Flow Separation and Control

The aerodynamics of a baseball are typically modeled as flow past a sphere with a Magnus effect lift contribution due to spin. However, the Magnus effect assumes uniform surface roughness, whereas the raised seams of a baseball are notable nonuniform elements. Recent research has proposed the existence of a “seam-shifted wake” (SSW), a phenomenon in which certain seam orientations alter boundary layer separation and consequently the wake structure. Certain pitches, especially 2-seam fastballs and knuckleballs, can exhibit behavior inconsistent with Magnus-only models, suggesting that seam geometry may significantly influence the aerodynamic forces on the ball at some point during its flight. The current study investigates the SSW effect using a 3D-printed baseball mounted on a custom high-rotation-rate rig in UCLA’s wind tunnel. The rig achieves spin rates and freestream velocities comparable to in-game conditions, and additional spin axes can be achieved by reprinting the ball. A laser sheet and high-speed camera captured smoke flow in the equatorial plane of the ball, allowing direct observation of separation and wake asymmetry. This setup  allows precise control over seam orientation, spin rate, and flow speed. Thousands of images were collected, capturing smoke-traced flow fields across a range of spin rates, seam orientations, and freestream velocities. In these images, flow features such as boundary layer separation, wake orientation, and seam location are clearly visible. Post-processing of these images in MATLAB has revealed a dependency of the separation point on seam orientation, particularly in 2-seam fastball cases. While further research is necessary to quantify forces from the SSW phenomenon, these findings offer experimental support for the existence of the SSW and highlight its potential as a mechanism for improving pitch manipulation at the highest levels of baseball.