  The transport of virus-laden aerosols from a host to a susceptible person is
governed by complex turbulent airflow, and physics related to breathing,
coughing and sneezing, mechanical and passive ventilation, thermal buoyancy
effects, surface deposition, masks, and air filtration. In this paper, we study
the infection risk via airborne transmission on an urban bus using unsteady
Reynolds-Averaged Navier--Stokes equations and a passive-scalar model of the
virus-laden aerosol concentration. Results from these simulations are directly
compared to the widely-used well mixed model, and show significant differences
in the concentration field and number of inhaled particles. Specifically, in
the limit of low mechanical ventilation rate, the well-mixed model will
over-predict concentration far from the infected passenger, and substantially
underpredict concentration near the infected passenger. The results reported
herein also apply to other enclosed spaces.
