John Bluck

Sept. 6, 2000

NASA Ames Research Center, Moffett Field, CA

Phone: 650/604-5026 or 604-9000


NASA aerodynamics technology may well help create more competitive tennis
matches between the world’s top players while stimulating student interest
in science and engineering.

In recent years, improved racquet technology and faster surfaces have led
to an emphasis on the serve and shorter rallies in professional tennis
matches. To slow the game, the International Tennis Federation, London,
England, recently approved the testing of a new ball, 6.5 percent larger in
diameter, during exhibition play. They also reviewed data of Dr. Rabi Mehta
and the wind tunnels at NASA Ames Research Center in the heart of
California’s Silicon Valley.

“The concern is that today’s top pros can serve a tennis ball at
almost 150 miles per hour. On faster surfaces, such as Wimbledon, that
ensures an increasing number of shorter rallies and tie-breaker sets,” said
Mehta, a world authority on the aerodynamics of sports balls. “A larger
ball will slow things down; the trick is to figure out how much. That was
the objective of experimental testing conducted in England and at Ames,” he

To inspire school students to learn physics and engineering, Mehta
began working with an engineering consulting firm, Cislunar Aerospace,
Inc., Napa, CA, about two years ago. Together, they demonstrated tennis
ball aerodynamics to students in order to pique their interest. Recently,
Mehta explained the complex airflow around big and small tennis balls that
he and his students have discovered to a Tennis Federation convention in
Roehampton, England. In particular, he noted, wind tunnel tests have shown
that ‘fuzz’ affects the flight of a tennis ball far more than previously

“Cislunar got a NASA grant from the Learning Technologies Project to
develop a web site for kids from kindergarten through grade eight
(,” Mehta said. Cislunar CEO, Dr. Jani
Macari Pallis, made an ‘Aeronautics Internet Textbook’ that includes a
tennis section in the sports ball area, the most popular part of the web
site, according to Mehta. “The first part of the student work was a flow
visualization study of a tennis ball in a NASA-Ames 3-foot by 4-foot smoke
tunnel two years ago. The data from those tests are on the web site.
Mainly, we performed the study to show the kids the basic principles of
fluid mechanics,” Mehta added. Fluid mechanics is the study of fluid flow
(gas or liquid), its properties, characteristics and behavior.

More recently, the investigators measured the drag on regular as
well as new, larger tennis balls over a wide range of flow speeds in the
NASA-Ames 15-inch by 15-inch wind tunnel. “With the help of data collected
by two college summer students, I think, for the first time, I understand
the full aerodynamics of a tennis ball in flight,” Mehta said.

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

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