Airflow and aerosol dynamics of breathing and coughing

Abstract

The small scale, transient fluid dynamics of human respiratory exhalations govern the dispersion, transport and inhalation of infectious aerosols, yet the complex physical interactions between glottal forcing, mask boundaries, and aerodynamic drag remain poorly understood. The present work aims to provide a better understanding of these interactions through a comprehensive experimental study using high resolution particle image velocimetry (PIV) and pressure field reconstruction to characterize the airflow within the first few centimeters to meters from the respiratory source. We first quantify the influence of face masks on airflow kinematics and droplet transport. While 2 ply cloth masks are observed to reduce mean exhalation velocities by 70% and peak jet velocities from 4.2 m/s to under 2 m/s, valved FFP2 masks redirect the flow downward at approximately 330◦. By adapting multiphase puff models to these breathing scenarios, we find that the inclusion of experimentally derived drag and entrainment coefficients is essential for accurate transport prediction; neglecting these factors overestimates the travel distance of 5 𝜇m droplets by up to 1.5 orders of magnitude (e.g., 15 m vs. 1.5 m). To provide a physically basis for these effects, a novel methodology is presented to recover the instantaneous pressure field from Eulerian velocity data by solving the Pressure Poisson Equation. This allows for a direct empirical estimation of the aerodynamic drag coefficient (𝐶𝐷 ) for a deformable respiratory puff and reveals the underlying pressure velocity coupling that dictates its deceleration.. During the high momentum phase of exhalation, we find that 𝐶𝐷 remains relatively stable, fluctuating between 0.2 and 1.5. However, as the convective momentum decays, a dramatic spike in 𝐶𝐷 reaching 5.0 is observed. This provides direct evidence of a phase lag where persistent vortical pressure cores in the wake continue to exert a drag force after the driving velocity has subsided, a phenomenon characteristic of unsteady, bluff body like respiratory flows. Finally, we shift from tidal breathing to a fundamental analysis of the human coughing. By applying high resolution kinematic tracking and topological vortex identification, we identify a fundamental glottal pulsation frequency of 3.7 Hz. We find that these rhythmic exhalations utilize aerodynamic wake shielding to sustain local velocities at 160% of the initial expulsion velocity. This pulsatile engine drives a linear volumetric expansion of the contaminated cloud through pronounced lateral entrainment, preventing the rapid homogenization typical of steady jets. These results provide a rigorous, data driven foundation for refining airborne transmission risk assessments and improving the design of respiratory protection in both healthcare and public settings.