New Scientist: Reverse Swing (Extracts) (21Aug93)

Reverse Swing.

For the full article, pictures, graphs, charts, colour paper, order a back copy of New Scientist, 21 August 1993, No 1887.

I have included only a few extracts here.

As a ball moves faster through the air, the sheer speed begins to cause turbulence in the laminar boundary layer, as it would even if the ball were a perfectly smooth sphere. The turbulence begins towards the middle of the ball, moving forwards as the ball's speed increases. If a ball is bowled fast enough, the boundary layer will trip into turbulence even before it reaches the seam. Now the aerodynamics are completely changed. The seam acts like a ramp, pushing the air on its side away from the ball. This makes the boundary layer thicken and separate more quickly on the seam side, which creates a side force pushing the ball in the "wrong" direction - from the seam side to the smooth side.

ball bowled in the conventional way needs to reach between 80 and 90 miles an hour before it will perform a reverse swing. Because few swing bowlers have ever matched [Imran] Khan's ability to reach this pace, there the matter might have rested with reverse swing remaining an oddity unique to one or two exceptionally fast bowlers. But while scientists were experimenting, so were the Pakistani bowlers. They found a trick that lowers the speed at which reverse swing takes hold.

The trick is simply to allow the ball to scuff up on one side and then bowl with this side forward rather than the smooth side. The boundary layer becomes turbulent more easily on the rougher surface than it would on a smooth leading surface. This means that turbulence starts in front of the seam at a lower speed. So, with the seam still acting like a ramp to thicken the layer and cause separation, reverse swing becomes effective at a lower speed.