Effectiveness of RANS in predicting indoor airborne viral transmission: A critical evaluation against LES

We investigate the dispersal of droplet nuclei inside a canonical room of size 10×10×3.2m3 with a four-way cassette air-conditioning unit placed at the center of the ceiling. We use Reynolds averaged Navier–Stokes (RANS) simulations with three flow rates corresponding to air changes per hour (ACH) values of 2.5, 5, and 10. The room setup as well as the operating conditions are chosen to match those of a recent high-fidelity large eddy simulation (LES) study. We use statistical overloading with a total of one million droplet nuclei being initially distributed randomly with uniform probability within the room. Six nuclei sizes are considered ranging in radius from 0.1 to 10μm (166,667 nuclei per size). The simulations are one-way coupled and employ the Langevin equations to model sub-grid motion. The flow and particle statistics are compared against the reference LES simulations, and we find that the RANS k−ɛ realizable model may be used as a computationally cheaper alternative to LES for predicting pathogen concentration in confined spaces albeit, with potentially increased statistical discrepancy. © 2023 Elsevier Ltd

Sensitivity of puff dynamics and airborne droplet nuclei distribution to variations in violent expiration events

We use large eddy simulations to investigate the puff and droplet dynamics from violent expiratory events such as coughs and sneezes in the first few seconds following an ejection. For each of the eleven simulations considered, over 60,000 droplets are ejected and individually tracked using the point-particle Euler–Lagrange approach. We test the sensitivity of the puff and droplet dynamics to various parameters including the ejection volume, momentum, and orientation. We also explore the effect of the mouth shape on the aforementioned dynamics by considering elliptical and circular inlet cross-sections. The results from the simulations compare favorably with a recent theoretical framework put forth by Balachandar et al. (2020) in terms of the puff size and propagation velocity. More importantly however, the theory is able to accurately predict the number and size spectra of the potentially virus-laden droplet nuclei that remain airborne within the puff. We observe that the ejection angle and mouth shape do not significantly affect the puff and droplet dynamics. Additionally, we quantify the carrying capacity of the detached puff portions in terms of the number and size spectra of droplets/droplet nuclei suspended within. © 2022 Elsevier Ltd

ON THE EJECTION SCALE PROBLEM OF EXPIRATORY EVENTS FROM THEORY AND SIMULATIONS

The overall purpose of this study is to investigate expiratory events such as coughs and sneezes in the ejection scale framework, i.e. within a short time span immediately after the expiration process. We conducted large eddy simulations (LES) and compared the results with a recent theoretical model put forth by Balachandar et al. [2]. The theoretical model [2] has been formulated to estimate the evolution of expiratory events such as coughs and sneezes. Some of the key features of the model include estimates for the time evolution of the puff centroid, its size, as well as the number and size of droplets suspended within. The theoretical model includes closure parameters that have been obtained from LES [6, 7]. The simulations cover a wide range of parameters, such as the ejection volume of the puff, its momentum, the ejection angle (whether horizontal, inclined, or vertical), and the ambient humidity. One of the important findings is that while certain aspects such as the front-most location and the lateral extent of the puff, show large variability from one realization to the other, global parameters, such as the centroid location, total volume, and buoyancy show are much less sensitive to turbulent fluctuations. The results also indicate that humid ambient conditions favor stronger gravitational settling of the ejected virus-laden droplets, thus decreasing the risk of infection from the dominant airborne route. Furthermore, the simulations highlight a mechanism for transporting a relatively large amount of droplets over distances upward of 2 meters in a time span on the order of one second. This mechanism, which is also observed in experiments, consists of fast moving detached vortex rings that propagate in a seemingly random direction. We further quantify the size and viral content of the detached portions. © 2022 Begell House Inc.. All rights reserved.

Insights into solvation effects, spectroscopic, Hirshfeld Surface Analysis, reactivity analysis and anti-Covid-19 ability of doxylamine succinate: Experimental, DFT, MD and docking simulations

In the present work, the experimental and theoretical reports on electronic and vibrational features of doxylamine succinate (DXS) are presented. The vibrational spectra were documented and wavenumbers were obtained theoretically assigned by means of potential energy distribution. In DXS, N-H…O and C-H…O intermolecular hydrogen bonding contacts are associated with O…H/H…O interactions. Solvation free energy (SFE) for DXS in water, methanol and DMSO, are -10.67, -10.95 and -10.61 eV/mol respectively. Interpretation of electrostatic potential, electron localization function (ELF), localized orbital locator (LOL) as well as atoms-in-molecules (AIM) analysis is also performed. Presence of non-covalent interactions is evident from the non-covalent interaction (NCI) isosurface. Molecular docking and simulations were used to determine the binding energy of DXS in order to investigate its potential activity against the SARS-CoV-2 protease.

Peering inside a cough or sneeze to explain enhanced airborne transmission under dry weather.

High-fidelity simulations of coughs and sneezes that serve as virtual experiments are presented, and they offer an unprecedented opportunity to peer into the chaotic evolution of the resulting airborne droplet clouds. While larger droplets quickly fall-out of the cloud, smaller droplets evaporate rapidly. The non-volatiles remain airborne as droplet nuclei for a long time to be transported over long distances. The substantial variation observed between the different realizations has important social distancing implications, since probabilistic outlier-events do occur and may need to be taken into account when assessing the risk of contagion. Contrary to common expectations, we observe dry ambient conditions to increase by more than four times the number of airborne potentially virus-laden nuclei, as a result of reduced droplet fall-out through rapid evaporation. The simulation results are used to validate and calibrate a comprehensive multiphase theory, which is then used to predict the spread of airborne nuclei under a wide variety of ambient conditions.

Host-to-Host Airborne Transmission As a Multiphase Flow Problem For Science-Based Social Distance Guidelines

COVID-19 pandemic has strikingly demonstrated how important it is to develop fundamental knowledge related to generation, transport and inhalation of pathogen-laden droplets and their subsequent possible fate as airborne particles, or aerosols, in the context of human to human transmission. It is also increasingly clear that airborne transmission is an important contributor to rapid spreading of the disease. In this paper, we discuss the processes of droplet generation by exhalation, their potential transformation into airborne particles by evaporation, transport over long distances by the exhaled puff and by ambient air turbulence, and final inhalation by the receiving host as interconnected multiphase flow processes. A simple model for the time evolution of droplet/aerosol concentration is presented based on a theoretical analysis of the relevant physical processes. The modeling framework along with detailed experiments and simulations can be used to study a wide variety of scenarios involving breathing, talking, coughing and sneezing and in a number of environmental conditions, as humid or dry atmosphere, confined or open environment. Although a number of questions remain open on the physics of evaporation and coupling with persistence of the virus, it is clear that with a more reliable understanding of the underlying flow physics of virus transmission one can set the foundation for an improved methodology in designing case-specific social distancing and infection control guidelines.