclusions about spinning spheres, dimples and lift were made
after tests inside a wind-tunnel (a special tunnel for testing wind
pressure on objects). A hollow ball (with a motor inside) is hung by a
wire from the top of the tunnel and another wire from the side, to hold
it steady. The wires provide power to the motor to make the ball spin
around a vertical axis. Gauges measuring lift and drag are attached.
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