From the pitch to the lab and back: Making science work in cricket
A brief guide on using scientific discovery in sport, and a few things I have learnt as a scientist working in cricket
This week, the first paper from my PhD research was published. For those of you not familiar with the world of academia, this is really good news. It means the work we have done has been reviewed by other experts, and is available for everyone to read. If you are interested in spending an afternoon learning about the history of swing research, you can find it here.
The paper is what is known as a ‘review paper’, where we collate all of the previous work on a subject, and discuss areas that are well understood, versus those where there are gaps in the collective knowledge. The ‘reassessment’ part of the title is where we update the models for swing, redefine some technical language, and show experimental results we have collected ourselves.
A lot of the work appears in the Literature Review in my thesis, but a special mention has to go to Sam for his tireless work in finding all the papers I had missed, rewriting the prose, and drawing the excellent cartoons. The paper is long, but comprehensive, and sets the foundation for our future publications on the subject; we hope there will be several more!
So, what is the purpose of this blog post other than blatant self-promotion? Well I thought I would use the contents of the paper as a mechanism to discuss the intersection of science and cricket. This will allow me to highlight cool bits of the article for those of you less keen to read the whole thing, and hopefully showcase how understanding science can benefit sport. So, let’s dive in.
Science and cricket give two answers to the same question
For scientists, the real question isn’t ‘How do you get a ball to swing?’, rather it is ‘Why doesn’t the ball always swing in every match?’
Since finishing my PhD in late 2022, I have been trying to make a career of being a scientist in cricket. Thankfully, as my PhD was funded by the England and Wales Cricket Board (ECB), I was given the opportunity to work part-time in their Performance Analysis department, which I still do today.
Part of my role is to provide scientific context to some of cricket’s ‘lore’, and use this to help make smarter and more reliable decisions across English cricket (basically I am a glorified myth-buster). Some of this is swing bowling related, but I also work on pitch behaviour, as well as other data analysis.
In the world of aerodynamics, we use a wind tunnel to replicate bowling, as blowing air past the ball recreates similar conditions to the ball flying at high speed through the air in a match. Now it turns out, if you put a new cricket ball in a wind tunnel, angle the seam and turn it up to 80mph, it will almost always swing. You can set your watch to it.
This idea of ‘questioning what we think we know’ brings me to the opening of the paper, and the first figure I am going to show. For scientists, the real question isn’t ‘How do you get a ball to swing?’, rather it is ‘Why doesn’t the ball always swing in every match?’
The figure above is from our paper, and shows the percentage of deliveries which swing in Test cricket. The measure of swing used, Cs, is akin to the curvature of the delivery which I’ve talked about previous: larger values = more swing.
It is clear from the left hand figure, which only contains deliveries from opening overs bowled by pace bowlers, that more deliveries don’t swing than do. This highlights the current discrepancy between research and practice. The scientists will tell you it’s easy to swing the ball, and the players will tell you it isn’t!
The lack of impact of scientific research into the cricket world is why questioning what we know is so important. Why aren’t the players able to recreate the type of swing you see in a wind tunnel? Are scientists actually simulating swing bowling correctly?
To answer these questions, we need a full picture of both scientific knowledge, and what happens in professional cricket. Our paper lays the scientific groundwork, and in my current role I try to merge this with cricket by talking to players and coaches, where I can both ask and answer questions, and everyone comes away with new ideas.
The seam is not a rudder
Players may not care if the seam is a rudder, a trip, or a vortex generator, but they do care about prolonging conventional swing with the new ball.
The most commonly used word I hear when discussing swing is ‘rudder’. Now I wouldn’t be a very good scientist if I wasn’t at least a bit pedantic about my specialism, so I’ll say it here: if you use that analogy, either you don’t fully understand how swing works, or you don’t know what a rudder is!
While dunking on non-specialists isn’t always the most honourable of pastimes, it does raise an important point on how the seam actually causes swing. This has often been a topic of discussion in scientific circles and one which I think we have cracked. The basic mechanism is determined by how the air flow separates from either side of the ball. You can find a refresher in my first ever blog post.
Just for clarity, a rudder works by physically deflecting the water behind a boat, causing the boat to move in the opposite direction. While the seam does change the air flow past a ball, it does so by altering the separation point on one side of the ball, creating asymmetric flow, and moving the ball sideways.
However, the exact way in which this happens is also important. What we found is that the seam works as a vortex generator, rather than as a trip to turbulence. These technical terms may not mean much to you, so I’ll show you some cool infrared (IR) images instead.
These IR images show you how the air flows around each side of the ball by highlighting where heat is transferred from the surface. Bright white patches indicate where the air is trapped close to the surface. Dark patches indicate turbulent flow, where the heat is transferred more effectively from the surface of the ball. The red lines on the diagram indicate where the flow separates on either side, and the speed of the air flow is increasing from left to right for each vertical pair.
The most important thing to note is the the presence of streaks (‘coherent streamwise structures’) on the front half of the ball in the bottom row of images, and the white separation bubble that is labelled. While bowlers may not really care about the presence of a bubble compared to fully turbulent flow, it does change how a cricket ball behaves on a pitch, most notably in how ball condition affects swing.
