Aerodynamic Mechanism of the Dual Lock-in Regions of Vertical Vortex-Induced Vibration of the Triple-Box Girder

The triple-box girder has attracted significant attention due to its excellent aerodynamic stability and passage capacity. However, this type of box girder may exhibit dual vortex-induced vibration (VIV) lock-in regions with identical vibration modes and frequencies, which deviates from the Strouhal relationship between vortex shedding frequency and wind speed. To elucidate the aerodynamic evolution mechanism underlying the dual vertical VIV lock-in regions of the triple-box girder, a systematic investigation was conducted using segment model wind tunnel tests and computational fluid dynamics (CFD) simulations. This study comprehensively analyzed the VIV response characteristics, flow field features, pressure distribution, and energy evolution characteristics of distributed aerodynamic forces within the dual lock-in regions, thereby revealing the fluid–structure interaction (FSI) mechanism. The findings indicate that the flow patterns around the girder and aerodynamic mechanism within the dual lock-in regions differ fundamentally. In the first lock-in region, the maximum VIV amplitude is larger than that in the second lock-in region. The flow at the trailing edges of the upper and lower surfaces of the three boxes serves as the primary driving force for the generation and amplification of VIV amplitude in the first lock-in region, whereas the trailing edges of the upper and lower surfaces of the downstream box act as the driving sources for vertical VIV in the second lock-in region. The vertical VIV in the first lock-in region is induced by Kármán vortex shedding at the wake region of the downstream box, and its development is dominated by the coupling of the instability of multiple shear layers and the motion of the girder. The vertical VIV in the second lock-in region is induced by the instability of the shear layer, and its development is governed by the coupling of a single separation shear layer on the upper and lower surfaces of the downstream box and the motion of the girder. In the actual aerodynamic optimization of the vertical VIV performance of triple-box girder, attention should be paid to the applicability of the same aerodynamic measure to the dual lock-in regions.