Modelling particulate dispersion in the wake of a vehicle

There is growing evidence to suggest that particulates within road vehicle exhaust emissions are a serious health risk. It is important to understand how these particles are dispersed in the wake of the vehicle, to enable roadside pollutant levels, etc. to be accurately predicted. In the past, the dispersion of gaseous exhaust pollutants has been studied in detail usually using a Gaussian Plume approach. In contrast, Eskridge and Hunt studied gaseous emissions within vehicle wakes using a boundary-layer approach to obtain a simple formulation for the turbulent velocity distribution of the air behind a vehicle in the direction of travel in the absence of wind effects. In subsequent work, they go on to derive a method for predicting gaseous pollutant dispersion behind a series of vehicles. In this present work, the wake formulation has been extended to give the velocity distribution in the cross-wake and vertical directions before moving on to investigate the dispersion of gaseous pollutants behind a single vehicle. The movement of particles within the wake is investigated by considering the mechanics of individual particle behaviour within the wake. This method makes use of the wake velocities and particle trajectories obtained by numerically solving a system of ordinary differential equations. Results are presented for particle trajectories for different particle sizes and different initial exhaust conditions together with concentration profiles for the case of solutes. This paper also investigates the changes in predicted wake behaviour of a single vehicle experiencing a crosswind as well as the effect of including a viscous sub-layer.