Dispersion of Contaminants In Urban Areas: Coupling Dispersion and CFD Models

Title: DISPERSION OF CONTAMINANTS IN URBAN AREAS: COUPLING DISPERSION AND CFD MODELS.

Authors: ^1*Perales, J.F.; ¹Gallego, E.; ¹Roca F.X ; ²Guardino X.

¹Laboratori del Centre de Medi Ambient. Universitat Politècnica de Catalunya

(LCMA-UPC). Avda. Diagonal, 647. E 08028 Barcelona. Phone: 34934016683, Fax: 34934017150, e-mail: Lcma.info@upc.edu

²Centro Nacional de Condiciones de Trabajo. INSHT. C/Dulcet, 2-10. E 08034

Barcelona. Phone: 34932800102, Fax: 34932803642, e-mail: cnctinsht@mtas.es

* Author to whom correspondence should be addressed

e-mails: jose.francisco.perales@upc.edu

            eva.gallego@upc.edu

            fco.javier.roca@upc.edu

            cnctinsht@mtin.es

Air dispersion models are used to obtain the inmission concentration values of atmospheric contaminants emitted by well defined focuses. There are normative models as AERMOD and CALPUFF in USA for example that must be applied before new industrial activities begin to operate in order to asses that the maximum legislated inmission levels are not surpassed. These models give the concentration values of contaminants for hexahedral nodes over the territory. They give only one averaged concentration value for each of the nodes.  These nodes over the terrain have a size of hundreds or thousands of meters, so the information they give is scarce when referred to an urban zone. For these dispersion models the characterization of the terrain is made through the ‘land use' file data which is then introduced in the calculation process with values of terrain roughness, that are used to predict the wind speed profile over this terrain and these wind profile is used later to calculate the transport of atmospheric contaminants and then theirs concentrations.

In order to obtain more accurate results when urban zones are studied a new approach is used in this work: the coupling of tridimensional wind fields calculated for the areas of interests using the meteorological part of atmospheric dispersion models and the use of Computational Fluid Dynamic (CFD) software to take into account the elements of the urban configuration: streets, buildings, parks, etc. It uses the meteorological results from The Atmospheric Pollution Model (TAPM, SCIRO, Australia) which is an Eulerian model that solves approximations of the fundamental fluid dynamics and scalar transport equations to predict meteorology and pollutant concentrations from mesoscalar meteorological data. Nesting is used to achieve the detailed wind field for the area of interest.

Examples of wind fields calculated with TAPM at three different territorial scales are shown in figure 1. As can be observed, the orography of the zone is taken into account and influences the wind trajectories calculated.

These calculated wind field is then used as initial and boundary conditions for the CFD software, COMSOL Multiphysics, where urban elements could be very well defined, as seen in figure 2, that shows the effect of the emission of a chimney over a regular distribution of buildings.  

Figure 1. Wind field examples obtained with TAPM model with the three inner grids of the four nested grid used. Size of terrain in top two pictures: 100x100 Km (nodes 5x5 Km). Middle pictures: 30x30 Km (nodes 400x400m). Bottom pictures: 8x8 Km (nodes 400x400 m).    

Figure 2. CFD results for the study of the impact of a chimney emission on an array of buildings in a hexahedral node whose size is similar to the nodes of the bottom of figure 1.