Host-to-host airborne transmission as a multiphase flow problem for science-based social distance guidelines

Abstract

COVID-19 pandemic has strikingly demonstrated how important it is to develop fundamental knowledge related to generation, transport and inhalation of pathogen-laden droplets and their subsequent possible fate as airborne particles, or aerosols, in the context of human to human transmission. It is also increasingly clear that airborne transmission is an important contributor to rapid spreading of the disease. In this paper, we discuss the processes of droplet generation by exhalation, their potential transformation into airborne particles by evaporation, transport over long distances by the exhaled puff and by ambient air turbulence, and final inhalation by the receiving host as interconnected multiphase flow processes. A simple model for the time evolution of droplet/aerosol concentration is presented based on a theoretical analysis of the relevant physical processes. The modeling framework along with detailed experiments and simulations can be used to study a wide variety of scenarios involving breathing, talking, coughing and sneezing and in a number of environmental conditions, as humid or dry atmosphere, confined or open environment. Although a number of questions remain open on the physics of evaporation and coupling with persistence of the virus, it is clear that with a more reliable understanding of the underlying flow physics of virus transmission one can set the foundation for an improved methodology in designing case-specific social distancing and infection control guidelines.

A Physics Modeling Study of COVID-19 Transport in Air

“…Numerical simulations show that during propagation the puff edge grows ∝ 1/4 [16]. After a100 s the puff would grow by a factor of 3 [26], in agreement with our analytical estimates. In closing, we note that if we ignore the motion of the air puff carrying the aerosols, as in the analysis of Wells (1934) [28], it is straightforward to see substituting by / ∼ 10 −12 into Equation 10 that the individual aerosols would not travel more than a few cm away from the exhaler, even under conditions of fast ejections, such as in a sneeze.…”

“…At the point of almost complete evaporation the settling velocity of the aerosols is sufficiently small that they can remain trapped in the puff and get advected by ambient air currents and dispersed by ambient turbulence. The size of the puff then continuously grows in time [26]. Our result can equivalently be interpreted in terms of the effective coherence length of the turbulent cloud assuming ∼ .…”

“…The aerosols and droplets are initially transported as part of a coherent gas puff of buoyant fluid. The ejected puff of air remains coherent in a volume that varies from 0.00025 to 0.0025 m 3 [26]. This corresponds to a puff size 0.78 ≲ / ≲1.68, where following [26] we have taken an entrainment coefficient of = 0.1 [27].…”

“…The ejected puff of air remains coherent in a volume that varies from 0.00025 to 0.0025 m 3 [26]. This corresponds to a puff size 0.78 ≲ / ≲1.68, where following [26] we have taken an entrainment coefficient of = 0.1 [27]. The puff is ejected with 1 ≲vV,0,∥/(m/s)≲10 [26].…”

“…This corresponds to a puff size 0.78 ≲ / ≲1.68, where following [26] we have taken an entrainment coefficient of = 0.1 [27]. The puff is ejected with 1 ≲vV,0,∥/(m/s)≲10 [26]. The turbulent puff cloud consists of an admixture of moist exhaled air and mucosalivary filaments.…”