Analysis of vortex-induced vibration characteristics of separated twin box girder based on high-order dynamic mode decomposition

The separated twin box girder is widely used in long-span bridges due to its excellent flutter performance. However, its unique aerodynamic profile may induce vortex-induced vibrations (VIV) which may threaten the bridge operational safety. This study systematically investigates the vertical and torsional VIV characteristics of separated twin-box girders with central gap ratios ( W/H ) ranging from 0.690 to 3.448 through wind tunnel tests and numerical simulations. To clarify the excitation mechanism of VIV under different gap widths, the High-Order Dynamic Mode Decomposition (HODMD) method is applied to analyze the pressure data, while standard DMD was used to analyze the simulated vorticity fields of representative cases, revealing the underlying flow-structure coupling. HODMD analysis identifies pronounced pressure-coefficient variations in the critical leading-edge regions of the downstream box. The first two DMD modes show a sharp increase in pressure coefficients in these regions, with larger increases correlating with higher VIV amplitudes, indicating that these areas significantly influence torsional vibration. Additionally, the alternating fluctuation of vortices shed from the trailing edge of the upstream box onto the downstream box also contributes notably to torsional VIV. In the stable stage, the flow field is dominated by the fundamental torsional frequency and its harmonics DMD modes, and the first seven DMD modes can accurately reconstruct the vorticity field. These findings provide valuable insights for VIV suppression of separated twin box girders.