With the cricket season in full flight, you may find yourself asking an increasingly common question: just what is reverse swing?
Most cricket fans are broadly familiar with the dark arts of swing bowling. But the practice is replete with myriad complexities, and the reverse-swing discipline in particular can seem perplexing to the uninitiated.
According to Nasa scientist Rabindra Metha, the explanation for why cricket balls swing is to be found in flow physics. As the ball moves through the air, it is effectively cushioned by a "boundary layer", which separates from one part of the ball but not the other. The separation point creates a pressure system. If there are different levels of pressure on either side of a ball, it will swing while in flight.
While hard and relatively new, the ball will swing in a predictable fashion. Cricketers, and fast bowlers in particular, are adept at keeping one side of the ball shiny. A bowler bowling to a right-handed batsman can get the ball to swing towards him if he points the seam of the ball towards fine leg with the shiny side positioned to the left.
The ball swings towards the batsman in the direction of the non-shiny side because the air pressure is different on either side of the ball. In aerodynamic terms, the shiny side has laminar air flowing past it, while the non-shiny side is affected by turbulent airflow.