Dispersion of a Point-Source Release of a Passive Scalar Through an Urban-Like Array for Different Wind Directions
- 436 Downloads
The dispersion of a point-source release of a passive scalar in a regular array of cubical, urban-like, obstacles is investigated by means of direct numerical simulations. The simulations are conducted under conditions of neutral stability and fully rough turbulent flow, at a roughness Reynolds number of Re τ = 500. The Navier–Stokes and scalar equations are integrated assuming a constant rate release from a point source close to the ground within the array. We focus on short-range dispersion, when most of the material is still within the building canopy. Mean and fluctuating concentrations are computed for three different pressure gradient directions (0°, 30°, 45°). The results agree well with available experimental data measured in a water channel for a flow angle of 0°. Profiles of mean concentration and the three-dimensional structure of the dispersion pattern are compared for the different forcing angles. A number of processes affecting the plume structure are identified and discussed, including: (i) advection or channelling of scalar down ‘streets’, (ii) lateral dispersion by turbulent fluctuations and topological dispersion induced by dividing streamlines around buildings, (iii) skewing of the plume due to flow turning with height, (iv) detrainment by turbulent dispersion or mean recirculation, (v) entrainment and release of scalar in building wakes, giving rise to ‘secondary sources’, (vi) plume meandering due to unsteady turbulent fluctuations. Finally, results on relative concentration fluctuations are presented and compared with the literature for point source dispersion over flat terrain and urban arrays.
KeywordsDirect numerical simulation Dispersion modelling Urban array
Unable to display preview. Download preview PDF.
- Hilderman T, Chong R (2007) A laboratory study of momentum and passive scalar transport and diffusion within and above a model urban canopy final report. Contract Report DRDC Suffield CR 2008-025, 70 ppGoogle Scholar
- Jerram N, Perkins RJ, Fung JCH, Davidson MJ, Belcher SE, Hunt JCR (1995) Atmospheric flow through groups of buildings and dispersion from localised sources. In: Cermak JE, Davenport AG, Plate EJ, Domingos X (eds) Wind climate in cities. Kluwer, Dordrecht, pp 109–130Google Scholar
- Mavroidis I (2000) Velocity and concentration measurements within arrays of obstacles. Int J Glob Nest 2: 109–117Google Scholar
- Soulhac L (2000) Modelisation de la dispersion atmospherique a l’interieur de la canopee urbaine. Ph.D. thesis, Ecole Centrale de LyonGoogle Scholar
- Sykes RI, Henn DS (1992) Large-eddy simulation of concentration fluctuations in a dispersing plume. Atmos Environ 17: 3127–3144Google Scholar
- Vincent JH (1978) Model experiments on the nature of air pollution transport near buildings. Atmos Environ 11: 765–774Google Scholar