Natural Ventilation in Urban Buses: From Negative Pressure To Velocity-Driven Dispersion of Aerosols

Abstract

In this report we analyze the air streams formed across the open windows (natural ventilation) of an urban bus model and the resulted dispersion of aerosols emitted in the passenger area. The methods include computational fluid dynamics simulations and three ways to characterize the dispersion of passive tracers: a continuous concentration-based model, a discrete random model and a parametric scalar based on the so-called mean age of air. We also conducted experiments using a 1:10 scale bus model and CO2 as a passive tracer to assess the ventilation characteristics. We found that dispersion and expulsion of aerosols is driven by a negative pressure field in the standard bus design equipped with lateral windows. Also, the average age of air is 6 minutes while the air streams promote aerosol accumulation at the front (driver’s area). To speed up the expulsion of aerosols and reduce their in-cabin accumulation, we proposed a bus bodywork prototype having a frontal intake of air. The expulsion of aerosols in this case is driven by a frontal velocity field while the average age of air is reduced to 50 seconds which manifests as an increase of the aerosols expulsion rate.