Passive Flow Control on a Circular Cylinder Using a Small Spoiler – A Window Into the Effect of Baseball Seams

Recent research has shown that baseball seams behave as “spoilers” by promoting boundary-layer separation at the seam when those seams lie near a region of adverse pressure gradient (Smith et al., How seams alter boundary layer separation points on baseballs. Exp Fluids 65, 27 (2024)). This presentation will show the early results of a wind tunnel study in which a cylinder of similar size to a baseball (about 70 mm) is placed in the tunnel with similar velocity to a baseball pitch (about 40 m/s). The spoiler is isosceles-triangular-shaped, protruding 1 mm from the cylinder surface and 4 mm in the streamwise direction. The main measurements are the surface pressure distribution around the cylinder. The present study shows that when the spoiler is in the region 80º < θ < 120º, boundary-layer separation occurs at the spoiler, in advance of the position near  θ = 120º where separation normally occurs for these turbulent boundary layers. A similar range was reported for baseballs. Further, the present study shows that location of the separation point, determined by the spoiler location, alters the pressure distribution around the cylinder. Specifically, the pressure upstream of the spoiler increases compared to the non-spoiler side, with this behavior increasing according to decreasing spoiler angle. However, the base pressure, separation on the non-spoiler side, and non-spoiler side pressure remain constant across observed spoiler locations. Thus, spoiler location determines the magnitude of lift and drag compared to the cylinder with no spoiler.