## 5 Conclusions and Outlook  
1. A three-dimensional gas-solid-coupling calculation model of spalling blocks, a train, a tunnel, and air was established on the basis of the basic theory of gas-solid interaction and the numerical simulation software, FLUENT. The transient coupling between a spalling block of a tunnel lining and turbulent train wind, as well as the numerical simulation of the entire falling process, were realized. The simulation results were consistent with the test results.
2. The aerodynamic evolution mechanism governing the falling motion of a flaky spalling block associated with a high-speed-railway tunnel lining operated as follows: The falling process of the block was affected by the transient coupling of its characteristics and the transient train wind. The train wind acted directly on the block, leading to horizontal movement of the block and consequent phenomena such as circumventeduce and vortex that further changed the motion state of the block. Due to its changed motion state, the spalling block affected the flow field structure.
3. The impact of train wind was more significant for a flaky spalling block than for a blocky spalling block. The spalling blocks that peeled off the tunnel vault risked colliding directly with the train and had a considerable effect on driving safety. The falling lining blocks that peeled off the tunnel's arched waist were attracted to the  
Figure 18: Impact partitions of falling blocky lining block on driving safety (unit: meters).  
train wind and risked colliding with the side of the vehicle body or becoming involved with the wheel and rail, thereby affecting driving safety. The falling lining blocks that peeled off the tunnel side wall were repelled by the wind of the train and moved away from the vehicle body; hence, they had no effect on driving safety.
4. The impact partitions of the flaky blocks on driving safety were divided into three areas along the circular direction of the tunnel lining: dangerous zone, risk zone, and safety zone. The dangerous areas of flaky and blocky blocks were 5.5 and 7.5 m above the ground in the vault area, respectively, indicating that flaky blocks are more dangerous. Thus, during the operation of a tunnel, attention should be paid to the maintenance of the weak area above 5.5 m above the ground to ensure driving safety.  
Although conclusions about safety zone were drawn based on a specific case, the research method adopted in this study may open a path for the further optimization of safety partitions in train tunnels. To obtain a comprehensive safe driving zone, we must consider the different densities, shapes, and sizes of lining blocks along with vehicle speeds and other factors. Such requirements were not part of the contents presented and should be a part of future work.  
Conceptualization, C.-H.S.; methodology, W.-C.Y.; software, A.W.; validation, X.-H.S. and A.W.; formal analysis, A.W.; investigation, A.W.; resources, W.-C.Y.; writing--original draft preparation, A.W.; writing--review and editing, X.-H.S.; visualization, A.W.; supervision, C.-H.S.; project administration, W.-C.Y.; funding acquisition, W.-C.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Natural Science Foundation of China, grant number: 51978670.  
Not applicable.  
Some or all data, models, or codes that support the findings of this study are available from the corresponding author upon reasonable request.  
The authors declare no conflict of interest.