Effects of Wind Direction on the Airflow and Pollutant Dispersion inside a Long Street Canyon

This study investigates the effects of wind direction on the airflow and pollutant dispersion inside a long street canyon using computational fluid dynamics (CFD). A 3D CFD model for predicting the flow and dispersion in a canyon is first developed using the FLUENT code and then validated against wind tunnel experiments. Then, the airflow and traffic pollutant dispersion in an isolated canyon with a street-length-to-building-height ratio of 10 are numerically simulated for seven wind directions (alpha = 0 degrees, 15 degrees, 30 degrees, 45 degrees, 60 degrees, 75 degrees and 90 degrees, where alpha is the angle between the approaching flow and street axis). The results demonstrate that the mean ((ACH) over bar) and turbulent (ACH') air exchange rates (ACHs) for the canyon are close when alpha = 0 degrees, 15 degrees, 30 degrees, 45 degrees and 60 degrees, whereas the magnitude of ACH' is significantly greater than that of (ACH) over bar when alpha = 75 degrees and 90 degrees. Additionally, the ACH reaches its maximum and minimum values when alpha = 30 degrees and 90 degrees, respectively. The computed velocity and concentration fields clearly reveal the variation in the in-canyon flow pattern and pollutant distributions on the canyon walls and footpaths due to the wind direction. Evaluating the maximum, minimum and average concentrations on the canyon walls and footpaths for each of the seven wind directions, we determine: On the leeward-oriented wall, the wall-averaged concentration increases greatly with alpha, and the values of the wall-maximum are highest when alpha = 75 degrees and lowest when alpha = 0 degrees. By contrast, on the windward-oriented wall, both the wall-averaged and wall-maximum concentrations are highest when alpha = 0 degrees. Finally, at the human respiration height, the highest concentration on the footpath next to the leeward-oriented wall occurs when alpha = 75 degrees, whereas the highest concentration on the footpath close to the windward-oriented wall is observed when alpha = 0 degrees.