Effect of harmonic inflow perturbation on the wake vortex dynamics of a 2-DOF cylinder undergoing vortex-induced vibration near a plane boundary

The flow development over a fixed circular cylinder is highly sensitive to the presence of harmonic waves superposed on the inflow. Vortex-shedding ‘lock-on’ occurs when the wave frequency (f_p) is within about 1.8-2.2 times the natural Strouhal vortex shedding frequency (f_o). In contrast, the effect of incipient harmonics waves on an elastically mounted circular cylinder remains largely unexplored. In this work, vortex-induced vibration (VIV) of a 2DOF cylinder with superposed harmonic waves is investigated numerically at a constant Reynolds number of 200 for 1.5f_o < f_p < 3.5f_o. The results show that adding harmonic waves to the inflow can induce a type of ‘lock-on’ involving excitation of a new mode involving highly complex and irregular vortex interactions. The Strouhal frequency, the system’s natural frequency and their inter-harmonics persist in the structural response for all cases, demonstrating that the observed dynamics differ fundamentally from those observed for a fixed cylinder. The harmonic wave frequency is detectable only in the x-direction vibration spectrum, indicating that the inline degree of freedom plays a critical role in triggering nonlinear interactions between perturbations and the shedding dynamics.