Droplet Nuclei Caustic Formations In Exhaled Vortex Rings

Abstract

Vortex ring structures occur in light or hoarse cough configurations. These instances consist of short impulses of exhaled air resulting to a self-contained structure that can travel large distances. The present study is the first implementation of the second order Fully Lagrangian Approach (FLA) for three-dimensional realistic flow-fields obtained by means of Computational Fluid Dynamics (CFD) and provides a method to calculate the occurrence and the intensity of caustic formations. The carrier phase flow field is resolved by means of second order accurate Direct Numerical Simulation (DNS) based on a Finite Difference approach for the momentum equations, while a spectral approach is followed for the Poisson equation using Fast Fourier Transform (FFT). The effect of the undulations of the carrier phase velocity due to large scale vortical structures and turbulence is investigated. The evaluation of the higher order derivatives needed by the second order FLA is achieved by prefabricated least squares second order interpolations in the three dimensions. The method allows for the simulation of the clustering of droplets and droplet nuclei exhaled in ambient air in conditions akin to light cough. Given the ambiguous conditions of vortex-ring formation during cough instances, three different formation numbers n = UT /D are assumed, i.e. underdeveloped (n = 2), ideal (n = 3.7) and over-developed VRs (n = 6). The formation of clusters results in the spatial variance of the airborne viral load. This un-mixing of exhumed aerosols is related to the formation of localised high viral load distributions that can be linked to super-spreading events.

Numerical model for cough‐generated droplet dispersion on moving escalator with multiple passengers

“…The jet strength (at t = 0.2 s) does not appear to be affected by the direction of movement. Once the coughing is completed, the jet starts to form a vortex ring, 39 and this is more prominent in the downwards movement (Figure 7C,D). As expected, large droplets fall faster than small ones due to the gravity and inertia force from flow fields in accordance with Equation (7).…”