Physics has been surprisingly prominent in the flurry of scientific activity to understand COVID-19 and its real and potential impacts.
Researchers have, for example, used computational fluid dynamics simulations to study how far droplets fly when we cough, and mathematical models to investigate how long these droplets take to dry in different situations and even different cities.
Now a Chinese team has used a computer simulation to provide the best encouragement yet to put the toilet lid down.
It shows how a flushing toilet can create a cloud of virus-containing aerosol droplets that is large, widespread and long-lasting enough for the droplets to reach and be breathed in by others.
Writing in the journal Physics of Fluids, Yangzhou University’s Ji-Xiang Wang and colleagues describe using a standard set of fluid dynamic equations known as the Navier-Stokes equations to simulate flushing in two types of toilet – one with a single inlet for flushing water, and another with two inlets to create a rotating flow.
They also used a discrete phase model – similar to that used for studies of how far droplets fly – to simulate movement of the numerous tiny droplets likely to be ejected from the toilet bowl into the air. The results were sobering.
As water pours into the toilet bowl from one side, it strikes the opposite side, creating vortices. These vortices continue upward into the air above the bowl, carrying droplets to a height of nearly a metre, where they might be inhaled or settle onto surfaces.
These droplets are so small, the researchers say, that they float in the air for over a minute. A toilet with two inlet ports for water generates an even greater velocity of upward flowing aerosol particles.
“One can foresee that the velocity will be even higher when a toilet is used frequently, such as in the case of a family toilet during a busy time or a public toilet serving a densely populated area,” says Wang.
The simulations show that nearly 60% of the ejected particles rise high above the seat for a toilet with two inlet ports.
The findings are general but relevant to the current times, the researcher add, because studies showing the novel coronavirus that causes COVID-19 can survive in the human digestive tract and show up in faeces.
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