# Experimental and Numerical Investigation on Ballast Flight from Perspective of Individual Particles
## 4 Conclusions  
The occurrence and development mechanisms of ballast flight were investigated from the particle perspective of aerodynamics. A wind tunnel experiment and CFD numerical simulation of ballast particles were utilized to analyze the movement characteristics of ballast particles under wind load, the flow around ballast particles, and the influence of the ballast mass and shape, as well as windward direction and wind speed on ballast flight. Besides, a series of preventive suggestions of ballast flight  
Figure 18: Air pressure difference on the ballast surfaces. were proposed, which had been adopted in the high-speed railway line of Beijing-Zhangjiakou in China and had a good effect. The main conclusions are as follows:  
1. The local features of ballast particles can be obtained accurately and efficiently based on the capture, reconstruction, and analysis method, which provides reliable technical means for investigating the mechanism of ballast flight in a wind tunnel experiment and CFD numerical simulation. 2. The phenomenon of slight vibration, violent vibration, roll, slow movement, and fast movement occur successively in the initial stage of ballast flight. The massive ballast is less likely to fly. The ballast of 5 g was blown off first at the wind speed of 14.22 m/s, and the ballast of 200 g was blown off last at the wind speed of 29.89 m/s. 3. The oblate ballast is the least prone to fly under wind load, which has the smallest flow separation, wake flow, and surface pressure difference; in contrast, the ellipsoid ballast is the easiest to fly, followed by the spherical and prolate ballast particles. A ballast particle that has its short side facing the wind is less likely to fly than one that has its long side facing the wind. The pressure difference of the ballast is proportional to the square of the wind speed. The average pressure difference of the ballast increases 14.9 times when the wind speed increases from 10 m/s to 40 m/s. 4. In order to avoid ballast flight, the ballast gradation could be adjusted by limiting the ballast particles with small size as well as with the shape of ellipsoid, and the ballast particles with high density or large size could be laid on the top of ballast bed before the operation of a high-speed railway line. During the operation, the ballast bed should be cleaned in time to reduce the ballast breakage and contamination caused by deterioration. The compaction operation should be carried out in a timely manner after tamping to improve the stability of the ballast bed. It should be noted that the intensity and distribution of the airflow underneath a high-speed train varies significantly over time. The purpose of this paper is to investigate the influence of the particle properties on the ballast flight. For this reason, the equivalent wind load was applied to the model in the wind tunnel experiment and the CFD simulation, rather than the actual wind load underneath high-speed train. Moreover, the test bench instead of the ballast bed was set up to place the ballast samples in this paper, which could eliminate the interference between particles on the ballast bed surface and focus on the influence of quality and shape on ballast flight. However, the mechanical characteristics of the ballast particles under wind load on the test bench were different from those of the ballast bed. Therefore, future work should consider the particles in the ballast bed under the actual wind load underneath a high-speed train on the basis of this paper. Furthermore, the ballast flight is the phenomenon of ballast particles flying off the track. The future research could focus on the coupling method of CFD and DEM (discrete element method), which could simulate and analyze the flying behavior of ballast particles directly. Conceptualization, H.Y. and L.G. ; Funding acquisition, L.G. ; Investigation, H.Y. ; Supervision, L.G. ; Writing--Review and editing, H.Y. and L.G. All authors have read and agreed to the published version of the manuscript.