Mass ratio effects on the vortex induced vibration of cylinders in tandem arrangement

Abstract

Vortex-Induced Vibration (VIV) is often regarded as the most complex fluidstructure interaction problem that is yet to be fully understood. This research investigated the impact of mass ratio on the mechanism of VIV of closely spaced cylinders. The mass ratio is a vital parameter which affects the VIV of a circular cylinder. There are few studies that discussed mass ratio effect on VIV of single cylinder and no significant research has been conducted to study the effect of mass ratio on VIV of closely spaced cylinders. In the study, numerical simulations were carried out to understand the nature of VIV of two cylinders with equal-diameter for different mass ratios in tandem configuration. VIV characteristics of two mass ratios were compared. Cylinder with mass ratio 2 represents lighter cylinder whereas mass ratio 8 represents a heavier cylinder. Only the upper or super upper response branch normally found between reduced velocities 5 to 8 was studied. The cylinders were exposed to uniform flows in subcritical flow regime and shear stress transport detached eddy turbulence model was employed for simulating the turbulent flow around these cylinders. The center to center spacing between cylinders was four times of cylinder diameter. A series of tests were conducted to validate the present numerical study. Vital VIV parameters with detailed discussions of flow patterns to scrutinize the influence of upstream cylinder's mass ratio on the VIV of the rear cylinder at resonance zone were presented. It was found that oscillation frequency of the upstream cylinder plays a significant role in the nature of VIV of downstream cylinder. For a relatively heavier upstream cylinder, VIV amplitude of downstream cylinder escalates at the lower limit of resonance zone. Noticeable VIV increment of rear cylinder can be found when natural frequency of the upstream cylinder is at least 14% lower than that of the downstream cylinder. The study of the mass ratio effect on VIV of closely spaced cylinders is significant in terms of designing aquatic clean energy converter widely known as VIVACE converter and assessing the collision risk and fatigue of cylindrical-shaped risers located close to each other.