Turbulence-sand Particle Motion Characteristics In Wind Field And Protection Optimization Of Railway Engineering

With the continuous improvement of the railway transportation network in the central and western regions of China,railway projects inevitably need to pass through desertification areas where there are various kinds of wind-blown sand disasters.Windblown sand disaster is one of the main environmental problems affecting railway operation and safety,causing many problems such as rail abrasion,sand buried,train rollover and so on.The movement of aeolian sand flow is mainly due to the transport of sand particles by the turbulent structure of atmospheric boundary layer(ABL),which is closely related to the interaction mechanism between turbulence and sand particles in the wind field.However,the current research on the characteristics of the flow field and particle movement of railway engineering in the wind-blown sand area,as well as the wind-prevention and sand-fixation researches do not reflect well the turbulent movement of the ABL and the discrete properties of sand particles,and cannot get a more suitable turbulence model for the actual environment wind-sand movement characteristics.Therefore,the thesis is based on the construction of the wind-sand flow field model,the characteristics of wind-sand flow field in typical working conditions,the optimization of protective measures design parameters as well as the advantages,disadvantages and application conditions of the model,and the main research contents and achievements are as follows:(1)Large Eddy Simulation(LES)and Lagrangian Discrete Phase Model(DPM)are used to simulate the fluid phase and the discrete phase in the wind-sand flow field respectively,and turbulent motion is introduced into the model to realize the influence of turbulent structure on particle motion,and a wind-sand flow field model close to the actual wind-sand environment was established.The wind-sand flow field model of a railway project is constructed.After meeting the requirements of the target turbulent wind field characteristics of the ABL,the calculation results can better reflect the pulsation and multi-scale characteristics of the turbulent wind field of the ABL.The motion trajectory of the particle phase strongly depends on the flow characteristics of the turbulent flow field,and the accumulated particles on the ground show the shape of sand ripples.The particle movement is more consistent with the actual sand.All these are consistent with the actual characteristics of aeolian sand flow.(2)Based on the wind-sand flow field model,the numerical simulation of the common transverse dune and embankment combination in desert area is carried out.It can be divided into low dune condition lower than embankment height and high dune condition higher than embankment height.The turbulent structure characteristics and particle movement rules under two types of working conditions are given,and the optimal design of protective measures under dune-embankment conditions is improved according to the distribution and movement rules of wind-blown sand caused by turbulent structure.(3)The numerical simulation of railway cutting is carried out based on wind-sand flow field model.The characteristics of vortex structure in cutting condition and embankment condition are compared.The influence of turbulent structure on particle motion and the distribution of particles in flow field are discussed.Then,according to the characteristics of the wind-sand flow field and the actual project,the optimized design parameters of the two protective measures,single-row sand retaining wall and double-row sand retaining wall,are proposed.(4)The characteristics of turbulence and particle movement in the transition section between embankment and bridge are discussed.It is revealed that the blocking effect of the embankment side and the diversion effect of the bridge side are the causes of the "detour" of the air carrying sand at the junction to the beam bridge side.It is pointed out that the protective measures should prevent the deflection of wind-blown sand flow and reduce the sediment accumulation at the foot of the embankment and under the bridge.According to the characteristics of turbulent flow and particle flow field,the "T" type sand retaining wall is designed to provide three-dimensional protection for the sand accumulation area of the embankment and bridge transition section.(5)The Reynolds Average Navier-Stokes(RANS)commonly used in engineering is compared with the turbulence model of wind-sand flow field based on LES-DPM proposed in the thesis.and the effects of the two algorithms on the flow field simulation results are discussed in terms of both particle transport and flow field characteristics.The results show that the LES-DPM model can obtain the turbulent pulsation characteristics,and the flow field characteristics can show more details.After interacting with the particle motion,the particle motion and deposition distribution rules can better satisfy the actual wind-sand flow field.However,in the actual engineering,it is necessary to select the appropriate algorithms in the design of windsand protection by referring to the complexity of the working structure,the accuracy required by the flow field information and the calculation time.