The science of swing bowling

So I was watching a prominent  sports channel the other day and watched the sultan of swing of Pakistan bowling, and a couple of ex english captains discussing the mechanics of reverse swing. The undisputed king of reverse and authority on the subject was asked why the ball does what it does after it gets old.

The legendary W often commenting for sky sports said there is no rocket science behind it, it simply is a matter of ball getting heavier on the side where it starts swinging against the conversational swing direction, and folks nothing could be farther from the fact. Indeed the science of reverse swing is rocket science and not merely ball getting heavier on one side.

The ball reverses due to changes in the aerodynamics of the ball. The essence of swing whether conventional or revere is air pressure causing side way forces and not the weight of the ball in the case of reverse.

The often heard terms on TV commentaries with respect to swing of the ball are “rough side” and “shiny side”, but the question is when the ball is new with both sides of the seam equally shiny what is causing the swing?

Conventional swing

With a new ball swinging conventionally it is the seam position which manipulates the aerodynamics of the ball. So for a lefty like myself holding the ball with the seam pointing to the leg side of a right handed batsman the ball will swing towards the direction of the seam or an in swinger from the perspective of a Right hand bat. As the ball travels through air, the air is split in two as the ball makes it way towards the batsman, now what makes the ball swing into the right handed batman is the position of the seam that causes the air on the seam side to become turbulent or lets say causing tiny storms on the side of the seam and moving faster while the air on the non seam side is laminar i.e. it kind of sticks to the ball like when two plastic sheets become stuck with each other due to their smooth surfaces. This causes the air to travel faster on the seam side because of the turbulent flow (tiny fast moving storms) caused by the seam and where the air travels faster the air pressure is reduced thus pulling the ball towards the side where the seam is, Roughing up the side where the seam would be pointing only helps in the turbulent flow (fast air movement) and enhances normal swing.

Reverse

Now we hear during TV commentaries oh the ball is old and will start reversing. What really happens is that during the play the previously shiny side gets rough as well. With both sides of the ball rough we now have turbulent air flow (fast moving tiny storms) on both sides of the ball, I guess now we can’t swing the ball any more as the air pressure will be equal on both sides so keeping the ball straight. Now here’s where the seam of the ball again comes into play. The seam this time around helps keep the turbulent air flow longer on it’s opposite side or separates the turbulent air (tiny fast moving storms) flow earlier on the seam side of the ball. The longer the turbulent air flow remains on one side the lower the air pressure thus the swing now goes against the seam side i.e. reverse swings.

And that folks is how the ball reverses, it’s all got to do with the separation point of turbulent flow of air, the earlier the separation point from the ball (helped by the seam) the higher the pressure thus the ball swinging opposite where the seam is pointing.

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