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

Abstract

Thermal stratification (neutral, unstable and stable) plays an important role in determining the transport processes in and above urban street canyons. This paper summarizes the recent findings of the effect of thermal stratification on the transport of momentum, heat, and pollutants in the two-dimensional (2D) urban street canyons in the skimming flow regime. Special attention is paid to the results from large-eddy simulations (LESs), while other experimental and numerical results are referred to when necessary. With increasing Richardson number, Ri, the drag coefficient of the 2D street canyon as felt by the overlying atmosphere decreases in a linear manner. Under neutral and stable stratification, a nearly constant drag coefficient of 0.02 is predicted by the LESs. Under unstable stratification, the turbulent pollutant transport is dominated by organized turbulent motions (ejections and sweeps), while under stable stratification, the unorganized turbulent motion (inward interactions) plays a more important role and the sweeps are inhibited. The unstable stratification condition also enhances the ejections of turbulent pollutant flux, especially at the leeward roof-level corner, where the ejections dominate the turbulent pollutant flux, outweighing the sweeps. With increasing Ri, both the heat (area active scalar source) and pollutant (line passive scalar source) transfer coefficients decrease towards a state where the transfer coefficients become zero at Ri ≈ 0.5. It should be noted that, due to the limit of the 2D street canyon configuration discussed in this paper, great caution should be taken when generalising the conclusions drawn here.

Large eddy simulation of reactive pollutants in a deep urban street canyon: Coupling dynamics with O3-NO -VOC chemistry

“…Future studies may consider the effects of other factors, such as complex urban configurations, wind speed, oblique wind directions, emissions and thermal/shading effects (e.g. Li et al, 2015;Li et al, 2016;Cheng and Liu, 2011b), on both the dynamic and chemical processing of reactive pollutants.…”

Modeling the Airflow and Particle Dispersion in Street Canyons under Unsteady Thermal Environment with Sinusoidal Variation

“…Li et al (2010Li et al ( , 2014 have investigated the flow field and pollutant dispersion characteristics inside urban street canyons with ground heating at different Richardson (Ri) numbers. Increasing ground temperature substantially enhanced the mean flow, turbulence, and pollutant flux inside the street canyons, but weakened the shear at the roof level.…”

Impacts of Urban Layouts and Open Space on Urban Ventilation Evaluated by Concentration Decay Method

“…Large eddy simulations (LES) are known to perform better in predicting turbulence than the Reynolds-Averaged Navier-Stokes (RANS) approaches. However, there are still some challenges involved with widely applying LES because of the strongly increased computational requirements, the development of advanced sub-grid scale models, and the difficulty in specifying appropriate time-dependent inlet and wall boundary conditions [16,17,20,26,38,39]. Therefore, in quantitative work one has to adopt time-averaged turbulence models (i.e., RANS approaches).…”

“…For three-dimensional UCL geometries, the plan area index λ p and frontal area index λ f [4][5][6]9,10] are regarded as the key urban parameters to influence the flow and pollutant dispersion in urban areas. In addition, the other factors including overall urban form and ambient wind directions [2,3,[6][7][8][9], building height variations [5,[20][21][22][23][24][25], thermal buoyancy force for weak-wind atmospheric conditions [10,18,[26][27][28][29][30], etc. also play significant roles in the flow and pollutant dispersion in UCL models.…”