Large-eddy simulation of wind flow and air pollutant transport inside urban street canyons of different aspect ratios

Abstract

of thesis entitled Large-Eddy Simulation of Wind Flow and Air Pollutant Transport inside Urban Street Canyons of Different Aspect Ratios submitted by Li, Xianxiang for the degree of Doctor of Philosophy at the University of Hong Kong in June 2008 The characteristics of the wind flow and air pollutant transport inside urban street canyons are of fundamental importance to the air quality monitoring and improvement. An investigation of these characteristics was performed in this study using both experimental and numerical techniques. The focus is on the mechanisms of pollutant transport and removal inside urban street canyons of high aspect ratios (AR, ratio of the building height to the street width). A physical model in water channel was first developed to study the wind flow in street canyons of different ARs of 0.5, 1.0, and 2.0. The velocity and turbulent fluctuation were measured by a Laser-Doppler Anemometer (LDA). The measured velocity and turbulent fluctuation at various locations were validated with several experimental datasets available in literature. The measured results in most locations were also in good agreement with previous numerical results. The comprehensive measurement data can provide a validation database for the numerical model development. To take into account the detailed transient turbulent processes, a large-eddy simulation (LES) model was developed based on a one-equation subgrid-scale (SGS) model and finite element method (FEM). This model was validated and fine-tuned by applying to an open channel flow at Reτ = 180. By comparing the calculated velocity and fluctuations with those obtained from experiment and direct numerical simulation (DNS), a set of model constants was determined for the LES model. A 1/7th wall model was further incorporated into this LES model to mitigate the strict near-wall resolution requirement. To validate the newly developed LES model for street canyons, the LES results for the street canyons of AR 1 and 2 were compared extensively with the waterchannel experimental data and previous LES results. The good agreement showed that the newly developed LES model was capable of predicting the complicated flow patterns and pollutant dispersion in street canyons. The validated LES model was then employed to simulate the street canyons of AR 3, 5, and 10. Three, five, and eight vertically aligned primary recirculations were found for the three cases, respectively, which showed decreasing strength with decreasing height. The very small ground-level wind speeds made the ground-level pollutants extremely difficult to disperse. Local maxima of the turbulence intensities were found at the interfaces between the primary recirculations and the free surface layer. The pollutant followed the trajectories of the primary recirculations. High pollutant concentration and variance were found near the buildings where wind flowed upward. Large gradients of pollutant concentration and variance were also observed at the interfaces between the primary recirculations and the free surface layer. Detailed analyses of concentration budget terms showed that the advection terms were responsible for pollutant redistribution within primary recirculations, while the turbulent transport terms were responsible for pollutant penetration between primary recirculations and pollutant removal from the street canyon. Based on the LES results, several quantities were introduced to compare the pollutant removal capability of different street canyon configurations. It was found that these quantities were all non-linear functions of the street canyon AR. Large-Eddy Simulation of Wind Flow and Air Pollutant Transport inside Urban Street Canyons of Different Aspect Ratios