{"id":4903,"date":"2019-07-31T20:30:53","date_gmt":"2019-07-31T09:30:53","guid":{"rendered":"https:\/\/www.computationalfluiddynamics.com.au\/?p=2953"},"modified":"2019-07-31T20:30:53","modified_gmt":"2019-07-31T09:30:53","slug":"how-can-cfd-help-us-better-understand-the-physics-of-reverse-swing-bowling","status":"publish","type":"post","link":"https:\/\/www.leapaust.com.au\/blog\/cfd\/how-can-cfd-help-us-better-understand-the-physics-of-reverse-swing-bowling\/","title":{"rendered":"How can CFD help us better understand the physics of reverse-swing bowling?"},"content":{"rendered":"
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\u2026and can Australia harness reverse swing effectively in the 2019 Ashes?<\/em><\/strong><\/h3>\n

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In the world of cricket, reverse swing bowling is a much talked-about topic but often misunderstood. With the much anticipated 2019 Ashes cricket series between Australia and England beginning this week, and likely bowling conditions (and with changes to the seams on the new Duke balls) expected to be conducive to reverse-swing, at LEAP we are particularly excited to see which team can best harness reverse-swing to optimal effect.<\/p>\n

The aerodynamics of a cricket ball has been well studied over many years \u2013 among many papers, R.D. Mehta provides an excellent summary of the \u2018Fluid Mechanics of Cricket Ball Swing\u2019 at this link<\/a> as well as a growing body of work using computational fluid dynamics (CFD) software<\/a> such as ANSYS.<\/p>\n

In a nutshell, swing bowling is the ability for medium and fast pace bowlers to make a cricket ball deviate laterally in fight to either beat the batsman\u2019s bat completely or move the ball enough to only contact the edge of the bat (which can then be caught by the wicketkeeper or within the slips cordon).\u00a0 Conventional swing bowling is most effective while the ball is still relatively new and the seam remains prominent (typically within the first 20-30 overs) \u2013 an angled seam trips the airflow on that side to form a turbulent boundary layer, while on the other side the flow remains laminar\u2013 this leads to a difference in flow separation points on each side, a difference in pressure and thus a lateral side force.\u00a0 Key variables include the prominence of the seam, the angle of the seam to the flow and (over time) roughing of the ball on one side (which adds further to the turbulence and the uneven separation points \/ pressure on opposing sides).\u00a0 The net effect is that the trajectory of swing is towards the seam direction (and towards the roughest side of the ball).<\/p>\n

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Historically, once the ball was over 30 overs old, it softened and the seam flattened, providing more attractive conditions for a batsman (until the ball is then replaced after 80 overs). In more recent times, entire cricket series have been decided by the ability of a team to get an older ball to reverse-swing \u2013 where a bowler can approach with the same bowling action but get the ball to move in completely opposite lateral direction to which the batsman expects it \u2013 and in some cases this has made the period where a batsman is facing an older roughed-up ball to be even more menacing than facing a brand new ball.\u00a0 So how does reverse swing work exactly?<\/p>\n

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When using an older ball that is in suitable condition to allow reverse swing, the lateral movement will be in the opposite direction to that expected by the batsman (for a typical seam position used in conventional swing bowling).\u00a0 In this case, the important physics is that you will now have sides of the ball which are rough and very<\/u> rough \u2013 so there is no longer any significant laminar flow, but instead turbulent flow on both<\/u> sides of the ball.\u00a0 The seam again trips the air into a turbulent state, which is amplified further by the very rough surface on that side of the ball and flow separation occurs even earlier.\u00a0 On the other side (merely \u2018rough\u2019 side), the turbulent air has a relatively more delayed separation point, which creates the uneven separation and uneven pressure to a side-force and the conditions for swing – but this time the lateral force is acting in the opposite direction to that seen in conventional swing (away from the seam). Thus, in reverse swing the trajectory of swing is now away from the seam direction (and away from the roughest side of the ball).<\/p>\n

So in summary, the differences between conventional and reverse swing lie in differences in flow separation on each side of the ball (for the same seam position) as the ball deteriorates during the game:<\/p>\n

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In practice, this means that in the right conditions and with a suitable scuffed-up ball (due to natural \u2018wear and tear\u2019 only<\/u> of course!), a fast bowler can bamboozle the batsman by delivering a series of balls exhibiting more subtle conventional swing (typically these are gentle outswingers to lull the batsman into a false sense of security) which are then followed (without any warning or visible deviation in bowling action) by an almost unplayable ball that reverse swings aggressively in the opposite direction (typically an in-swinger attacking the stumps).<\/p>\n

Here is one example (painful as it may be for Australian cricket fans):<\/p>\n