Effects of aspect ratio on the drag of a wall-mounted finite-length cylinder in subcritical and critical regimes

The effect of cylinder aspect ratio (a parts per thousand H/d, where H is the cylinder height or length, and d is the cylinder diameter) on the drag of a wall-mounted finite-length circular cylinder in both subcritical and critical regimes is experimentally investigated. Two cases are considered: a smooth cylinder submerged in a turbulent boundary layer and a roughened cylinder immersed in a laminar uniform flow. In the former case, the Reynolds number Re (d) (a parts per thousand U (a) d/nu, with U (a) being the free-stream velocity and nu the fluid viscosity) was varied from 2.61 x 10(4) to 2.87 x 10(5), and two values of H/d (2.65 and 5) were examined; in the latter case, Re (d) = 1.24 x 10(4)-1.73 x 10(5) and H/d = 3, 5 and 7. In the subcritical regime, both the drag coefficient C (D) and the Strouhal number St are smaller than their counterparts for a two-dimensional cylinder and reduce monotonously with decreasing H/d. The presence of a turbulent boundary layer causes an early transition from the subcritical to critical regime and considerably enlarges the Re (d) range of the critical regime. No laminar separation bubble occurs on the finite-length cylinder immersed in the turbulent boundary layer, and consequently, the discontinuity is not observed in the C (D)-Re (d) and St-Re (d) curves. In the roughened cylinder case, the Re (d) range of the critical regime grows gradually with decreasing H/d, while the C (D) crisis becomes less obvious. In both cases, H/d has a negligible effect on the critical value of Re (d) at which transition occurs from the subcritical to critical regime.Engineering, MechanicalMechanicsSCI(E)EI0ARTICLE2423-4365