Dispersion of a Point-Source Release of a Passive Scalar Through an Urban-Like Array for Different Wind Directions

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.

Are Urban-Canopy Velocity Profiles Exponential?

“…1. The data of Coceal et al (2006) and Branford et al (2011) are also considered; they performed DNS on both staggered and aligned (sometimes called 'square') arrays of cubes having λ p = 0.25 in channels with H = 8h using DNS but with a somewhat coarser resolution (Δ = h/32). A similar set of flow data derives from the recent study of Cheng and Porte-Agel (2015) (see also Cheng and Porte-Agel 2016), who used LES to compute a spatially-developing boundary-layer flow, in a domain of height H = 12.6h.…”

“…Within the canopy these are extrinsic spatial averages-i.e. averages over the total volume, rather than the fluid volume only, (2006); LC (black, blue, red, purple), Leonardi and Castro (2010); CXFRCHHC (green), Castro et al (2016); BCTB (black), Branford et al (2011); CP-A (green), Cheng and Porte-Agel (2016) which would yield intrinsic averages. Böhm et al (2013) outline the differences between these two averaging methods and the topic has recently been explored more fully by Xie and Fuka (2017, in press).…”

Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation

“…Many of the most detailed numerical studies have been limited to flow perpendicular to a group of regular obstacles [66]. Some recent numerical studies have attempted to address this lack of realistic incoming wind and turbulence (e.g., [67,68]).…”

Modelling Street-Scale Flow and Dispersion in Realistic Winds—Towards Coupling with Mesoscale Meteorological Models

“…Hence in the −90 • wind the street flows are likely to be dominated by channelling along the streets and recirculations in the perpendicular streets, whereas in the −51 • wind the branching of the flow around the buildings, which leads to the so-called topological dispersion, is likely to be important. These are intensively investigated, discussed or reviewed in literature for flows and dispersion over a group of regular obstacles, for example, Davidson et al (1996), Belcher (2005), Claus et al (2009) and Branford et al (2011). However, the geometry of the DAPPLE site is more complicated than 'regular'.…”

“…Certainly, the large-time scale fluctuations of wind direction strengthened the effects of the secondary sources. Using direct numerical simulations, Branford et al (2011) recently investigated the dispersion of a point-source release in a regular array of cubes in three different pressure gradient directions (i.e. 0 • , 30 • and 45 • ).…”