Dynamics of human speech as aerosol transmission sources: Implications for sustainable urban health based on impulsive jet theory

In the context of sustainable urban environments and public health, this study investigates exhaled airflows during vocalization to understand their flow dynamics and transmission characteristics. Using illuminated high-speed imaging and PIV, the research characterizes exhaled flows through comprehensive parameters including jet flow stages, injection times, mouth scales, directions, distances, velocities, Reynolds numbers, and dimensionless time parameters, establishing baseline data for flow description and modeling. Through analogy with impulsive jets, two distinct flow regimes were identified based on formation time parameter (t*_inj) and formation number (Fn). Regime A (t*_inj < Fn) exhibits single vortex ring evolution, while Regime B (t*_inj > Fn) features additional trailing jets. The formation number (Fn = 5.5–9.0) serves as a critical transition parameter between these regimes, with peak vortex ring vorticity reaching approximately 70 s^-1. Analysis suggests that bulk velocity and injection time may serve as key parameters for simplified CFD modeling, pending validation. As expelled droplets tend to concentrate and penetrate in vortices, disrupting these structures may reduce aerosol penetration and transport. These findings contribute to the theoretical framework for respiratory flow modeling and disease transmission control.