The flow and saliva particulate transport dynamics during normal human breathing through the mouth are simulated numerically using an Eulerian large-eddy simulation (LES) approach for the flow coupled with a Lagrangian approach for the transport of saliva particles. The coupled Eulerian–Lagrangian (EL) simulation results reveal new striking insights into the rich dynamics of the Lagrangian coherent structures (LCS) that arise from saliva particles during normal breathing. Specifically, they uncover a new time-periodic mechanism via which particles are introduced into the flow as individual breathing pulses and accumulate to form a slowly propagating vortex front that persists long distances away from the source. The simulated LCS reveal a wealth of recurrent material motion through which the biosols propagate forward while their cloud expands laterally with a slowly evolving vortex front. Also, the finite-time Lyapunov exponent (FTLE) field of human breathing was calculated using the flow map from the LES velocity field. The ridges of the calculated FTLE field revealed distinct hyperbolic LCS, which closely resemble trajectories of saliva particles obtained from the coupled EL simulation. Finally, simulation results for normal breathing with a non-medical face mask show that the mask can effectively disrupt the formation of coherent particle surfaces and, thus, effectively limit saliva particle propagation.

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In this study, we investigated the transport of contaminants in the southern tip of Manhattan, New York City, under prevailing wind conditions. We considered a hypothetical contaminant particle release on the east side of the New York Stock Exchange at 50 m above the ground level. The transport of individual particles due to the wind flow in the city was simulated by coupling large-eddy simulations (Eulerian) with a Lagrangian model. The simulation results of our coupled Eulerian and Lagrangian approach showed that immediately after the contaminant particles are released, they propagate downwind and expand in the spanwise direction by ~0.5 km. Specifically, approximately 15 min after the release, the contaminant particles reach the end of the 2.5-km-long study area with a mean velocity of 1.8 m/s, which is approximately 50% of the dominant wind velocity. With the cessation of the particle release, the contaminant particles start to recede from the urban area, mainly owing to their outflux from the study area and the settling of some particles on solid surfaces in the metropolitan area. More specifically, the study area becomes clear of particles in approximately 48.5 min. It was observed that some particles propagate with a mean velocity of 0.6 m/s, i.e., ~17% of the dominant wind velocity. We also conducted a detailed investigation of the nature of particle transport patterns using finite-time Lyapunov exponents, which showed that dynamically rich Lagrangian coherent structures are formed around the buildings and off the tops of the skyscrapers.

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