Kwangmin Son, Jin Choi, Woo-Pyung Jeon, and Haecheon Choi
In this paper, the effect of free-stream turbulence (FST) on the flow over a sphere is experimentally investigated at the Reynolds numbers of 0.5×105–2.8×105. Three levels of FST are generated in a wind tunnel by installing three different types of grids upstream of the sphere. It is found that FST triggers the boundary layer instability above the sphere surface and delays the separation. Once laminar separation occurs, the disturbances both from the boundary-layer instability and FST trigger the shear-layer instability. Then, high momentum is entrained toward the sphere surface and the separated flow is reattached, forming a separation bubble on the sphere surface. Due to high momentum near the surface, the main separation is delayed and the drag is reduced. The critical Reynolds number, at which the drag coefficient decreases rapidly, decreases as the FST intensity increases. With increasing Reynolds number, the first separation point moves downstream, but the reattachment and main separation points are nearly fixed, resulting in a constant drag coefficient. As the Reynolds number further increases, the separation bubble finally disappears but the main separation point is still nearly unchanged, resulting in a constant drag coefficient within the Reynolds number range considered. Therefore, the formation, regression, and disappearance of a separation bubble on the sphere surface are the key mechanisms of the drag change by FST.