Swing Bowling - Physics of Swing Bowling

Physics of Swing Bowling

The essence of swing bowling is to get the cricket ball to deviate sideways as it moves through the air towards or away from the batsman. In order to do this, the bowler makes use of five factors:

• The raised seam of the cricket ball
• The wear and tear on the ball
• The polishing liquid used on the ball
• The speed of the delivery
• The bowler's action

The asymmetry of the ball is encouraged by the constant polishing of one side of the ball by members of the fielding team, while allowing the opposite side to deteriorate through wear and tear. Over time, this produces a marked difference in the aerodynamic properties of the two sides.

The main theory of swing bowling surrounded turbulent and laminar airflow. Laminar air separates from the surface of the ball earlier than turbulent flow air, so that the separation point moves toward the front of the ball on the laminar side. On the turbulent flow side it remains towards the back; inducing a greater lift force on the turbulent airflow side of the ball. The calculated net lift force is not enough to account for the amount of swing observed: Additional force is provided by the pressure-gradient force.

To induce the pressure-gradient force the bowler must create regions of high and low static pressure on opposing sides of the ball. The ball is then "sucked" from the region of high static pressure towards the region of low static pressure. The Magnus effect utilises the same force but by manipulating spin across the direction of motion. A layer of fluid, in this case air, will have a greater velocity when moving over another layer of fluid, than it would have had, if instead it were moving over a solid, in this case the surface of the ball. The greater the velocity of the fluid, the lower its static pressure.

When the ball is new the seam is used to create a layer of turbulent air on one side of the ball - by angling it to one side and spinning the ball along the seam. This changes the separation points of the air with the ball, this turbulent air creates a greater coverage of air; providing lift. The next layer of air will have a greater velocity over the side with the turbulent air due to the greater air coverage and as there is a difference in air velocity, the static pressure of both sides of the ball are different and the ball is both 'lifted' and 'sucked' towards the turbulent airflow side of the ball.

When the ball is older and there is an asymmetry in roughness the seam no longer causes the pressure difference, and can actually reduce the swing of the ball. Air turbulence is no longer used to create separation point differences and therefore the lift and pressure differences. The rough side of the ball, features scratches and pits in the surface of the ball. These rough features act in the same manner as the dimples of a golf ball. They trap the air, creating a layer of trapped air next to the rough side of the ball, which moves with the surface of the ball; the shiny smooth side doesn't trap a layer of air. The next air layer will have a greater velocity over the rough side, due to its contact with a layer of trapped air (rather than solid ball), thereby lowering the static pressure relative to the shiny side, which swings the ball. If the scratches and tears completely cover the rough side of the ball, the separation point on the rough side will move to the back of the ball, further than that of the turbulent air, thereby creating more lift and faster air flow. This is why a slightly older ball will swing more than a new ball. If the seam is used to create the turbulent air on the rough side, the tears won't fill as quickly as they would with laminar flow, dampening the lift and pressure differences.

Reverse swing occurs in exactly the same manner as conventional swing, despite popular misconception. Over time the rough side becomes too rough and the tears become too deep - this is why golf ball dimples are never below a certain depth, and so "conventional" swing weakens over time; the separation point moves toward the front of the ball on the rough side. When polishing the shiny side of the ball, numerous liquids are used; sweat, saliva, sunscreen, hair gel (which is why bowlers may do their hair before a game) and other illegal substances like Vaseline (applied to the clothing where the ball is polished). This doesn't disappear; it penetrates the porous surface of the leather ball. Over time this expands and stretches the surface of the ball (which increases the surface area meaning more lift) and creates raised bumps on the polished side, due to the non-uniform nature of the expansion. These raised bumps invariably creates valleys, which hold the air in the same manner as the tears on the rough side. Thereby creating a layer of air over the shiny side, moving the separation point towards the back of the ball on the shiny side. The greater air coverage is now on the shiny side meaning more lift and faster secondary airflow, and therefore lower static pressure on the shiny side - swinging the ball towards it instead.

The rough side tears hold the air more easily than the shiny side valleys and so to maintain the air within the valleys the initial air layer must have a very high velocity, which is why 'reverse' swing is primarily, but not necessarily, achieved by fast bowlers. Due to the less static nature of the initial air layer it takes longer for the swing to occur, which is why it occurs later in the delivery, this is also why conventional and reverse swing can occur in the same delivery.

All other things being equal, cold and humid weather enhances swing, due to the lift force increasing with the density of the air. Colder air is more dense than warm air. The humidity is a measure of the water content in the air, and although water vapour does not increase the mean density of the air since water molecules weigh less than nitrogen or oxygen molecules, tiny suspended droplets of condensed liquid water will certainly do so since water is a fluid one-thousand times more dense than air.