Central vertical stabilizers are widely employed to enhance the aerodynamic stability of long-span bridges with streamlined box girders, yet the fundamental mechanisms underlying their effectiveness, particularly regarding the critical role of solidity ratio, remain incompletely understood and debated. This study comprehensively investigates the flutter control mechanisms of central vertical stabilizers, explicitly evaluating the impact of solidity ratio. By integrating the bimodal flutter analysis with surface pressure measurements, the research provides novel insights into the spatiotemporal distribution of self-excited forces during coupled deck motion. The results demonstrate that central vertical stabilizers increase the uncoupled torsional aerodynamic damping significantly, effectively suppressing coupled flutter. However, an insufficient solidity ratio (below 50% in this case) drastically diminishes this stabilizing effect. Pressure distribution analysis demonstrates that self-excited forces are mainly generated by the windward edge of the deck section, and the properties of self-excited forces generated by heaving and torsional motions differ primarily in their phase relative to the motion. Further, it is found that central vertical stabilizers fundamentally alter the flow pattern over the deck: they disrupt the large-scale separation bubble, splitting it and delaying reattachment downstream. This vortex restructuring induces a critical phase shift (30°–40° lag in this case) in self-excited forces behind the central vertical stabilizers. This phase shift is identified as the primary mechanism driving the observed increase in local aerodynamic damping and reduction in stiffness within that region, thereby enhancing the overall flutter stability.
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