Physicists devise precise laser-based method to measure a baseball’s drag

Baseball has long been a popular topic of research for physicists, largely because of the complex aerodynamics of a baseball in flight. Traditionally, scientists relied upon wind tunnel experiments to measure key properties like speed, spin, lift, and drag, but this approach can’t quite precisely capture tiny shifts in drag. And even small shifts in drag can have large effects—like a dramatic increase in the number of home runs.

That’s why two physicists have developed a laser-guided speed measurement system to measure the change in speed of a baseball mid-flight and then used that measurement to calculate the acceleration, the various forces acting on the ball, and the lift and drag. They described their approach in a recent paper published in the journal Applied Sciences and suggested it could also be used for other ball sports like cricket and soccer.

Any moving ball leaves a trail of air as it travels; the inevitable drag slows the ball down. The ball’s trajectory is affected by diameter and speed and by tiny irregularities on the surface. Baseballs are not completely smooth; they have stitching in a figure-eight pattern. Those stitches are bumpy enough to affect the airflow around the baseball as it’s thrown toward home plate. As a baseball moves it creates a whirlpool of air around it, commonly known as the Magnus effect. The raised seams churn the air around the ball, creating high-pressure zones in various locations (depending on the pitch type) that can cause deviations in its trajectory.

Modern baseball physics arguably began with the efforts of a physicist named Lyman Briggs in the 1940s. Briggs was a baseball fan who was intrigued by whether a curveball actually curves. Initially, he enlisted the aid of the Washington Senators pitching staff at Griffith Stadium to measure the spin of a pitched ball; the idea was to determine how much the curve of a baseball depends on its spin and speed.

Briggs followed up with wind tunnel experiments at the National Bureau of Standards (now the National Institute of Standards and Technology) to make even more precise measurements since he could control most variables. He found that spin rather than speed was the key factor in causing a pitched ball to curve and that a curveball could dip up to 17.5 inches as it travels from the pitcher’s mound to home plate. 

Physicists have been enthusiastically studying various aspects of baseballs ever since. For instance, in 2006, mathematicians studied the effects of elevation in Major League Baseball (MLB) slugging percentages (the total number of bases divided by the number of at-bats) by building a statistical model. They found that the slugging percentage at Coors Field in Denver, Colorado (aka the “Mile-High City”), was about 9.2 percent higher than at middle elevations (between 500 and 1,100 feet) and 12.5 percent higher than at elevations below 500 feet. Small wonder the stadium has a reputation of being home-run friendly.

In 2018, we reported on a Utah State University study to explain the fastball’s unexpected twist in experiments using Little League baseballs. The USU scientists fired the balls one by one through a smoke-filled chamber. Two red sensors detected the balls as they zoomed past, triggering lasers that acted as flashbulbs. They then used particle image velocimetry to calculate airflow at any given spot around the ball.

The current study was inspired by an unusual recent shift in home run percentages in the MLB. Home runs are typically tracked by a metric known as the HR/BB (home runs per batted ball). According to the authors, from 1960 until 2015, the HR/BB ratio generally fell between 0.03 to 0.04. That changed dramatically in the 2015 season when the HR/BB ratio increased rapidly, hitting 0.053 in 2017. It was sufficiently alarming that the MLB actually commissioned a panel to investigate. The panel issued its report in 2018, concluding that a small decrease in the aerodynamic drag on the baseballs was the culprit.

That, in turn, has focused attention in recent years on developing better methods for measuring the drag on a baseball in flight. As we’ve previously reported, the drag coefficient describes how much the flowing air “sticks” to the ball’s surface. The faster the ball moves, the less “sticky” the ball becomes. Typically wakes are larger, and drags are higher, at slow speeds. But if the ball hits a critical speed threshold, it experiences a so-called “drag crisis.” The wake shrinks suddenly, and the drag plummets as the airflow shifts abruptly from laminar (smooth) to turbulent.

These kinds of experiments have typically been done in wind tunnels. But that method has some pronounced shortcomings in precisely measuring drag. “You have to hold on to the ball in some way and that means there’s always going to be some imperfection when you’re using a wind tunnel to measure drag,” said co-author Lloyd Smith of Washington State University.

https://arstechnica.com/?p=1846726