We describe a comprehensive study of the statistical characteristics of concentration fluctuations in a neutrally buoyant tracer plume dispersing through a large array of building-like obstacles, each of which measured 12.2 m x 2.42 m x 2.54 m. The plumes were released both upwind and within the obstacle array for a range of source heights between 0.15 and 5.2 m. Detailed flow field and instantaneous plume concentration data were obtained from a comprehensive series of tracer experiments that utilized a large number of high-resolution concentration detectors, accompanied by the simultaneous acquisition of meteorological and turbulence measurements with sonic anemometer/thermometers. Extensive analyses are performed on the plume concentration data, and results are presented for a number of concentration statistics such as the mean plume lateral and vertical spreads, mean concentration, fluctuation intensity, peak concentration to concentration standard deviation ratio, concentration probability density function (pdf), concentration power spectra, and various concentration time and length scales of dominant motions in the array plume (e. g., integral scale, Taylor microscale). For the range of downwind distances from the source examined, the lateral mean concentration profiles are well approximated by a Gaussian distribution. The vertical profiles of mean concentration develop in a rather complex manner with downwind distance, with the result that the reflected Gaussian form is generally a less than ideal description of the mean array plume in the vertical direction. A comparison of the array plume with an open-terrain plume as a function of downwind distance indicates that the obstacle array significantly increases the lateral and vertical plume spreads and decreases the magnitude of the plume centreline mean concentration. The small-scale, high-intensity turbulence generated in the obstacle array results in a drastic reduction in the concentration fluctuation level in the array plume compared to an open-terrain plume under similar conditions. The evolution of the concentration pdf at a fixed range, but with decreasing height from above and into the obstacle array is similar to that obtained at a fixed height but with increasing downwind distance from the source. The integral and Taylor microscale time and length scales of the plume increase significantly within the obstacle array. Concentration power spectra measured within the array had a greater proportion of the total concentration variance in the lower frequencies ( energetic subrange), with a correspondingly smaller proportion in the higher frequencies ( inertial-convective subrange). It is believed that these effects result from the rapid and efficient stirring and mixing of plume material by the small-scale, high-intensity turbulence within the array.
