Last Updated 20th August, 2002
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Normal Swing
The leading part of the ball is covered by a film of fast moving air, called the boundary layer. About halfway round the ball, the boundary layer separates from the surface. On the non-seam side the boundary layer peels away before the halfway mark. But on the seam side the flow is disrupted by the protuberance of the seam, the boundary layer is tripped into a chaotic turbulence and peels away after the halfway mark. The effect is to make the air pressure on the seam side of the ball lower and this pushes the ball towards the seam side, away from the batsman.
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The figure shows the standard out-swinger; for the in-swinging delivery the ball is reversed, with the seam pointing to the leg-side. This delivery requires a different action to the out-swinger, allowing the (good) batsman to recognise the deliveries as they are delivered.
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Contrary to intuition and popular belief, the swing is not primarily due to the difference in friction caused by the rough and smooth sides of the ball - although it is this difference that helps generate turbulent flow on one side and laminar flow on the other. For a blunt object like a sphere the main contributor to aerodynamic drag is the point of separation of the boundary layer, and turbulent flow holds the layer on to the surface longer, reducing the pressure on that side of the ball.
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{This is the same principal that allows a dimpled golf ball to fly further than a smooth one. In this case the dimples promote turbulent flow which reduces the pressure drop behind the golf-ball, thus reducing drag.}
The ideal ball for normal swing is highly polished on one side with a prominent seam delivered at an angle of about 20 degrees to the direction of flight, and with about 11 revs per second spin about an axis perpendicular to the seam
There is no simple linear relationship between the speed of delivery and the amount of sideways movement for conventional swing. Up to a certain limit - dependent of the atmospheric conditions and the condition of the ball - the amount of swing increases with the speed of delivery. As the ball's speed increases past this limit however turbulence will start to develop on the shiny side reducing the net side force. This is why medium pace bowlers can often generate more swing than fast bowlers. If the ball is bowled even faster still, the turbulence may begin before the seam causing reverse swing! (Turbulence is initiated at the back of the ball and moves forward as the speed increases).
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Reverse Swing
With a turbulent boundary layer on both sides of the ball, the effect of the seam is reversed. It now acts as a ramp, pushing the turbulent air away from the ball and causing the boundary layer to peel away sooner. That makes the pressure on that side higher, forcing the ball to swing towards the batsman.
To get reverse swing with a new ball, smooth on both sides, experiments show that the bowler has to reach 80-90 miles per hour to get appreciable movement. This kind of speed has only ever been achieved consistently by a few bowlers. A scuffed ball however can generate substantial reverse swing at speeds well within the capabilities of any medium-paced bowler.
The ideal ball for reverse swing has one side rough, the other smooth, with a prominent seam in between. The seam should be angled at about 15 degrees to the direction of flight, pointing away from the desired direction of movement. The ball can then be swung both into and away from the batsman depending solely on which side of the ball is delivered at the front - generating either normal or reverse swing. Because the bowler does not need to change either his grip or his action, the batsman will have no clue which way the ball is likely to move.
For reverse swing the amount of sideways movement is related to the speed of delivery, making this a particularly effective delivery for fast bowlers.
