Study of rectangular 5:1 cylinder vortex-induced vibration based on the mode decomposition method

Vortex-induced vibration (VIV) is a complicated fluid-solid coupling phenomenon. VIV of the rectangular section has drawn a lot of investigation because its aerodynamic properties offer useful models for high-rise buildings and bridge decks. This study aims at an in-depth analysis of VIV of 5:1 rectangular sections by using mode decomposition. The 2-D Unsteady Reynolds-Averaged Navier-Stokes (URANS) and 3-D Detached Eddy Simulation (DES) calculations were performed. Then, this study assessed four vortex identification methods, including the vorticity method, the Q criterion method, the λ 2 criterion method, and the Ω method, and discovers that Ω = 0.62 is most suitable for identifying vortices in rectangular sections. Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) methods are applied for contrastive analysis. The results demonstrate that there are two types of VIV for rectangular sections: resonance type and competition-induced type. For the resonance type, the vortex-shedding frequency and its harmonic mode dominate the initial growth stage. Thereafter, the structural mode and its harmonic dominant states rapidly emerge. In the case of the competition-induced type, the structural natural frequency mode grows much more quickly than the vortex departure mode and ultimately causes the flow mode to vibrate at the same frequency.