Flow and dispersion in ground vehicle wakes

This paper considers the flow velocities and the dispersion of pollutants in the wake of a number of different types of ground vehicles. It does this mainly through a collation of the results of a number of experimental, numerical and analytical investigations carried out by the author and his co-workers over the last few years, and a comparison of these results with the work of other investigators. It is shown that the wakes of ground vehicles may be conveniently taken to consist of two regions: a near wake and a far wake. The near wake is characterised by large scale recirculation and longitudinal vortex structures, with unsteady fluctuations caused by a variety of effects, including instability of the separated shear layer and wake pumping. In the far wake there are no discernible flow structures with a steady decay of the velocity field, with the major component of wake unsteadiness being at large scales. The effect of cross-winds is to translate and diffuse the wake, with the balance between the two effects changing depending upon the nature of the surrounding topography. Only a relatively few measurements have been made of dispersion within vehicle wakes, other than in the rather complex case of vehicles in street canyons. However, there are a number of analytical solutions of wake dispersion that have, to some extent, been validated by comparison with full-scale experiments. On the basis of these investigations, it is suggested that the lower frequency fluctuations in vehicle wakes may have an effect on the dose of pollutants received by pedestrians at the roadside, and more work is suggested to quantify this further. (C) 2001 Academic Press.