Numerical Study Of Flow And Dispersion In Urban Street Canyons

The airflow and dispersion of a pollutant in a complex urban area of Yuzhong, Gansu, China, were numerically examined by coupling a Computational Fluid Dynamics (CFD) model with a mesoscale weather model. The models used were Fluent software package and Weather Research and Forecasting (WRF) model. Fluent was firstly validated against field experiment data of Yuzhong city. Then, the WRF model was integrated, and the results were used to drive Fluent calculating by one-way nesting method. In WRF simulation, four nested computational domain were considered, and the innermost domain with a horizontal grid size of1km covers the Yuzhong area. The NCEP final analysis data were used as initial and boundary conditions for WRF model. WRF was integrated for24h starting from08:00LST07October2011, and the results were simulated well compared with the observations of synoptic weather and some meteorological elements. The hit rate (HR) of2-m temperature,10-m wind direction and speed was0.54,0.46, and0.64, respectively. The criteria for HR calculation were model-observation agreement within1℃,30°for wind direction, and1.5m/s for wind speed. WRF-Fluent coupled model was integrated for5h starting from15:00LST07October2011, and15:15-18:45was period of pollutant releasing. Results from the coupled WRF-Fluent model simulation shown that the flow in the presence of real building clusters can change significantly as the ambient wind speed and direction in the present simulation. Some characteristic flows, such as the flow eddy, channeling flow, and vertical recirculation vortex were simulated. Pollutant dispersion pattern and the degree of lateral pollutant dispersion were shown to be complicated in the presence of real building clusters and under varying ambient wind speed and direction. This suggest that because of the sensitive dependency of urban flow and pollutant dispersion on variations in ambient wind, time-dependent boundary conditions should be used to better simulate or predict them when the ambient wind varies over the period of Fluent model simulation. Result of present study also suggested that the coupled WRF-Fluent model is an important tool that can be used for studying and predicting urban flow and dispersion in densely built-up area.