Flow dynamics of the transversely oscillating tapered circular cylinder under vortex-induced vibrations at low Reynolds number

This study numerically investigates the influence of the taper on the flow-induced vibrations of an elastically mounted circular cylinder under vortex-induced vibrations (VIV). The dynamic response of three different taper ratios [ TR defined as TR=l/(d(2)-d(1)) = 12 (highly tapered cylinder), 20 (medium tapered cylinder), and 40 (low tapered cylinder)], where d(1) and d(2) are the largest and smallest diameter of the tapered cylinder, respectively, with length l is studied at a fixed Reynolds number [ ReD, defined based on the averaged cylinder diameter, D=(d(1)+d(2))/2] of 150. The amplitude and frequency response of the tapered cylinder is characterized by a low mass ratio (defined as the ratio of the total oscillating mass to the displaced fluid mass) of m* = 2 over the wide range of reduced velocity ( 3 <= U-r <= 16) covering the full amplitude-response spectrum (based on the oscillation amplitude) of the VIV. The results show the existence of difference in the spanwise shedding of vortices owing to the poor spanwise pressure correlation. The flow field analysis in the wake of the oscillating cylinder reveals the dominance of the three-dimensional structures in the wake (near the top end with the larger diameter) behind the cylinder with the increase in the taper ratio (even at such low ReD where the uniform cylinder exhibits the two-dimensional wake). Also, the tapered cylinder exhibits a wide range of frequency synchronization (i.e., wide lock-in area) compared to the uniform cylinder. Tapering the cylinder results in the shift of the peak of the max oscillation amplitude or, in turn, the shift in the transitioning of the response branches. Further, force decomposition, energy transfer, and phase dynamics are also discussed for the taper cylinders.