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## 13. Golf Balls, Cricket Balls, and Tennis Balls

Tripping the boundary layer to reduce drag on spheres is widely used in sports. For example, it is the reason why golf balls are dimpled. The dimples act like a very effective trip wire, and the reduction in drag due to the delayed separation allows the ball to travel further for the same effort. A driver shot in golf can easily make a golf ball carry 250 yards, but the same shot using a smooth ball will only carry about 100 yards. Similarly, a tennis ball has a textured surface with a convoluted seam, much like a baseball. Figure 13.1 shows how effective different degrees of roughness can be in reducing the drag on a sphere.

 Figure 13.1 Drag coefficient as a function of Reynolds number for spheres with different degrees of roughness. From Munson, Young & Okiishi, Fundamentals of Fluid Mechanics, John Wiley & Sons, 1998.

Cricket balls are slightly different to golf, tennis and baseballs. A cricket ball has a single, circumferential seam which looks remarkably like a trip wire. If a cricket ball is launched without spin so that the seam is tilted forward on the top of the ball, then the boundary layer over the top surface becomes turbulent, whereas the boundary layer on the bottom surface remains laminar. The wake becomes asymmetric, and a downward force is produced so that the ball dips sharply. A similar effect can be obtained with a baseball if the seam is held correctly.

The addition of spin complicates this picture enormously. Spin can produce a side force on a ball due to the Magnus effect. The effect of spinning the ball can depend strongly on the orientation of the seam. Knuckle ball pitchers typically pitch a baseball with very little spin, and they rely largely on the uneven tripping of the boundary layer. However, even a little spin will make the direction of the side force change dramatically, and it is no wonder that the behavior of a knuckle ball is highly unpredictable.

To learn more about these matters, see the article by Dr. Rabi Mehta of NASA-Ames, entitled ``Aerodynamics of sportsballs,'' Annual Review of Fluid Mechanics, 17:151--189, 1985.

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