Don’t Walk So Close to Me. CFD Redefines Coronavirus Safe Distance
Roopinder Tara posted on May 14, 2020 |
Safe distance of 6 ft? Try over 60 ft if you're cycling.
How close should you be to a moving person? Dr. Blocken’s CFD simulation suggests a linear relationship between airspeed of a moving individual, either walking, running or cycling, and another.

COVID-19 is respiratory ailment that spreads primarily by saliva sent into the air from an infected person. Droplets of saliva laden with the SARS-CoV-2 virus are expelled by sneezing, coughing—and possibly by breathing. They are inhaled, the primary method of transmission. The secondary method is virus spread via saliva that lands on a surface we touch and transfer to our mouth, nose and possibly eyes.

A 2015 study found people touch their face an astounding 23 times an hour. Mucous membranes are the most susceptible to contagion and the most popular touches (mouth, 36%, nose, 27% and eyes, 6%). 

You may be touching your face right now.

Countries have suggested, and in many cases declared, a rule of “social distance” of 5 ft to 6 ft, or 1.5 to 2 meters, to be kept in between individuals. The droplets fall to the floor and evaporate after traveling that distance. However, this only works when individuals are sitting or standing. This presents a question, how effective are the rules when regarding exercising outdoors? Exercising outdoors, such as hiking, running, and cycling is encouraged nowhere as much as it is in California, as long as “social distance” is maintained.

Across the Atlantic, in Belgium, the little country that has produced cycling’s greatest star (Eddy Merckx, aka the Cannibal), also includes Dr. Bert Blocken, a Belgian professor working in the civil engineering departments at TU Eindhoven in the Netherlands and KU Leuven in Belgium. Dr. Blocken, an avid cyclist, wondered about the proper social distance while people are active, possibly after finding himself in another cyclist’s wake and seeing their breath come his way in the cold and rain of the Northern European spring.

Dr. Blocken’s expertise in the rarified intersection of aerodynamics and cycling has helped professional and amateur cyclists alike, as well as sharing his findings and being curated on engineering.com. Rather than wonder how far behind a cyclist he should be, Dr. Blocken took it upon himself to employ ANSYS Fluent CFD to find out. He measured the proper and ideal safe distance as a function of speed of our most popular forms of outdoor exercise which includes walking, running, and cycling. His aerodynamics study, published here as a preprint, investigates whether individuals moving encounter the droplets of another. 

The droplets exhaled from a runner travel backwards relative to the runner. In this example, the runner is going 4 m/s or 14.4 kph, says Dr. Blocken. [That's a 6:42 mile pace which would result in a very respectable marathon time of 2:56, Ed.] A runner behind him going at the same pace should avoid the potentially germ-laden slipstream by remaining 10 meters (30 ft) behind. A walker, slower and less athletic, behind another walker going a moderate 4 kph (2.5 mph) should be 5 meters (15 ft) away. Cyclists going 30 kph (19 mph, quite respectable for amateur cyclists over distance) should be 20 meters. That is 65 feet! That is the length of a bowling lane, how far a major league pitcher is from home base. In other words: pretty damn far. These distances apply to if the trailing individual is in the slipstream and both are moving at similar velocities.

Bad Breath

Our breath comes out white clouds

Mingles and hangs in the air

Speaking strictly for me

We both could have died then and there

--Joan Baez, Diamonds in Rust, 1975

Two individuals moving through the air at the same speed or relative to each other must increase the safe distance, thought Dr. Blocken. A lot of it has to do with your breath and the size of droplets in exhalation, mostly from your mouth during exercise, essentially airborne saliva.

Airborne saliva droplets vary in size. A previous study cited by Dr. Blocken had found that the total average size distribution of the droplets from people was 0.58 to 5.42 μm (10-6 m) with 82% of droplets 0.74 to 2.12 μm. Droplets from coughs were larger, as big as 15.9 μm. Another study showed a 4x to 6x increase in concentration of droplets with deep exhalation but no increase in concentration with rapid breathing. A study of healthy people talking and coughing found considerable subject variability and an average size of droplets of about 50 – 100 μm.

The smaller the droplets, the longer they stay at the height from which they were expelled. A cough will release more than 6.7 mg of saliva in droplets of varying size at a speed of 22 m/s. Small droplets (less than 30 μm) can remain suspended in air and float along with air currents. Droplets of 50–200 μm fall harmlessly to the floor, far from our lungs. Larger droplets, those of more than 300 μm had enough momentum to go furthest. The smaller the droplets, the longer they stay at the height from which they were expelled. A cough could release more than 6.7 mg of saliva in droplets of varying size at a speed of 22 m/s. Small droplets (less than 30 μm) could remain suspended in air and float along with air currents. Droplets of 50–200 μm generally fall harmlessly to the floor, far from us if at social distance. Larger droplets, those of more than 300 μm generally have enough momentum to go furthest.

Hence the 6 ft distance required by law or by suggestion in the US, and 1.5 m or 2 m elsewhere.

While that might be a safe distance from an infected person if that person is stationary, if they are moving, all bets are off. Not only are more droplets expelled because the person is exerting themselves but the particles are expelled along a line of movement. If the weather is cold enough, one can visually see the white plume behind a walker or runner.

