Protection Effect Of Wind Barrier On High Speed Railway And Its Wind Loads

With the development of high-speed and light-weight of trains, the security of the train becomes a problem of increasing concern. In order to create a local environment which has a relatively lower wind speed for trains, the wind barriers either one side or both sides are installed on the railway, which is an effective measurement to protect the trains from the strong cross wind. Therefore, in this study, the method of theory analysis, wind tunnel tests and numerical simulations are used to investigate the protection effects of railway wind barrier and its wind loads.Firstly, the wind tunnel tests are conducted to investigate the reasonable aperture of circle-hole wind barrier in scaled model, and study the similarity between circle-hole wind barriers with grid wind barriers. Under the condition of the same grid, the wind barriers are simulated by the boundary of wall and porous jump in FLUENT, and compare to the results of wind tunnel tests. Based on wind tunnel tests, the effects of Reynolds number of vehicles, wind field distribution, vehicle wind loads, and vehicle dynamic responses are studied. In the aspect of the vehicle Reynolds number, the curves of the aerodynamic force coefficients to Re are obtained by the numerical simulation method, and it shows that the drag coefficient satisfies the exponential law except for the sudden change region and transition region; furthermore, the relationship between the lift-drag ratio of vehicle and the flow pattern of the vehicle roof are discussed. In the aspect of wind field distribution, the profiles of wind pressure coefficient above track are measured by wind tunnel tests, and the aerodynamic mechanism is analyzed. In the aspect of vehicle wind loads, the height and porosity of wind barriers, track positions, and two trains meeting each other et al. are discussed, and the lift mechanism of railway wind barrier is proposed, i.e. after the airflow bypassing the wind barriers with certain height, the accelerated airflow (strong shear layer) will act on the roof of the vehicle, consequently, increase the local Reynolds number of vehicle roof, and lead to some differences of protection effect of wind barriers between the vehicle roof and bottom, which increase the lift-drag ratio of vehicle, and also may be increased the lift coefficient. In the aspect of vehicle dynamic responses, the coupled vibrations of wind-vehicle-bridge system method are adopted to calculate the dynamic responses when the wind barriers installed on the railway.Secondly, a theoretical relationship of moving vehicle aerodynamic force coefficient under the cross wind is proposed, and the physical meanings of theoretical relationship are analyzed; meanwhile, the reliability is verified by the experimental data in literature. In terms of the wind barrier drag mechanism, Baker’s hypothesis and the theoretical relationship, an approximate calculation formula of moving vehicle aerodynamic characteristics when the wind barriers are installed on one side is proposed, and it be verified by CFD. On this basis, a formula of additional aerodynamic force effect is suggested, which is only suitable for the last vehicle running on the ground roadbed, and it be verified by CFD in the multiple wind barrier cases. Further, the applicability of theoretical relationship, the approximate calculation formula, and the additional aerodynamic action in the different line structure forms are studied by CFD method.Thirdly, the cross wind loads of wind barriers are measured by wind tunnel tests, and the effect of height, porosity, and line structure forms are discussed. Furthermore, under the action of train-induced wind, the empirical fitting formula of the peak drag coefficients for wind barrier changed with distance is proposed, which is verified by numerical simulation method, the effects of distance, porosity, and vehicle speeds on the drag coefficient of wind barrier are discussed respectively. A finite element model of wind barrier is used to analyze the stress distribution, and the fatigue characteristics of wind barrier under the action of train-induced wind are further analyzed to determine whether the transient wind load will be considered as an evaluation index.Finally, in terms of the definition of vehicle aerodynamic force coefficients and the results of wind tunnel tests, it is presented that the effect of sudden change of vehicle wind loads can be measured by aerodynamic force coefficients of the single train. Because of the output indexes are the same, the magnitudes of sudden change of vehicle wind loads, and the vehicle dynamic responses are considered as the input indexes, respectively, and the method of DEA (Data Envelopment Analysis) is used to evaluate the performance of wind barrier. According to the evaluation results of wind load and vehicle response, the consistency between the index of wind loads and the index of responses are discussed.The results demonstrated as follows:the aerodynamic characteristics of moving vehicles under the cross wind were consisted of the movement item, static item, and boundary item, When the wind barrier installed the railway, the aerodynamic characteristic is also satisfied this theoretical relationship. The protection effect of wind barrier was the result of the combination effects of drag mechanism and lift mechanism, the lateral drag coefficient when the wind barrier installed on the railway could be better estimated by the approximate calculation method based on the drag mechanism, but the lift coefficient and side roll moment coefficient were somewhat less compared with CFD. The line structure forms had significantly effect on the wind field distribution, wind load of static vehicle and moving vehicle, and wind load of wind barrier under the cross wind. Under the action of train-induced wind, the transient wind loads of wind barrier decreased rapidly with the increasing of wind barrier distance, which led to a smaller stress range for the practical wind barrier. When the porosity is0%, the height is2.05m (the height above the track plane), the protection effect of wind barriers installed on ground roadbed was better, and the sudden change of vehicle wind loads was in good agreement with the vehicle response index.