# Numerical Investigation on the Droplet Dispersion inside a Bus and the Infection Risk Prediction
###### Abstract  
COVID-19 can be easily transmitted to passengers by inhaling exhaled droplets from the infected person in a bus. Therefore, studying droplet dispersion would provide further insight into the mechanism of virus transmission and predict the risk of infection among passengers on a bus. In this research, a bus equipped with air-conditioning was employed as the research object. To determine the dispersion path, concentration distribution, and escape time of the droplets, computational fluid dynamic (CFD) was applied to simulate the flow field and the droplets' dispersion. The effect of the air supply rate, the location of vents, and the location of infected persons on the dispersion were discussed. Based on the distribution of droplets in the cabin calculated by CFD, a superposition method was used to determine the number of virus particles inhaled by every individual passenger over a four-hour journey. Then, infection risk was assessed by the Wells-Riley equation for all the passengers in the cabin after the whole journey. The results show that the distribution of droplets in the cabin is greatly influenced by the location of the infected person, and the airflow pattern is highly associated with the air supply rate and the location of vents. The infection risk of passengers located at the droplet dispersion path and the distance from the infected persons less than 2.2 m is over 10%. The increase in the air supply rate could speed up the spread of the droplets but at the same time, it could reduce the infection risk. bus; droplet; infection risk; CFD; Wells-Riley equation +
Footnote †: journal:As early as 2012, a study by [PERSON] et al. [11] showed that the probability of infection of passengers close to an infected person and in the airflow path was up to 27.2%. Since the outbreak of the COVID-19, several studies have been conducted studies on the dispersion of droplets in the bus. The role of air supply directions and relative humidity was studied by [PERSON] et al. [12]. The results showed that the droplet dispersion was mainly influenced by gravity, airflow field, and upward thermal body plume. [PERSON] et al. [13] tested four cases with different boundary conditions to investigate the coughed droplets' transport characteristic from an infected person in a rail cabin. Their results showed that the droplets remove more efficiently and spread much farther with the flow from the front to back. [PERSON] et al. [14] studied the effects of ventilation systems, opening windows, and wearing masks, and proposed strategies to reduce the infection risk. They found that opening windows could help lower the droplets concentration by almost 50%, and wearing a mask can significantly minimize the infection risk. [PERSON] et al. [15] studied the effects of the window, contaminant source, and output flow location on the dispersion of droplets in the train cabin. Their study showed that opening a window reduces the duration of particles remaining in the cabin, and the best position for the infected person was near the exit. However, the effect of operation and structural parameters of the bus on droplet spread, such as air supply rate and location of vents, have not been revealed yet. Thereby, it's of great necessity to conduct further research. Generally, virus particles are always released with the droplets exhaled by the infected person when they are coughing or sneezing [16; 17; 18]. Thus, an appropriate risk assessment model may be established based on the number of inhaled droplets to estimate the risk of infection [19; 20]. Therefore, it is important to obtain accurate distribution of droplet. Computational fluid dynamic (CFD) is an effective and economical approach to modeling the transport of droplets in enclosed setting [21]. Hence, the transport and distribution of droplets from the coughing of an infected person in a bus were investigated using CFD method in this paper. It was possible to determine how many droplets passengers inhaled.