Cricket

A cricket ball is made of either 2 pieces or 4 pieces, stitched to form a seam. The seam has 6 rows of stitches with 60 to 80 stitches each row. This is called as the Primary seam. In a 4 piece ball, each hemispheres are stitched together by an internal stitching. This forms the secondary seam.
Flow Dynamics:
Lets first understand the basic flow dynamics of a sphere. When a ball is flying through air, it forms a layer around the ball surface called'The Boundary Layer'. This layer cannot travel all over along with the ball. So it has to separate at a point where it is called as 'Flow separation'. This flow separation determines the pressure. The early flow separation means more pressure and late separation means less pressure. This difference in pressure makes the ball to swing...!

So it is clear that there must be a difference in pressure on either side of the ball to make it swing..!

Now lets see the Boundary Layer more closely...!
The boundary layer has two states. One is laminar flow and other is turbulent flow. The laminar flow is smooth flow with out any disturbance or turbidity. When a cricket ball flies in air, if it does not has any deviation in the position of seam ,then both sides of the ball may have a laminar air flow pattern.

Let us understand the swing of the cricket ball with the seam position being straight...!
This kind of swing does not require expertise bowling skills but of course practice is essential. Observing the above picture, by positioning the seam straight, the ball being rough on one side and smooth on other side, with the average bowling speed less than 70 miles per hour, the ball starts to swing automatically. When we look at the figure, the rough side creates turbulence and thus delays the flow separation. This causes the pressure to reduce on the rough side. When looking at the smooth side, the laminar flow exists, and the flow separation point is seen early than the rough side. This imparts more pressure on the smooth side. Thus the ball swings from high pressure side to low pressure side.

Now lets say the bowler positions the seam towards the slips or away from the right hand batsman, as shown in the above figure. The rough side and the seam produces turbulence, thus delaying the flow separation, causing lesser pressure. On the other side, the smooth surface, renders a laminar flow, with out any disturbance. This creates early flow separation and thus high pressure. So the ball is being pushed from high pressure to low pressure side, i.e , the ball swings away from the batsman, called 'OUT SWINGER'. When the seam in positioned towards the right hand batsman, then the ball swings into the batsman, called as 'IN SWINGER'.

So now wondering what is a Reverse Swing...?

The reverse swing is mostly produced only by few bowlers who bowl at a speed of around 85 to 90 miles/hr. This happens because, at such high speeds, the laminar flow becomes turbulent before reaching the ball surface. Thus when the flow hits the seam, the boundary layer becomes more turbulent, making the layer thick and cause early flow separation when compared to bottom side. Thus this again causes the pressure difference, which makes the ball to swing. But to be observed in depths, that the swinging direction is opposite to the seam position...! Thus it is called as reverse swing...!

Thus the science(aero-dynamics) behind the cricket ball is clear and appreciated.