Propagation characteristics of exhaled flow and its role in indoor airborne transmission-A review

Human talks, coughs, and sneezes are the primary routes for airborne diseases such as COVID-19. The understanding of the propagation characteristics of these respiratory events is essential to estimate the infection risk of indoor airborne transmission. Turbulent vortex ring structures have been found in the human exhaled flow by using schlieren imaging technique and Direct Numerical Simulation (DNS). In this context, the traditional circular free air jets formula could not accurately predict the transmission distance of the turbulent flows and carried droplets. This paper systematically reviews the correspondences between jet morphologies and stroke ratios. Based on theories of traditional jets and vortex motion, theoretical analysis is conducted to compare the propagation characteristics of exhaled flow with steady jet, starting-interrupted jet, puff and vortex ring morphologies. Calculation results confirm that the vortex ring exhibits lower energy loss, enhanced resistance to disturbances, and longer transmission distances compared to traditional jets, which introduces new characteristics in terms of short-range transmission caused by exhaled flows. Future study could incorporate numerical simulation methods to further investigate the internal structural evolution of exhaled flow to deepen the understanding of the mechanisms of indoor airborne transmission.