Wake dynamics and heuristic modelling in the desynchronization region of 1-DOF VIV

Vortex-induced vibrations of a circular cylinder has been investigated experimentally using a cyber-physical apparatus with m* = 8 and zeta = 0.005. The Reynolds number is held fixed at Re = 4000, with the reduced velocity being modified via a change in the structural natural frequency in a cyber-physical controller. The study focus is on a detailed analysis of the wake dynamics in the desynchronization region of VIV, where the cylinder motion is no longer synchronized to the wake vortex shedding. The analysis is presented using Proper Orthogonal Decomposition (POD), Phase Averaging, and a novel technique that improves the POD approach by shifting the reference frame of the velocity field data. Each analysis approach is targeted towards efficiently extracting the coherent motions from the wake flow and developing a heuristic model that empirically links the dynamically relevant modes. The results demonstrate that coherent motions persist in the desynchronization region and involve the interaction of two frequencies: the vortex shedding frequency, f(VS) and the lock-in oscillation frequency of the cylinder, f(o). The interaction between these two frequencies produces changes in the fluctuating field dynamics that are centered on the beat frequency, f(VS) -f(o). When the vortex shedding frequency and oscillation frequency are in phase, the cylinder exhibits a strong dynamic response. As they move out of phase, the cylinder response decays. These dynamics of the wake are captured in the first six POD modes. Moreover, the amplitude of these modes is shown to be linked to POD modes representing low frequency variations in the base flow with time.