Physics of cricket

                  Swing bowling

A cricket ball can swerve to the left or the right as it moves through the air, either because it spins about a vertical axis or because it spins about an axis perpendicular to the seam. Vertical axis spin is commonly used by spin bowlers by not by fast bowlers. Fast bowlers prefer to swing the ball by making sure the seam is inclined at an angle of about 20 degrees to the direction that the ball is headed, in such a way that about 3/4 of the front of the ball is smooth. That way, the air flows smoothly around the smooth half but it becomes turbulent on the other side since it has to flow past the seam. Turbulent air is at a lower pressure than smooth flowing air, so the ball gets pushed sideways. It is almost impossible to eliminate backspin as the ball leaves the bowler's hand, but if the spin axis is perpendicular to the seam then it will help to keep the seam aligned at a fixed angle.

The sideways force on the ball peaks at about 110 km/hr, drops to zero at about 120 km/hr and then reverses direction. Reverse swing arises because the air flow on the smooth side becomes turbulent at sufficiently high ball speeds. The smooth side then becomes the low pressure side so the ball swings in the opposite direction. Normally, this effect is significant only at speeds above about 140 km/hr. However, the effect can occur at lower speeds if the ball has a roughened side and if the roughened side faces forward. Conventional swing bowlers polish the ball so one side is as smooth as possible. Reverse swingers like to make sure the other side is as rough as possible. The best ball to swing is therefore one that stays smooth on one side and roughens up during normal play on the other side.

Details of the aerodynamics involved are described on my home page under the heading Ball Trajectories where you can find several pdf files to download on the subject, including one called Sports Balls.pdf and one called Fluidflow Photos.pdf. The secret behind swing bowling lies in the way that the thin boundary layer of air near the ball surface can separate from the ball either early or late depending on whether the air flows smoothly over the surface or is tripped into turbulence by the seam or by roughness of the surface, or both. Those boundary layers were revealed many years ago by the marvelous smoke tunnel photos shown in the Photos.pdf file. Here is one taken by the late Professor F. Brown from University of Notre Dame showing how air flows around a sphere when part of the bottom half is covered in a rough grit. Air separates early over the smooth portion, becomes turbulent over the rough portion and separates later, so the air is deflected upward, resulting in an equal and opposite downward force on the ball. That is the secret that lies behind almost all aerodynamic flows, and it is what determines both the lift and drag coefficients acting on an object. Note how air backflows into the low pressure  “hole” left behind the ball, forming a turbulent wake