Large-eddy simulation of street canyon flow and pollutant transport in neutral and unstable stratifications

Large-eddy simulation (LES) with the one-equation subgrid-scale (SGS) model was performed to examine the flow and pollutant removal patterns of urban street canyons. An idealized street canyon with unity building-height-to-street-width (aspect) ratio was studied in both neutral and unstable stratifications. To capture the correlations among the eddies/transports in the street canyon, downstream and prevalent flow aloft, the computational domain consisted of three identical street canyons and the vertical extent of shear layer equals five times the building height.

In neutral condition, the LES shows a primary recirculation in the core and three small recirculations at the ground-level windward, ground-level leeward and roof-level leeward corners. Heating up the street leads to unstable stratification (bulk Richardson number, Rb = gh(Θ_h-Θ₀)/(Θ_refU_h²) = -10, where g is the gravitational acceleration, h the height of building, U_h the roof-level wind speed, Θ_ref the reference temperature, Θ_h the roof-level temperature, and Θ₀ the street-level temperature). The small recirculation at the ground-level windward corner enlarges while the other two shrink. Buoyancy increases the recirculating speed of the primary recirculation and raises it up. As a result, the primary recirculation in the street canyon is no longer isolated from the shear layer but is partly over the roof level immersing in (the lower part of) the prevalent flow.

LES results in neutral stratification were compared with other data available in literature. It was found that the shear layer thickness is an important factor affecting the accuracy of the turbulence statistics inside the street canyons. For street canyons with a shallow shear layer, excessive turbulence kinetic energy (TKE) is observed at both roof and ground levels, which is a drawback of (too) rapid velocity gradient enforced under a thin shear layer. Eventually, the turbulence in the street canyons is over estimated.

Pollutant was modeled as a gaseous and passive scalar. Uniform pollutant concentration at the street level was used to model traffic emission in urban street canyons. Significantly different pollutant transports are observed between the neutral and unstable situations. In neutral stratification, the pollutant is only able to stay in a thin layer over the street canyon implying poor pollutant removal. In contrast, under unstable stratification, the pollutant is able to move upward into the shear layer that demonstrates a more effective pollutant dilution and removal compared with its neutral counterpart.