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 Re d (≡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, Re d = 1.24 × 104–1.73 × 105 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.
Experiments in Fluids – Springer Journals
Published: Apr 7, 2012
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