Dispersion of Airborne Respiratory Saliva Droplets by Indoor Wake Flows

Abstract

A new analysis of the interaction between vortical wake structures and exhaled saliva droplets is conducted in the presented study. We demonstrate how wake flows may alter the droplet and aerosol dispersion, exceeding previously reported settling distances. A dipolar vortex is used to model the wake flow, self-propelling through a cloud of micron-sized evaporating saliva droplets, which are tracked using a Lagrangian method. The droplet's spatial location, velocity, diameter, and temperature are traced, while coupled to their local flow field thereof. Contrary to previous studies of droplet dispersion by vortical flows, a vortex viscous decay model is included. This proves to be essential for accurately predicting the dispersion and settling distances of droplets and aerosols. The non-volatile saliva components are also considered, allowing for properly capturing the dropletaerosol transition while predicting the equilibrium diameter of post evaporation residual aerosols. Moreover, the model is verified with previously published results, yielding an accurate prediction of the droplet's evaporationfalling curves. Using this model, our theoretical analysis reveals non-intuitive interactions between wake flows, droplet relaxation time, gravity, and mass transfer rates. The underlying physical mechanism responsible for the increased dispersion is studied, where aerosols originating from saliva droplets entrap within the vortical structure and subsequently translate to distances two orders of magnitude larger than the size of the vortex. Thus, a new mechanism enhancing the transmission of airborne pathogens is suggested, offering a new outlook on the spreading of airborne-carried pathogens.