PhD defence of Vijaya Esther Veeravalli: Airflow and Aerosol Dynamics of Breathing and Coughing

Abstract: 

Traditional respiratory pathogen transport models frequently rely on a simplified, binary rule assuming that respiratory droplets either settle ballistically within two meters or drift passively as isolated aerosols. However, real-world human exhalations emerge as highly transient, momentum-driven turbulent puff clouds that trap pathogens within a warm, high-humidity micro-climate, significantly extending droplet suspension lifetimes. This dissertation addresses critical gaps in current predictive safety frameworks by providing the first coupled, high-resolution quantitative mapping of the velocity profiles, spatial pressure landscapes, and multi-pulsed architectures of human breathing and coughing.

Using high-fidelity particle image velocimetry (PIV) inside a controlled stagnant air chamber, this work systematically quantifies how face coverings modify exhalation dynamics. The results demonstrate that while cloth and FFP2 masks achieve significant forward velocity reductions (55 and 45%, respectively), they fundamentally act as flow redirectors, channeling the risk envelope sharply upward or downward . By applying mathematical reconstructions via the Pressure Poisson Equation (PPE) to the experimental velocity data, the unmapped spatial static pressure fields of expiratory flows were unveiled. This analysis captured a distinct spatiotemporal phase lag where structural low-pressure vortical cores persist even after exhalation terminates. This decoupling drives a dramatic, unsteady aerodynamic drag coefficient (CD$) spike to 5.0, identifying a natural "dynamic braking" mechanism that dictates far-field cloud deceleration.

Finally, this research rejects standard continuous-jet assumptions by isolating the multi-pulsed nature of human coughs, resolving a distinct 3.7 Hz glottal modulation frequency. This discontinuous pulse train triggers a profound "wake shielding" effect, where successive velocity bursts exploit the pre-accelerated wake of preceding pulses, bypassing conventional drag constraints to carry aerosols deeper into the ambient environment. Ultimately, the experimental coefficients and physical mechanisms established in this dissertation bridge the gap between flow kinematics and dynamic forces, providing a realistic foundation for next-generation indoor HVAC simulations and public health safety guidelines.