Abstract
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.
Similar content being viewed by others
References
Barlow JF, Belcher SE (2002) The rate of exchange of passive scalar between streets and the boundary layer aloft. Boundary-Layer Meteorol 104: 131–150
Belcher SE (2005) Mixing and transport in urban areas. Philos Trans R Soc 363: 2947–2963
Belcher SE, Jerram N, Hunt JCR (2003) Adjustment of the atmospheric boundary layer to a canopy of roughness elements. J Fluid Mech 488: 369–398
Boppana VBL, Xie Z-T, Castro IP (2010) Large-eddy simulation of dispersion from surface sources in arrays of obstacles. Boundary-Layer Meteorol 135: 433–454
Britter RE, Hanna SR (2003) Flow and dispersion in urban areas. Annu Rev Fluid Mech 35: 469–496
Carpentieri M, Robins AG, Baldi S (2009) Three-dimensional mapping of air flow at an urban canyon intersection. Boundary-Layer Meteorol 133: 277–296
Castro IP, Robins AG (1977) The flow around a surface-mounted cube in uniform and turbulent streams. J Fluid Mech 79: 307–335
Coceal O, Belcher SE (2004) A canopy model of mean winds through urban areas. Q J Roy Meteorol Soc 130: 1349–1372
Coceal O, Thomas TG, Castro IP, Belcher SE (2006) Mean flow and turbulence statistics over groups of urban-like cubical obstacles. Boundary-Layer Meteorol 121: 491–519
Coceal O, Dobre A, Thomas TG, Belcher SE (2007) Structure of turbulent flow over regular arrays of cubical roughness. J Fluid Mech 589: 375–409
Davidson MJ, Mylne KR, Jones CD, Phillips JC, Perkins RJ (1995) Plume dispersion through large groups of obstacles a field investigation. Atmos Environ 29: 3245–3256
Davidson MJ, Snyder WH, Lawson RE, Hunt JCR (1996) Wind tunnel simulations of plume dispersion through groups of obstacles. Atmos Environ 30: 3715–3725
Dejoan A, Santiago JL, Martilli A, Martin F, Pinelli A (2010) Comparison between LES and RANS computations for the MUST field experiment. Part II: Effects of incident wind angle deviation on the mean flow and plume dispersion. Boundary-Layer Meteorol 135: 133–150
Fackrell JE, Robins AG (1982) Concentration fluctuations and fluxes in plumes from point sources in a turbulent boundary layer. J Fluid Mech 117: 1–26
Hamlyn D, Hilderman T, Britter R (2007) A simple network approach to modelling dispersion among large groups of obstacles. Atmos Environ 41: 5848–5862
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 pp
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 109–130
Kim J-J, Baik J-J (2004) A numerical study of the effects of ambient wind direction on flow and dispersion in urban street canyons using the RNG k-epsilon turbulence model. Atmos Environ 38: 3039–3048
Macdonald RW, Griffiths RF, Cheah SC (1997) Field experiments of dispersion through regular arrays of cubic structures. Atmos Environ 31: 783–795
Macdonald RW, Griffiths RF, Hall DJ (1998) A comparison of results from scaled field and wind tunnel modelling of dispersion in arrays of obstacles. Atmos Environ 32: 3845–3862
Mavroidis I (2000) Velocity and concentration measurements within arrays of obstacles. Int J Glob Nest 2: 109–117
Milliez M, Carissimo B (2007) Numerical simulations of pollutant dispersion in an idealised urban area, for different meteorological conditions. Boundary-Layer Meteorol 122: 321–342
Moin P, Mahesh K (1998) Direct numerical simulation: a tool in turbulence research. Annu Rev Fluid Mech 30: 539–578
Robins A, Savory E, Scaperdas A, Grigoriadis D (2002) Spatial variability and source-receptor relations at a street intersection. Water Air Soil Pollut Focus 2: 381–393
Snyder WH, Castro IP (2002) The critical Reynolds number for rough-wall boundary layers. J Wind Eng Ind Aerodyn 90: 41–54
Soulhac L (2000) Modelisation de la dispersion atmospherique a l’interieur de la canopee urbaine. Ph.D. thesis, Ecole Centrale de Lyon
Soulhac L, Garbero V, Salizzoni P, Mejean P, Perkins RJ (2009) Flow and dispersion in street intersections. Atmos Environ 43: 2981–2996
Sykes RI, Henn DS (1992) Large-eddy simulation of concentration fluctuations in a dispersing plume. Atmos Environ 17: 3127–3144
Theurer W, Plate EJ, Hoeschele K (1996) Semi-empirical models as a combination of wind tunnel and numerical dispersion modelling. Atmos Environ 30: 3583–3597
Vincent JH (1978) Model experiments on the nature of air pollution transport near buildings. Atmos Environ 11: 765–774
Wood CR et al (2009) Dispersion experiments in Central London. Bull Am Meteorol Soc 90: 955–969
Xie Z-T, Castro IP (2006) LES and RANS for turbulent flow over arrays of wall-mounted obstacles. Flow Turbul Combust 76: 291–312
Xie Z-T, Hayden P, Voke PR, Robins AG (2004) Large-eddy simulation of dispersion: comparison between elevated source and ground-level source. J Turbul 5: 1–23
Xie Z-T, Hayden P, Robins AG, Voke PR (2007) Modelling extreme concentrations from a source in a turbulent flow over a rough wall. Atmos Environ 41: 3395–3406
Xie Z-T, Coceal O, Castro IP (2008) Large-eddy simulation of flows over random urban-like obstacles. Boundary-Layer Meteorol 129: 1–23
Yao YF, Thomas TG, Sandham ND, Williams JJR (2001) Direct numerical simulation of turbulent flow over a rectangular trailing edge. Theor Comput Fluid Dyn 14: 337–358
Yee E, Biltoft CA (2004) Concentration fluctuation measurements in a plume dispersing through a regular array of obstacles. Boundary-Layer Meteorol 111: 363–415
Yee E, Gailis RM, Hill A, Hilderman T, Kiel D (2006) Comparison of wind-tunnel and water-channel simulations of plume dispersion through a large array of obstacles with a scaled field experiment. Boundary-Layer Meteorol 121: 389–432
Yeung PK, Xu S, Sreenivasan KR (2002) Schmidt number effects on turbulent transport with uniform mean scalar gradient. Phys Fluids 14: 4178–4191
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Branford, S., Coceal, O., Thomas, T.G. et al. Dispersion of a Point-Source Release of a Passive Scalar Through an Urban-Like Array for Different Wind Directions. Boundary-Layer Meteorol 139, 367–394 (2011). https://doi.org/10.1007/s10546-011-9589-1
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s10546-011-9589-1