The aerodynamic mechanism of the effect of gap width on the vertical vortex-induced vibration of the triple-box girders

The triple-box girder is prone to vortex-induced vibration (VIV). However, the effect of gap width on vertical VIV and its underlying aerodynamic mechanism remains unclear. Based on wind tunnel tests and computational fluid dynamics (CFD) simulations, this study investigates the aerodynamic mechanism of the effect of gap width on the vertical VIV performance of the triple-box girders. Four groups of girder models with varying gap widths were tested experimentally to elucidate the relationship between gap width and the vertical VIV performance. The CFD simulations were used to explore the effects of gap width on flow characteristics and pressure distribution under peak amplitude of VIV conditions, while the energy distribution of distributed aerodynamic forces was analyzed to systematically discuss the aerodynamic mechanisms involved. Results indicate that gap width significantly influences the vertical response of VIV of the triple-box girder. The peak amplitude of VIV increases with gap width, though the growth rate diminishes as gap width expands. Increasing gap width enhances the intensity of vortex shedding between gap regions, leading to a marked rise in fluctuating pressures on the local surfaces of midstream and downstream boxes, while mean pressure distributions remain relatively stable. As the gap width increases, the positive energy contribution region progressively expands from the lower surfaces and the trailing edges of the upper surface of the downstream box to encompass the lower surfaces and the trailing edges of the upper surface of the upstream and midstream boxes, thereby enhancing the peak amplitude of VIV. These findings provide valuable insights for understanding the aerodynamic mechanisms of triple-box girders subject to VIV and optimizing the aerodynamic shape.