Numerical Evaluation of the Effect of Building Layout on Turbulent Flow Field and Particulate Matter Transport in Street Canopies

The main objectives of this study are to establish trajectory tracking technique for different particle sizes and to compare the effects of different building layout (i.e., opened and staggered street canopies) on turbulent flow field and particulate matter transport in urban street canopies.
To simulate the three-dimensional turbulent flow field under different urban street canopies, we select the finite volume method to discretize the governing equations of our numerical model. SIMPLE scheme is used to adjust flow field to satisfy the continuity equation. Large eddy simulation (LES) together with the subgrid-scale stress model (SGS) and the wall function are adopted to model the turbulence flow field in the study. The Lagrangian particle trajectory tracking technique is adopted to investigate particulate matter transport behavior in urban street canopies. The effects of the drag force, gravitational force, Brownian motion, and Saffman lift force on particles are all considered in the particle tracking technique. A sensitive analysis is firstly conducted to assess how many particles are necessary to be released in the simulated 3-D turbulent flow field for maintaining in a stable concentration profile. The analysis indicates that as the released particles are more than 10000, the particle and number concentration distributions approach to a steady profile.
The simulated results show that, in the opened street canopy at the pedestrian wind airflow range, PM10 concentration in the gap area is 10% higher than that in the wake zone. However, it increases to 70% in the staggered street canopies. Also, in the gap area, the removal efficiency for PM2.5 and PM1 is better than that for PM10, especially in the staggered street canopies.
The larger particles are easier to deposit than smaller ones. Once the large particles are transported to the vortex region behind the buildings, it is hard for them to escape from the region. The smaller particles need longer time to deposit. In the staggered street canopies, these small particles would follow low-velocity airflow and wander in the wake zone. This phenomenon results in accumulations of PM2.5 and PM1 in this region.