Cough generated infectious aerosols are of interest while developing strategies for the mitigation of disease risks ranging from the common cold to SARS. In this work, the velocity field of human cough was measured using particle image velocimetry (PIV). The project subjects (total 29) coughed into an enclosure seeded with stage fog. Cough flow velocity profiles, average widths of the cough jet, and maximum cough velocities were measured. Maximum cough velocities ranged from 1.5 m/s to 28.8 m/s. The average width of all coughs ranged between 35 to 45 mm. Wide variability in the data suggests that future cough simulations consider a range of conditions.

https://www.tandfonline.com/doi/pdf/10.1080/02786826.2010.542785

Particle Image Velocimetry of Human Cough

There is recent interest in the dynamics of pulsed jets in uniform crossflow. A classification scheme for fully pulsed Jets in crossflow is proposed based on the stroke ratio of the jet pulse and the duty cycle of the pulse train. Scaling relations for penetration and dilution of fully pulsed jets'in crossflow are derived from the self-similar scaling of turbulent vortex rings and puffs in quiescent media. Individual turbulent structures are assumed to drift freely with the crossflow in the proposed scaling. The penetration of fully pulsed jets scales with the fourth root of the velocity ratio, stroke ratio, and axial distance. The decay of mean concentration scales with the velocity ratio and axial distance to the -3/4th power. These scaling relations apply to distinct turbulent structures before any interaction between successive structures. The criteria for interaction among flow structures near the nozzle and in the far field are also presented.

Scaling of Fully Pulsed Jets in Crossflow

The mixing characteristics of a round, turbulent, unsteady jet were studied experimentally. A gravity-driven flow was created by releasing dyed fluid from a vertical tube into a large water tank. The jet velocity increased from zero to a maximum and then decreased continuously such that each run lasted about  s. The jet dilution was examined by an optical absorption technique that measured the line integral of concentration across the jet diameter. These measurements revealed that the portion of the unsteady jet corresponding to the deceleration phase dilutes more than the steady jet. The molecular scale mixing, as deduced from an acid-base neutralization reaction, corroborated the finding that the jet mixes in a shorter distance than the steady jet.

Dilution and mixing in an unsteady jet

We study the effects of fully modulated pulsing on the penetration, entrainment, and mixing characteristics of transverse jets in a crossflow. Of particular interest are the specific effects of the pulsing parameters, especially the duty cycle, i.e., the jet-on fraction for each cycle

Duty-Cycle Effects on Penetration of Fully Modulated, Turbulent Jets in Crossflow

Effect of post injections on mixture preparation and unburned hydrocarbon emissions in a heavy-duty diesel engine

The structure and mixing of impulsively started jets have heen studied in a water tank utilizing an acid-base reaction. The flow consists of a starting vortex that separates from the rest of the jet in the near field. Penetration of the jet tip scales with the square root of time, normalized by the nozzle diameter and velocity. The celerity of the jet tip is approximately one-half of the centerline velocity of a steady jet, with the same nozzle exit velocity, at the same location. Results of chemically reactive experiments indicate that the fluid in the vicinity of the jet tip mixes with the ambient fluid faster than the rest of the jet. The extent of the region near the jet tip with improved mixing becomes larger as the jet travels further downstream. The more rapid mass mixing at the jet tip implies faster momentum diffusion, which corroborates the slowing down of the jet tip in comparison with the steady jet.

S. M. Bourque

Papers

Impulsively started turbulent jets