  The dispersion of viral droplets plays a key role in the transmission of
COVID-19. In this work, we analyze the dispersion of cough-generated droplets
in the wake of a walking person for different space sizes. The air flow is
simulated by solving the Reynolds-Averaged Navier-Stokes equations, and the
droplets are modelled as passive Lagrangian particles. Simulation results show
that the cloud of droplets locates around and below the waist height of the
manikin after two seconds from coughing, which indicates that kids walking
behind an infectious patient are exposed to higher transmission risk than
adults. More importantly, two distinct droplet dispersion modes occupying
significantly different contamination regions are discovered. A slight change
of space size is found being able to trigger the transition of dispersion modes
even though the flow patterns are still similar. This shows the importance of
accurately simulating the air flow in predicting the dispersion of viral
droplets and implies the necessity to set different safe-distancing guidelines
for different environments.
