I was recently contacted by Dr. James A Scobie, a Lecturer at the University of Bath in England. He straightened me out on a few points about Cricket from my previous post. Specifically, he says:
- In cricket the bowler isn’t allowed to deliberately alter the ball. It’s true that the ball will wear as the game progresses, but this is one of the many nuances that causes the game to evolve over the course of play. (NOTE, I am told bowlers do polish the ball on their pant leg and occasionally deliberately scuff it).
- It’s also not true that the entire match is played with two balls. This depends on the form of the game being played but generally the ball is replaced after 80 overs (480 deliveries) or at the start of each innings (in Test cricket each team bats for two innings). Deciding when to take the new ball, when it’s available, can be an important decision for the captain to make.
- The seams of a cricket ball are raised – the picture of the ball you have shown has been cropped to a perfect circle. The attached picture shows the raised seams much more clearly.
- The primary seam goes around the hemisphere but high quality cricket balls are actually made from four pieces of leather so there is a quarter seam too (you can see these in the pictures of the balls you have posted).
With the exception of my italicized text, I take James’ points. The point about how raised the seams are is quite important and he’s right, the photo I used minimized that.
James also pointed me to a very interesting article he has published on cricket ball aerodynamics (“Fluid dynamics of cricket ball swing” Journal of Sports Technology and Engineering, by James A Scobie, Simon G Pickering, Darryl P Almond and Gary D Lock, Journal of Sports Engineering and Technology, 2012.)
His work used a very clever technique to show regions of the ball where the boundary layer is separated. He found that, for a non-spinning ball in a wind tunnel, a “laminar separation bubble” forms on the smooth side of the ball, resulting in the “reverse swing” phenomenon that Mehta claimed came from a different effect.
Laminar separation bubbles are a very interesting effect, most often found on low-speed wings. They occur when laminar flow separates from a curved surface leading to transition to turbulent flow. In some cases, the turbulent flow entrains external air so well that the flow reattaches, as shown in the sketch below (not my sketch). For a wing, this enhances the lift, since the wing appears to have more camber. For a ball, a laminar separation bubble will lead to a forced toward the side with the bubble.
So, while I have poo-pooed wind tunnel studies, there is no doubt that finding a phenomenon like this is potentially very useful. I’ve never seen anything like this on a baseball. As always, I’ll say I have never tested pristine MLB balls (but will soon). And I’ll be watching for laminar separation bubbles thanks to James and his co-workers.
More on CFD
A colleague of mine, who is very experienced with CFD, has been messing around with a model of flow over a baseball. He spent some time trying to get an answer form two different popular codes (Open Foam and Star) and has been dismayed that very small changes in the model (e.g. turbulence models, cell refinement) lead to radically different answers. I’ve been trying to talk him into writing an article entitled “The futility of modeling baseballs with CFD” but no dice.