One of the things that I have always wondered in my life is that why does a cricket ball swing? So, In this post I will try to understand the mechanics of ball swinging and decipher this mystery in the best way possible.
To do that, we need to first understand, what is swing? So, swing is the art of turning the ball into or away from the batsmen after the bounce. There are 3 major types of swing deliveries.
Outswing : This delivery involves the lateral movement to the batsman’s offside in the air after the bounce.
Inswing : This delivery involves the lateral movement to the batsman’s legside in the air after the bounce.
Reverse Swing : This delivery involves swinging of the ball that looks like inswing or outswing when the ball is released but gets completely opposite by the time it reaches the batsman.
Reverse Swing Video-> https://youtu.be/E5Em0LEbhPQ
Boundary Layer Theory
To look at this aerodynamically, one must understand the concept of boundary layer formation and separation. Boundary Layer is a layer of fluid (in this case, Air) which tends to stick to the surface of the object (The ball) when the object tries to move through the fluid. The boundary layer does not cause any effect on the object’s flow through the fluid (except for resistance) until it separates. Boundary layer separation occurs when the flow velocity increases to such an extent that the pressure difference increases drastically in the flow direction. Boundary Layer separation causes formation of eddies which then resist the movement of the flowing object. The Boundary Layer formed on any surface can be either Laminar or Turbulent. In Laminar Flow, the flow of the fluid is in organised parallel lines. Whereas, the Turbulent Boundary Layer the fluid particles move in disorganised and intersecting paths.
In swing bowling, this phenomenon is harnessed by making use of the seam. The ball tends to swing because of the difference in the surface roughness of the ball on either side of the seam. You might have noticed that before bowling, a bowler sometimes rubs the ball on his trousers. The rubbing action produces a smooth and shiny surface.
To execute a swinging delivery, the bowler holds the ball with seam pointing in the direction he wants the ball to swing ensuring that the rougher and smoother sides are aligned as shown in the above figure. So, for an inswinging delivery to a right handed batsman, the bowler will hold the ball with the seam pointing slightly toward the batsman, and for an outswinging delivery, the seam will be pointing away from the batsman.
The nature of the boundary layer on the ball surface directly affects the way the ball swings. When the ball is thrown from the bowler’s hands, it encounters air resistance. Now, due to the grip on the ball when it is thrown, air flows around the ball in 2 different paths. On one path, air flows over the seam, forming a turbulent boundary layer while on the the other path, air flows over the smooth ball surface, forming a laminar boundary layer. Due to the laminar boundary layer, the air encounters more resistance on the flow with the seam side than on the smoother side. Due to the difference in the resistances, the ball tends to rotate about the axis midair, while exerting a force towards the rougher side. This causes the magical lateral movement of the ball towards the rougher side.
So, now what about reverse swing? Well. Reverse swing as you might have known, happens when fast bowlers bowl with a somewhat used ball. When such a ball is thrown at high speeds, the ball encounters greater air resistance as before, and due to the speed of the ball, the airflow immediately produces a Turbulent Boundary layer on either side,with the boundary layer on the seam side being more turbulent. Now, the more turbulent the boundary layer gets, the quicker it tends to separate from the surface. The quicker separation on the seam side causes the ball to swing in the opposite direction then it would do conventionally. So, the Outswing becomes Inswing and vice-versa.
Due to the inherent physics, reverse swing can occur if the ball is bowled at very high speeds or if the ball is worn enough on both sides of the seam. Using a new ball, reverse swing can be achieved above 160kmph and hence, reverse swing is generally not seen at the start of a match because bowling at 160kmph is not an easy task even for the best of the bowlers. The adequate bowling speed for swing to occur with a worn ball is around 110-120 kmph and reverse swing occurs above 120 kmph.
Some factors such as the climate and height of the pitch from Mean Sea Level also affect the magnitude of swing produced. In humid climates, there is more water vapour in air, thus the air is much less dense. Due to this, the resistance encountered by the ball in flight is reduced and ultimately, the swing magnitude is reduced.The opposite happens in drier and hotter climates.
The higher the stadium is from Mean Sea Level, the air density decreases and hence, the resistance to the ball decreases, reducing the magnitude of swing.
That’s it! I hope that you now have a good understanding of why swing happens. If not, feel free to contact me.