Fluid Dynamics of Cricket Ball

Position – Graduate Research Assistant

Host – Department of Mechanical Engineering,  Indian Institute of Science, Bangalore, India.

Adviser – Dr. J. Srinivasan 

Duration – May 2008 – July 2009

Motivation

Cricket is a very popular game in the world, especially in the former British commonwealth. In several ways it is similar to baseball, wherein a pitcher (bowler) throws a ball toward a batter (batsman).  A 20 m long pitch of the cricket field, a speed of up to 170 km/hr of the ball thrown, along with several other factors like spin imposed on the ball, roughness of the ball surface and stitches on the surface of the ball (Figure shows a model of the ball, where stitches protrude from the surface) play their roles in determining the

Model of a cricket ball

trajectory of the ball as it reaches the batsman. Conventional swing of the cricket ball involves, the flow on the side of the ball with stitches facing the oncoming airflow tripping to turbulence, thereby creating a low pressure zone on that side, deflecting the ball in the same direction. However, reverse swing of the same ball, when it is rough occurs when the ball deflects to the other side, where the stitches are not present.

The reverse swing of a cricket ball has been considered by many to be a  mystery that cannot be explained easily from the known laws of fluid mechanics. The bowlers are unable to achieve the reverse swing consistently in the cricket field.  There have been several attempts to measure the drag and side force of the cricket ball experimentally. However, difference in experimental observations and actual practice lies in the method used to hold the ball during the measurement. However, we do not have a widely accepted theory to explain the reverse swing of a rough cricket ball. There has been no study of the reverse swing of a cricket ball swing using the modern tool of computational fluid dynamics (CFD), which was the main focus of this work.

Objectives

  1. Develop CFD (ANSYS FLUENT) models for standard bluff bodies for a range of fluid velocities and validate them with existing experimental studies.
  2. Develop CFD models for the cricket ball by using experimentally measured surface roughness values, top spin (Magnus effect) and simulate normal and swing of the cricket ball
  3. Predict trajectories of the ball thrown with known parameters.

Major Results

In case of normal swing, where the ball is considered smooth, the only factor responsible for the differences in fluid flow on either side of the ball was the inclined stitches of the ball.

Normal swing of the Cricket ball when seen from the top using vorticity contours. The ball moves from right to left (Air flows from left to right).

The stitches facing the ball trip the flow on the bottom side to turbulence, while the top side sees a regular laminar separation. This ball will turn along the direction of the seam (left when seen from the bowlers point of view and toward bottom in this figure).

In case of reverse swing, the rough surface on the top becomes a competition to the side with the seam facing the flow, and both the factors are trying to trip the flow to turbulence. However, for the reverse swing to occur, the top side with rough surface should undergo the transition as shown in Figure. In this case, the ball will turn right as seen by the bowler (opposite to the seam direction therefore reverse swing) and toward top in the figure.

Reverse swing of the cricket ball.

If side forces in all cases are measured, trajectories of the Cricket ball can be plotted as shown in Figure below.

Cricket ball trajectories with normal and reverse swing modes. The difference between the side force for a smooth sphere and that with a cricket ball is evident.

More information can be found here – Pahinkar D. G. and J. Srinivasan (2010), “Simulation of Reverse Swing of the Cricket Ball”, International Journal of Sports Science and Engineering, V04, No.1, pp. 053-064. http://www.worldacademicunion.com/journal/SSCI/sscivol04no01paper07.pdf