An analysis of asymmetrically rough sphere aerodynamics with application to sports balls
Spherical aerodynamics has a rich history of observation and empirical data collection. A sphere’s somewhat unintuitive drag coefficient behavior with respect to Reynolds number is something taught to all aerodynamics undergraduates. NASA has a wonderful page about the Drag of a Sphere that dives into some of the flow physics of why a sphere has lower drag coefficients at higher Reynolds numbers. It also includes some discussion about how the surface roughness of the sphere affects the critical speed at which a given sphere would trip flow and severely alter flow behavior.
In 2015, while taking classes to obtain my Masters degree, I was fascinated by a catcher in the Major League Baseball World Series who was obviously roughing up one side of a baseball before throwing it back to the pitcher. It was an age-old trick used by pitchers to throw pitches with more movement; illegal in the rulebook, but nevertheless was able to fly under the radar during the World Series!
Upon some further digging, sphere aerodynamics happened to be something not heavily yet studied. In theory, it seemed as though the asymmetric pressure causing a scuffed baseball to move was likely due to the Reynolds number being just perfect such that the rough side of the baseball was tripping the flow to be turbulent, and the smooth side was leaving the flow as laminar. But that was only a theory.
I was able to convince my wonderful Advisor that studying the aerodynamic behavior of asymmetrically rough spheres, specifically as applied to sports balls, was a worthwhile-enough endeavor to receive independent study credits. My advisor had a contact at NASA Ames who happened to be the World’s foremost expert on sports ball Aerodynamics who agreed to help me with the research project, so I was off to the races! I also had a contact at the University of Washington’s Kirsten Wind Tunnel who graciously granted me (otherwise impossibly expensive) wind tunnel test time between a couple other higher profile customers, so we were able to develop a theory as well as test it.
The PDF below is the result of the investigation. In summary, scuffed sports balls certainly have the ability to “curve”, and baseballs, especially at Major League Baseball pitch speeds of 90+ miles per hour, are certainly in a sweet spot to dramatically be affected by scuffing.
The study was not perfect – in hindsight, I certainly should have added flow visualization runs to better understand where flow was becoming turbulent and where it was separating on the baseball. That being said, it was a lot of fun, I learned a lot, and I think the investigation yielded some interesting results.
I hope you enjoy reading the investigation, and think the most interesting portion of the paper is the general overview of Sports Ball Aerodynamics in Section I.
Disclaimer: The work above was never published, and my advisor, NASA scientists, and the Kirsten Wind Tunnel in no way endorse the conclusions of this study.