Wind Tunnel Modelling of Flow and Pollutant Dispersion in a Heterogeneous Urban Area

Understanding air flow and pollutant dispersion in heterogeneous urban environments is essential for improving urban climate models and air quality predictions. This study presents results from wind tunnel experiments on investigating dispersion patterns within a scaled physical model (1:200) of an area of central Bristol, UK. The city’s complex urban morphology, including varying building heights and street layouts serves as a case study for heterogeneous urban environments. Experiments were conducted in the EnFlo wind tunnel at the University of Surrey to examine the influence of urban form and source locations on flow dynamics and pollutant transport, utilising a fast-response flame ionisation detector (FFID) and three-component laser Doppler anemometry (LDA) to measure the concentration and velocity fields, respectively. Various wind directions, including southerly, south-westerly, and westerly winds, were tested to capture the variability in pollutant dispersion and momentum transport. Pollutant sources were placed at different locations, including elevated release points (rooftop) and near-ground sources within street canyons, to assess sensitivity to emission height and location. Flow measurements and concentration distributions were analysed to identify key dispersion mechanisms, including building-induced wake effects, recirculation zones, and channelling along major street canyons. The results highlight significant variations in dispersion patterns depending on wind direction and urban geometry. Pollutant retention within deep street canyons and leeward of large buildings was observed under certain conditions, while open intersections and gaps between buildings facilitated its dispersion. This work underscores the importance of integrating wind tunnel experiments alongside computational modelling to enhance predictive capabilities for urban air quality and microclimate assessments. Future research will extend this approach to assess the impact of terrain variations and patterns on dispersion and flow dynamics.