Fluid dynamics expert explains how to reduce the risk of airborne COVID-19 transmission inside a car



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Editor’s Note: Varghese Mathai is a physicist at the University of Massachusetts at Amherst who studies the flow of fluids and gases. He conducted a study using computer fluid dynamics simulations to understand how air circulates inside a car and its implications for airborne transmission of COVID-19. In this interview, he explains the optimal ways to ensure maximum airflow inside a car.

Varghese Mathai from the University of Massachusetts at Amherst explains and shows how air circulates inside cars and how to reduce the risk of airborne transmission of COVID-19.

What can be done to reduce the risk of airborne transmission inside a car?

It is important to have good ventilation. This means you get as much outside air as possible to mix with the air inside the cabin and then flush it out.

You can do this in several ways. The first is to turn on the heating system, which draws in cool air from outside, and open the windows through which it can be flushed. Another way is to just open the windows. The advantage of having the windows open is that if you are going 20 miles an hour or more, a lot of air is exhausted just by the speed of the car.

Having the windows open allows more air to escape than simply turning on the heating or air conditioning.

Which windows should be kept open and closed to ensure optimal air circulation?

Numerical simulation of fluid dynamics of air flow in cars with two windows, one at the rear, one at the front, open. Source: Varghese Mathai CC-BY-ND

We believe that the best configuration is to have all windows open, and if possible fully open. If that’s not practical, then it would be good to have two windows open. Preferably one at the back and one at the front.

What we discovered from computer simulations is that air enters through the rear window, turns around behind the rear passenger, and exits through the front window. In this way, many of these aerosol particles inside the cabin can be vented.

What about barriers and screens between passenger and driver?

Many taxis and ride-sharing services like Uber and Lyft use a barrier or screen between the front and back areas of the cab. These help reduce transmission through larger droplets. These are the types of droplets that are released by coughing, sneezing, or loud conversations. Surface decontamination helps control fomite transmission. But air transmission would not be greatly reduced by these barriers as there are always spaces and holes in the barriers through which air can pass.

How did you conduct this study?

For this study, we used computer simulations, in particular computer fluid dynamics simulations, which are widely used to study flows around cars and airplanes. We used it because of its fast turnaround time so that we could compare different configurations of open and closed windows and qualitatively predict what might be better in terms of removing these airborne particles. .

Sequence on one of the field tests performed to understand airflow in a car. Source: Varghese Mathai CC-BY-ND

After this post was released, we went in and did a number of field tests to get some sort of validation of the simulated airflows. We released smoke in various places inside the car and examined the pathways of the smoke as it was released inside the car. It was more or less the same as what we found from the computer simulations.

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