Comparison of CFD model and one-compartment materials balance model for predicting 8-hr exposure to pathogen-laden expiratory droplets in a two-person office

Abstract: In this study, we estimate predicted levels of 8-hr exposure to pathogen-laden expiratory droplets within a representative two-person workspace. The scenario modeled included one asymptomatic infected individual present in the simulated workspace. Inhalation and fomite exposure was estimated at adjacent office workstations , separated by a cubical divider. The pathogen release is represented by the quanta emission rate, where one quanta is the “dose of airborne droplet nuclei required to cause infection in 63% of susceptible persons”, using SARS-CoV-2 as an example (Buonanno et al. 2020). The quanta emission rate was divided into several particle sizes based on size distributions found in the scientific literature. The computational fluid dynamic (CFD) approach with the use of the three–equation (k-kl-w) turbulence model of Walters and Cokljat (2008) was used in the simulations. The infected droplet concentration in the office space was evaluated with a user-define-function (UDF) accounting for the turbulent diffusion and gravitational sedimentation. Size-resolved deposition rates for the materials balance modeling were obtained from the indoor aerosol literature. For the same baseline scenario, the average concentration of 1-micron particles predicted by the CFD model and the materials balance model is approximately the same.Both models estimate 0.35 quanta for an 8-hr emission and intake. The CFD and materials balance model results diverge for the larger particle sizes. For the small two-person office, both increases in the ventilation rate and face coverings worn by the infected and susceptible individuals reduced 8-hr exposure estimates dramatically. Because many exposure models are based on the materials balance modeling approach for which instantaneous mixing is a basic assumption, a comparison with the CFD modeling approach quantifies the result of this assumption for time-averaged exposure estimates.

Impact/Purpose

Computational fluid dynamics (CFD) can be used to evaluate infectious droplet transport from an infected person, in this case in an office environment, to other people and their respective exposure level at different work stations. CFD modeling of a baseline scenario was compared to the materials balance model of the same scenario, and the results are approximately the same. 

Citation

Rosati, J., P. White, M. Singh Rawat, S. Obeid, G. Ahmadi, AND A. Ferro. Comparison of CFD model and one-compartment materials balance model for predicting 8-hr exposure to pathogen-laden expiratory droplets in a two-person office. American Association for Aerosol Research (AAAR) 39th Annual Conference, Albuquerque, NM, October 18 - 22, 2021.