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

Abstract

The effect of cylinder aspect ratio (≡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 Red (≡U∞d/ν, with U∞ being the free-stream velocity and ν the fluid viscosity) was varied from 2.61 × 104 to 2.87 × 105, and two values of H/d (2.65 and 5) were examined; in the latter case, Red = 1.24 × 104–1.73 × 105 and H/d = 3, 5 and 7. In the subcritical regime, both the drag coefficient CD 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 Red 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 CD–Red and St–Red curves. In the roughened cylinder case, the Red range of the critical regime grows gradually with decreasing H/d, while the CD crisis becomes less obvious. In both cases, H/d has a negligible effect on the critical value of Red at which transition occurs from the subcritical to critical regime.