| As the global climate is fast changing,the possibility of thunderstorm downburst has gradually increased in the worldwide.Thunderstorm downburst is a weather phenomenon that the sinking air currently hits the ground in a thunderstorm and then quickly spreads and causes short-term strong winds near the ground.At present,research on the thunderstorm downburst is mostly aimed at transmission line towers and building engineering structures in flat terrains,while research on bridge structures under the action of the thunderstorm downburst is relatively rare.In recent years,many scholars’ experimental data of measuring wind speed has shown that the traditional definition of wind characteristics will no longer be applicable under the complex terrains(such as mountainous areas and gorge terrains).With the increase in the number of long-span bridges in mountainous areas,these bridges will inevitably straddle the mountaingorge terrains.Under the actions of thunderstorm downbursts,the wind-induced vibration responses of long-span bridges straddling the complex mountain-gorge terrains will be extremely complicated,and will affect the safety of wind-resistance during the construction and operation of the bridge.This paper discusses the wind field characteristics of flat and gorge terrains under the actions of thunderstorm downbursts,which is based on an actual engineering case and the SST k-co turbulence model combining with the simulation method of impinging jet,and then analyze the impacts of thunderstorm downbursts on the buffeting responses of bridges.The main works are as follows:(1)This paper introduced the characteristics of thunderstorm downburst wind fields and the current research situation,and determined the research content of this paper in view of the existing problems in the current research.(2)Based on the SST k-ω turbulence model,a three-dimensional numerical simulation of the static thunderstorm downburst was carried out by using the impinging jet model.By comparing with the previous studies,the accuracy of the numerical simulation method was verified.Based on the above,the wind field characteristics at the bridge site in the flat terrain and gorge terrain were calculated and analyzed.The research results show that the wind attack angle values at each typical measurement point of the gorge terrain are generally larger than those of the flat terrain,but the fluctuation ranges are much smaller than the those of the flat terrain.(3)Based on the verified numerical simulation method and the dynamic mesh technology,the wind field characteristics of flat terrain and gorge terrain under the impacts of the moving thunderstorm downburst were compared and analyzed.The research results show that the gorge terrain has a certain degree of acceleration effects on the wind speeds.The wind attack angles of the flat terrain are mostly negative,while the gorge terrain can cause very large positive wind attack angles.(4)The weighted amplitude wave superposition(WAWS)method was used to simulate the nonstationary fluctuating wind speed fields of the moving thunderstorm downburst at the bridge site.Considering a long-span suspension bridge as the engineering background,the buffeting responses of the main girder in the flat and the gorge terrains under the actions of the thunderstorm downburst were analyzed.The research results show that the buffeting displacement responses of the main girder are mainly affected by the time-varying average wind fields,but the aerodynamic loads caused by the fluctuating wind speeds cannot be ignored.The components of vertical and lateral buffeting responses of the main girder under the two terrains both are dominated by their first-order mode shapes.For torsional responses,first-order torsional modes are dominated in the flat terrain,while the first-order torsional,second-order torsional,and higher-order torsional modes are mostly occurred in the gorge terrain. |
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