CFD-based Study On Flow-driving Mechanism Of VIV And Flutter In Typical Section Of Steel Box Girder

Because of the flexibility and low damping ratio,the long-span steel bridge is prone to wind-induced vibration.Inverted trapezoidal box section with projecting slabs(ITBSPS)and separated twin-box section are common forms of steel box girder,but their VIV(vortex induced vibration)performance is not good.Although appropriate aerodynamic measures can improve their VIV performance,the flow driving mechanism of VIV and the anti-vibration mechanism of aerodynamic measures are still unclear.Relevant research can deepen the understanding of the formation mechanism of VIV and lay a foundation for the design and optimization of aerodynamic measures.Besides,the aerodynamic measures of the separated double-box section may deteriorate the flutter performance,so the comprehensive control effect of aerodynamic measures on both VIV and flutter needs to be further studied.Based on CFD(Computational Fluid Dynamics),the following work has been carried out in this paper:(1)Aerodynamic force and flow characteristics of ITBSPS in static state are calculated.Reynolds number effect of Strouhal number and vortex shedding law of original section and section with wide spoiler are compared and analyzed.According to the scale and distribution of vortexes,it is inferred that the original section is more prone to VIV.(2)Based on the dynamic grid technology,a free vibration simulation method for the VIV and flutter of the box girder section is established,and verified by the wind tunnel test results.It is found that small time step and initial excitation are needed to ensure the accuracy of the calculation.(3)The vertical bending and torsional VIV of ITBSPS found in wind tunnel tests are simulated and reproduced,and the phenomena of "frequency locking" and "phase switch" are found.The flow driving mechanism of vertical bending and torsional VIVs is analyzed.It is found that the regular large-scale vortex at the top of the section is the main cause of VIV.(4)The vibration response of ITBSPS is calculated after adding wide spoiler.It is found that the wide spoiler can effectively suppress the vertical bending and torsional VIVs.The mechanism of vibration suppression of wide spoiler is discussed.It is found that it reduces the scale of vortexes at the top of the section and changes the trajectory of vortexes by means of high-speed jet.(5)The flow driving mechanism of vertical bending VIV of the separated twin-box section is studied.It is found that the large scale vortex at the central slot is the main cause of VIV.The control effect of the top grille at the central slot is reproduced.The vorticity contour shows that it effectively breaks the vortex at the tail of the upstream box.(6)The flutter performance of the separated twin-box section before and after adding the central grille was tested,and it is found that the central grille does not significantlydeteriorate the flutter performance of the section.