Spray dispersion regimes following atomization in a turbulent co-axial gas jet

A canonical co-axial round-jet two-fluid atomizer where atomization occurs over a wide range of momentum ratios: $M=1.9 - 376.4$ is studied. The near field of the spray, where the droplet formation process takes place, is characterized and linked to droplet dispersion in the far field of the jet. Counterintuitively, our results indicate that in the low-momentum regime, increasing the momentum in the gas phase leads to less droplet dispersion. A critical momentum ratio of the order of $M_c=50$ , that separates this regime from a high-momentum one with less dispersion, is found in both the near and far fields. A phenomenological model is proposed that determines the susceptibility of droplets to disperse beyond the nominal extent of the gas phase based on a critical Stokes number, $St=\tau _p/T_E=1.9$ , formulated based on the local Eulerian large scale eddy turnover time, $T_E$ , and the droplets’ response time, $\tau _p$ . A two-dimensional phase space summarizes the extent of these different regimes in the context of spray characteristics found in the literature.

Efficacy of SG Shield in reducing droplet contamination during collection of oropharyngeal swab culture specimens

Introduction: Oropharyngeal swabs for diagnosis of COVID-19 often induce violent coughing, which can disperse infectious droplets onto providers. Incorrectly doffing personal protective equipment (PPE) increases the risk of transmission. A cheap, single-use variation of the face shield invented by a Singaporean team, SG Shield, aims to reduce this risk. This manikin study aimed to study the efficacy of the SG Shield in combination with standard PPE. Methods: A person attired in full PPE whose face and chest was lined with grid paper stood in front of an airway manikin in an enclosed room. A small latex balloon containing ultraviolet fluorescent dye was placed in the oral cavity of the manikin and inflated until explosion to simulate a cough. Three study groups were tested: (a) control (no shield), (b) face shield and (c) SG Shield. The primary outcome was droplet dispersion, determined quantitatively by calculating the proportion of grid paper wall squares stained with fluorescent dye. The secondary outcome was the severity of provider contamination. Results: The SG Shield significantly reduced droplet dispersion to 0% compared to the controls (99.0%, p = 0.001). The face shield also significantly reduced droplet contamination but to a lesser extent (80.0%) compared to the control group (p = 0.001). Although the qualitative severity of droplet contamination was significantly lower in both groups compared to the controls, the face shield group had more contamination of the provider’s head and neck. Conclusion: The manikin study showed that the SG Shield significantly reduces droplet dispersion to the swab provider’s face and chest.

Passive Tracer Visualization to Simulate Aerodynamic Virus Transport in Noninvasive Respiratory Support Methods

<b><i>Background:</i></b> Various forms of noninvasive respiratory support methods are used in the treatment of hypoxemic CO­VID-19 patients, but limited data are available about the corresponding respiratory droplet dispersion. <b><i>Objectives:</i></b> The aim of this study was to estimate the potential spread of infectious diseases for a broad selection of oxygen and respiratory support methods by revealing the therapy-induced aerodynamics and respiratory droplet dispersion. <b><i>Methods:</i></b> The exhaled air-smoke plume from a 3D-printed upper airway geometry was visualized by recording light reflection during simulated spontaneous breathing, standard oxygen mask application, nasal high-flow therapy (NHFT), continuous positive airway pressure (CPAP), and bilevel positive airway pressure (BiPAP). The dispersion of 100 μm particles was estimated from the initial velocity of exhaled air and the theoretical terminal velocity. <b><i>Results:</i></b> Estimated droplet dispersion was 16 cm for unassisted breathing, 10 cm for Venturi masks, 13 cm for the nebulizer, and 14 cm for the nonrebreathing mask. Estimated droplet spread increased up to 34 cm in NHFT, 57 cm in BiPAP, and 69 cm in CPAP. A nonsurgical face mask over the NHFT interface reduced estimated droplet dispersion. <b><i>Conclusions:</i></b> During NHFT and CPAP/BiPAP with vented masks, extensive jets with relatively high jet velocities were observed, indicating increased droplet spread and an increased risk of droplet-driven virus transmission. For the Venturi masks, a nonrebreathing mask, and a nebulizer, estimated jet velocities are comparable to unassisted breathing. Aerosols are transported unboundedly in all these unfiltered therapies. The adequate use of protective measures is of vital importance when using noninvasive unfiltered therapies in infectious respiratory diseases.