Aerodynamics study on sweeper

Recent developments in professional baseball have introduced the “Sweeper” pitch, a variation of the traditional slider that exhibits pronounced lateral movement and minimal vertical drop. Despite its increasing use, the aerodynamic mechanisms behind its distinctive flight remain unclear. This study investigates the unsteady aerodynamic forces acting on the Sweeper, with a focus on the effects of seam geometry and spin efficiency. Using high-resolution computational fluid dynamics simulations based on the lattice Boltzmann method and a 3D-scanned model of an official Major League Baseball, we analyze the airflow and wake behavior around the ball. Our results show that asymmetric seam placement leads to asymmetric boundary layer separation on the upper and lower surfaces, resulting in a diagonally deflected wake and a longitudinal lift force not explained by the traditional Magnus effect. Simulated flight trajectories closely match MLB tracking data when the spin efficiency is between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>50</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>65</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> . These findings highlight the critical role of seam-induced flow structures in shaping pitch behavior and provide new insights into the aerodynamic design and control of breaking pitches in baseball.