Numerical investigation of the pollution dispersion in an urban street canyon

The pollution levels in a real urban street canyon are determined numerically, using the Reynolds Averaged Navier-Stokes (RANS) approach, in the framework of Remote Optical Measurement Techniques (ROMT) evaluation under realistic conditions, in order to identify the most appropriate placing of the ROMT instruments to be tested in the street canyon. As the operation of instruments is based on optical absorption along a beam, a balance must be sought between beam-path length and height and the inherent concentration characteristics of the pollution dispersion field in the canyon. For modelling, a Computational Fluid Dynamics (CFD) code is utilised, which solves the 3D unsteady Reynolds Averaged Navier-Stokes (RANS) within a computational domain that includes all buildings and streets in the vicinity of the canyon. Different scenarios are examined as regards wind direction, resulting in highly varying in-canyon concentration field characteristics. Different instrument placements are also investigated, as well as beam-lengths and heights, to establish the most appropriate location and instrument configuration for the forthcoming field campaign. Results show that although oblique-to-street-axis placement of the beam-path ensures a higher overall concentration-indication due to longer beam-path, the high dependence of indication on the wind direction makes this case less favourable compared to the perpendicular-to-street-axis placement.