HRMODURB · High resolution models for flow and pollutant dispersion in urban areas

High resolution models for flow and pollutant dispersion in urban areas

The research work within the HRModUrb project falls mainly within the field of dispersion modelling, i.e. linking emissions to concentrations. Despite apparent public perception to the contrary, there is much uncertainty in our understanding of the air pollution system and the way emissions and impact are related. Nevertheless, air quality modelling is currently employed routinely as an air quality management and planning tool. Recent EU legislation recognise modelling in this role for assessing air quality in Europe. Historically and understandably, dispersion models had been applied to quite simple situations, often just a single emission point (industrial stack) in a relatively simple (from the geometrical point of view) environment. Applying dispersion models in urban areas has proved particularly difficult due to the increased geometrical complexity, the number and the variety of emission sources, the range of different pollutant species and their complex chemical interactions, the particular microclimate and the interaction between different length scales. Complex urban models are currently able to simulate emission, dispersion, chemical reactions and deposition of a range of pollutants at spatial scales from regional (i.e. hundreds of km) to city (i.e. tens of km). Currently, at the neighbourhood and street scales (i.e a few metres to 1-2 km), the most advanced numerical models (CFD, Computational Fluid Mechanics models) are capable of solving nearly all the complexities, but their use is limited by the amount of computational resources needed, which make their operational use in air quality management impracticable. Simpler operational models have been developed, but their performance is limited when applied to complex geometries and meteorological conditions like those that can be found in a real urban environment. Several key physical phenomena have yet to be completely understood, and, in order to develop new high resolution (neighbourhood scale) models, new parameterisations have to be found. Wind tunnels, which have been the main experimental device in environmental fluid mechanics research for several decades, play an important role in developing this understanding. Large facilities, like the EnFlo wind tunnel at the University of Surrey, are though needed to simulate adequately atmospheric flows with their large range of turbulence scales. Current operational canopy (i.e. valid for flow and dispersion within the streets) models usually adopt simple approaches, deriving flow and dispersion fields from basic parameters, such as wind speed and directions and a few additional meteorological conditions above the canopy, building plan area density of the neighbourhood and mean building height, as well as, obviously, the pollutant emissions. Previous studies, however, have demonstrated the influence specific geometrical features, such as the presence of a single building much taller than its neighbours and building height variability. These two features affect the turbulence field, and thus also the turbulent characteristics in their vicinity, and play an important role in shaping the three-dimensional wind field within the canopy and the exchange of pollutants between the canopy and the free flow above. This area was the main focus of the HRModUrb project.

Data: CORDIS, © European Union