Large-eddy simulation of airborne disease transmission in thermally stratified indoor environments

A Eulerian-Lagrangian large-eddy simulation approach is used to investigate exhaled particle movement and dispersion during speaking under different combinations of thermally uniform, stratified, with and without displacement ventilation indoor conditions. A polydisperse particle size distribution is released, with every particle trajectory tracked in space and time by a Lagrangian Particle Tracking algorithm. The model captures the difference between smaller and larger particles based on their physical behaviour, whereby smaller particles stay suspended in the air for longer, reach further forward, can follow air currents, and their upward movement is trapped by thermal stratification while larger particles fall out immediately and are unaffected by thermal stratification. The introduction of cooler air inflow initially shows turbulent and chaotic mixing, with particles resuspended back into the breathing zone before stratification effects are countered. Furthermore, it reduces the concentration of the smallest particles within 0.5 m of the infected individual by half.