Field and wind tunnel investigations of plume dispersion around single surface obstacles

Abstract

The near-wake entrainment behaviour of a limited range of isolated obstacles, representative of those found in urban areas, was investigated in the field at a nominal scale between 1/10 and 1/20. Tracer gas was released from a continuous point source located upwind of the obstacles. It is shown that concentrations in the lee of different obstacles depend on the proportion of the plume entrained and on the dimensions of the region of recirculating flow. The presence of taller obstacles results in a reduction of ground level concentrations. The effect of longitudinal, lateral and vertical source displacement was also examined for the different obstacles in the field. The field experimental programme was supported by physical modelling in the wind tunnel at a nominal scale of approximately 1/100. Dispersion around a single cube was mainly investigated in the wind tunnel and the results confirm in general the findings of the field trials. However, comparisons between wind tunnel and field results clearly show that the plume is more dispersed in the field, attributable to the effect of additional wind meander occurring in the atmosphere. Accordingly, centreline concentrations are higher in the wind tunnel and the effect of lateral source displacement is more pronounced in the wind tunnel than in the field.