The pioneering work of Taylor on the turbulent dispersion of aerosols is one century old and provides an interesting way to introduce both diffusive processes and turbulence at an undergraduate level. Low mass particles transported by a turbulent flow exhibit a Brownian-like motion over time scales larger than the velocity correlation time. Aerosols and gases are, therefore, subjected to an effective turbulent diffusion at large length scales. However, the case of a source of pollutant much smaller than the integral scale is not completely understood. Here, we present experimental results obtained by undergraduate students in the context of the COVID-19 pandemic. The dispersion of a fog of oil droplets by a turbulent flow is studied in a wind tunnel designed for pedagogical purposes. It shows a ballistic-like regime at short distance, followed by Taylor's diffusive-like regime, suggesting that scale-free diffusion by the turbulent cascade process is bypassed. Measurements show that the dispersion of CO2 emitted when breathing in a natural, indoor air flow is not isotropic but rather along the flow axis. The transverse spread is ballistic-like, leading to the concentration decaying as the inverse-squared distance to the mouth. The experiment helps students understand the role of fluctuations in diffusive processes and in turbulence. A Langevin equation governing aerosol dispersion based on a single correlation time allows us to model the airborne transmission risk of pathogens, indoors and outdoors. The results obtained in this study have been used to provide public health policy recommendations to prevent transmission in shopping malls.
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November 2022
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November 01 2022
Turbulent dispersion of breath by the wind
Florian Poydenot;
Florian Poydenot
a)
Laboratoire de Physique de l'École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité
, F-75005 Paris, France
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Ismael Abdourahamane;
Ismael Abdourahamane
Laboratoire de Physique de l'École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité
, F-75005 Paris, France
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Elsa Caplain;
Elsa Caplain
Laboratoire de Physique de l'École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité
, F-75005 Paris, France
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Samuel Der;
Samuel Der
Laboratoire de Physique de l'École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité
, F-75005 Paris, France
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Antoine Jallon;
Antoine Jallon
Laboratoire de Physique de l'École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité
, F-75005 Paris, France
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Inés Khoutami;
Inés Khoutami
Laboratoire de Physique de l'École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité
, F-75005 Paris, France
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Amir Loucif;
Amir Loucif
Laboratoire de Physique de l'École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité
, F-75005 Paris, France
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Emil Marinov;
Emil Marinov
Laboratoire de Physique de l'École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité
, F-75005 Paris, France
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Bruno Andreotti
Bruno Andreotti
b)
Laboratoire de Physique de l'École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité
, F-75005 Paris, France
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a)
ORCID: 0000-0002-9552-4055.
b)
Electronic mail: andreotti@phys.ens.fr, ORCID: 0000-0001-8328-6232.
Am. J. Phys. 90, 826–832 (2022)
Article history
Received:
July 27 2021
Accepted:
July 04 2022
Citation
Florian Poydenot, Ismael Abdourahamane, Elsa Caplain, Samuel Der, Antoine Jallon, Inés Khoutami, Amir Loucif, Emil Marinov, Bruno Andreotti; Turbulent dispersion of breath by the wind. Am. J. Phys. 1 November 2022; 90 (11): 826–832. https://doi.org/10.1119/5.0064826
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