Variability in the Short-Range Dispersion of Passive, Short-Duration Emissions

The objective of this analysis was to use wind tunnel concentration measurements to describe the structure of dispersing clouds from an elevated source in a deep turbulent boundary layer and to develop scaling rules that reduce the results to a universal form. The experiments were carried out in a 1 m deep simulated atmospheric boundary layer in the EnFlo meteorological wind tunnel at the University of Surrey. Ensembles of between 100 and 200 repeat emissions were used, with emission durations between 0.067 and 1.02 s with the reference flow speed at the boundary edge of 2 ms-1. The fetch studied extended to about six source heights downwind (roughly, two boundary layer depths). The structure of the evolving clouds was analysed to determine time of flight, along-wind spread and dosage and to compare the dosage behaviour with the concentration field in a plume from the same source. This illustrated how the two were related and therefore how cloud dosage statistics (mean and standard deviation) could be derived from plume data. It also demonstrated that much larger ensembles were required to reduce the statistical uncertainty in the mean cloud properties. Consequently, the proof of the near-universal scaling presented is somewhat compromised. However, it is believed to be both relatively simple and adequate for practical applications. An application to a sequence of short- term releases at full-scale is demonstrated. The next steps involve testing these conclusions in a wider range of flow and dispersion conditions, namely in the presence of obstacles or complex urban areas.

Keywords: Boundary layer; Dosage; Mean; Short-Range Dispersion.