The results of an initial validation study of an urban dispersion model

DERA Porton Down has produced an Urban Dispersion Model (UDM), which accounts for the interactions between a dispersing cloud and both individual obstacles and groups of obstacles. It is an empirical Lagrangian puff model that is based on an extensive wind tunnel database. Interactions between puffs and obstacles are modelled explicitly using a simple procedure if the puff is of comparable size to the obstacle or smaller. Our experiments have shown that if a puff interacts with groups of obstacles that are too close together to be considered individually, then the dispersion is Gaussian (when averaged over time) and the bulk characteristics of the surface obstacles determine the rates of dispersion. In this Urban Array Regime, the new empirically derived equations for urban dispersion rates are employed. For longer-range dispersion the model follows conventional Gaussian dispersion modelling practice, but with some development of the model to deal with specific aspects of urban dispersion that are not usually well covered: the variation of both dispersion rates and atmospheric stratification with surface roughness. Attention is also given to determining the appropriate local wind speed for calculating the puff advection and concentrations. A single-zone building infiltration model and a wind-flow module for estimating the wind-field over moderately complex terrain have been incorporated into the model. There is no limit on the number of obstacles that can be included in a scenario and model runs have been conducted using several thousand buildings that execute faster than real time on a PC. The figure shows a sample output from the model.

A verification and validation programme for the UDM has been initiated. Initially the model will be validated against the extensive scaled field experimental data that DERA Porton Down have collected. Macdonald conducted continuous ground-based releases in an urban array constructed from almost 100 cubic obstacles each of dimension 1.2m. There is a total of ten array configurations, covering a range of idealised urban types, instrumented with between eight and twelve photo-ionisation detectors (PIDs). Griffiths’s experiments involved puff releases using individual obstacles and obstacle arrays with between 15 and 30 PIDs deployed. Seven configurations were used and more than 1000 individual puff releases were made. Further model validation will be carried out against other scaled-field data and wind-tunnel data.

The presentation will concentrate on describing the results of the initial validation study of the UDM against the Macdonald and Griffiths datasets. The UDM will also be available for demonstration during the conference.