We provide research findings on the physics of aerosol and droplet dispersion relevant to the hypothesized aerosol transmission of SARS-CoV-2 during the current pandemic. We utilize physics-based modeling at different levels of complexity, along with previous literature on coronaviruses, to investigate the possibility of airborne transmission. The previous literature, our 0D-3D simulations by various physics-based models, and theoretical calculations, indicate that the typical size range of speech and cough originated droplets ( ) allows lingering in the air for ) so that they could be inhaled. Consistent with the previous literature, numerical evidence on the rapid drying process of even large droplets, up to sizes , into droplet nuclei/aerosols is provided. Based on the literature and the public media sources, we provide evidence that the individuals, who have been tested positive on COVID-19, could have been exposed to aerosols/droplet nuclei by inhaling them in significant numbers e.g. . By 3D scale-resolving computational fluid dynamics (CFD) simulations, we give various examples on the transport and dilution of aerosols ( ) over distances in generic environments. We study susceptible and infected individuals in generic public places by Monte-Carlo modelling. The developed model takes into account the locally varying aerosol concentration levels which the susceptible accumulate via inhalation. The introduced concept, ’exposure time’ to virus containing aerosols is proposed to complement the traditional ’safety distance’ thinking. We show that the exposure time to inhale aerosols could range from to or even to depending on the situation. The Monte-Carlo simulations, along with the theory, provide clear quantitative insight to the exposure time in different public indoor environments.

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Measurements with both two-dimensional (2D) two-component and three-component stereo particle image velocimetry (PIV) and computation in 2D and three-dimensional (3D) using Reynolds stress turbulence model with commercial code are carried out in a square duct backward-facing step (BFS) in a turbulent water flow at three Reynolds numbers of about 12,000, 21,000, and 55,000 based on the step height h and the inlet streamwise maximum mean velocity U0. The reattachment locations measured at a distance of Δy=0.0322h from the wall are 5.3h, 5.6h, and 5.7h, respectively. The inlet flow condition is fully developed duct flow before the step change with the expansion ratio of 1.2. PIV results show that the mean velocity, root mean square (rms) velocity profiles, and Reynolds shear stress profiles in all the experimental flow cases are almost identical in the separated shear-layer region when they are nondimensionalized by U0. The sidewall effect of the square BFS flow is analyzed by comparing the experimental statistics with direct numerical simulation (DNS) and Reynolds stress model (RSM) data. For this purpose, the simulation is carried out for both 2D BFS and for square BFS having the same geometry in the 3D case as the experimental case at the lowest Reynolds number. A clear difference is observed in rms and Reynolds shear stress profiles between square BFS experimental results and DNS results in 2D channel in the spanwise direction. The spanwise rms velocity difference is about 30%, with experimental tests showing higher values than DNS, while in contrast, turbulence intensities in streamwise and vertical directions show slightly lower values than DNS. However, with the modeling, the turbulence statistical differences between 2D and 3D RSM cases are very modest. The square BFS indicates 0.5h–1.5h smaller reattachment distances than the reattachment lengths of 2D flow cases.

Microbial aerosols can be used as model particles for examining the dispersion and deposition of particles as well as assessing the reliability of the simulation methods. For example, the computational fluid dynamics model (CFD) can be used in the evaluation of indoor microbial contamination and the possible spread of harmful microbes in spaces with high densities of people or in special hermetic environments. The aim of this study was to compare the results of the CFD simulation, which predicts the deposition of biological particles on the surfaces of a spacecraft, and real particle deposition, using Bacillus licheniformis/aerius bacterium particles as the model organism. The results showed that the particles were mainly deposited on floor surfaces, but also onto the supply air diffusers, where bacterial concentrations were higher than on the wall and ceiling surfaces. The CFD simulation showed similar trends with actual particle dispersal, conducted in this experiment with Bacillus particles.

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