Flow and Pollutant Transport in Urban Street Canyons of Different Aspect Ratios with Ground Heating: Large-Eddy Simulation

Abstract

A validated large-eddy simulation model was employed to study the effect of the aspect ratio and ground heating on the flow and pollutant dispersion in urban street canyons. Three ground-heating intensities (neutral, weak and strong) were imposed in street canyons of aspect ratio 1, 2, and 0.5. The detailed patterns of flow, turbulence, temperature and pollutant transport were analyzed and compared. Significant changes of flow and scalar patterns were caused by ground heating in the street canyon of aspect ratio 2 and 0.5, while only the street canyon of aspect ratio 0.5 showed a change in flow regime (from wake interference flow to skimming flow). The street canyon of aspect ratio 1 does not show any significant change in the flow field. Ground heating generated strong mixing of heat and pollutant; the normalized temperature inside street canyons was approximately spatially uniform and somewhat insensitive to the aspect ratio and heating intensity. This study helps elucidate the combined effects of urban geometry and thermal stratification on the urban canyon flow and pollutant dispersion.

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

“…For a street canyon of h/b = 0.5, when ground heating is introduced (Ri ranging from −0.9 to −2.9), along the roof level, both the mean flow and turbulence contribute to pollutant removal [55], which is consistent with Kang et al [58], while under neutral stratification condition, the mean flow contributes little to pollutant removal.…”

“…Li et al [54,55] have performed LES studies of flow and pollutant dispersion in street canyons of h/b = 0.5, 1, and 2 with ground heating (Ri ranging from −0.5 to −2.9), where "ground heating" is the term used for heating or cooling of the air near the ground by heat transfer to or from the ground. Their results showed that the ground heating significantly enhanced the mean flow, turbulence, and turbulent pollutant flux inside street canyons, but weakened the shear layer at the roof level.…”

“…2). With ground heating, the flow in the street canyon of h/b = 0.5 can change from wake interference to skimming flow regime, while the two vertically-aligned vortices in the street canyon of h/b = 2 can merge into one large vortex [55].…”

“…With decreasing Ri (increasing instability), the pollutant removal from the street canyon is greatly enhanced, while the non-dimensional temperature difference (the temperature difference between the air inside the street canyon and approaching air, non-dimensionalized by T ) inside the street canyon only increases slightly [55]. The important difference between "no- heat-flux" or "adiabatic" and "fixed-temperature" boundary conditions for temperature at solid walls has also been demonstrated [55]. In the case with adiabatic boundary conditions, the non-dimensional temperature difference inside the street canyon is much higher.…”

“…On the other hand, the profile of turbulent heat/pollutant flux can vary depending on the location of the source. For the cases with flux/pollutant source at the ground level, the flux values usually have two maxima: near the source and at the roof level (see [54,55]). However, above the roof level, the transport processes of these fluxes are quite similar under near-neutral conditions [65].…”