Photo/Illutration A simulated image by the supercomputer Fugaku shows how droplets spread when a passenger coughs in a bus. A partition prevents droplets from reaching the bus driver. (Provided by Isuzu Motors Ltd. in cooperation with the Riken research institute, National Institute of Advanced Industrial Science and Technology, Espec Corp., Kanomax Japan Inc., Toyohashi University of Technology, Kyushu University and the transport ministry)

A partition separating the driver and passenger seats in a bus can prevent large droplets from reaching either side when someone coughs, according to simulations by the world’s fastest supercomputer.

An air conditioning system can also efficiently ventilate the air within minutes, while droplet-catching filters effectively remove particles even when the windows are closed.

A team led by the government-affiliated Riken research institute used Japan’s supercomputer Fugaku to run simulations of the spread of droplets from coughs in a local bus.

The bus has two doors, one at the front and another at the back, and was carrying 63 passengers, or a passenger occupancy rate of 80 percent. It was traveling at a speed of 20 kph. 

Two ventilators, one at the front and another at the back, were turned on, with the air conditioner set to circulate air in the vehicle.

If there is a partition between the driver and passenger seats, large droplets won’t travel to the other side when either the driver or a passenger coughs, while small droplets will spread across the interior.

Even if the windows are closed, the ventilators can pump in the same volume of fresh air as the air volume of the interior in three and a half minutes. When each of the five windows was cracked open by 5 centimeters, it took two and a half minutes.

And if droplet-catching filters are installed inside the air conditioner, they grab the same amount of droplets whether the windows are open or closed.

According to a representative of Isuzu Motors Ltd., which participated in the study, the filters used for the simulation are for experimental use, while coarser, dust-catching filters are used for local buses currently in operation.

The automaker is preparing to commercialize filters that can catch small droplets in an effective manner.

The researchers also ran simulations on a taxi with a driver and two passengers traveling at 40 kph in a city.

When the air conditioner is running on fresh-air mode and is set to half its maximum airflow volume with the windows closed, it took 85 seconds for the taxi to be fully ventilated.

With the windows on the driver’s side and on the left rear passenger side cracked open by 5 cm, it took 68 seconds to completely replace the air, beating the closed-window scenario by just a little.

The air fully ventilates within 45 seconds after closing the windows and putting the air conditioner on its maximum airflow setting.

There is almost no benefit in opening the windows if the vehicle slows to 20 kph.

When the driver coughs without wearing a mask, small droplets will spread throughout the inside of the car in the direction of the air conditioner’s airflow.

The droplets will effectively exit the vehicle if partitions are set up between the driver and passenger seats while the windows on the driver’s side and the left rear passenger side are open. The amount is reduced by half 20 seconds later, with fewer droplets reaching the passenger seats.

Opening the windows has a limited effect when a passenger coughs without a mask, and the partitions have limited usefulness when it comes to preventing small airborne droplets.

The team concluded it would be better to reduce droplets by wearing a mask.

The results can be applied to private vehicles. When sharing a car with people other than family members and other close people, wearing a mask is recommended.

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