Unravelling the Influence of Taper Ratio on 2-DOF Vortex-Induced Vibration Characteristics of a Circular Cylinder*

The present study discusses the effect of the taper of the circular cylinder on its vortex-induced vibration characteristics when subjected to an incoming flow with a Reynolds number (based on the average diameter) of 500.  The tapered cylinder (with a mass ratio of 10) is mounted with the help of linear springs (with a natural frequency of 1.0) and the dampers (with a damping coefficient of 0.02) such that it is free to oscillate in both the inline and the cross-flow direction. The three-dimensional numerical simulations are performed over taper ratios (defined as: τ=l/(d₂ - d₁): 12, 20, and 40) for a range of reduced velocities (3 ≤ U_r ≤ 16). The findings indicate that increasing the taper ratio leads to delayed branch transition between the initial and lower branches with higher peak oscillation amplitude than the uniform cylinder. The combined study of the vortex distribution and the time-frequency spectrum of the probe data reveals the presence of oblique shedding and vortex stretching/dislocations. The pressure-driven secondary motion on the taper cylinders at the front stagnation regime travels from the wide end towards the narrow end and becomes weak near the narrow end, while behind the cylinder, it doesn’t have any preference for the flow-direction in the near or far wake of the cylinder.