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https://scitechdaily.com/droplet-spread-from-humans-doesnt-always-follow-airflow-important-implications-for-spread-of-covid-19/
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A mathematical model of droplet migration may have important
implications for understanding the spread of airborne diseases, such as
COVID-19.
The World Health Organization has warned that aerosol transmission of
COVID-19 is being underestimated. If aerosol spread is confirmed to be
significant, as suspected, we will need to reconsider guidelines on
social distancing, ventilation systems, and shared spaces.
A group of researchers from Heriot-Watt University and the University of
Edinburgh in the U.K. believes a better understanding of different
droplet behaviors and their different dispersion mechanisms based on
droplet size is also needed.
In Physics of Fluids, from AIP Publishing, the group presents a
mathematical model that clearly demarcates small-, intermediate- and
large-sized droplets. Simple formulas can be used to determine a
droplet’s maximum range.
Droplet paths from patient to extraction device: small, medium, and
large droplets. Credit: Cathal Cummins
This has important implications for understanding the spread of airborne
diseases, such as COVID-19, because their dispersion tests revealed the
absence of intermediate-sized droplets, as expected.
“The flow physics of someone coughing is complex, involving turbulent
jets and droplet evaporation,” said Cathal Cummins, of Heriot-Watt
University. “And the rise of COVID-19 has revealed the gaps in our
knowledge of the physics of transmission and mitigation strategies.”
One such gap in the physics is a clear, simple description of where
individual droplets go when ejected.
“We wanted to develop a mathematical model of someone breathing that
could be explored analytically to examine the dominant physics at play,”
Cummins said.
As a person breathes, they emit droplets of various sizes that don’t
necessarily follow the airflow faithfully.
Maximum Horizontal Distance Exhaled Water Droplets
Maximum horizontal distance covered by exhaled water droplets for
various droplet diameters: heavy vs. quiet breathing. The red dot
indicates the global minimum in distance covered. In this case, the
minimum is attained for droplets of diameters between 50 and 80 microns.
Credit: Cathal Cummins
“We represent breathing as a point source of both air and droplets and
include a point sink to model the effect of extraction of air and
droplets,” Cummins said. “To take their size and density differences
into account, we use the Maxey-Riley equation, which describes the
motion of a small but finite-sized rigid sphere through a fluid.”
This work gives researchers a general framework to understand the
droplet dispersion. The model simplicity demonstrates that bimodality
could actually be a property of the droplets themselves, and the group
provides formulas to predict when such droplets will have short ranges.
“Our study shows there isn’t a linear relation between droplet size and
displacement — with both small and large droplets traveling further than
medium-sized ones,” said Felicity Mehendale, co-author and academic
surgeon at the University of Edinburgh. “We can’t afford to be
complacent about small droplets. PPE is an effective barrier to large
droplets but may be less effective for small ones.”
As a solution, Mehendale came up with the idea of creating an aerosol
extractor device. The team is working on plans to manufacture the
aerosol extractor to keep clinicians safe during a wide range of
aerosol-generating procedures routinely performed in medicine and
dentistry. Extraction units placed near the droplet sources can
effectively trap droplets, if their diameters fall below that of a human
hair.
“This has important implications for the COVID-19 pandemic,” said
Cummins. “Larger droplets would be easily captured by PPE, such as masks
and face shields. But smaller droplets may penetrate some forms of PPE,
so an extractor could help reduce the weakness in our current defense
against COVID-19 and future pandemics.”
Mehendale said a better understanding of the droplet behavior will help
“inform the safety guidelines for aerosol-generating procedures, and it
will be relevant during the current and future pandemics, as well as for
other infectious diseases. This mathematical model may also serve as the
basis of modeling the impact on droplet dispersion of ventilation
systems existing within a range of clinical spaces.”
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