# Study on the Impact of Ventilation Methods on Droplet Nuclei Transmission in Subway Carriages
## 3 Results
### Spatiotemporal Distribution of Cough Droplets Under the MV System  
When an infected person coughs, a large number of droplets are generated, and both the air supply velocity and ventilation method in enclosed spaces significantly affect droplet distribution. Figures 6 and 7 show the spatial distribution of droplets and droplet nuclei in Case 4 and Case 6. The central patient expels cough droplets forward. Figure 6 shows that the air supply velocity in the MV system is 0.8 m/s. At t = 1 s, droplets with varying initial diameters can be observed, and by t = 5 s, the particle travel distance expands outward under the influence of the initial kinetic energy. Droplets measuring 50-80 \(\upmu\)m diminish to below 50 \(\upmu\)m as a result of evaporation, transforming into droplet nuclei. During the dispersion of cough droplets, the water content serves as an evaporative component, and decreases significantly, particularly for particles around 20 \(\upmu\)m. Droplets of 100 \(\upmu\)m are deposited on the patient or the floor due to gravity. At t = 20 s, particles spread significantly forward at the initial speed of 13 m/s, with some larger particles continuing to move in the direction of the cough. However, the droplet size has noticeably reduced to below 50 \(\upmu\)m. The concentration range of the tracer gas N\({}_{2}\) shows that a significant portion of particles is concentrated in the direction of the cough. The movement of these droplets is predominantly affected by the patterns of indoor airflow, causing them to gradually move toward nearby passengers, the sidewalls, and the ceiling, before dispersing outward. At t = 120 s, the droplet dispersion range remains in the direction of the cough, with minimal impact on the breathing zones of passengers seated in the opposite direction.  
Figure 7 illustrates the scenario in Case 6, where the air supply velocity of the MV system is 1.6 m/s. It can be observed that, unlike at lower wind speeds, the droplets disperse more quickly and broadly at t = 1 s. At t = 5 s, the largest droplets complete a forward and downward movement cycle, ultimately depositing on the ground. In contrast to Case 6, at t = 20 s, the droplets disperse more widely and over a larger range, with a variety of particle sizes still present. In Case 6, due to the combined effect of the exhaled airflow and thermal plume, the droplets move upwards and disperse upon reaching the ceiling. The droplet nuclei move forward with the airflow. At t = 120 s, the particulate matter in the rear area of the carriage is almost completely removed, with no extensive dispersion into the breathing zones of other individuals in the carriage, contrasting sharply with the low wind-speed scenario. Notably, most of the droplet nuclei are concentrated  
Figure 5: Comparison of experimental and simulation results: Axial distribution of velocity and temperature.  
within a range of 0-4 m in front of the patient, forming a high concentration area. The dispersion pattern is similar to that observed in Case 4. However, at higher wind speeds, the distribution of droplet nuclei is more uniform, and they are more rapidly drawn into the return air vents, resulting in more uniform and swift dispersion due to the carrying effect of the airflow supply. Low wind speed is not ideal for preventing cross-infection.  
Figure 6: Spatiotemporal distribution of cough droplets in the carriage with an air supply velocity of 0.8 m/s under the MV system.  
Figure 7: Spatiotemporal distribution of cough droplets in the carriage with an air supply velocity of 1.6 m/s under the MV system.