Secondary lock-in observation of vortex-induced vibration of a two-degree-of freedom circular cylinder oscillating in the crossflow direction *

We have experimentally studied vortex-induced vibration (VIV) of a flexibly-mounted circular cylinder, when it was given two degrees of freedom to oscillate in its first two natural modes in the crossflow direction. Dynamic response of the system was studied for a reduced velocity range of U* = 4.0 - 28, corresponding to a Reynolds number range of Re = 1,030-6,930. The natural frequency ratio between the first two modes was varied from 1.3-3.0 and the VIV response was studied for oscillations of the cylinder at its first and second mode, as well as the reduced velocity ranges over which transition between modes occurs. Flow evolution around the cylinder was qualitatively and quantitatively studied using a hydrogen bubble imaging and a time resolved volumetric particle tracking velocimetry technique, respectively. The VIV response of the system consisted of oscillations at a combination of the first two natural modes of the system with varying contributions from each mode. While the lock-in range at the first mode excitation stayed the same for all frequency ratios, as the frequency ratio increased, the lock-in range was extended to higher reduced velocities. By increase in frequency ratio, the reduced velocity range over which multi-modal oscillations were observed, decreased.