how cricket bowlers get swing

In cricket you will hear the commentators saying that the conditions are perfect for swing. This ‘swing’ they are talking about can be normal swing or reverse swing. But not all bowlers can perform reverse swing. The ball will need to be propelled above 80mph so it will be able to move in the air.

Normal swing will mainly occur when fielding team have a new ball. As the ball wears the aerodynamics of the ball will change and it will hard to have a large amount of movement for the swing. But when the ball becomes much older, around 40 overs it will begin to swing towards the ‘shiny’ side. This is known as reverse swing. This means that a normal in-swinging ball will become an out-swinger and an out-swinging ball will become an in-swinger. So, both side have turbulent flow, but the seam will cause the airflow to separate earlier on one side of the ball.

As the ball becomes rougher, it will have different characteristics when it moves in the air after during the bowing motion. 

To get swing on a cricket ball, you have to have a ball that is asymmetric. So you have a smooth side, a rough side. When the ball is angled slightly like the ball in the figure the incoming airflow will go both sides of the ball creating the ball to swing.  There will be laminar flow around the smooth side of the ball and will separate from the ball quite early creating a wake at the tail of the ball where it had come from. On the other side however, you have flow that starts off laminar, until it reaches the seam of the ball then tripping it into a turbulent flow sticking to the ball for longer and separate later from the ball. Therefore it will swing in the direction the seam is pointing.

You can bowl the ball in exactly that same way and get reverse swing on the ball. The only difference is that the smooth side has more wear as it has been used for longer as each over goes past. Now the ball is getting a turbulent flow on both sides of the ball. Now the smooth side has become rougher it will have a turbulent flow traveling round the ball and it will come off the ball fairly late. On the other side of the ball, the flow is turbulent before it reaches the seam. The seam causes the turbulent boundary layer to thicken. Because this is a thinker layer the flow of the air will leave earlier, so the wake is nearing the middle of the ball. This causes resultant force acting upon the ball making the ball swing towards the smooth side.

In some games of cricket the ball will swing more than in other games. This is caused by the humidity over the pitch. If there is a high level of humidity that is when the ball swings the most.