Large-eddy simulation study of Reynolds number effects on the flow around a wall-mounted hemisphere in a boundary layer

Abstract

Large-eddy simulations were used to investigate unsteady flows around a wall-mounted hemisphere as the Reynolds number (Re, based on the diameter of the hemisphere D) increased from 7 × 104 to 7 × 105. The hemisphere was immersed in a low-turbulence-intensity boundary layer with a thickness of δ/D = 0.5. Strong Re dependence was confirmed to be present even for the flow around a wall-mounted obstacle after systematic examination of aerodynamic forces, local pressures, and flow structures. Drag and lift crises were observed simultaneously, with the critical Re noted at approximately 3 × 105. As with circular cylinders and spheres, a laminar-turbulent transition and induced flow separation delay were observed in the supercritical Re regime. Flow separation occurred on the sides of the body later than on the top, regardless of whether Re was subcritical or supercritical. The spatial and temporal features of flow structures at different scales were described in detail based on the present high-resolution simulations. The coexistence of lateral oscillations and arch-type vortex shedding occurred throughout the subcritical and supercritical Re range. However, both of these motions diminished in scale and strength at supercritical Re. Flow motion frequencies were also quantified. The frequency ratio of arch vortex shedding to lateral oscillation was approximately 4 at subcritical Re but decreased to 3 at supercritical Re.