Mechanism of suppression of vortex-induced vibrations of a streamlined closed-box girder using additional small-scale components

Abstract

VIV (vortex-induced vibration) suppression using aerodynamic countermeasures are critical for design of long-span bridges. Strong vertical VIVs were observed in a streamlined closed-box bridge girder subject to wind at an initial attack angle of +3°. CFD simulation, and pressures and displacements measured in wind tunnel tests on a large-scale model (approximately 1:20) were analyzed to reveal the VIV-triggering mechanism for a bridge girder, as well as the VIV-suppression mechanism for bridge girders modified with typical small-scale additional components such as spoilers on crash barriers or guide vanes near maintenance traces. Spoilers can almost eliminate VIVs, while guide vanes can moderately mitigate them. Large-scale vortexes generated from flow separation at leading barriers and leading maintenance trails, referred to as separated vortexes and secondary vortexes, respectively, and called "double vortex mode" together, are responsible for VIVs of an unmodified girder. When spoilers were set on the barriers, the separated vortexes, and then the "double vortex mode" observed on the unmodified bridge girder, could be broken. This greatly reduced the correlation between local aerodynamic forces and general vortex-excited forces (VEFs), and thus the contribution of local aerodynamic forces to general VEFs on the model surface, which caused the VIVs to disappear. As for the unmodified girder, the "double vortex mode" is also responsible for vertical VIVs of a girder modified with guide vanes. This is due to the strong correlation between local aerodynamic forces and general VEFs. However, the phase lags between them have an overall offset of approximately 90° from the approximate synchronous action. Furthermore, a significant decrease in RMS values of pressure coefficients on the model surface dramatically reduce the direct contribution of local aerodynamic forces to general VEFs, especially in the downstream region of the upper surface, thus clearly mitigating VIVs.