Direct numerical simulation of turbulent dispersion of evaporative aerosol clouds produced by an intense expiratory event

Abstract

Airborne particles are a major route for transmission of COVID-19 and many other infectious diseases. When a person talks, sings, coughs, or sneezes, nasal and throat secretions are spewed into the air. After a short initial fragmentation stage, the expelled material is mostly composed of spherical particles of different sizes. While the dynamics of the largest droplets are dominated by gravitational effects, the smaller aerosol particles, mostly transported by means of hydrodynamic drag, form clouds that can remain afloat for long times. In subsaturated air environments, the dependence of pathogen-laden particle dispersion on their size is complicated due to evaporation of the aqueous fraction. Particle dynamics can significantly change when ambient conditions favor rapid evaporation rates that result in a transition from buoyancy-to-drag dominated dispersion regimes. To investigate the effect of particle size and evaporation on pathogen-laden cloud evolution, a direct numerical simulation of a mild cough was coupled with an evaporative Lagrangian particle advection model. The results suggest that while the dispersion of cough particles in the tails of the size distribution are unlikely to be disrupted by evaporative effects, preferential aerosol diameters (30–40 μ m) may exhibit significant increases in the residence time and horizontal range under typical ambient conditions. Using estimations of the viral concentration in the spewed fluid and the number of ejected particles in a typical respiratory event, we obtained a map of viral load per volume of air at the end of the cough and the number of virus copies per inhalation in the emitter vicinity.

A model to predict the short-term turbulent indoor dispersion of small droplets and droplet nuclei released from coughs and sneezes

“…For the other group of particles, their diameters were kept constant (non-evaporative particles). According to Fabregat et al, 6 the overall characteristics of the cloud constituted by small particles (4, 8, 16 and 32 μm) were essentially independent of the evaporation of particles. Evaporation was found to effectively decrease the diameter of smaller particles, as reported elsewhere, 7,47–51 but the shape of the cloud of small droplets and droplets nuclei (<20–30 μm) was insensitive to the evaporation (see, for example, Figure 8 and 9 in Fabregat et al).…”

“…34 LES and DNS are far more expensive with about some hundreds of thousands CPU hours. 6,30 Table 1 also indicates the physical situation considered in each study, the size of the computational domain and the number of mesh elements used. Most of the studies indicated in Table 1 include computational grids that resolve the exit geometry of the mouth with elements sizes in the range of a few millimetres in this region and a significantly coarser grid in the far-field.…”

Covid-19 Exposure Assessment Tool (CEAT): Easy-to-use tool to quantify exposure based on airflow, group behavior, and infection prevalence in the community

“…Exhalation plumes are typically anisotropic jets and have a buoyant nature due to their relative warmth and higher humidity. CFD modeling has captured these dynamics (Fabregat, Gisbert, Vernet, Ferré, et al 2021;Fabregat, Gisbert, Vernet, Ferré, et al 2021;Li et al 2022). To compensate for the fact that CEAT assumes plumes are nonbuoyant, which likely results in CEAT over-predicting concentrations at breathing heights, we adjust the height of the near-field volume to be equal to the distance between the source and the receptor, mixing the emission in all directions and in a larger near-field volume.…”

“…Rigorous study of the physics of aerosol behavior in indoor spaces have also been accomplished using both experiments and computational fluid dynamic (CFD) numerical simulations. These studies analyzed key aspects of the hydrodynamics produced by expiratory events, including sneezing, coughing, talking, singing and breathing, and the dispersion processes of the resulting aerosol cloud Fabregat, Gisbert, Vernet, Ferré, et al 2021;Li et al 2022). While these experiments and modeling studies produce important understanding of the aerosol behavior in the environment and the respiratory system, their results are specific to the defined scenarios that were modeled and are too computationally intensive to be used directly and routinely by non-experts.…”

Reducing Virus Transmission from Heating, Ventilation, and Air Conditioning Systems of Urban Subways

“…coupling this with the ideal gas equation of state gives (10) where R is 287 J/(kg K). In order to track the aerosol cloud in the context of diffusion through the urban subway, the energy equation was solved to find the thermal gradient and its effects on aerosol motion.…”