Airborne transmission of respiratory diseases has been under intense spotlight in the context of coronavirus disease 2019 (COVID-19) where continued resurgence is linked to the relaxation of social interaction measures. To understand the role of speech aerosols in the spread of COVID-19 globally, the lifetime and size distribution of the aerosols are studied through a combination of light scattering observation and aerosol sampling. It was found that aerosols from speaking suspended in stagnant air for up to 9 h with a half-life of 87.2 min. The half-life of the aerosols declined with the increase in air change per hour from 28 to 40 min (1 h −1 ), 10–14 min (4 h −1 ), to 4–6 min (9 h −1 ). The speech aerosols in the size range of about 0.3–2 μm (after dehydration) witnessed the longest lifetime compared to larger aerosols (2–10 μm). These results suggest that speech aerosols have the potential to transmit respiratory viruses across long duration (hours), and long-distance (over social distance) through the airborne route. These findings are important for researchers and engineers to simulate the airborne dispersion of viruses in indoor environments and to design new ventilation systems in the future.

Purpose This paper aims to report on the physical distortions associated with the use of additive manufactured components for wind tunnel testing and procedures adopted to correct for them. Design/methodology/approach Wings of a joined-wing test aircraft configuration were fabricated with additive manufacturing and tested in a subsonic closed-loop wind tunnel. Wing deflections were observed during testing and quantified using image-processing procedures. These quantified deflections were then incorporated into numerical simulations and results had agreed with wind tunnel measurement results. Findings Additive manufacturing provides cost-effective wing components for wind tunnel test components with fast turn-around time. They can be used with confidence if the wing deflections could be accounted for systematically and accurately, especially at the region of aerodynamic stall. Research limitations/implications Significant wing flutter and unsteady deflections were encountered at higher test velocities and pitch angles. This reduced the accuracy in which the wing deflections could be corrected. Additionally, wing twists could not be quantified as effectively because of camera perspectives. Originality/value This paper shows that additive manufacturing can be used to fabricate aircraft test components with satisfactory strength and quantifiable deflections for wind tunnel testing, especially when the designs are significantly complex and thin.

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The impact of skewness angle on the effectiveness of vortex generators (VGs) and the behavior of streamwise vortices on flow separation behind a backward-facing ramp (BFR) with a sharp transition were experimentally investigated using surface oil flow visualizations, planar and stereoscopic particle image velocimetry measurements. Counter/corotating streamwise vortices were generated by a set of boundary layer-type rectangular VG located upstream of the BFR that comprised a flat- and 30-inclined sections with different skewness angles of 10°, 20°, and 30°. Local Reynolds number based on the VG location was Rex ≈ 3 × 106. Results show that the reattachment length was reduced by ∼45% when the VG was located five times its height ahead of the transition. Additionally, the behavior of the vortex core generated by the left vane displayed strong dependence on the skewness angle, whereby its vorticity magnitude and vortex instability increase with the skewness angle. Circulation magnitude and vortex radius of the left vortex core are also observed to be physically larger and less stable. In contrast, the vortex core produced by the right vane displays opposite behavior as the skewness angle increases. Lastly, the vortex core location is observed to fluctuate more in the vertical direction than horizontal direction.