Abstract
Previous studies reported that specially designed ventilation systems provide good air quality and safe environment by removing airborne droplets that contain viruses expelled by infected people. These water droplets can be stable in the environment and remain suspended in air for prolonged periods. Encounters between droplets may occur and droplet interactions should be considered. However, the previous studies focused on other physical phenomena (air flow, drag force, evaporation) for droplet transport and neglected droplet interactions. In this work, we used computational fluid dynamics (CFD) to simulate the transport and fate of airborne droplets expelled by an asymptomatic person and considered droplet interactions. Droplet drag with turbulence for prediction of transport and fate of droplets indicated that the turbulence increased the transport of 1 µm droplets, whereas it decreased the transport of 50 µm droplets. In contrast to only considering drag and turbulence, consideration of droplet interactions tended to increase both the transport and fate. Although the length scale of the office is much larger than the droplet sizes, the droplet interactions, which occurred at the initial stages of release when droplet separation distances were shorter, had a significant effect in droplet fate by considerably manipulating the final locations on surfaces where droplets adhered. Therefore, it is proposed that when an exact prediction of transport and fate is required, especially for high droplet concentrations, the effects of droplet interactions should not be ignored.
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Abbreviations
- C D :
-
Drag coefficient
- C L :
-
Lagrangian time scale coefficient
- d p :
-
Droplet diameter
- e :
-
Elementary charge
- ε :
-
Dissipation rate of turbulent kinetic energy
- ε 0 :
-
Permittivity of vacuum
- μ :
-
Air dynamic viscosity
- η :
-
Kolmogorov’s length scale of turbulence
- F C :
-
Coulomb force
- F D :
-
Drag force
- F LJ :
-
Lennard — Jones force
- i :
-
Particle i
- j :
-
Particle j
- k :
-
Turbulent kinetic energy
- l e :
-
Turbulent dissipation length scale
- m p :
-
Particle mass
- Re p :
-
Particle Reynolds number
- r i :
-
Position vector of the ith particle
- r j :
-
Position vector of the ith particle
- ρ :
-
Air density
- ρ p :
-
Particle density
- s :
-
Interaction strength
- St :
-
Stokes number
- σ LJ :
-
Distance of closest approach between particles
- t :
-
Simulation time
- τ e :
-
Eddy lifetime
- τ c :
-
Eddy crossing time
- τ i :
-
Eddy interaction time
- τ L :
-
Lagrangian time scale
- τ p :
-
Particle velocity response time
- u:
-
Averaged air velocity
- uf :
-
Air velocity vector
- urms :
-
Root mean square of air velocity (Turbulent air velocity perturbation)
- u*:
-
Friction velocity of air at wall
- μ :
-
Dynamic viscosity of air
- v:
-
Particle velocity
- x :
-
Wall normal direction
- y + :
-
Wall lift — off
- Z p :
-
Particle surface potential = zeta potential
- ξ :
-
Vector of random numbers
- 1 :
-
Streamwise direction (parallel) to wall
- 2 :
-
Spanwise direction (orthogonal to streamwise and normal)
- 3 :
-
Normal direction to wall
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Acknowledgements
This work was supported by the Brain Pool Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT, Republic of Korea (No. 2019H1D3A 2A02101993).
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Highlights
• Coulomb and Lennard-Jones forces were considered for droplet interactions.
• The net droplet interactions were repulsive.
• Repulsive droplet interactions increased the transport of droplets.
• Repulsive droplet interactions significantly modified the fate of droplets.
Conflict of Interest
The authors do not have conflicts of interest to declare.
Author Contributions
Allan Gomez-Flores: Methodology, Investigation, Software, Writing — Original draft preparation. Gukhwa Hwang: Writing — Original draft preparation. Sadia Ilyas: Writing — Reviewing and Editing. Hyunjung Kim: Conceptualization, Supervision, Writing — Reviewing and Editing.
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Gomez-Flores, A., Hwang, G., Ilyas, S. et al. A CFD study of the transport and fate of airborne droplets in a ventilated office: The role of droplet-droplet interactions. Front. Environ. Sci. Eng. 16, 31 (2022). https://doi.org/10.1007/s11783-021-1465-8
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DOI: https://doi.org/10.1007/s11783-021-1465-8