Effect of gap on vortex-induced vibration characteristics and interference mechanisms in tandem-arranged arch ribs under construction

Separated twin rectangular cross-sections are widely used in arch ribs. However, the aerodynamic interference effects and mechanisms between tandem sections—specifically how the gap governs vortex-induced vibration (VIV) evolution and flow regime transitions—remain insufficiently understood due to the influences of spacing, cross-sectional shape, and complex aerodynamic characteristics. This study experimentally investigates the influence of spacing ratios ( L/B = 2.1, 2.9, 3.6, 4.3, and 5.0, where L is the gap distance and B represents the width) on spacing-dependent VIV characteristics and flow-structure interactions. The primary focus is to elucidate the effect of spacing ratio on flow patterns and aerodynamic forces, and to establish their interconnections, thereby providing a foundation for engineering applications. The results reveal that the spacing ratio critically controls gap vortex formation and development, leading to markedly different evolutionary trends in vertical VIV responses. Two distinct flow regimes are identified and quantitatively characterized: the Shear-layer Reattachment Pattern and the Co-shedding Pattern. The findings establish a fundamental framework that systematically links spacing ratio effects with VIV characteristics and flow field evolution, providing new insights into the aerodynamic design of multi-section bluff body structures.