# Experimental and Numerical Investigation on Ballast Flight from Perspective of Individual Particles
## 1 Introduction  
The ballast track is one of the most important tracks of high-speed railway. It has been commonly applied in the world and has great potential for the advantages of low cost, small vibration and noise, and easy maintenance [1; 2]. In addition, the ballast track could be used in areas such as large span bridges and poor geologic zones, which are inaccessible for other track structures. However, with the increase of the operation speed of trains, the problem of ballast flight becomes more and more significant [3; 4; 5]. The ballast flight is the phenomenon by which the ballast particle goes out of the ballast bed due to train aerodynamic behavior mainly. The flying ballast may strike the rail, train body, and the infrastructures near the railway line, which will increase the maintenance cost and threaten the safety of operation. There are some prevention methods of ballast flight in the market, such as using ballast glue and a protection net. However, these measures will also increase the construction cost and the maintenance difficulty. The ballast flight limits the further development of the ballast track in high-speed railway and it is necessary to be investigated. The diverse reasons of ballast flight are complex, and some considerable research papers based on experiments and numerical simulations have been published recently. The experimental research includes field tests and laboratory tests, which concentrate on the flow above the ballast bed mainly. [PERSON] [4; 5] set up some strips of lightweight plastic tape on the ballast shoulder of a railway line, and the movement of these strips before and after the train passage was monitored by a high-frequency video camera. The airflow direction and its rate of change were investigated qualitatively. [PERSON] [6] set a view area of small ballast particles in a high-speed railway line to observe the occurrence anddevelopment of ballast flight at the train speed of 350 km/h. A set of sensors of air velocity and pressure were set up in the horizontal and vertical space above the ballast bed in a field experiment to monitor the flow during the train passage and to analyze the areas where ballast flight may occur easily [7; 8; 9; 10; 11; 12; 13]. The wind tunnel experiments of the ballast bed of both model scale and full scale were carried out to study the influence of track parameters on ballast flight [13; 14; 15; 16; 17]. The influence of sleeper type, track vibration, and snow on ballast flight was considered based on laboratory tests as well [17; 18; 19; 20; 21; 22; 23]. Moreover, numerical simulation models of the train and ballast bed were established with the method of computational fluid dynamics (CFD). The distribution characteristics of airflow above the ballast bed and the influence of the parameters such as the computational mesh size and track type on flow were discussed [6; 16; 17; 24; 25; 26; 27; 28]. The essence of ballast flight is the ballast movement of individual particles. Most previous studies on ballast flight have analyzed the aerodynamic feature of flow from the macroscopic properties of the ballast bed, which ignored the dynamics characteristics of individual ballast particles under wind load. Besides, the ballast bed is formed by granular ballast particles with different shapes and sizes. If using the integral model of a ballast bed [6; 24; 25] to investigate the mechanism of ballast flight, the influence of particle shape, mass, volume, or other factors could not be considered properly. Therefore, some researchers investigated the mechanism of ballast flight from the perspective of individual particles. [PERSON] and [PERSON] [7] classified the ballast particles as flat, half-spherical, and spherical by shape. The influence of shape on ballast flight was investigated based on a wind tunnel experiment of individual particles. [PERSON] [29] pointed out that the contact of the ballast and the platform had a marginal effect on ballast flight based on the CFD simulations of an individual ballast.