Physicists design an accurate laser method to measure the drag of a baseball

Enlarge / Scientists have developed a new method to determine the aerodynamics of baseballs in free flight.

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Baseball has long been a popular research topic for physicists, largely because of the complex aerodynamics of a baseball in flight. Traditionally, scientists have relied on wind tunnel experiments to measure key properties such as speed, spin, lift and drag, but this approach cannot accurately capture minute changes in drag. And even small changes in drag can have big effects, like a dramatic increase in the number of circuits.

That’s why two physicists developed a laser-guided velocity measurement system to measure the change in velocity of a baseball in midair, then used that measurement to calculate acceleration, the different forces acting on the ball, as well as 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 football.

Any moving ball leaves an air trail as it moves; the inevitable drag slows the ball. Ball trajectory is affected by diameter and velocity and by tiny irregularities on the surface. Baseballs are not completely smooth; they have figure-eight seams. These points are bumpy enough to affect the airflow around the baseball as it is thrown toward home plate. When a baseball moves, it creates a whirlwind of air around it, commonly known as the Magnus Effect. Raised seams move the air around the ball, creating areas of high pressure in various places (depending on the type of pitch) that can cause deviations in its trajectory.

Lyman Briggs conducted wind tunnel experiments in the 1940s to determine if (and why) a curveball really bends.
Enlarge / Lyman Briggs conducted wind tunnel experiments in the 1940s to determine if (and why) a curveball really bends.

NIST

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 the actual curve of a curveball. Initially, he enlisted the help of the Washington Senators pitching staff at Griffith Stadium to measure the spin of a pitched ball; the idea was to determine to what extent the curve of a baseball depends on its spin and speed.

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

Since then, physicists have enthusiastically studied various aspects of baseball. For example, in 2006, mathematicians studied the effects of raising Major League Baseball (MLB) slugging percentages (the total number of bases divided by the number of at-bats) by constructing a statistical model . They found that the slugging percentage at Coors Field in Denver, Colorado (aka “Mile-High City”), was about 9.2% higher than at average altitudes (between 500 and 1,100 feet) and 12.5% ​​higher than at altitudes below 500 feet. . No wonder the stadium has a reputation for being home-run friendly.

In 2018, we reported on a Utah State University study to explain the unexpected turn of the fastball in experiments using Little League baseballs. USU scientists fired the bullets one by one through a smoke-filled chamber. Two red sensors picked up the bullets as they passed, firing off lasers that acted like flashes. They then used particle image velocimetry to calculate the airflow at any given location around the ball.

Airspeed measurement system at Washington State University that measures drag.
Enlarge / Airspeed measurement system at Washington State University that measures drag.

Washington State University

The current study was inspired by a recent unusual change in home run percentages in MLB. Home runs are usually tracked by a metric known as HR/BB (home runs per batted ball). According to the authors, from 1960 to 2015, the HR/BB ratio generally fell between 0.03 and 0.04. That changed drastically in the 2015 season when the HR to BB ratio rose rapidly, reaching 0.053 in 2017. It was alarming enough that MLB actually commissioned a panel to investigate. The panel released its report in 2018, concluding that a slight decrease in baseball aerodynamic drag was the culprit.

This, in turn, has focused attention in recent years on the development of better methods for measuring the drag of a baseball in flight. As we have already pointed out, the drag coefficient describes how well the flowing air “sticks” to the surface of the ball. 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 bullet reaches a critical speed threshold, it experiences a so-called “drag crisis”. The wake suddenly narrows and the drag drops as the airflow suddenly changes from laminar (smooth) to turbulent.

These types of experiments have usually been performed in wind tunnels. But this method has pronounced shortcomings in the accurate measurement of drag. “You have to hold the balloon somehow and that means there will always be imperfections when you use a wind tunnel to measure drag,” said co-author Lloyd Smith of the University of Washington State.

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