Abstract
Violent expiratory events, such as coughing and sneezing, are highly nontrivial examples of a two-phase mixture of liquid droplets dispersed into an unsteady turbulent airflow. Understanding the physical mechanisms determining the dispersion and evaporation process of respiratory droplets has recently become a priority given the global emergency caused by the SARS-CoV-2 infection. By means of high-resolution direct numerical simulations (DNS) of the expiratory airflow and a comprehensive Lagrangian model for the droplet dynamics, we identify the key role of turbulence on the fate of exhaled droplets. Due to the considerable spread in the initial droplet size, we show that the droplet evaporation time is controlled by the combined effect of turbulence and droplet inertia. This mechanism is clearly highlighted when comparing the DNS results with those obtained using coarse-grained descriptions that are employed in the majority of the current state-of-the-art investigations, resulting in errors up to $100\%$ when the turbulent fluctuations are filtered or completely averaged out.
Highlights
Turbulent transport of droplets in a jet or puff is a problem of paramount importance in science and engineering that nowadays has become even more important given the global emergency caused by the COVID-19 infection
As a first step in our analysis, we provide an overview of the observed dynamics by quantifying the number of airborne transmitted droplets and of those settling on the ground
We investigated the physical mechanisms involved in violent expiratory events such as coughing and sneezing, focusing on the evaporation and consequent airborne spread of small exhaled droplets
Summary
Turbulent transport of droplets in a jet or puff is a problem of paramount importance in science and engineering that nowadays has become even more important given the global emergency caused by the COVID-19 infection (for a recent review see, e.g., Refs. [1,2,3]). We attack the problem on the numerical side by performing accurate direct numerical simulations (DNSs) for the fluid flow and humidity field, complemented by a Lagrangian solver for the droplet dynamics including a dynamical equation for the evolution of the droplet radii modeling the evaporation-condensation process (see Fig. 1). Such an accurate description is nowadays possible thanks to the deep understanding achieved in the microphysics of small liquid droplets under different ambient conditions [12].
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