Wind tunnel simulations of plume dispersion through groups of obstacles

In this paper we present the results of two wind-tunnel simulations of dispersion from upwind point sources through a large group of obstacles, and compare these with an associated field study (Davidson et al., 1995, Atmospheric Environment 29, 3245-3256), Detailed flow-field and plume concentration data were obtained from simulations at scales of 1:20 and 1:200. With these data we are able to provide experimental confirmation of many of the ideas developed during the field study and to confirm the experimental results obtained in the field. In doing so, we show that the upstream flow-field parameters are the most effective means of scaling the three data sets to achieve broad quantitative agreement. Measurements and flow visualisation of the flow-field confirm that there are a number of mechanisms influencing the behaviour of a plume as it passes through an obstacle array: in particular the divergence and convergence of streamlines and changes to the structure of the turbulence within the array, However, although the turbulence within the array is shown to be of greater strength and smaller scale than at corresponding locations outside the obstacle array, it is found that there is little change in the transverse diffusivity (and therefore in the lateral plume width). The concentration data confirm that the divergence of streamlines near the upstream end of the obstacle array has a significant effect on the vertical width of a plume (sigma(2)). Changes to the structure of the turbulence appear to have little effect, however, since the transverse diffusivities within the obstacle array are unchanged. Thus, the mean lateral spread and decay of mean concentration of the plume with downstream distance resemble that of a control plume; that is; a plume released under identical conditions where the obstacle array is not present. We also confirm that the mean structure of a plume has a Gaussian form as it passes through an array of obstacles. By contrast, concentration measurements with a high-frequency-response detector confirm that the small-scale, high-strength turbulence rapidly mixes the plume internally, dramatically reducing the strength of concentration fluctuations within the plume. Since the wind tunnel is shown to be an effective means of modelling this type of field situation, with the appropriate scaling, these studies were extended to consider the effects on plume behaviour of changes in source position, array configuration and array height. Copyright (C) 1996 Elsevier Science Ltd