Droplet nuclei caustic formations in exhaled vortex rings.

Area of Science:

• Fluid dynamics
• Aerosol science
• Computational physics

Background:

• Vortex rings (VRs) are formed by exhaled air during coughing, creating structures that travel significant distances.
• Understanding VR formation is crucial for analyzing airborne disease transmission, particularly concerning droplet nuclei and viral load distribution.

Purpose of the Study:

• To implement a second-order Fully Lagrangian Approach (FLA) for simulating three-dimensional realistic flow-fields of cough-generated vortex rings.
• To develop a method for calculating the occurrence and intensity of caustic formations within these vortex rings.
• To investigate the impact of different vortex ring development stages (under-developed, ideal, over-developed) on aerosol clustering and viral load distribution.

Main Methods:

• Utilizing Computational Fluid Dynamics (CFD) with second-order accurate Direct Numerical Simulation (DNS) for the carrier phase flow field.
• Employing a spectral approach with Fast Fourier Transform (FFT) for solving the Poisson equation.
• Applying pre-fabricated least squares second-order interpolations for evaluating higher-order derivatives required by the FLA.

Main Results:

• The study successfully simulates the clustering of exhaled droplets and droplet nuclei in conditions resembling a light cough.
• Analysis reveals that the formation of clusters leads to spatial variance in airborne viral load.
• Different vortex ring development parameters influence the degree of aerosol un-mixing and localized high viral load distributions.

Conclusions:

• The Fully Lagrangian Approach (FLA) provides a robust method for simulating cough-generated vortex rings and their associated aerosol dynamics.
• Aerosol clustering within vortex rings is a key factor in creating localized high viral load zones, potentially explaining super-spreading events.
• This research offers insights into the mechanisms of airborne transmission and can inform strategies for mitigating the spread of respiratory infections.