Why I don’t want to talk about cricket balls, wind tunnel studies or CFD Post #18

This may sound cranky. I love hearing ideas from others, but some I am rather dismissive of, and I want to explain why.

Cricket balls

Let’s talk about the 40 ways cricket balls are nothing like baseballs.

  • The leather of a cricket ball is covered in shellac which is easily scratched. The ball hits the ground every time it is bowled (is that the right verb?). The bowler is allowed deliberately alter the ball. The entire match is played with 2 balls, one for each team. So, except at the start, only God and the bowler knows what that ball looks like.
  • The seams of a cricket ball are not raised. They may be “roughness” but they are not bumps.
  • The seams go around the hemisphere only.
  • OK, that’s only three, but they are important.

So, while it is smart to look to other sports balls for ideas, it’s a mistake to look for a one-to-one correspondence with cricket phenomena. I hear it frequently suggested that seams upstream of the hemisphere line (or equator) of the ball cause the flow to become turbulent on the backside. This notion seems to come from Mehta’s description of cricket ball swing. Below is a flow vis photo from his article “Sports Ball Aerodynamics”.

I have seen many similar photos and they have common features:

  1. The boundary layer separates near the hemisphere line
  2. The separated boundary layer rolls up into periodic vortices

Mehta claims that cricket balls may “swing” when the seam is near the front but on one side of the ball, causing the flow over that side of the ball to be turbulent while the other side is laminar, as show in his flow vis photo below from the same article.

Note that 17 m/s is 38 mph. That’s not very relevant to baseball. But, we have made measurements close to that speed (45 mph), and guess what? The separation was laminar–actually on both sides. The PIV data below is a fastball (note the Fergie Jenkins signature!). It’s easy to see the same periodic vortex structure visible in the smooth sphere flow vis from Mehta’s Figure 2.5.

So, in my view, laminar separation should come with ring vortices in the wake. I have never seen this in a baseball at baseball speeds. Some of our knuckleball data (70 mph) does have separation off the hemisphere line, similar to the top of the ball in Mehta’s Figure 3.2. I don’t think this is laminar, it’s just less turbulent.

Take a look at the PIV data set below:

The ball is smooth everywhere, yet the flow remains attached well beyond the hemisphere and is clearly turbulent. Baseballs live in the turbulent regime. I won’t speak for cricket balls. I’ve only ever seen one in my life.

Wind tunnel studies

I am often asked why I don’t do some of this work in a wind tunnel. We have a brand new ELD 2′ X 2′ wind tunnel at USU, and it would be an easier measurement. Repeating conditions would be simple.

BUT, I have several concerns about this approach. Mostly, they have to do with boundary layer separation, which is the whole game here.

  1. Wind tunnel walls alter the pressure field around the object. The ball blocks part of the test section, and that causes the flow around the ball to have to accelerate more than in real life. This alters the pressure distribution which, in turn, alters boundary layer separation and wake formation.
  2. The ball has to be held in place and probably spun at 2000 rpm or more. Any way this is done will cause a disturbance and may alter boundary layer separation. Note Mehta used a very stout “sting” on one side of the ball.

Maybe these issues have not affected previous studies, but there is no way to know for sure. I see a role for wind tunnels (like CFD, as described below) but they should not be our primary source of information.

Computational Fluid Dynamics (CFD)

I have a complicated opinion of CFD. It’s an incredibly powerful tool and we have a lot of good things to thank for it. But, I find most lay-persons are surprised to learn that it has major flaws, even today. In fact, no one believes these flaws will be fixed in my lifetime. Horace Lamb, a famous Fluid Dynamiscist said in 1932 “I am an old man now, and when I die and go to Heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics and the other is the turbulent motion of fluids. And about the former I am really rather optimistic.” It hasn’t gotten much better since then. Here’s a good article on the challenge from a few years ago.

One area that CFD is particularly bad is boundary layer separation from smooth curved objects. And that is exactly what we are interested in.

All that said, I often think of questions that can, at least qualitatively, be answered by CFD. For example, does the point where the pressure gradient switch signs shift from the hemisphere line if the ball is spinning? But any attempt to compute drag from CFD, or any other quantitative study, is very unlikely to be successful.

 

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