Tennis Balls and Wind Tunnels

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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,"

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 believed.

"Cislunar got a NASA grant from the Learning Technologies Project to develop a web site for kids from kindergarten through grade eight (http://wings.ucdavis.edu/Tennis)," 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.