Secrets of COVID-19 transmission revealed in turbulent puffs
By breathing new life into an old model, scientists explore the behavior
of turbulent puffs, and what it means for the spread of COVID-19

Date:
August 26, 2021
Source:
Okinawa Institute of Science and Technology (OIST) Graduate
University
Summary:
Researchers have developed a new model that explains how
turbulent puffs, like coughs, behave under different environmental
conditions. They found that at environmental temperatures 15DEGC
or lower, the puffs behaved with newly observed dynamics, showing
more buoyancy and traveling further. Their findings could help
scientists better predict how turbulence and the environment affect
airborne transmission of viruses like SARS-CoV-2.



FULL STORY ========================================================================== Turbulence is everywhere -- in the movement of the wind, the ocean waves
and even magnetic fields in space. It can also be seen in more transient phenomena, like smoke billowing from a chimney, or a cough.


========================================================================== Understanding this latter type of turbulence -- called puff turbulence --
is important not only for the advancement of fundamental science, but
also for practical health and environmental measures, like calculating
how far cough droplets will travel, or how pollutants released from a
chimney or cigarette might disperse into the surroundings. But creating
a complete model of how turbulent puffs of gases and liquids behave has
so far proven elusive.

"The very nature of turbulence is chaotic, so it's hard to predict,"
said Professor Marco Edoardo Rosti, who leads the Complex Fluids and
Flows Unit at Okinawa Institute of Science and Technology Graduate
University (OIST). "Puff turbulence, which occurs when the ejection
of a gas or liquid into the environment is disrupted, rather than
continuous, has more complicated characteristics, so it's even more
challenging to study. But it's of vital importance -- especially right
now for understanding airborne transmission of viruses like SARS-CoV-2."
Until now, the most recent theory was developed in the 1970s, and focused
on the dynamics of a puff only at the scale of the puff itself, like
how fast it moved and how wide it spread.

The new model, developed in a collaboration between Prof. Rosti from OIST, Japan and Prof. Andrea Mazzino from the University of Genova in Italy,
builds on this theory to include how minute fluctuations within the puff behave, and how both large-scale and small-scale dynamics are impacted
by changes in temperature and humidity. Their findings were published inPhysical Review Letters on August 25th 2021.

Interestingly, the scientists found that at cooler temperatures (15DEGC
or lower), their model deviated from the classical model for turbulence.



==========================================================================
In the classical model, turbulence reigns supreme -- determining how all
the little swirls and eddies within the flow behave. But once temperatures dipped, buoyancy started to have a greater impact.

"The effect of buoyancy was initially very unexpected. It's a completely
new addition to the theory of turbulent puffs," said Prof. Rosti.

Buoyancy exerts an effect when the gas or liquid puff is much warmer than
the temperature of the immediate surroundings it is released into. Warm
gas or fluid is much less dense than the cold gas or fluid of the
environment, and therefore the puff rises, allowing it to travel further.

"Buoyancy generates a very different kind of turbulence -- not only
do you see changes in the large-scale movement of the puff, but also
changes in the minute movements within the puff," said Prof. Rosti.

The scientists used a powerful supercomputer, capable of resolving
behavior of the puff at the large-scale and the small-scale, to run
simulations of turbulent puffs, which confirmed their new theory.



==========================================================================
The new model could now allow scientists to better predict the movement
of droplets in the air that are released when someone coughs or speaks unmasked.

While larger droplets fall quickly to the ground, reaching distances of
around one meter, smaller droplets can remain airborne for much longer
and travel further.

"How fast the droplets evaporate -- and therefore how small they are
-- depends on turbulence, which in turn is affected by the humidity
and temperature of the surroundings," explained Prof. Rosti. "We can
now start to take these differences in environmental conditions, and
how they affect turbulence, into consideration when studying airborne
viral transmission." Next, the researchers plan to study how puffs
behave when made of more complicated non-Newtonian fluids, where how
easily the fluid flows can change depending on the forces it is under.

"For COVID, this could be useful for studying sneezes, where non-Newtonian fluids like saliva and mucus are forcefully expelled," said Dr. Rosti.

This work used computational resources provided by HPCI under the
grant hp200157 of the "HPCI Urgent Call for Fighting against COVID-19." ========================================================================== Story Source: Materials provided by Okinawa_Institute_of_Science_and_Technology_(OIST)
Graduate_University. Original written by Dani Ellenby. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Andrea Mazzino, Marco Edoardo Rosti. Unraveling the Secrets of
Turbulence
in a Fluid Puff. Physical Review Letters, 2021; 127 (9) DOI:
10.1103/ PhysRevLett.127.094501 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210826111729.htm

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