Influence of borehole diameter and attack angle on the flow around a perforated sphere

Abstract Motivated by the potential safety hazards posed by cracks in flow around bluff bodies in ocean engineering applications, this study investigates incompressible viscous flow around a sphere containing a central cylindrical bore at Re = 300. The effects of borehole diameter and attack angle on vortex shedding, flow separation, and drag coefficients are systematically explored. Results indicate that at low attack angles, increasing the borehole diameter markedly influences the primary shedding frequency of vortex, thereby modifying their structural characteristics. Moreover, total drag is observed to increase with larger borehole diameters. At elevated attack angles, the borehole diameter affects the initiation of flow separation due to its proximity to the separation point. The impact of attack angle is most significant for larger borehole diameters, where pronounced alterations in both vortex shedding and flow separation are evident. Additionally, the magnitude of the borehole diameter amplifies the influence of the angle of attack on the drag coefficient. Due to the jet-like flow induced by the bore, flow instability around the sphere is intensified when the borehole diameter is sufficiently small. This instability initially escalates with increasing angle of attack, peaking at 45 degrees, and then diminishes as the angle of attack continues to increase.