The bubble on the seam side is why you get the most swing when you shine both sides of the ball. The seam acts as a vortex generator, meaning the difference in separation between the two sides is not simply laminar versus turbulence flow. To get the most swing, you want the biggest separation bubble, and for that you need the seam side to be smooth as well. If you are swinging a new ball, roughening the seam side will reduce the size of the bubble, and actually make the ball swing less.
This is an example of a complex scientific phenomenon translating to a simple on-field advantage. Understanding the exact mechanics of the seam can help you reduce inefficiencies in on-field tactics. And while the players may not care if the seam is a rudder, a trip, or a vortex generator, they do care about prolonging conventional swing with the new ball.
Reverse swing is all about the rough side
Infrared imaging also helps us to unlock how reverse swing really works. While the general effect of the ball aging is to make one side rougher, exactly how that impacts swing, specifically reverse, is less clear. So, cut to more IR images of cricket balls that I lovingly degraded to get them to reverse (spoilers: no sandpaper was used in the process).
Here we have 3 balls tested at similar speeds, but with different surface conditions. The first ball, on the left, is new, but stops swinging because the speed is too high. Here you can see a separation bubble on both sides, which is not what you want for swing (I would recommend polishing the logo off this ball to restore a laminar separation on the non-seam side).
The third ball on the right hand side is rough all over. Here, even with the seam angled to one side, there is no difference in air flow between the two sides of the ball, so there is no swing. The rough surface is generating a turbulent boundary layer on both sides, which is not what you want for swing (I hope you are seeing the pattern here).
The middle ball, however, is reversing a lot. This ball is new on one side, and rough on the other; unrealistic for a match, I know, but a useful case study. If you compare it to the left hand ball, you can see it is the rough side that has caused the change in separation, becoming turbulent and causing an earlier separation on the seam side, which is exactly what you want for swing.
Practically, this tells you that no matter how much you polish the shiny side, if your rough side isn’t rough, the ball won’t reverse. Now there are more nuances around reverse swing not covered by this example (we’ve saved them for the next paper), but this fundamental idea is really useful when managing an old ball.
These ideas around ball management are pounced upon by players and coaches. You give them a simple idea (ball management is inefficient), briefly explain the science behind it (creating different airflow on either side) and give them a method to try (shine both sides). If you are engaging, answer questions honestly, and take feedback on board, then even the dullest science can become gold dust when combined with a player’s experience.
Understanding differences between the lab and the pitch
I’m often asked how realistic can a lab experiment actually be? In this section I’ll consider this question and discuss two aspects which we consider carefully in the paper. The first is the importance of on-field conditions, often overlooked in applying science to cricket. Section 5 of our paper looks at realistic boundary conditions, which include the effects of weather and the rotation of the ball. The aim was to look at all of the assumptions in place for wind tunnel tests, and outline where real cricket differs, in an attempt to resolve the discrepancies I mentioned earlier.
The plots below show the changing atmospheric conditions I measured at cricket grounds during lockdown. You can see the variety between days and venues, but also from hour to hour. Understanding how this fits into the context of your experiments is crucial for applying any results you find.
The main takeaway is that swing is statistical, not deterministic. For any given delivery, there are many variables, all of which fluctuate slightly, from seam angle to the wind speed across the pitch. Therefore, we shouldn’t look to say exactly what is going to happen, rather what is most likely to happen. This is a trap that pundits often fall into, assuming the outcome of every ball is pre-determined based on a small number of variables. In reality, if any one of the bowler, the ball, or the conditions aren’t perfect for a given delivery, the chances are it won’t swing.
The second point of this section is to remind you that cricket is a game played by humans. While this is exceedingly obvious, it is also incredibly important to remember when trying to apply science and data to sport. Humans aren’t perfectly repeatable. Sometimes they won’t believe the science. Other times they will do the opposite to try and prove you wrong. These are all variables you need to consider.
The psychology of an athlete often trumps any science you can present to them. Even after 4 years of study and a year working in cricket, if a player has a gut feeling that the ball won’t swing, they won’t try and swing it, no matter how perfect the science says the conditions are. This is where humility and soft skills play a massive role, and something I am learning all the time; I’m sure there are at least 3 more papers in that alone!
Get yourself in, then go big
This paper marks the culmination of a lot of work, but is also only the beginning. There are several more interesting articles we are looking to write, and hope to continue moving the field of cricket aerodynamics forward for others to follow. Again, Sam has been instrumental in keeping our academic pursuits moving since my PhD has finished, especially as I often push them to the bottom of my to-do list! Here are his excellent diagrams showing a summary of the reverse swing IR images, looking top-down on a ball travelling right-to-left.
I also feel like there is a long way to go for scientists and data analysts in cricket. I’ve had the privilege to work with lots of amazing people at the ECB, the first-class counties and other parts of cricket science and technology, but I still have lots to learn about how to really influence the sport at any level. At any rate, I hope that the few people who read this blog will have been prompted to think a bit differently about the game.
At the same time, it is important that we keep evolving as scientists too, and always question our assumptions. One of the core principles of the Whittle Laboratory is to solve real world problems, rather than doing science for the sake of it. For sport, that means constantly considering the practical application of our work, always looking for new areas to investigate, and taking on feedback from players, coaches and analysts.
That’s all from me for now, please go and read the full paper if you are interested, and send me any questions you have. I’ll be back at the start of the English Test summer to discuss some more topical storylines. Go well!