Abstract
Air exchange between people has emerged in the COVID-19 pandemic as the important vector for transmission of the SARS-CoV-2 virus. We study the airflow and exchange between two unmasked individuals conversing face-to-face at short range, which can potentially transfer a high dose of a pathogen, because the dilution is small when compared to long-range airborne transmission. We conduct flow visualization experiments and direct numerical simulations of colliding respiratory jets mimicking the initial phase of a conversation. The evolution and dynamics of the jets are affected by the vertical offset between the mouths of the speakers. At low offsets the head-on collision of jets results in a `blocking effect', temporarily shielding the susceptible speaker from the pathogen carrying jet, although, the lateral spread of the jets is enhanced. Sufficiently large offsets prevent the interaction of the jets. At intermediate offsets (8-10 cm for 1 m separation), jet entrainment and the inhaled breath assist the transport of the pathogen-loaded saliva droplets towards the susceptible speaker's mouth. Air exchange is expected, in spite of the blocking effect arising from the interaction of the respiratory jets from the two speakers.
Highlights
As the world emerges from the COVID-19 pandemic, many people are resuming ordinary activities
We focus on short-range airborne transmission via unmasked face-to-face conversations that occur in meetings, social gatherings, restaurants, etc
We study the interaction between two colliding jet-like flows, produced due to breathing and speaking, through direct numerical simulations (DNS) and flow visualization experiments
Summary
As the world emerges from the COVID-19 pandemic, many people are resuming ordinary activities. In COVID-19 the viral load from the upper airways of infected people is high the day before the onset of symptoms, so asymptomatic spreading accounts for a significant share of transmissions (Anderson et al 2020; Prather et al 2020) These facts emphasize the need for analysing the local flow environment and air exchange during a conversation. We focus on short-range airborne transmission via unmasked face-to-face conversations that occur in meetings, social gatherings, restaurants, etc Such a configuration is complex, and our present understanding is impeded by the spectrum of physical phenomena involved, ranging from microbiology, human phonetics and aerosolization of droplets (Pöhlker et al 2021), apart from the fluid mechanics that involves exhaled jets, turbulent interaction of colliding jets and interaction with background flows. Phonetic features of speech, such as plosives, likely extend the interaction distance and reduce the necessary contact time
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