The last year and a bit has seen Jasprit Bumrah’s meteoric rise as one of the world’s best fast bowlers. Much of the Indian team’s success on recent overseas tours can be attributed to the brilliance of its pace spearhead. The fact that Bumrah is now the no.1-ranked ODI bowler in the world and played a leading role in Mumbai Indians winning its fourth Indian Premier League title is testimony to his prowess.

 

It is interesting to investigate the reasons for Bumrah’s dominance and what sets him apart from contemporaries. The primary weapons in a fast bowler’s arsenal are pace, action and the ability to move the ball, both laterally and vertically. Higher pace ensures a smaller reaction time for batsmen which in turn makes it more difficult for them to hit the ball. The action of a bowler determines how effectively he can hide the seam position, the point and height of the release, and the backspin he can impart to the ball.

 

Finally, a fast bowler’s most lethal weapon is his ability to move the ball in order to deceive the batsman. This is also the primary indicator of the bowler’s wicket-taking ability.

 

Bumrah can consistently bowl at speeds in excess of 145 kmph which makes him one of the quickest bowlers in world cricket at the moment. In addition, his unconventional high-arm action has proved extremely difficult for batsmen to read. Bumrah also possesses the ability to swing the ball both ways and is known to extract a great deal of seam movement off the pitch as well. Apart from conventional swing, his high speed also allows him to reverse-swing a relatively new ball. All these are important factors in making Bumrah one of the top fast bowlers of his generation.

 

However, one of the most overlooked components of his bowling and the skill that makes him so dangerous is his ability to rapidly dip the ball at great pace. In order to understand how Bumrah accomplishes this, it is important to take a look at the aerodynamics of a cricket ball.

 

The movement of the cricket ball causes a thin layer of air — called the boundary layer — to form across the surface of the ball. This boundary layer “separates” from the surface of the ball at some point. The location of this point governs the pressure on the surface of the ball with a late separation on a particular side associated with low pressure on that side.

 

Another important characteristic of fluid flow that is relevant for this discussion is turbulence. At slower speeds, any fluid flows with a smooth and steady stream. This kind of flow is called laminar flow. As the speed of the flow is increased, the irregularity increases. Beyond a certain critical speed, the flow transitions into a time-varying and chaotic state known as turbulence. By virtue of its increased energy, a turbulent boundary layer stays attached to a surface longer than a laminar boundary layer.

 

Fast bowlers release the ball with their fingers along the seam of the ball which imparts backspin to the ball. Backspin can have a very profound effect on the turbulence in certain conditions. This can be seen from the illustrations in Figure 1 (see box). The figure is for a ball that is moving from right to left. The airflow, therefore, with respect to the ball is from left to right. Fig.1 (a) shows that the upper half of the rotating cricket ball (called the retreating side) moves in the direction of the air flow. This causes the air flow to stay attached with the cricket ball on the upper half and the flow separation is delayed till Point A. On the other hand, the flow on the bottom half (called the advancing side) separates earlier, at Point B.

 

 

 

Reverse Magnus effect

 

Earlier flow separation on the advancing side results in high pressure below the ball and low pressure above it. This results in an upward force on the ball, and this phenomenon is called Magnus effect. The Magnus force on a cricket ball moving with backspin keeps the ball afloat in the air longer which makes it easier for batsmen to hit it.

 

As the rotational speed of the ball is increased — that is, if there’s more backspin on the ball — a very interesting phenomenon is observed. The increased rotational speed induces turbulence on the advancing side (that is, the lower side) of the spinning cricket ball. As noted earlier, turbulence causes delayed flow separation and hence the point of flow separation moves further downstream on the advancing side as shown in Fig.1 (b). This reverses the high and low pressure regions on either side of the ball. Consequently, the direction of the Magnus force is reversed and this phenomenon is termed reverse Magnus effect.

 

The direction of the Magnus force is dependent on a parameter known as spin ratio. It is the ratio of the tip speed of the ball due to rotation (equal to the ball’s angular speed multiplied by its radius) to its linear speed (delivery speed in common terminology). The reverse Magnus force on a cricket ball moving with backspin has the effect of making the ball dip quicker. A dipping ball moving at great pace makes it extremely difficult for batsmen to judge its trajectory and hit it. In fact, it is a wicket-taking ball. It is thus useful for bowlers to have the spin ratio in the regime of the reverse Magnus effect. Only fast bowlers who can impart strong backspin can achieve this.

 

Bumrah can bowl at speeds of around 145 kmph coupled with a rotational speed of 1000 RPM and a very stable seam position. This gives a spin ratio of nearly 0.1 for the cricket ball. Experiments on a rotating sphere at the National Wind Tunnel Facility, IIT Kanpur, conducted by this writer and his students, have revealed that this spin ratio puts the ball into the reverse Magnus effect regime. A downward force on a cricket ball moving at Bumrah’s pace causes it to dip very sharply and therefore batsmen find it difficult to pick his deliveries.

 

(Sanjay Mittal is Professor, Aerospace Engineering, at IIT Kanpur)