Vortex-induced vibrations of a circular cylinder in unsteady freestream conditions

The transient response of an elastically-mounted rigid cylinder subjected to unsteady incoming flow conditions is experimentally examined through simultaneous displacement, force, and two-component velocity field measurements. The incoming flow was varied at two distinct freestream accelerations including a “fast ramp”, where the change in freestream velocity occurs within 45 cylinder oscillation cycles, and a “slow ramp” where the freestream velocity change is within 900 oscillation cycles. A total of six cases were considered with the freestream velocity varied between end states corresponding to adjacent response branches. The results indicate that the wake and structure respond relatively quickly to the changing freestream (within 20 oscillation cycles in all cases); however, the transient progression varied significantly between the considered cases. The transient dynamics are closely related to the energy transfer between the fluid and structure, with differences in energy transfer across cases evidenced in the rates of cycle-to-cycle amplitude change. The slowest rates of oscillation amplitude change were observed at higher reduced velocities near the lower branch and desynchronization, while the fastest rates of amplitude change occurred near the initial-upper branch transition. Overall, in contrast to quasi-steady response, where the system is allowed to reach equilibrium at each reduced velocity, the transient dynamics for the fast ramp deviated significantly. The distinct transient dynamics across the examined cases are linked to differences in the forcing characteristics, primarily the phase difference between forcing and cylinder oscillation. Analysis of the slow ramp cases revealed the transient dynamics approaching quasi-steady behaviour, while the nature of the responses remained similar to those from the fast ramp. A comparison of the present results and those from a limited number of available transient studies highlights the importance of both the mass ratio and the mean acceleration rate to the transient system response characteristics.