The dynamics of vortex induced vibrations starting from rest

The present study investigates the structural and wake dynamics of a 1-DOF elastically mounted rigid cylinder released from rest in a freestream. The mass-damping (m*ζ=0.025), and Reynolds number (Re=4400) are fixed, and the reduced velocity is varied from U*=4 to 11. The results show that the system response can be subdivided into three stages. The first stage comprises small cylinder oscillations (A*<0.05 D) that are desynchronized from the Strouhal shedding. The second stage involves cycle-to-cycle increases in oscillation amplitude until the system attains the third stage: steady-state oscillations. Both the duration of the first stage, as well as the rate of amplitude changes in the second stage are related to the difference in Strouhal shedding frequency (f_vs), and natural frequency (f_n). For around |f_n-f_vs|/f_vs<0.2, the system bypasses the first stage and exhibits rapid changes in amplitude response in the second stage. For |f_n-f_vs|/f_vs>0.2, the first stage comprises multiple shedding cycles, and the second stage amplitude response changes are comparably slower. Planar particle image velocity at the midspan is used to explore the wake dynamics at each stage.