Vortex-induced vibration characteristics of parallel II-shaped composite girders

A series of sectional model testing were conducted to investigate the vortex-induced vibration (VIV) characteristics of parallel II-shaped composite girders under different spacing ratios and wind attack angles. To enhance the understanding of the aerodynamic interference effects on the VIV, particular attention is devoted to the vibration amplitude, lock-in wind speed, phase difference, and spectrum characteristics of the upstream and downstream girders. Results showed that the aerodynamic interference has significant effects on the downstream girder, resulting in the VIV amplitude amplified obviously. Nevertheless, the upstream girder is less affected, with its amplitude slightly smaller or almost equivalent to that of a single-II-shaped girder. The lock-in range and vortex shedding frequency of the upstream and downstream girders are almost unaffected by aerodynamic interference. The phase difference phi between the vibrations of upstream and downstream girders diminishes in a nearly linear manner as wind speed rises in the lock-in region for all spacing ratios investigated, except at the end of the lock-in region. Additionally, the initial phase difference is directly related to the spacing ratio. In addition, the fluid mechanism of VIV was analyzed by computational fluid dynamics (CFD) technique. It was found that the downstream girder experiences the alternating flow fields ' ZS ' and ' S2 ' generated by the upstream wake vortices, leading to VIV amplitude amplification of downstream girder.