Mechanism And Preventive Measures Of Ballast Flight For Ballast Track In High-speed Railway

Ballast track is one of the main track structure forms of high-speed railway.It has the advantages of low cost,low vibration and noise,easy maintenance and strong applicability.Ballast track has to be used in long-span bridges,poor geology zones.Its outstanding advantages have been paid attention to and widely used by most countries in the world,and it has tremendous potential for development.With the increase of running speed,ballast flight occurs from time to time.When the train passes at high speed,ballast particles on the surface of the ballast bed will fly away and hit the track,train or the facilities near the railway line,which seriously increases the maintenance cost of the track and train,endangers the operation safety and restricts the further development of ballast track of high-speed railway.However,the occurrence and development mechanism of ballast flight in high-speed railway is extremely complex,and its obvious uncertainty make it difficult to use traditional experimental and theoretical methods for analysis.Aiming at the above problems,the ballast flight analysis model system for high-speed railway was established,and the wind tunnel test of ballast flight was carried out.The occurrence mechanism of ballast flight was studied by analyzing the airflow of ballast particles and ballast track.The development mechanism of ballast flight was studied by analyzing the movement and dynamic response characteristics of ballast flight.The preventive measures based on the influence of ballast bed parameters and sleeper parameters on ballast flight were studied.And suggestions on scientific,reasonable and effective preventive measures of ballast flight were proposed.The main research contents and results are as follows:(1)The flow analysis model and dynamic analysis model of ballast flight were established,which formed the ballast flight analysis model system for high-speed railway.The flow analysis model of ballast particles was established based on the refined acquisition,analysis and reduction method of particle shape,by which model the airflow of ballast particle was analyzed.The ballast bed was considered as a porous medium structure,the flow analysis model of ballast track was established based on the sliding grid analysis method to investigate the airflow of ballast track.The dynamic analysis model of ballast flight was established based on the CFD-DEM-MBD coupling method.It could intuitively analyze the movement and dynamic response characteristics of ballast flight under wind-vibration condition.And it could reflect the shape and gradation of ballast,the influence of airflow on ballast particles,and the interaction between sleeper and ballast bed.(2)The wind tunnel test of ballast flight was carried out to analyze the airflow of ballast particles and ballast track.And the ballast flight analysis model system was validated.The influence of particle properties and external factors on ballast flight,as well as the airflow strength and distribution of ballast track were analyzed based on the wind tunnel test of ballast particles and ballast track.The ballast flight analysis model system was verified based on the self-developed ballast particle wind pressure testing device and the wind velocity measuring points above ballast track.Research shows,the ballast with a large mass is not easy to fly.The ellipsoid ballast is the easiest to fly,followed by sphere,prolate and oblate ballast with similar mass.The higher the distance from the surface of the ballast bed is,the closer to the center of the railway line is,the greater the wind velocity becomes,and the smaller the variation range of the wind velocity gets.The wind velocity decreases by 63.9% from the rail head to the surface of the ballast bed.And it decreases by 62.1% from the center of the railway line to the vicinity of the steel rail.(3)The occurrence mechanism of ballast flight was analyzed based on the flow analysis model,which investigated the flow filed characteristics of ballast particles and ballast bed.The influence of mass,shape,wind direction and wind velocity on airflow characteristics of ballast particles were studied based on the airflow analysis model of ballast particle.The distribution characteristics and time-history characteristics of the wind pressure and wind velocity above the track during the train running were studied based on the airflow analysis model of ballast track,as well as the influence of the running speed and cross wind conditions on the ballast track airflow.Research shows,the wind pressure on the windward side of the ballast is positive,while the top,side and back is negative.During the train passage,the wind pressure of ballast track presents the fluctuation characteristics of "positive-negative-negative-positive ".The wind velocity on the ballast bed is maximum when the head and rear of the train pass through.The pressure difference caused by the head train is 54.6% higher than that caused by the tail.The maximum pressure difference at the center is 1.4-1.7 times than that at the edge.The probability of ballast flight at the center of ballast bed is 95.3% than that of the sleeper,and 2.2 times than that of the shoulder.The maximum pressure difference is proportional to the square of running speed.The maximum pressure difference is 46.7% higher when the cross wind speed is 30 m/s than there is no cross wind.(4)The development mechanism of ballast flight was analyzed based on the dynamic analysis model,which investigated the behavior and dynamic response characteristics of ballast flight.The movement and dynamic response characteristics of ballast flight were studied under the condition of the train aerodynamic load,track dynamic load and the coupling load respectively,based on the dynamic analysis model with CFD-DEM-MBD coupling algorithm.The effects of train speed and axle load on ballast flight were also studied.Research shows,ballast flight mainly occurs on the inner surface of ballast bed,and is concentrated in the central area of the railway line.The ballast particles will not fly under the track dynamic load only.However,the track dynamic load has a significant aggravating effect on ballast flight,which could not be neglected.Under the condition of wind and vibration,the critical running speed of ballast flight decreases from 350 km/h to 300 km/h,and the number of flying ballast is 8.3 times than that under the train aerodynamic load only.The number of flying ballast increases gradually with the increase of running speed and axle load.(5)The influence of ballast bed parameters and sleeper parameters on the airflow and dynamic characteristic of ballast flight were studied,based on the ballast flight analysis model system.And suggestions on preventive measures of ballast flight were proposed.The influence of ballast bed parameters and sleeper parameters on the preventive effect of ballast flight were studied based on the flow field characteristics and dynamic characteristics of ballast flight.Suggestions on effective preventive measures of ballast flight only by adjusting the existing track parameters were proposed.Research shows,reducing the height of ballast bed,shoulder and slope can prevent ballast flight effectively,and the effect of adjusting ballast bed is the best.Reducing sleeper spacing and smoothing sleeper corner can also reduce ballast flight significantly.The effect of sleeper lengthening on ballast flight control is poor,and the effect of sleeper lengthening is better.In order to carry out comprehensive preventive measures of ballast flight and deterioration,the small diameter ballast of 22.4 mm to 31.5 mm can be reduced on the surface of ballast bed,and the wide grade ballast of 22.4 mm to 63 mm can be restored below the sleeper.The influence of proposed preventive measures on the construction,operation and maintenance of railway lines should also be considered comprehensively.