Science of Cricket - Genuine Reverse Swing

The Science of Cricket series continues with the second type of swing bowling, reverse swing. This series will gradually increase in complexity, so you can learn more and understand better by reading them in order. The first in the series is The Science of Cricket - Conventional Swing. Despite common misconceptions, reverse swing is not bowled with a straight seam. That form of swing bowling is contrast swing, to be discussed in the next post. True reverse swing is performed with a similar action to conventional swing but with the rough side at the front of the ball. The seam is still angled, but in this case away from where the bowler wants it to go.

Again, we will start with another diagram from Mehta, who is the media's go-to man when it comes to swing science (and I can't really be bothered drawing my own diagrams, when he has the basic flow ideas right).
So again, we have the ball flowing through a fluid (air). But on this occassion, the ball has the rough side oriented towards the flow. The ball is bowled at a combination of higher speed and/or more roughness at the front of the ball. This causes the boundary layers (layers of fluid near the surface of the ball) at the top and the bottom of the ball to transition to turbulence (irregular, chaotic flow) before the flow hits the seam. In this case, the seam cannot kick the flow into turbulence as it does in conventional swing bowling, as the flow is already turbulent. So the seam strips away the base of the boundary layer (visualise it as a thin stream of fluid flowing over the ball), leaving a thinner boundary layer. Now as discussed in the conventional swing article, any
asymmetry in flow will produce a difference in the separation point of the boundary layer. In this case, as the boundary layer over the top is thinner and has less energy holding it to the ball, it will separate more easily. From this point, the macroscopic consequences of this are a mirror image of conventional swing, with the fluid flowing over the top side of the ball exerting more pressure once the boundary layer has separated. This moves the ball towards the bottom, swinging the ball away from where the seam is directed.

Reverse swing appears to be later than conventional, but paradoxially, this is because it begins earlier. The only sideways (technically perpendicular to the direction of travel) forces which will act on the ball are those provided by swing, assuming there is no sideways spin on the ball, and any backspin is minimal (we'll get into the effects of that later, in the article on spin "drift" and dip). These forces keep working over time, and so the sideways speed keeps increasing all the way to the keeper. In most (elite level at least) cases of conventional swing bowling the ball is not travelling slow enough to generate any swing before it is close to, or even after bouncing. This means the ball can only generate side force during around the last 8 metres or so of its flight (number will vary rapidly betwen bowlers and conditions, but 8 is good for an example). This is why a conventionally swinging ball will often move rapidly after it has passed the batsman. But a reverse swinging ball can move from the moment it leaves the bowler's hand. This enables it to build up to a high sideways speed by the time it reaches the batsman (where it may sometimes have stopped accelerating sideways). For the same reason, all swinging balls will move faster later in their flight.

If the flow state is between reverse and conventional swing (ie the flow just before the seam (top of the diagrams used) is laminar and the flow over the bottom is turbulent then no swing will result, as both sides will have turbulent boundary layers of similar thickness.

In reverse swing, the smoothness of the "smooth side" is less important than the roughness of the rough side. If this smooth side is too rough, the boundary layer over this section after the seam will increase in thickness, negating the advantage gained by trimming the boundary layer with the seam. This will reduce the amount of movement, and combined with lessening seam protrusion as the ball wears is responsible for the ball stopping genuine reverse swing after a time.

The next post in this series will deal with Contrast Swing, aka "backyard reverse swing".

A "technical" aside, for those who want to know more: The flow will transition from laminar to turbulent at a certain "Reynolds number", smaller for a rougher surface. The Reynolds number is (ball speed*distance travelled along ball/kinematic viscosity) where kinematic viscosity is a property of the atmospheric environment. For example, the kinematic viscosity of the atmosphere is generally 8% higher at 1000m above sea level (lower than the Potchefstroom ground in South Africa) than at sea level, meaning a bowler must bowl 8% faster to get the saame reverse swing. But conventional swing can also be achieved at a speed of 8% faster at this altitude than at sea level. The kinematic viscosity varies all over the Earth too, and varies depending on weather conditions (if anyone reading this has links to measurements of kinematic viscosity, density or dynamic viscosity in sunny compared to overcast conditions, or England compared with Australia (to identify why swing bowling was so prevalent in the Ashes series of 2005 compared with 06/07) then I would appreciate the data) So to allow a ball to reverse swing, you can increase your speed, increase the surface roughness or increase your seam angle (although diverting this from the optimal will reduce the amount of movement, some movement is better than none). If you are trying to swing the ball conventionally then you need to keep the flow below transition over the bottom of the ball until separation (generally somewhere around 90 degrees around the ball). This means you may need to reduce your speed or change the atmospheric conditions, but changing the seam angle will not affect the transition speed. It will still affect the amount the ball swings.

Non-technical aside: Wow, that was a long post.

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