Swing Bowling

The mystery of the swinging ball in cricket has only recently been understood from a scientific perspective. Physicists have discovered some of the secrets that make a cricket ball swing in the air as it approaches the batsman. The air flow around the ball, the speed of delivery, the angle and height of the seam and even the weather contribute to making batting difficult and bowling — when the ball swings — an enjoyable experience.

Air Flow
The resistance of air molecules when a cricket ball passes through the air on its way to the batsman creates what Rabindra Mehta, a NASA scientist, calls “a boundary layer” around the ball’s surface. In the ESPN article “The Science of Swing Bowling,” he says that the “boundary layer cannot stay attached to the ball’s surface all the way around the ball and it tends to leave or ‘separate’ from the surface at some point … A side force or swing will only be generated if there is a pressure difference between the two sides of the ball.” The seam of a cricket ball, or the deliberate roughing up of one side and polishing of the other, creates the variation in air flow. This variation increases the likelihood of air flow separation on one side of the ball that causes the movement in the air.

The Speed of Delivery
When a right-handed bowler is running in toward a right-handed batsman and wants to swing the ball from right to left away from the batsman, he must keep the smooth side of the ball to the right. Bob Woolmer, former South African head coach and author of “The Art and Science of Cricket,” states that a ball traveling at a speed of more than 112 km/h “will become turbulent, and the boundary layer separation point on the smooth side will move further towards the back of the ball so that the separation points become more similar, reducing swing.” This scientific discovery explains why medium-paced bowlers always have been able to swing the ball more than fast bowlers.swingbowling

Height of the Seam
Because of the new string seam that runs around its circumference, a new cricket ball always moves more in the air than an old ball. The old ball, having been used for two hours or 150 deliveries, has a seam that has been flattened into the ball. This means the ball has become smooth, causing a perfect “boundary layer” and reducing the chances of swing. The new ball, used at the beginning of an innings, causes greater turbulence, a disturbed “boundary layer” and an increased likelihood of swing.

The Angle of the Seam
Research at Melbourne University titled “An Experimental Study of Cricket Ball Aerodynamics” by Alam, La Brooy, Watkins and Subic discovered that “the seam angles have negligible effect on drag coefficients,” thus reducing the focus bowlers place on aiming the angle of the seam away from the batsman if they want to generate away swing. However, Bob Woolmer claims that if the seam maintains “an optimum angle of attack throughout the delivery,” the maximum swing can be achieved. The specific angle of attack should be a seam tilted 20 degrees either way with a “rate of back-spin between 11 and 14 revolutions per second.”

References
• “Bob Woolmer’s Art and Science of Cricket”; Bob Woolmer, Professor Tim Noakes and Dr Helen Moffett; 2008
• ESPN: The Science of Swing Bowling [http://www.espncricinfo.com/magazine/content/story/258645.html] • RMIT University of Melbourne: An Experimental Study of Cricket Ball Aerodynamics [http://www.buet.ac.bd/me/icme/icme2007/Proceedings/PDF/ICME07-FL-17.pdf]

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