It is this plume behind walkers, runners and cyclists, visible or not, that you need to avoid for safe distancing if you are following them, doing the same exercise.

The Fluid Simulation

High-resolution computational grid on the runner and in the vertical center plane. With cells as small 50 mm and 40 layers of near the surface of the runner, the total number of cells for the single runner model reached about 6 million cells. (Picture courtesy of Dr. Bert Blocken.)
High-resolution computational grid on the runner and in the vertical center plane. With cells as small 50 μm and 40 layers of near the surface of the runner, the total number of cells for the single runner model reached about 6 million cells. (Picture courtesy of Dr. Bert Blocken.)

The study was performed using computational fluid dynamics software (CFD, Fluent by ANSYS) and the model, if not the results, were validated with wind tunnel results done previously by Dr. Blocken and others.

The wind tunnel model of the runner was ¼ scale and NC machined from a 3D scan of an actual runner. The CFD model was full size, composed of single runner was 6 million cells and the two-runner model, 12 million cells. The high-resolution grids with the cells on the surface of the runners were as small as 50 μm to resolve the viscous sublayer. The modeling and solution was done with ANSYS Fluent. The 3D steady state Reynolds-averaged Navier-Stokes (RANS) equations were solved using with the shear stress transport (SST) k-w model. Pressure-velocity coupling was calculated by the coupled algorithm with pseudo-transient under-relaxation and a pseudo-transient time step of 0.01 s. Pressure interpolation was second order, gradient interpolation was performed with the Green-Gauss node based scheme and second-order discretization schemes were used for both the convection terms and the viscous terms of the governing equations. Simulations were run for a total of 5000 timed steps.

Properties of water were used for saliva, and the size of droplets was distributed between 40 μm and 200 μm diameter, with an average diameter of 80 μm.

There is no wind from any direction. The exhaling velocity is 2.5 m/s relative to the movement of the walker or runner.

Safe Running

Running 1.5 m, some regions’ idea of a safe distance, behind another runner will have you breathing many of the smaller diameter droplets exhaled by the leading runner, but being 1 m off to the side makes and you encounter no droplets. (Picture courtesy of Dr. Bert Blocken.)
Running 1.5 m, some regions’ idea of a safe distance, behind another runner will have you breathing many of the smaller diameter droplets exhaled by the leading runner, but being 1 m off to the side makes and you encounter no droplets. (Picture courtesy of Dr. Bert Blocken.)

From the figure above, the CFD results are quite clear: running 1.5 m (5 ft) directly behind another runner will have you breathing in many of the smaller diameter droplets exhaled by the leading runner. That is also a little too close in most recreational running situations – not mention creepy. Increasing the gap to 6 ft, the commonly dictated safe distance in the US, does little to reduce the number of small droplets inhaled.

However, move just 1 meter (3 ft) to one side, next to or behind the runner in front, and it appears you will be completely out of harm’s way – as long as there is no crosswind from the leading runner’s side.

Therefore, if you are running with a friend, run either next to them or stagger yourself to one side. Avoid their slipstream. If you are passing a slower runner, get to one side as you approach them. If a runner has passed you and then moves in front of you, you would do well to move off to the side until you are out of their slipstream. Same for walking – though what kind of friends would do that for long? For cycling, where riding next to each other is generally not advisable and staggering impossible unless you are on a road closed to traffic, the safest approach is simply to be behind the rider by over 6o ft, the length of a bowling lane, if you are going moderately fast (near 20 mph).

(Image courtesy of Dr. Bert Blocken.)
(Image courtesy of Dr. Bert Blocken.)

The image above demonstrates a runner behind another feels their slipstream even at distance of 4.5 m (15 ft). Dr. Blocken’s study found no droplets reached the body of trailing runner if their gap was 10 m (33 ft). For walkers, no droplets reached the body of the trailing walker with a gap of 5 m (16 ft). The study on cycling was so involved, Dr. Blocken will devote a whole study to it. He reveals that the gap is 60 ft for the cyclist in the slipstream.

Aftermath

When your language is math and simulation and you express yourself with differential equations and CFD code with your colleagues, it can get difficult to explain the obvious. Dr. Bert Blocken, as with most researchers, will struggle to explain their findings on popular media. This most recent study, still to be published, about safe distances between walkers and runners, which initially included cyclists as well, has been thoroughly misunderstood by a vocal few. Rather than appreciate the amended safe distances as necessary for moving people, a few have interpreted it as how much more dangerous runners and cyclists can be, spreading their germs over a wide swath, and endangering the public.

We catch up to Dr. Blocken, at his home near University of Eindhoven, where he teaches in the Civil Engineering department and he comes up with an example that might have worked.

“When you spray windshield wiper fluid when the car is moving, it goes up and over the car,” says Dr. Blocken. “When the car is stationary, it does not.”

Works for us. Should work for an angry mob that looking at cyclists as if they were madmen with flame throwers.

We ask when the study will be published? And the cycling study?

“Soon. I would have time to finish it off if it wasn’t for all the media interviews,” says Dr. Blocken. “I’d also like to get back on my bike.”

That would be our cue to exit. We wish Dr. Blocken well and, cyclist to cyclist, to be safe out there, a farewell that, in the age of the Coronavirus, takes on more meaning.

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