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They want to look as if they are adultsSurface area is increased. I would guess this does several things - it allows the racket to get a better grip for spin although this can be done with both squash balls and snooker balls - it will slow the ball down because of increased air turbulence - it does change other aerodynamic properties; the seams stabilize the ball Other things are possible but it is beyond me
The tennis ball cover is rough but not like sandpaper. Looking closely at the felt you can see that air flows through those raised fabric filaments - better known as fuzz. So the surface of a tennis ball is a "porous surface." It's more like dealing with the wind blowing through leaves on a tree and just as complicated. The fuzz filaments act like tiny cylinders each having their own drag component. In addition to the skin friction drag from the cover itself, drag is created from the airflow over these fuzz fibers interacting with all the other fibers behind it. Rabi Mehta dubbed this phenomenon "fuzz drag."Experiments Mehta conducted also showed that as the ball speed increases the fuzz filaments lay down on the ball and the fuzz drag declines. Here are two pictures from the wind tunnel test. You can see in the picture on the right that the fuzz is closer to the ball's surface. The air passing over the ball on the left is about 45 mph, while on the ball on the right the air is 135 mph.
Initially, we could not determine why the drag on tennis balls is so much higher than that on other sports balls, he said. "Then we realized that the 'fuzz' on the ball plays a much larger role in the aerodynamics than had been anticipated in the past," Mehta said."If you have a smooth ball, such as a ping pong ball, it produces a large air wake, like that of a motor boat. The ball's large wake creates drag that slows the ball's flight," Mehta said. "If you add roughness, like the dimples on a golf ball, air disturbance near the ball's surface actually helps produce a smaller air wake that creates less air drag, and the ball can go farther," he explained. A smooth golf ball might only go about 100 yards compared to the 300 yards covered by today's dimpled golf balls, he added."Even though a tennis ball does not have a smooth surface, you get a bigger wake because of the very rough surface, plus the effect of additional drag from each fuzz filament, which I have termed fuzz drag," he said. "Fuzz drag makes the aerodynamics of the tennis ball even more interesting since the fuzz elements change orientation with increased velocity and the fuzz wears off during play."Mehta said the complex interactions of air density, air ‘stickiness,’ air speed and physical size and surface roughness, normally are major factors in determining how sports balls fly through the air.Air is a bit viscous or ‘sticky,’ resulting in ‘skin-friction’ drag, he explained. When a smooth ball flies through air at a slower speed, a layer of slow-moving air forms around the ball's front. The sticky, smooth-flowing air layer separates from the ball's surface, forming a wake that begins in a circle like the edge of a grapefruit that has been sliced in half. The wake behind the smooth ball is almost as wide as the ball, creating a great deal of ‘pressure’ drag that adds to the sticky air drag on the front side of the flying ball.Surface roughness, such as dimples on a golf ball, produce turbulence in the slow-moving air close to the ball and the more energetic layer separates much later, thus leading to a smaller wake."The two types of flow can easily be demonstrated at home," Mehta said. "Go to a water faucet, turn it on at a slow rate, and you get a smooth stream of water almost to the bottom of the sink. Increase the flow rate, and you get a splashy, chaotic flow; this is turbulent flow," he concluded.
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 6 shows how effective different degrees of roughness can be in reducing the drag on a sphere.Figure 6. 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.
Wow !!..Thank You....thank you all for your fantastic speculations, opinions theories and quoted texts. I really do appreciate the time taken to answer and have enjoyed reading the comments and posts. Excellent texts above George...cheers.You have all handled my hairy balls very well !!........................(sorry....couldn't help myself) 
You will not be able to play tennis correctly if you use old tennis balls. The balls are made up of a pressurized rubber core covered in felt. When this felt becomes worn & smooth, it will not react properly when it is struck by the racquet. The ball will tend to skid when it hits the court.
You have all handled my hairy balls very well !!
Quote from: neilep on 01/08/2007 03:00:11You have all handled my hairy balls very well !!I knew there was a good reason for delaying my response to this thread........................
(How sad am I that I find amusement in my own threads ?)