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Whenever you sneeze or cough, clouds of respiratory droplets are sprayed into the air in an explosive manner. These pulsatile events create turbulent free-shear jets that move differently compared to a continuous flow. With the lingering COVID-19 pandemic, a better understanding of how these jets work to distribute the aerosols is especially timely.
In real-life scenarios, coughs are rarely solitary, and usually occur as a series of pulses. Monroe et al. studied the flowing pattern of these sequences and showed how particle dispersion and penetration depth are affected by the number and frequency of pulses.
Based on numerical simulation data, they found higher entrainment for multi-pulse events compared to single-pulse events, meaning droplets can travel further due to the vortical structure created by consecutive pulses. Even as coughs weaken later in the series – as the cougher runs out of breath – droplets from the latter coughs could still travel almost as far as the first cough by riding on the vortices created by the pulsatile event.
This finding, combined with previous studies that suggest later coughs contain higher viral concentrations since they originate deeper within the lung, should be considered in transmission models.
Their results also reveal another advantage of mask wearing besides the filtering of droplets – masks can dampen the airflow and prevent the vortices from forming.
“The way I see it, a mask does three things: It captures a portion of the particles leaving the mouth; it reduces the momentum of the airflow; and it increases humidity which hinders evaporation, reducing the generation of small aerosols,” said author Jesse Capecelatro.
Source: “Role of pulsatility on particle dispersion in expiratory flows,” by K. Monroe, Y. Yao, A. Lattanzi, V. Raghav, and J. Capecelatro, Physics of Fluids (2021). The article can be accessed at https://doi.org/10.1063/5.0048746.
This paper is part of the Flow and the Virus special collection, learn more here.
  1. © 2021 Author(s). Published by AIP Publishing (https://publishing.aip.org/authors/rights-and-permissions).

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