Long Streams of Virus-Loaded Droplets Can Track Behind Infected Individuals

The cough-generated droplets of a walking individual disperse differently in a narrow corridor and free area. In narrow corridors, the droplets concentrate in a small bladder and remain further back. Credit: Xiaolei Yang

Fast walking in narrow corridors can be increased COVID-19 transmission risk.

Long streams of virus-laden droplets can track behind infected individuals walking through a narrow corridor, with an impact on safe social distancing guidelines.

Computer simulations were used to accurately predict airflow and droplet diffusion patterns in situations where COVID-19 could be propagated. In the magazine Fluid Physics, from AIP Publishing, results show the importance of the shape of space in modeling how virus-laden droplets move through the air.

The simulations are used to determine flow patterns behind a migrating individual in spaces of different shape. The results reveal a higher transmission risk for children in some cases, such as behind people moving quickly in a long narrow corridor.

Previous research using this simulation technique has helped scientists understand the influence of objects, such as glass barriers, windows, air conditioners and toilets, on airflow patterns and the spread of viruses. The previous simulations usually assumed a large open interior space but did not take into account the impact of nearby walls, such as those that could exist in a narrow corridor.

Cough Generated Droplets Open Space

The cough-generated droplets of a migrating individual disperse differently in a narrow corridor and free area. In free space, the droplets are scattered in a wide range linked to the person. Credit: Xiaolei Yang

If someone walking in a corridor coughs, their breath expels droplets that travel around and behind their body, forming a wake in the way a boat forms a wake in water as it travels. The investigation revealed the existence of a “recirculating bladder” directly behind the person’s torso and a long wake flowing out behind them at about waist height.

“The flow patterns we found strongly relate to the shape of the human body,” said author Xiaolei Yang. “At 2 meters downstream, the wake is almost negligible at mouth height and leg height but is still visible at waist height.”

Once the airflow patterns were determined, the survey modeled the spread of a cloud of droplets expelled from the pretend person’s mouth. The shape of the space around the moving human is particularly critical for this part of the calculation.

Two types of diffusion regimes have been found. In one mode, the cloud of droplets detaches from the moving person and floats far behind that individual, creating a floating bubble of virus-laden droplets. In the other mode, the cloud is tied to the person’s back, trailing like a tail as they move through space.

Droplet cloud

In both modes, the cloud of droplets hovers at about half the height of the infected person before reaching the ground, indicating a higher risk for children to inhale the droplets. Credit: Xiaolei Yang

“For the detached regimen, the droplet concentration is much higher than for the bound regimen, five seconds after coughing,” Yang said. “This presents a major challenge in determining safe social distance in places like a very narrow corridor where a person can inhale male droplets even if the patient is far in front of him or her.”

The danger is especially great for children, because in both modes, the cloud of droplets hovers far above the ground, which is about half the height of the infected person – in other words, at the oral level for children.

Reference: “Effects of space sizes on the spread of cough-generated droplets of a walking human” by Zhaobin Li, Hongping Wang, Xinlei Zhang, Ting Wu and Xiaolei Yang, December 15, 2020, Fluid Physics.
DOI: 10.1063 / 5.0034874

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