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We use advanced experimental techniques to explore turbulence-induced deflagration-to-detonation transition (tDDT) in hydrogen–air mixtures. We analyze the full sequence of turbulent flame evolution from fast deflagration-to-detonation using simultaneous direct measurements of pressure, turbulence, and flame, shock, and flow velocities. We show that fast turbulent flames that accelerate and develop shocks are characterized by turbulent flame speeds that exceed the Chapman–Jouguet deflagration speed in agreement with the tDDT theory and direct numerical simulation (DNS) results. Velocity and pressure evolutions are provided to detail the governing mechanisms that drive turbulent flame acceleration. Turbulent flame speeds and fluctuations are examined to reveal flow field characteristics of the tDDT process. This work contributes to the understanding of fundamental mechanisms responsible for spontaneous initiation of detonations by fast turbulent flames. 

Abstract Background Airborne viral pathogens like severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can be encapsulated and transmitted through liquid droplets/aerosols formed during human respiratory events. Methods The number and extent of droplets/aerosols at distances between 1 and 6 ft (0.305–1.829 m) for a participant wearing no face covering, a cotton single-layer cloth face covering, and a 3-layer disposable face covering were measured for defined speech and cough events. The data include planar particle imagery to illuminate emissions by a light-sheet and local aerosol/droplet probes taken with phase Doppler interferometry and an aerodynamic particle sizer. Results Without face coverings, droplets/aerosols were detected up to a maximum of 1.25 m (4.1ft ± 0.22–0.28 ft) during speech and up to 1.37 m (4.5ft ± 0.19–0.33 ft) while coughing. The cloth face covering reduced maximum axial distances to 0.61 m (2.0 ft ± 0.11–0.15 ft) for speech and to 0.67 m (2.2 ft ± 0.02–0.20 ft) while coughing. Using the disposable face covering, safe distance was reduced further to 0.15 m (0.50 ft ± 0.01–0.03 ft) measured for both emission scenarios. In addition, the use of face coverings was highly effective in reducing the count of expelled aerosols. Conclusions The experimental study indicates that 0.914 m (3 ft) physical distancing with face coverings is equally as effective at reducing aerosol/droplet exposure as 1.829 m (6 ft) with no face covering.