Baseball physics: Measure drag, ball direction, speed,... with wind tunnels to lasers

The newly invented laser-based measurement tests appear to be more realistic and accurate than traditional wind tunnel drag measurements from scientists.

Baseball has long been a popular subject of study among physicists, largely due to the complex aerodynamics of a baseball in flight. Scientists have traditionally relied on wind tunnel experiments measuring key properties such as speed, rotation, lift, and drag, but this method cannot accurately capture small changes in force. barrier. And even small changes in drag can have big effects - like a dramatic increase in home runs.

That's why physicists have developed a laser-guided tachometer system to measure the change in speed of a baseball while airborne and then use that measurement to calculate the acceleration. speed, different forces acting on the ball, lift and drag. The new approach was presented in the newly published paper and they suggest that it could also be used for other ball sports such as cricket and football.

Any moving ball will leave a trail in the air as it travels, air resistance clearly having an effect on ball speed in flight. A ball's trajectory is affected by its diameter and speed, and by various features on its surface. Baseball surfaces aren't completely smooth either, they often have stitching in an 8 pattern, those stitches are bumpy enough to affect the airflow around the baseball when it's thrown on the field. When a baseball moves, it creates a swirling air around it, commonly known as the Magnus effect. The raised seams stir up the air around the ball, creating zones of high pressure in different locations, and also depending on the pitch type can cause misalignment of its trajectory.

Lyman Briggs conducted experiments with baseball in wind tunnels in the 1940s

Modern baseball physics is said to have begun with the efforts of a physicist named Lyman Briggs in the 1940s. Briggs was a baseball fan who was intrigued by the question of whether a ball was real. curvature or not. Initially, he enlisted the help of athletes at Griffith Stadium to measure the spin of a handball with the idea of ​​​​determining the curve of a baseball depending on its spin and speed.

Briggs monitored wind tunnel experiments at the Office of National Standards (now the National Institute of Standards and Technology) to make even more precise measurements because there he could control most of the time. all variables. He discovered that spin rather than speed was the key factor in a ball's curvature, a ball with a curve that could drop as much as 17.5 inches as it moved from the pitcher's position to the pitcher's position. home yard.

Physicists then enthusiastically studied various aspects of baseball ever since. For example, in 2006, mathematicians studied the effect of altitude on the miss rate of MLB - Major League Baseball (the total number of balls passing through the golf course divided by the number of hits) by constructing a statistical model. They found that the slip rate at Coors Field in Denver, Colorado was about 9.2% higher than average altitudes 152 meters to 335 meters, and 12.5% ​​higher than average altitudes. 152 meters. What is surprising is that this stadium is famous for being home-friendly.

In 2018, researchers reported on a Wyoming State University study to explain the sudden swing of the ball in experiments using Little League baseball. USU scientists fired balloons one by one through a chamber filled with smoke. The red sensors detect the balls as they pass, triggering lasers that act as flash bulbs. They then used the particle imaging velocity to calculate the airflow at any given point around the ball.

Tachometer system

The present study was inspired by the unusually recent variation in home operation percentages in MLB. Home runs are often tracked by a metric known as HR/BB (home runs per ball hit). According to the authors, from 1960 to 2015, the HR/BB ratio in general fell between 3 and 4%. But that changed dramatically in the 2015 season when the HR/BB ratio rose rapidly, hitting 0.053 in 2017. The fact that MLB actually commissioned an investigative panel raised alarm bells. The panel released its report in 2018, concluding that the small drop in aerodynamic drag on the baseball was the "culprit".

This, in turn, has led researchers to focus more attention in recent years on developing better methods for measuring the drag of a baseball in its flight. The drag coefficient describes the amount of air flowing "sticky" to the surface of the ball, where the faster the ball moves, the less "sticky" the ball is. Usually the ball will have more turbulence and higher drag at slow speeds. If the ball hits the critical speed threshold, it will experience what is known as a "crisis of drag". Air turbulence decreases dramatically and drag decreases sharply as the airflow changes from uniform to mixed.

These types of experiments are usually done in wind tunnels. But this method has certain limitations in accurately measuring drag. "You have to hold the ball in a certain way and that means there are always some imperfections when using wind tunnels to measure drag"

That's because the rack is used to keep the ball stationary, measuring aerodynamic forces that can alter the airflow as well as the associated aerodynamics. The scientists tried to solve this problem by mounting the brackets to the bottom, minimizing interference effects. However, drag is also affected by the direction of the ball. Tunneling experiments to study the ball's orientation required the ball to be supported on its axis of rotation, close to the path separation point. The authors write: 'It is unclear how these attachments affect the position of the path and drag separation measurements.

Smith and his co-author, Andrea Sciacchitano of Delft University of Technology in the Netherlands, set out to find a better method for measuring the resistance of a baseball in its free flight. They note that these first measurements were made during the 1996 Olympics, using two 120fps cameras to track the balls. In 2006, MLB installed video-based baseball tracking systems in every MLB park. "Systems are primarily intended to monitor pitch speed, field movement, and strike area position, but drag can also be extracted from publicly available matching parameters," the authors said. The authors also added that such measurements are subject to noise and influence of the football field environment.

All the baseballs used in their experiments came from the same manufacturer that supplies MLB: Rawlings Sporting Goods. Each ball has a rubber ball wrapped in 3 layers of wool, and all are upholstered in leather, with handcrafted seams. Material properties are carefully controlled to ensure uniformity in weight, size, hardness, elasticity, . to the color of the rubber. A lot of people, including experts, say it's hard to tell the difference between belly skin and back skin on a ball, but it's really two different skin types.

Graph comparing the MLB drag of non-spinning balls.

The researchers used a pitcher to shoot a baseball, placing two speed sensors to track the ball as it passed between screens of light about 0.4 meters apart. They used a cradle attached to the end of the pitcher's pneumatic piston to impart spin to the balls. Spin speed increases with pedestal length, they can control it by adjusting the ball's position in the rack before firing. They measure the direction of the ball by tracking three markers placed on each ball. All trajectories of the ball are captured by high-speed video. These experiments were conducted at WSU.

Sciacchitano then performed particle imaging velocity measurements in the Laboratory of Aerodynamics at TU Delft. The air in the laboratory is sprinkled with micrometer-sized water glycol particles, which are illuminated with a laser. The baseballs are thrown without any spin and their trajectories are also captured on high-speed video. This allowed the team to visualize the turbulence in the air of baseballs that don't spin in free flight.

Smith and Sciacchitano's method produced drag values ​​that were consistent with radar and video measurements, but significantly lower than reported from other wind tunnel measurements. They conclude that the free-fly tests provide a more realistic reconstruction of the flow physics around a moving baseball, suggesting their use to complement future wind tunnel experiments.

Smith said: 'We had to go back and look at what could be done to make this more precise, and we did. It's not that simple to do and we spend a lot of time calibrating every day just to make sure the sensors are telling us what we think, but we believe the drag measurements working and ongoing so that it can signal if there is a change. Very small differences can make a big difference'.