The “knuckleball” effect is believed to be caused by asymmetric flow separation over the baseball, but little is known about its flow physics. The baseball is gripped with the knuckles in a certain position and is pitched in a way that introduces nearly no rotation, resulting in erratic flight paths which confuse batters. In the experiment described in this paper, the flow near the seams of the baseball is visualized thoroughly and the velocity vector fields near the surface and in the wake are obtained with Digital Particle Image Velocimetry. Depending on its position, the seam is found to trigger the boundary layer transition thus delaying the separation, or to cause separation itself. Three-dimensional wake patterns associated with specific ball orientations are identified and related to the force variations on the ball.

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