Aerodynamic Mechanism And Optimization Of Wind Barrier On Highway-railway Double-deck Bridges

Along with the transportation network covering the strong wind regions, wind-induced vehicle accidents happen frequently. Because of the complexities of highway-railway double-deck bridge structures, the effect of accelerated flow around the structure is obvious.The driving environment of vehicles on the bridge will be more worsening. Setting of the wind barrier is one of the most effective means to improve the driving safety of vehicles. However,the wind barrier will also influence the wind-resistance performance of the bridge. So, the following studies are carried out to investigate the aerodynamic mechanism and optimization of wind barriers on highway-railway double-deck bridges.Firstly, the distribution characteristics of flow fields behind barriers and its wind loads are tested by means of a full-scale wind tunnel test. The effects of pore sizes and opening forms of wind barriers on the flow fields are analyzed. The protective regions of different wind barriers along the approaching flow and vertical direction are discussed. Based on the drag force and overturning moment equivalent principle, the equivalent wind speeds behind different wind barriers are obtained. Furthermore, the numerical simulation methods suitable for porous barriers and bar-type barriers are proposed, respectively. Then, the reliability of the two methods is verified, according to comparing with the results of the full-scale section model wind tunnel test. On this basis, the effects of pass parameters on the flow fields behind wind barriers are explored, such as the distribution of porosity, the rows and space shapes of barrier strips et al. Based on the above results, reasonable parameter values are suggested.Secondly, taking the wind barrier-vehicle-double-deck bridge system into consideration,the scale section model wind tunnel test is conducted to investigate the local wind field on the upper deck and lower deck, with and without installing wind barriers. The effects of wind barriers on the aerodynamic force coefficients of CRH2 train and truck are discussed. Based on the above, the wind-vehicle-bridge system coupled vibration method is used to calculate the dynamic response of the CRH2 train and the truck. Then, the impacts of lane positions, wind speed, vehicle speed, and wind barriers on the driving safety and driving comforts of the vehicles are analyzed.Thirdly, as to the highway-railway double-deck truss bridge, the static aerodynamic coefficients, flutter critical wind speed, and vortex vibration response of the main girder with and without installing wind barriers are tested. The effect of wind barriers on the static-dynamic aerodynamic characteristics of the truss bridge is discussed. Aiming at the separate-type double-deck box girder bridge, the aerodynamic force of the vehicle and wind fields distribution of the separate railway bridge and the highway-railway double-deck bridge are analyzed to explore the aerodynamic interaction between the upper deck and lower deck.Various height intervals have been taken into account to further explore the influence of the height interval between highway-railway double decks on the wind fields characteristics and aerodynamic effects of vehicles. Then, the reasonable height interval values are proposed, on the condition of installing wind barriers or not, respectively.Finally, comprehensively considered the safety of vehicles and bridges, the comprehensive optimization of the protection effect of wind barriers is converted into the multi-objective optimization. The multi-objective optimization genetic algorithm (NSGA-?) is employed as the optimization method to optimize wind barriers. Then, the most optimal wind barrier is achieved by using the Data Envelopment Analysis (DEA) to evaluate the relative efficiency of the Pareto solution set.The results demonstrated as follows: the full-scale section model wind tunnel test results can provide more standard reference data for optimization design and numerical simulation of this type of wind barriers. 2D numerical simulation proposed in this study can well simulate the porous wind barrier performance. Effects of the distribution of the porosity, the rows and space shapes of barrier strips on the flow fields downwind of wind barriers are relatively obvious. As to the optimization design of wind barriers in the future, effects of the pass parameters should be paid more attention. The research results of double-deck truss bridges indicate that, wind barriers will increase the drag coefficients of the main girder and decrease its lift coefficients. After installing wind barriers, the flutter critical wind speed of the main girder decreases significantly, which indicates that wind barriers will deteriorate the flutter stability of this type of bridges. However, wind barriers can be used as the aerodynamic measure to inhibit the vortex-induced vibration of this type of bridges. The study results of the separate-type highway-railway double-deck bridge show that, wind barriers will intensify the aerodynamic interaction between the upper deck and lower deck. The height interval just needs match to the basic structure gauge of railway bridges without wind barriers. When the height interval is more than 15m, the change of wind profile above the orbit of railway bridge deck and the aerodynamic forces of vehicles located in the upwind side gets flat under the wind barrier. The results of the multi-objective optimization problem of wind barriers show that it is feasible to add the NSGA-? and DEA hybrid algorithm to the multi-objective optimization of wind barriers. This optimization design method provides a new idea for the wind barrier optimization problem.