Coughing is one of the major routes for the transmission of respiratory diseases in indoor environments. Previous studies have shown that coughing can form vortex ring structures. However, the conditions under which cough-generated vortex rings form, as well as the influence of boundary condition on the formation and flow characteristics of these vortex rings, remain inadequately understood. This study investigates the effects of three velocity profiles (real-cough, sinusoidal, and pulsation) and five outlet shapes (real-mouth, circular, square, rectangular, and elliptical) on the flow dynamics of cough jets. The results show that the penetration distance of cough-generated vortex rings ranges from 0.56 to 0.68m in 0.5s. Separated (or deviated) vortex rings show a power-law distribution in the cough jets: s~ t2/3 (or s ~ t1/2) during the initial stage and s ~ t1/3 (or s ~ t1/5) during the interrupted stage. Due to the differences between the power-law and existing studies, short-distance infection risk assessment models require further revision. Furthermore, the real-mouth, circular, and square have a relatively minor effect on the formation of saturated vortex ring compared to velocity profiles. The formation conditions for saturated vortex rings can be determined by a dimensionless energy αlim= 0.2±0.06.
Li et al. (2026) studied this question.