Hot Bodies a Lure for Unseen Specks

Computing airflow dynamics

We can’t see them, but tiny particles—dust, pollen, microbes, and the like—swirl around us in complicated, turbulent pathways. New numerical simulations suggest that, at least in tiny indoor spaces, our body heat may pull them even closer, where they have a better chance of eventually landing in our lungs.

 

The positions of 2-micrometer particles inside a 20-degree-Celsius room with a mannequin heated to 25 degrees Celsius, three minutes after particles were released through a floor event. In this simulation, 31 out of 1000 particles fell directly onto the mannequin’s warm body; none managed to leave the room through the ceiling vent. Yet when the mannequin was the same temperature as the room, no particles fell onto the body, and 160 out of 1000 particles escaped. Results were similar for simulations with 10-micrometer-diameter particles.“The conventional wisdom is that the thermal plume from your body protects you from particles falling from above,” says John B. McLaughlin, PhD, professor of chemical and biomolecular engineering at Clarkson University and coauthor of the study. “We found that, in our small room at least, that is not true.” Such findings can help engineers design better ventilation systems, McLaughlin says. “Studies have shown that schoolchildren learn more and office workers are more productive in environments where the concentration of particles in the air is very low.”

 

Airflow dynamics are notoriously tough to model computationally, largely because of the huge range of physical scales in equations for turbulent fluids. McLaughlin and his colleagues used a direct numerical simulation approach that offers accuracy but requires intensive computational resources. Their computational models of airflow and particle paths were built in a 4.8-square-meter virtual room at two-centimeter resolution using three-millisecond time steps over about three minutes of total simulated time. In each simulation, a mannequin sits motionless in the middle of the room. A stream of air suffused with particles—each with the density of sand and about the size of a grain of pollen—shoots up through a floor vent in front of the chair. Particles fan out throughout the room, with a ceiling vent as the only exit.

 

In simulations where the mannequin was bestowed with realistic body heat, researchers could see the hot air surging off the body and interacting with particulates. This thermal plume pulled rising particles directly into the mannequin’s breathing zone. At the same time, the plume blocked the path of particles traveling near the ceiling, forcing them to fall down into the mannequin’s personal space, doubling the trapping effect of the plume. The work was presented in March 2010 at the American Physical Society meeting in Portland, Oregon.

 

“The computational and the experimental go hand in hand when studying complex turbulent flows such as those around human beings,” says Mark N. Glauser, PhD, professor of mechanical and aerospace engineering at Syracuse University, whose empirical results helped guide McLaughlin’s modeling. Fundamentally, experiments can help validate computational models and give physical insights that spur new simulations. “Then the simulation tools can be used to probe a broader range of parameter space ‘virtually,’ as well as look in more detail at flow physics,” Glauser says. For example, the models from McLaughlin’s team can track individual particles in a turbulent flow—a feat that’s nearly impossible in real-life experiments.



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