Fluid Dynamics Shows Why You Should Wear Masks Outdoors To Prevent Coronavirus Exposure

Large eddy simulation model showing the evolution of a cough ray. Credit: Indian Institute of Technology Bombay

Wind blowing in the same direction as a cough can increase viral transmission.

As the highly contagious delta strain of the coronavirus continues to spread across the United States, guidelines from the Centers for Disease Control and Prevention even recommend using vaccinated masks indoors to prevent exposure and transmission.

However, it is less clear what people should do outside.

In Physics of Fluids, by AIP Publishing, researchers at the Indian Institute of Technology Bombay found that when someone coughs outdoors, winds flowing in the same direction can spread the virus faster over longer distances than in calm conditions.

“The study is important because it points to the increased risk of infection that can cause coughing in the same direction as the wind,” said study co-author Amit Agrawal. “Based on the results, we recommend wearing masks outdoors, especially in windy conditions.”

Large eddy simulation model showing the evolution of a cough ray. Credit: Indian Institute of Technology Bombay

Other guidelines, such as coughing into an elbow or turning the face away while coughing, should be followed to reduce transmission when socializing outdoors.

Most studies model cough flow using air bubbles or a simple pulsating profile. But a true cough is more complicated, showing turbulent flow with prominent vortical structures swirling like mini vortices.

To investigate these vortices, the researchers used a large eddy simulation, a numerical model in computational fluid dynamics that simulates turbulence. They modeled cough rays in windy conditions and in calm conditions that represent a typical indoor climate.

These simulations show that even a light breeze of about 8 mph extends effective social distancing by about 20%, from 3-6 feet to 3.6-7.2 feet, depending on how strong the cough is. At 9-11 mph, the spread of the virus increases in distance and duration.

The researchers found that the vortices allow larger droplets to remain in the air for longer than commonly believed, increasing the time it takes to adequately dilute the viral load in the fresh air. As the cough jet evolves and spreads, it interacts with the wind flowing in the same direction, and the larger infected droplets get trapped in the jet’s vortices instead of falling to the ground relatively quickly under gravity.

“The increase in residence time of some of the larger droplets will increase the viral load transmitted by the cough ray, and therefore the likelihood of infection,” Agrawal said. “Overall, the study points to an increased chance of infection in the presence of even a light breeze.”

Reference: “Effect of co-flow on the fluid dynamics of a cough ray with implications for the spread of COVID-19” by Sachidananda Behera, Rajneesh Bhardwaj and Amit Agrawal, October 12, 2021, Physics of Fluids.
DOI: 10.1063/5.0064104