Abstract
Recently, the COVID-19 virus pandemic has led to many studies on the airborne transmission of expiratory droplets. While limited experiments and on-site measurements offer qualitative indication of potential virus spread rates and the level of transmission risk, the quantitative understanding and mechanistic insights also indispensably come from careful theoretical modeling and numerical simulation efforts around which a surge of research papers has emerged. However, due to the highly interdisciplinary nature of the topic, numerical simulations of the airborne spread of expiratory droplets face serious challenges. It is essential to examine the assumptions and simplifications made in the existing modeling and simulations, which will be reviewed carefully here to better advance the fidelity of numerical results when compared to the reality. So far, existing review papers have focused on discussing the simulation results without questioning or comparing the model assumptions. This review paper focuses instead on the details of the model simplifications used in the numerical methods and how to properly incorporate important processes associated with respiratory droplet transmission. Specifically, the critical issues reviewed here include modeling of the respiratory droplet evaporation, droplet size distribution, and time-dependent velocity profile of air exhaled from coughing and sneezing. According to the literature review, another problem in numerical simulations is that the virus decay rate and suspended viable viral dose are often not incorporated; therefore here, empirical relationships for the bioactivity of coronavirus are presented. It is hoped that this paper can assist researchers to significantly improve their model fidelity when simulating respiratory droplet transmission.
Highlights
As of August 2021, the COVID-19 virus pandemic has infected more than 216 484 958 people and caused more than 4 503 000 deaths worldwide.[1]
The COVID-19 virus pandemic has led to many studies on the airborne transmission of expiratory droplets
The mechanisms of transmission of droplets and aerosols in the indoor and outdoor spaces are poorly understood,[12] because their dispersion range depends on several factors such as the evaporation rate, flow pattern, initial particle size distribution (PSD) at the mouth/nose release point, the time-dependent velocity profile of exhaled jet during coughing and sneezing, thermal plumes associated with the warm body surface temperature,[13,14,15] and environmental relative humidity and temperature.[16]
Summary
As of August 2021, the COVID-19 virus pandemic has infected more than 216 484 958 people and caused more than 4 503 000 deaths worldwide.[1]. Virus infection can either be transmitted by larger droplets upon direct contact over short (i.e., 2 meter-scale) social distance or by smaller airborne droplets and droplet nuclei in the air over a much longer distance range ($10 m or more).[10] The mechanisms for long-range and short-range transmissions are different.[11] Disease transmission is a complex and interdisciplinary process involving microbiology, environmental science, and social science.[10] The mechanisms of transmission of droplets and aerosols in the indoor and outdoor spaces are poorly understood,[12] because their dispersion range depends on several factors such as the evaporation rate, flow pattern, initial particle size distribution (PSD) at the mouth/nose release point, the time-dependent velocity profile of exhaled jet during coughing and sneezing, thermal plumes associated with the warm body surface temperature,[13,14,15] and environmental relative humidity and temperature.[16] All these complexities present significant challenges when one attempts to perform numerical simulations of expiratory droplets and aerosols Another complexity for numerical modeling is the proper boundary conditions.
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