Sudden Change Mechanism Of Wind Forces On The Train And Running Safety Analysis In The Wake Of Bridge Tower

The dynamic behavior and safety of both the train and the bri dge in crosswinds are concerned increasingly.On one hand,it is due to the massive scale of high-speed railway(HSR)construction,which is inevitable in some populous nations for transporting their high density of population and for trade transactions.T he HSR lines are rapidly developing,leading to a great but steady increase in the operation speed of trains.The highly raised train speed,to some extent,adds to the risk of fatal accidents of trains under extreme windy environments.The bridge engineering community has presently entered a new era which the construction of bridges crossing mountainous and oceanic terrain or connecting islands needs to be addressed.Long-span bridges are favored by designers because of its strong crossing ability and conv enient navigation.The spans of the suspension and cable-stayed bridges have been challenging new limits.The size of the bridge tower also increases with a large proportion even reaching tens of meters along the longitudinal bridge direction.The existenc e of the bridge tower will change the windy environment around the deck.The authentic aerodynamic forces of the vehicle are determined by the flow field around it.In this scenario,the sudden change of transverse wind speed in the tower region is more li kely to induce the lateral instability of vehicles and handling/controllability problem for the driver.This lies in the dramatic change of aerodynamic forces acting on the vehicle as it passes through the bridge tower.When the vehicle drives through the tower,the vehicle will be tightly blocked.The wind loads acting on the vehicle will decrease as it enters the tower region,and then increase as it leaves the region,experiencing a sharp change with a high risk that the vehicle can be turned over.The f low field near the tower region is complicated.For better evaluation of the running safety of the vehicle and improving the ride comfort of passengers as well as to provide perspectives for accident prevention,the study on the aerodynamic characteristics of the vehicle crossing the wake of the bridge tower is of critical importance.This thesis mainly contains the following work:(1)The dynamic performance and safety of both the train and the bridge are great concerns for long-span bridges subjected to c rosswinds.For railway bridges,there is track laid on the bridge deck.The wind,train,track,and bridge subsystems form a complicated coupling system.This paper presents the dynamic responses of both the train and the bridge under crosswinds based on a wind-train-track-bridge coupled model,in which the vibration effect of the track structure involved in the coupling system is especially considered.The train adopts the railway vehicle dynamics model and the bridge adopts the three-dimensional finite el ement model,considering the interaction between the train and the track,the track and the bridge respectively.The wind action on the train and the bridge consists of a steady-state force and an unsteady-state force.Based on an in-situ test,the proposed numerical wind-train-track-bridge model involving the vibration effect of the track is validated.Then,the application of this proposed model on a long-span cable-stayed bridge on the Shanghai-Nantong HSR is presented as a case study.The running train and the track modeled in this study are typical types currently serving in the Chinese railway transportation system.Excitations from track irregularities are also taken into account.Finally,with the proposed wind-train-track-bridge coupled model,the d ynamic performance of both the train and the bridge subjected to crosswinds are investigated.(2)An experimental investigation into the changes in aerodynamic loads on a high-speed train due to the presence of the bridge tower is presented whilst also considering train motion.An innovative moving vehicle device which can apply measurements with shelters(e.g.the truss bridge,bridge tower,oncoming vehicle,wind barrier,and tunnel),compel the vehicle to a high driving speed,and adjust incoming wind directions,was developed.A truss girder and a typical high-speed train geometry were selected as the prototype with bridge towers at both sides of the deck with a scaled ratio of the bridge,train,and tower of 1:30.Systematic experiments were performed t o investigate the variation of aerodynamic forces on the train as well as its sudden change mechanism as the train passes through the wake of the bridge tower.This investigation considers the influences of various parameters such as the incoming wind spee d,train speed,and yaw angle which represents the first step towards a larger research project.The results show that the bridge tower has an apparent shielding effect on the train passing through it.The train speed is the main factor affecting the influ encing width of aerodynamic coefficients,and the mutation amplitude is mainly related to the yaw angle obtained by changing the incoming wind speed or train speed.The vehicle movement introduces an asymmetry in the process of approaching and leaving the wake of the bridge tower,and should not be neglected.(3)Through the systematic test,the change of train aerodynamic force s when it passes through the wind barrier in the bridge tower area is investigated,and the effects of various parameters are consi dered,such as wind speed,train speed,wind barrier form and wind barrier porosity.Based on the test data of train aerodynamic coefficient time histories when it passes through the wind barrier in the bridge tower area,the influence of sudden change win d loads due to the wind barrier under crosswind on the train running performance is discussed.The results show that wind barriers can decrease the wind load on the moving train effectively.In the wind barrier shielding area,the aerodynamic parameter cur ves are stable,and the drag coefficient of train is more sensitive to the change of wind speed than the lift and the moment coefficient,while the train speed has little impact on the average value of each coefficient.The influential range and extent of aerodynamic parameters of the train vary with different barrier porosities.When a wind barrier is installed near the pylon,the lateral acceleration of train decreases as well as the amplitude of the wheel-rail force,which illustrates that the wind barri ers can enhance the running safety of the train under crosswind.