# Numerical Investigation on the Droplet Dispersion inside a Bus and the Infection Risk Prediction
## 3 Geometry and Case Setups
### Case Setups  
To investigate the influence of the operation parameters and structural parameters on the droplet dispersion characteristics, 15 cases were discussed with different air supply rates, location of vents and location of an infected person (listed in Table 1). Although the air return vent is often located in the middle of the bus, some studies have shown that it may be locate at different positions. For example, the return air vent was located at the front of the bus in the studies by [PERSON] et al. (2018), [PERSON] et al. (2019), [PERSON] et al. (2019) Therefore, the effects of the location of the vent were considered in this study. The dispersion of coupled droplets in the cabin was studied when the infected passenger was located at Seat 2B, 6D, and 12C, respectively. Time steps was used to model the transient transport of droplets was 0.05 s. Moreover, Exhaled airflow from the mouth when the patient coughing was developed with user define functions (UDF). As shown in Figure 4, a cough lasts around 0.5 s, with a maximum velocity of approximately 9 m/s ([PERSON] et al., 2019; [PERSON] et al., 2020). What's more, there was a 30 degrees angle between the exhaled airflow and the horizontal plane. During a single cough, one hundred thousand 5 \(\upmu\)m-diameter droplets with a density of 980 kg/m\({}^{3}\) were released to ensure a stable distribution of droplets. Mouths and noses of other passengers were set as wall. As demonstrated in [PERSON] and [PERSON]'s study ([PERSON] and [PERSON], 2019), when the ambient humidity is less than 80%, the effect of evaporation on dispersion can be ignored for 5 \(\upmu\)m droplets. Hence, the evaporation of the droplets was neglected in this study. In addition, discrete random walk model was used to model turbulent dispersion of droplets. Figure 4: Flow velocity of a single cough with respect to time. Figure 3: Breathing area of Passenger. \begin{table}
\begin{tabular}{c c} \hline \hline
**Case Number** & **Case Description** \\ \hline Case 1.1 & Vents 1, Air supply velocity 3 m/s, index infected person is passenger 2B \\ Case 1.2 & Vents 1, Air supply velocity 3 m/s, index infected person is passenger 6C \\ Case 1.3 & Vents 1, Air supply velocity 3 m/s, index infected person is passenger 12C \\ Case 2.1 & Vents 2, Air supply velocity 3 m/s, index infected person is passenger 2B \\ Case 2.2 & Vents 2, Air supply velocity 3 m/s, index infected person is passenger 6C \\ Case 2.3 & Vents 2, Air supply velocity 3 m/s, index infected person is passenger 12C \\ Case 3.1 & Vents 3, Air supply velocity 3 m/s, index infected person is passenger 2B \\ Case 3.2 & Vents 3, Air supply velocity 3 m/s, index infected person is passenger 6C \\ Case 3.3 & Vents 3, Air supply velocity 3 m/s, index infected person is passenger 12C \\ Case 4.1 & Vents 4, Air supply velocity 3 m/s, index infected person is passenger 2B \\ Case 4.2 & Vents 4, Air supply velocity 3 m/s, index infected person is passenger 6C \\ Case 4.3 & Vents 4, Air supply velocity 3 m/s, index infected person is passenger 12C \\ Case 5 & Vents 3, Air supply velocity 2 m/s, index infected person is passenger 2B \\ Case 6 & Vents 3, Air supply velocity 3 m/s, index infected person is passenger 2B \\ Case 7 & Vents 3, Air supply velocity 4 m/s, index infected person is passenger 2B \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Case description.