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

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

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

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

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

In Physics of Fluids, by AIP Publishing, researchers from the Indian Institute of Technology Bombay found when a person coughs outdoors, wind flowing in the same direction can propagate the virus faster over longer distances than in calm conditions.

“The study is significant in that it points to the increased infection risk that coughing in the same direction as the wind could bring about,” co-author Amit Agrawal said. “Based on the results, we recommend wearing masks outdoors, particularly in breezy conditions.”

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

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

Most studies model cough flow using puffs of air or a simple pulsating profile. But a real cough is more complicated, exhibiting turbulent flow with prominent vortical structures swirling like mini whirlpools.

To investigate these vortices, the researchers used a large eddy simulation, a numerical model in computational fluid dynamics that simulates turbulence. They modeled cough jets in breezy conditions and in calm conditions representing a typical indoor environment.

These simulations show even a light breeze of about 5 mph extends effective social distancing by around 20%, from 3-6 feet to 3.6-7.2 feet, depending on cough strength. At 9-11 mph, spreading of the virus increases in distance and duration.

The researchers found the vortices enable bigger droplets to persist in the air longer than has been typically assumed, increasing the time it takes to adequately dilute the viral load in fresh air. As the cough jet evolves and spreads, it interacts with the wind flowing in the same direction, and the bigger infected droplets become trapped in the jet’s vortices instead of falling relatively quickly to the ground under gravity.

“The increase in residential time of some of the larger droplets will increase the viral load transmitting through the cough jet and, therefore, the chances of infection,” Agrawal said. “Overall, the study highlights increased chances of infection in the presence of even a light breeze.”

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

COVID-19Fluid DynamicsFluid MechanicsInfectious DiseasesPopularPublic Health
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  • Alexiev

    I quote from this article: “As the highly infectious delta variant of the coronavirus continues to spread across the United States… ”
    Should read: “… continues TO BE spread… ” It is all in the language of Cause and Effect: COVID (∆Variant or NOT) does not spread itself; unless it is MAGICAL. There must BE a BE-ing in there to have a distribution vector. OF COURSE (not of CAUSE), as it is said of the Lord: COVID SEEMS to work in MYSTERIOUS WAYS, DEFYING a broad-based VAC program, AFTER the fact. Hmmm… Might be some MAGIC there, after all. WHERE? Where EFFECT leads CAUSE in a Masquerade of CAUSALITY: DUALITY, in other words.
    SO BE IT.
    Shame on you. Wear a Mask, it will hide the BLUSH… of DECEPTION.
    (!)
    ☝️

  • SciSCHLECHTDaily

    You’re deranged. Masks? They aren’t effective. They haven’t worked ever in history, in 2020 nor 2021. But wear them outdoor because of this laboratory NONSENSE?. Absolutely pathetic propaganda. Pathetic.

    • Neil B

      I kind of enjoy the overlap between those who squeal “hire the best person for the job!” every time an individual of some minority subgroup gets promoted, and those who think that their bizarre opinions should be held at the same merit of trained & published career scientists.

  • Mandy

    I used to read Science Daily updates because I felt it was a source I could trust and expand my knowledge. Now I’m wondering how niave I am as it’s just regular humans reporting on the political quote of the day. If you want to be taken seriously actually get deep and report ALL the science, not just what fits your agenda. I now don’t even know what to put the grain of salt too as you’re just completely out there.

  • Peter Parker

    3 previous commentators: why bother reading a science news site if you are are only looking for articles confirming your previously held beliefs? How about refute the evidence presented with your own reproducible experiment. Otherwise stick with cult/religious sites. Or just Fox news.

    • Neil B

      I kind of want them to try Ars and see how long it takes for that crowd to get them to run for the razors.

  • The 10th Man

    Fact: Covid is a no more deadly than the common Flu. Fact Mask work. Fact If your afraid of the Flu, then be afraid of the Covid virus. Fact The media is hyping this to control the money. Fact The government is hyping the Covid virus to get more money. Its a scam and you know it.

  • Freedom Rules

    This article has nothing concerning the effectiveness of cloth masks, surgical masks which is what many persons are wearing. These types of fraud studies are worthless and unhealthy. Publications with an agenda prior to any study is fraud. In line one of any study and/or article should be the disclaimer “Our publication/group/organization have taken ‘advertising/sponsorship money’ from Pfizer/Johnson & Johnson/Moderna/big pharma – ie bribes” As educated Americans we no longer believe big media/corrupted studies/corrupt politicians that take these millions and millions of big pharma bribes. Sponsorship and advertising on “news” programs is bribery. Sponsoring studies by big pharma is bribery. Pay CDC millions from big pharma is bribery. The effect of keeping everyone scared of a manufactured virus pushes vax mandates with questionable results for profit. CDC own website states masks do not stop what your study shows in your graphics. Where is the study that proves if I get one droplet in my nose does that mean I will contract a virus? Where is the study that shows there may be people who will never catch this virus as they never catch a cold virus?