Large-Eddy Simulation for the Mechanism of Pollutant Removal from a Two-Dimensional Street Canyon

Abstract

Large-eddy simulation (LES) is conducted to investigate the mechanism of pollutant removal from a two-dimensional street canyon with a building-height to streetwidth (aspect) ratio of 1. A pollutant is released as a ground-level line source at the centre of the canyon floor. The mean velocities, turbulent fluctuations, and mean pollutant concentration estimated by LES are in good agreement with those obtained by wind-tunnel experiments. Pollutant removal from the canyon is mainly determined by turbulent motions, except in the adjacent area to the windward wall. The turbulent motions are composed of small vortices and small-scale coherent structures of low-momentum fluid generated close to the plane of the roof. Although both small vortices and small-scale coherent structures affect pollutant removal, the pollutant is largely emitted from the canyon by ejection of low-momentum fluid when the small-scale coherent structures appear just above the canyon where the pollutant is retained. Large-scale coherent structures also develop above the canyon, but they do not always affect pollutant removal.

Pollutant Dispersion in Boundary Layers Exposed to Rural-to-Urban Transitions: Varying the Spanwise Length Scale of the Roughness

“…Reynolds and Castro (2008) and Michioka et al (2011) investigated the occurrence of sweeps and ejections in the urban canopy by computing two-point correlations of u. However, the direct correlation between street-canyon ventilation and the flow was not made.…”

“…Tomas et al (2016) show that up to 14h downstream of a rural-to-urban roughness transition the advective pollutant flux remains significant. Furthermore, Michioka et al (2011Michioka et al ( , 2014 conclude that street-canyon ventilation mostly takes place when low momentum regions pass over the canyon, and they suggest that these regions are of small scale compared to the coherent structures in the outer region and are generated close to the top of the canopy.…”

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

“…Please note that the definitions of ejections and sweeps here for turbulent pollutant flux are different from those for momentum flux due to the different transport processes of pollutant and momentum. Quadrant analysis of turbulent pollutant fluxes have shown that the ejections and sweeps dominate while sweeps contribute slightly more than ejections near the roof level [38], while Michioka et al [39] indicated that a large amount of pollutant is removed from the canyon by ejection due to the large-scale coherent structures. The turbulent intensity of the approaching flow could also directly influence pollutant removal from street canyons, as demonstrated in wind-tunnel experiments [34] and numerically [40].…”

“…Michioka et al [39] showed that the coherent structures close to the plane of the roof strongly affect pollutant removal from the street canyon. Therefore, changes in turbulent structures upwind of a street canyon where pollutants are emitted may cause different mech-anisms of pollutant removal (i.e., removal majorly by the mean or turbulent pollutant flux) when the street canyon geometry remains the same [41].…”

“…Cheng and Liu [38] reported that their horizontal domain size (L x = 6h) is not enough for the two-point correlation to vanish and thus the turbulence development is limited. Kanda et al [71] and Michioka et al [39] pointed out that in order to simulate properly the coherent turbulent structures developed over the street canyon, L x should be larger than 10h. This rule should be followed in practice when dispersion and pollutant removal is of concern, as those coherent structures also contribute to the pollutant removal [39,41].…”