基于动态模态分解的Π形梁涡振流场特征分析

Analysis of vortex-induced vibration flow field evolution characteristics forΠ-shaped beam based on dynamic mode decomposition

为探索Π形桥梁断面的涡振机理及气动措施的抑振原理,采用动态模态分解方法,结合计算流体动力学数值模拟,对某Π形桥的不同工况进行流场解耦和对比分析.结果表明,主梁下表面方腔内的不稳定涡是导致Π形梁涡激振动的重要因素,增设2道四分点下稳定板将空腔分割成3个较小的区域,可减少下缘分离涡的形成空间并降低空腔内涡的强度和尺度,从而有效抑制涡振.第2阶分解模态与结构振动同频,是引起涡激振动的主导模态;此模态能够直观揭示流场中主涡的形成和发展,简化流场分析的复杂性.第3、4分解阶模态与结构的涡振频率存在明显的倍频关系,能够精确捕获涡激力中的高频成分,有利于深入理解涡激振动的瞬时变化特性,制定更为精细的抑振措施.

To explore the vortex-induced vibration(VIV)mechanism ofΠ-shaped bridges and the suppression principle of aerodynamic measures,the dynamic mode decomposition(DMD)method is introduced in conjunction with computational fluid dynamics(CFD)numerical simulation to decouple and comparatively analyze the flow fields under various operational conditions for a givenΠ-shaped bridge.The results demonstrate that unstable vortices within the square cavity beneath the main girder's lower surface play a significant role in triggering VIV ofΠ-shaped girders.The installation of two stabilizing plates at quarter points beneath the girder effectively divides the cavity into three smaller regions,thereby reducing the formation space for trailing-edge separation vortices,diminishing both the intensity and scale of vortices within the cavity and effectively mitigating VIV.The second-order DMD mode is found to be in-phase with the structural vibration,representing the dominant mode of driving VIV.This mode provides a direct visualization of the primary vortex formation and evolution within the flow field,greatly simplifying the complexity of flow field analysis.The third and fourth DMD modes exhibit clear harmonic relationships with the structure's VIV frequency and can accurately capture the high-frequency components of the vortex-induced forces,which is beneficial for a deep understanding of the instantaneous variation characteristics of VIV and the formulation of more refined damping strategies.