## 3 Results and Discussion
### Droplet Distribution at Different Times within the Carriage
This is the reason that the ventilation system can expel the droplets more quickly within 0-30 s (Figure 11d). From the proportion of droplets expelled by the ventilation system, even though the expulsion speed of the droplets is faster under the condition of 1 m/s, the final proportion of droplets expelled by the carriage ventilation system is essentially the same in both scenarios. Furthermore, the deposited proportion of droplets is larger when the initial velocity decreases. By further categorizing the droplets deposited on the carriage surfaces, we found that droplets primarily settle on the carriage's ceiling  
Figure 11: Droplet distribution at different times: (**a**) suspended droplets; (**b**) deposited droplets; (**c**) trapped droplets; (**d**) escaped droplets (case 1: 10 μm–10 m/s—D1; case 2: 50 μm–10 m/s—D1; case 3: 100 μm–10 m/s—D1; case 4: 10 μm–10 m/s—D2; case 5: 10 μm–10 m/s—D1; case 6: 50 μm–10 m/s—_RH_ 35%; case 7: 10 μm–10 m/s—4 p/m\({}^{2}\)). rather than the floor. According to the diffusion process of the droplets in Figure 8a,e, the droplets have already started to disperse at the shoulder height of the carriage passengers when the initial velocity is 1 m/s. When the droplets are released at 10 m/s, the droplets first sink to the floor, and then spread upward with the airflow. Droplets with low initial momentum are inclined to disperse faster than those with high initial momentum. The upward airflow inside the carriage also increases the possibility that droplets are captured on the ceiling during the expelling process. Figure 11 demonstrates that the \(RH\) does not impact the ratio of suspended and escaped droplets within the carriage. However, the \(RH\) notably influences the proportion of droplets deposited on the carriage surfaces and trapped by passengers' bodies (cases 2 and 6). At an \(RH\) of 35%, the proportion of trapped droplets is approximately twice that at 70% \(RH\). Conversely, at 70% \(RH\), the proportion of deposited droplets is roughly double that at 35% \(RH\). This outcome is linked to the droplet evaporation rate under varying humidity conditions. When the \(RH\) is at 70%, the time required for droplets of the same size to complete the evaporation process is longer than in an environment with 35% humidity. In environments with a higher humidity, the duration of gravity plays a dominant role in the droplet movement, resulting in more droplets being deposited on the carriage floor. It also means that under the present ventilation system, maintaining a higher \(RH\) can reduce the droplets' evaporation rate, and it facilitates the rapid settling of larger droplets on the carriage floor, ultimately contributing to a significant reduction in the possibility of droplets being inhaled or trapped by passengers. The proportion of suspended droplets remains relatively consistent as the passenger density increases from 2 p/m\({}^{2}\) to 4 p/m\({}^{2}\) (cases 5 and 7). However, with the number of passengers increasing, the airflow within the carriage is obstructed by the passengers' bodies, resulting in a notable reduction in the expulsion speed of the droplets. Furthermore, increasing the passenger density from 2 p/m\({}^{2}\) to 4 p/m\({}^{2}\) leads to more droplets being trapped by passengers' bodies, escalating from less than 20% to over 50% (Figure 11c). Therefore, appropriately limiting the number of passengers inside subway cars is also an effective way to reduce the transmission of virus-laden droplets. Overall, the suspended time of droplets in the carriage primarily depends on the initial droplet size. Additionally, when passengers are standing in the middle section of the carriage, where the droplet sizes are the same, the suspension time of the droplets is reduced. The proportion of droplets deposited on the carriage surfaces is closely related to the droplet size and significantly influenced by the initial release velocity of the droplets.