The Science of Swing Bowling
As all of you know, I have somewhat inexplicably become a huge cricket fan in the last year or so. I’ve given a lot of thought to why I’m so drawn to the sport and it seems like a combination of a few factors. First, the game is extremely elegant, from the crisp, white uniforms donned for Test cricket, to the strictly enforced rules of decorum, to the freedom conferred to the batsman to hit the ball in all directions (the full 360 degrees of the oval are in play). It’s also a very accessible game, where no height or weight minimums are required to participate on the highest level. Arguably the greatest Indian batsman of all time, Sachin Tendulkar, was 5' 5". In the a recent West Indies — India match, Carlos Brathwaite was bowling to Ajinkya Rahane. The contrast was staggering: Brathwaite, a giant at 6'4" and 260 pounds, bounding down the track, bowling at 90mph to Rahane, standing at a diminutive 5' 6". The game is also very fluid, where the batting side may be playing defensively to save their wickets (similar to outs in baseball) or swinging for the fences with reckless abandon to add runs quickly (think home-run derby). Finally, and the part I’ve recently found most intriguing, the game centers on something totally foreign from baseball: the bounce.
The biggest weapon that a bowler has at their disposal is the uncertainty created by the ball. For many strict pace bowlers (80 MPH+), the avenue to creating uncertainty is through the intensity of the bounce. For example, for taller pacers, they can attempt to “hit the deck” where their motion puts more downward force on the ball as it approaches the batsman and the bounce has more vigor than a typical ball. For a given length of the delivery (how far down the pitch the ball goes before bouncing), a ball that has “hit the deck” will bounce higher than expected. Especially if this surprises the batsman by careening towards his ribs or even head, he may swing wildly at the ball and lose control of it, often leading to an easy catch. On the other hand, a smaller bowler may try and bowl a “grubber,” a ball that skids on from its point of impact with the pitch. In this case, the ball could feasibly sneak under the bat and hit the wickets.
Another option for creating uncertainty in the batsman’s mind through the bounce is spin bowling. The concept is pretty straightforward — the bowler imparts some RPMs onto the ball as it leaves his hand, and when the ball bounces in an odd direction, the batsman may mis-hit it. This often can create an “edge,” where the ball nicks off an edge of the bat (think a foul ball) and leads to an easy catch for the wicketkeeper (think catcher) or a nearby fielder. It can also lead to an LBW (leg before wicket) where the ball would miss the bat and make contact with the batsman leg pads. If the path of the ball is judged to be hitting the wicket (the three stumps behind the batsman, think the strike zone) if it hadn’t hit the leg pad, then the batter is out as well.
Between pace and spin, there are various ways to create uncertainty through bounce. The most difficult form for a batsman to deal with is a combination of the two — a concept called swing bowling. Swing bowling occurs when a fast bowler utilizes the physics of ball flight and air flow to bowl a delivery that moves rapidly but also bounces in a crazy way. If pure pace bowling is like a fastball, and spin bowling is like a curveball, then swing bowling is the nasty slider.
Swing bowling can be explained through aerodynamics. A cricket ball (picture below) is made up of leather and a seam that splits the ball into two hemispheres. Unlike baseball, the cricket ball is not replaced constantly through the game — in test cricket, for example, it lasts for 80 overs (480 pitches) before the bowling team has the option to opt for a new ball. The newness of the ball has several impacts. A totally new ball is most advantageous for pace bowlers as it bounces with the most vigor to keep speed through the bounce. A much older ball is great for spinners as it has imperfections on the surface that grip the ground better and create more robust spin.
As the ball gets worn through impact with the ground, these imperfections start popping up. If the team has a good swing bowler in the side, you may see the captain or another fielder constantly shining the ball on their pants as it becomes older and older. In doing so, this player is trying to keep one hemisphere of the ball smooth while the other half gets roughed up. When this is accomplished to a certain threshold, the ability to swing the ball in the air is accomplished.
The reason for this comes from aerodynamics. When the ball is traveling through the air, a thin layer of air forms around the surface called the boundary layer. This boundary layer can’t be continuous around the entire surface of the ball, and has to separate from the surface at some point. The distance along the surface of this separation point determines the pressure imparted on that side of the ball — the further along the separation point, the lower the pressure. The other important piece of this boundary layer is that it can either demonstrate laminar (smooth/steady) flow or turbulent flow. The transition from laminar to turbulent flow has to do with the speed at which the ball is moving, as well as the homogeneity of the surface. Basically the more roughed up the surface is, the lower the critical speed becomes that is required to catalyze turbulent flow. Additionally, an irregularity in the surface (the seam for example) can create turbulent flow. Typically the speed required for a seam to trigger turbulent flow is lower than the speed required for a purely rough surface to trigger it.
So when you have a ball that has a rough side and a smooth side, and you can bowl it fast enough (but not too fast, see paragraph below), you can trigger swing bowling. To do so, you would angle the seam to the direction of the flight. As the air moves around the ball in the direction of the seam, it moves from laminar to turbulent flow, given that you are bowling fast enough. As the air transitions over the seam and onto the roughed up side, this turbulent flow is maintained because of the imperfections in the ball’s surface. Therefore, the separation point is far along the ball as the increased energy and activity of turbulent flow keeps the boundary layer attached to the surface for longer. On the other smooth side, there is no seam and a relatively homogenous surface, so the air moves along the surface in pure laminar flow. This laminar flow is uninterrupted, and the boundary layer separates from the ball at an earlier point. Because of the difference in separation point along the horizontal axis of the ball, the pressure on the rough side (top) is lower than the smooth side (bottom). This pressure differential rotates the ball in the direction of the rough side, and the ball will swing in the air.
This phenomenon looks amazing when you see it in action. Here are some famous examples of swing bowling — note how much horizontal deviation there is in the location of the bounce of the ball on the ground from the original release point. Hope you boys enjoyed this foray into cricket and aerodynamics.
Sources:
http://www.espncricinfo.com/magazine/content/story/258645
