Dr. Hongping Wang and his colleagues from the State Key Laboratory of Nonlinear Mechanics at the Institute of Mechanics of the Chinese Academy of Sciences and the University of Chinese Academy of Sciences have carried out lab experiments in a water tunnel to investigate the droplet dispersion exhaled from a person going up- or downstairs, with the particular focus on elucidating the effects of flow unsteadiness on droplet dispersion.
Visualization of the motion of particles expelled from the mouth of the mannequin with inclination angles of 60° (a) and (b) and 120° (c) and (d); panels (a) and (c) display the average intensity of particle fluorescence, and panels (b) and (d) show the instantaneous particle distribution at three times; the white arrows mark the ejection direction of the particles; the nondimensional time interval in these figures corresponds to a physical time interval of 0.1 s. Image credit: Wang et al., doi: 10.1063/5.0073880.
“Two different patterns of droplets dispersion are observed due to the different wake flows,” Dr. Wang said.
“These results suggest that we should cough with the head down toward the ground to ensure that most of the droplets enter the wake region.”
Dr. Wang and co-authors 3D-printed mannequins using white resin, each with a different inclination angle to represent the leaning in that we naturally do when going up stairs and the leaning back when we walk down.
After placing each mannequin in the water tunnel, they introduced hollow glass microspheres into the tunnel.
When illuminated by lasers, the glass microspheres provided a way to visualize the flow motion behind the mannequins.
This flow field, often called a wake, was studied using a technique called particle image velocimetry.
In computer simulations, particles lower than the head and moving toward the ground became caught in each mannequin’s wake and moved downward.
It appeared particles above the head were able to move relatively far distances horizontally as if they were emitted from the top of the head.
For the mannequins whose inclines reflected going up stairs, particles concentrated below the shoulder and moved downward with a short travel distance.
For simulating going down, particles dispersing over the person’s head were carried for a long distance.
“The major challenge is how to use particles in water to simulate the droplets in the air,” Dr. Wang said.
“The most surprising part was that the particles higher than the head can travel a much longer distance than those particles lower than the head due to the induction of the wake flow.”
The results are publsihed in the journal AIP Advances.
_____
Hongping Wang et al. 2022. Experimental study of the dispersion of cough-generated droplets from a person going up- or downstairs featured. AIP Advances 12 (1): 015002; doi: 10.1063/5.0073880
