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Turbulence
By Brad Rodgers (P1930) on Thursday, August 10, 2000 - 10:03 pm:

Why do the dimples on golf balls, the stitches on base balls, and the fabric on tennis balls all make them go farther? In other words, why does turbulence cause something to obtain less friction rather than more?

Brad

By Ian Robinson (P3243) on Friday, November 10, 2000 - 08:33 am:

It has a lot to do with the Reynold's number of small elements of the surface.
Add some dimples or hairs or other odd additions and the surface's effective Reynold's number is increased and as a result lift is increased (especially if the 'ball' is spinning - watch what cricket bowlers do to their balls (one side is more polished than the other - ie the smooth side has a lower Reynold's number than the other).
If you want a more precise explanation refer to fluid mechanics texts in particular those that deal with turbulent boundary layers and wakes about spheres.
Essentially, the roughened surface results in the early development of a turbulent boundary layer about the surface of the ball , substantially reducing the wake and thereby reducing the effect drag (by perhaps 50%)--- it flies farther for the same effort.

By Michael Doré (P904) on Friday, November 10, 2000 - 01:38 pm:

I think (but am not sure) that the reason is something like as follows.

If the ball is roughened then when it is moving it causes nearby air to rush away, therefore causing a pressure reduction nearby. Now when a golf ball flies through the air, due to its spin the relative speed between the upper surface and the air is greater than the relative speed between the lower surface and the air. This means that the pressure of air above the ball is lower than the pressure of the air below the ball, and this pressure gradient causes lift. This effect is obviously going to be exaggerated the more effect the moving ball has on nearby air, therefore it is better for it to be slightly roughened.

With cricket, the bowlers don't polish the ball to give it extra lift or make it fly farther - rather to make it swing to one side. Same principle though. In cricket, the question of why reverse swing works is still open - though there are many theories. Also the question of why swing bowling is more effective in overcast conditions - something which is empirically true - has not been answered.