Experimental investigation on the nodal-shaped oscillation in vortex-induced vibration of a rectangular cylinder

This paper presents a peculiar nodal-shaped oscillation in vortex-induced vibration (VIV). This response is distinct from the commonly observed limit cycle oscillation in VIV and other aeroelastic problems. To gain insight into the dynamics in nodal-shaped oscillation, we conduct wind tunnel tests to investigate the VIV response of a freely oscillating rectangular cylinder with various transverse structural natural frequencies fs and wind speeds. The evolution of the flow field during nodal-shaped oscillation was examined using time-frequency analysis and modal analysis. During the divergent stage of the nodal-shaped oscillation, two modes, namely, the wake mode with a frequency of f = 1.28f(s) and the structure mode with a frequency of f = f(s), were found to dominate the response of the flow. The two modes in VIV are coupled and compete with each other, diverging the oscillation of the structure. As the oscillation diverges and the amplitude increases to the maximum, the wake mode vanishes, which disengages the coupling of the fluid and structure modes. This stops the VIV and the oscillation decays until the wake mode reappears at f = 1.28f(s). The above processes cycle over and over again, resulting in nodal-shaped (divergent-decay-divergent-decay) oscillation cycles. This finding explains the underlying dynamical mechanism of nodal-shaped oscillation and demonstrates the strong relationship between the wake mode and VIV.