Boundary Layer Separation - A Batsman's Nightmare

Since my childhood, I have been fond of swing bowling. I feel that there is no better sight for a bowler than to see a batsmen being completely bamboozled by the swing. Watching Glenn McGrath, Zaheer Khan, James Anderson rip through some of the greatest batting line ups with a display of unplayable swing bowling was a sight to behold. Swing bowling is a sorcery which continues to fascinate me. Recently, I watched an NPTEL lecture by Prof. Suman Chakraborthy in which the fluid mechanics governing swing bowling was beautifully elucidated. Getting to know the science behind it made it lot more interesting. One of the fundamental concepts of fluid mechanics - 'Boundary layer separation' is responsible for the batsmen's nightmare. When a bowler bowls with a straight seam position with one side of the ball kept relatively smooth (shiny) while the other side is made rough by removing the shine, there exists a particular speed range in which the flow will be laminar in the boundary layer on the smooth half and a turbulent in the boundary layer over rough half of the ball. When the bowler bowls in this speed range, it is possible to swing the ball. In turbulent flow, the point at which the boundary layer separates from the solid is located farther away from the stagnation point than in laminar flow. So the jet of fluid leaving the surface of the ball make different angles on the smooth (laminar) side and rough (turbulent) side. As a result of this, there will be a force imbalance causing the ball to move laterally in the direction of the rough side. However, this is just one of the ways to swing the cricket ball. There are several other ways too! Watch the following lecture by Prof. Suman Chakraborthy form the 40th minute to learn more about the science behind swing bowling. 

https://www.youtube.com/watchv=8HfcskC9CQM&list=PL3zvA_WajfGCmUQch2UbOTu3ofmuEcnba&index=44&ab_channel=nptelhrd