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Why Forkballs Fall
Solution Found with Supercomputers by the Tokyo Institute of Technology, Kyushu University, and Keio University
"Negative Magnus Effect" a Major Factor

2021.06.09

A joint research team comprised of researchers from the Tokyo Institute of Technology, Kyushu University, and Keio University, and led by Professor Takayuki Aoki of the Global Scientific Information and Computing Center (GSIC), Tokyo Institute of Technology, has conducted a detailed computational fluid simulation of how the seams on a baseball rotate, using the Center’s Tsubame 3.0 supercomputer. By doing so, the team has, for the first time, successfully found that a major factor in why slow spinning two seam forkballs fall is the occurrence of a "negative Magnus effect" at a certain range of angles of the seams.

The negative Magnus effect on a two seam ball.
(Copyright: Takayuki Aoki, Tokyo Institute of Technology)

Due to their backspin, forkballs are expected to have a floating trajectory as a result of the Magnus effect, but in fact they float very little, and rather follow a trajectory close to a parabola. The reason was hitherto unknown. In this experiment, the team successfully carried out a simulation by efficiently placing a fine grid close to the ball’s surface, and then using the grid to calculate the motion of the seams correctly.

By carrying out a computational fluid simulation of a two seam forkball, a type of pitch in which the ball revolves slowly with a backspin, the team found that instead of approaching a parabolic trajectory due to the low speed and weak upward lift, the trajectory was pushed downward by a "negative Magnus effect" that occurred when the angle of the seams was within a certain angle (about 1/3 of rotation). The team also learned that the negative Magnus effect did not occur with four seam balls with the same pitch speed and number of rotations.

By carrying out a computational fluid simulation of a two seam forkball, a type of pitch in which the ball revolves slowly with a backspin, the team found that instead of approaching a parabolic trajectory due to the low speed and weak upward lift, the trajectory was pushed downward by a "negative Magnus effect" that occurred when the angle of the seams was within a certain angle (about 1/3 of rotation). The team also learned that the negative Magnus effect did not occur with four seam balls with the same pitch speed and number of rotations.

They further found that, were the pitcher able to know the speed and axis of rotation and the velocity of the ball immediately after releasing it, they would be able to reproduce the same trajectory again with a high degree of accuracy. They also learned that when a two seam and four seam ball were both pitched at a speed of 150 kph and 1,100 RPM (number of rotations per minute), the simple difference in stitch motion translated into a difference of 19 cm in the drop of the ball from the pitcher’s hand.

Pitcher Kodai Senga of the Fukuoka Softbank Hawks is famous for the large drop in his forkball, which is said to be caused not by the two seam spin, but by a gyro motion (in which the spin axis of the ball matches the direction of its trajectory, or the same spin as a rifle bullet). The team ran an additional simulation of the phenomenon and found that the drop in the trajectory was greater than that of a two seam ball rotating at 1,100 RPM

.

A Deeper Understanding of Ball Games in General

According to Aoki, "The airflow behind the ball is unstable and oscillates, and the speculation is that the backward airflow may be affecting the trajectory as well, such as with a knuckleball. We hope that by analyzing this in detail moving forward, we may gain a deeper understanding of ball games in general."

■ Two seams and four seams refers to the seams (revolutions) as seen from the point of view of the pitcher. A two seam spin is when only two seams can be observed for each spin of the ball. Accordingly, a four seam spin is when the pitcher can observe four seams for each spin of the ball. Straight pitches and some others have a four seam spin, and they are known for having a fast spin and straight trajectory that makes them appear to float.

■ Negative Magnus effect: This effect occurs when the speed of the ball (wind speed) increases and the boundary layer (a thin layer on the surface of the object) becomes turbulent because the faster velocity relative to the surface of the ball is more likely to adhere to the surface. This causes the boundary layer to peel off and move upstream on the upper surface, and downstream on the lower surface. Accordingly, the airflow after the ball passes is right-side up, creating a downward force (the negative Magnus effect) at a specific number of rotations.

Due to their backspin, forkballs are expected to have a floating trajectory as a result of the Magnus effect, but in fact they float very little, and rather follow a trajectory close to a parabola. The reason was hitherto unknown. In this experiment, the team successfully carried out a simulation by efficiently placing a fine grid close to the ball’s surface, and then using the grid to calculate the motion of the seams correctly.

By carrying out a computational fluid simulation of a two seam forkball, a type of pitch in which the ball revolves slowly with a backspin, the team found that instead of approaching a parabolic trajectory due to the low speed and weak upward lift, the trajectory was pushed downward by a "negative Magnus effect" that occurred when the angle of the seams was within a certain angle (about 1/3 of rotation). The team also learned that the negative Magnus effect did not occur with four seam balls with the same pitch speed and number of rotations.

They further found that, were the pitcher able to know the speed and axis of rotation and the velocity of the ball immediately after releasing it, they would be able to reproduce the same trajectory again with a high degree of accuracy. They also learned that when a two seam and four seam ball were both pitched at a speed of 150 kph and 1,100 RPM (number of rotations per minute), the simple difference in stitch motion translated into a difference of 19 cm in the drop of the ball from the pitcher’s hand.

Pitcher Kodai Senga of the Fukuoka Softbank Hawks is famous for the large drop in his forkball, which is said to be caused not by the two seam spin, but by a gyro motion (in which the spin axis of the ball matches the direction of its trajectory, or the same spin as a rifle bullet). The team ran an additional simulation of the phenomenon and found that the drop in the trajectory was greater than that of a two seam ball rotating at 1,100 RPM.

A Deeper Understanding of Ball Games in General

According to Aoki, "The airflow behind the ball is unstable and oscillates, and the speculation is that the backward airflow may be affecting the trajectory as well, such as with a knuckleball. We hope that by analyzing this in detail moving forward, we may gain a deeper understanding of ball games in general."

■ Two seams and four seams refers to the seams (revolutions) as seen from the point of view of the pitcher. A two seam spin is when only two seams can be observed for each spin of the ball. Accordingly, a four seam spin is when the pitcher can observe four seams for each spin of the ball. Straight pitches and some others have a four seam spin, and they are known for having a fast spin and straight trajectory that makes them appear to float.

■ sNegative Magnus effect: This effect occurs when the speed of the ball (wind speed) increases and the boundary layer (a thin layer on the surface of the object) becomes turbulent because the faster velocity relative to the surface of the ball is more likely to adhere to the surface. This causes the boundary layer to peel off and move upstream on the upper surface, and downstream on the lower surface. Accordingly, the airflow after the ball passes is right-side up, creating a downward force (the negative Magnus effect) at a specific number of rotations.

This article has been translated by JST with permission from The Science News Ltd.(https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.

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