Transient vortex-induced vibrations of a cylinder released from rest

The transient response of an elastically-mounted rigid cylinder released from rest in a steady freestream is investigated experimentally through simultaneous displacement and two-component velocity field measurements in conjunction with force estimation. The Reynolds number is maintained constant at $Re\approx$ 4,400, while the reduced velocity is varied between $U^{\ast }\approx$ 4.5 and 11.5. The amplitude response indicates distinct transient behavior across all response branches, including a notable amplitude overshoot in the initial branch and continuous amplitude growth to quasi-steady state in the upper and lower branches. Following cylinder release, forcing is shown to transition from purely von Kármán frequency to nonlinear forcing, and comparisons to a linear oscillator show key differences in system behavior during this transition. Following lock-in, the time taken to attain quasi-steady state increases, while the maximum amplitude growth rate decreases with $U^{\ast }$. The observed differences in the transient amplitude growth rate are linked to distinct changes in the forcing characteristics, primarily driven by the phase difference between forcing and cylinder displacement. The presented analysis of the transient flow development reveals a close relationship between the timing of vortex shedding and the forcing phase difference. Additionally, the mechanisms underlying the transition from initial von Kármán shedding to quasi-steady lock-in behavior, highlighted by notable changes in wake characteristics, are identified for transients in each response branch.