Numerical investigation of the effects of environmental conditions, droplet size, and social distancing on droplet transmission in a street canyon

This study investigated interpersonal droplet transmission between a healthy and an infected person in a two-dimensional (2D) street canyon using Computational Fluid Dynamics (CFD) simulation. The CFD simulation modeled four droplet sizes (D_p), two ambient wind speeds (U_ref), three relative humidity (RH) levels, and four social distances (D) to estimate their effects on interpersonal droplet transmission. The results revealed that under weak ambient winds (U_ref = 1.54 m/s), droplets had more upward movements and longer airborne time, but strong dispersions under high winds (U_ref = 6.68 m/s). RH has trifling influence on the dispersion of small droplets (D_p = 10, 25 and 50 μm), but significantly modify the dispersion of large droplets (D_p = 100 μm), especially under low ambient winds. Small droplets travel longer distances in dry air (RH = 35%) and either land on the surrounding buildings' walls or become suspended in the air. In contrast, 45% of the large droplets are inhaled by pedestrians or become suspended in the dry air. But when evaporation is slow at RH = 95%, they would rapidly fall to the ground or land on the pedestrians’ lower bodies due to heavy mass. With farther apart social distancing, small droplets pose little infection risks, but that risk of infection increased for large droplets due to complex interactions with the ambient airflow and gravity. A 2 m social distance is recommended for pedestrians in deep urban street canyons with high winds, and 4 m if winds and RH are low.