Large-eddy simulations on pollutant reduction effects of road-center hedge and solid barriers in an idealized street canyon

This study conducted large-eddy simulations (LES) on the pollutant reduction effects of hedge and solid barriers in a three-dimensional idealized street canyon with an aspect ratio of 0.5. The wind direction was perpendicular and oblique (45°) to the street. The results were validated with data from wind tunnel experiments. LES accurately predicted the concentration distribution in the barrier-free case and reproduced well the barrier-induced concentration reduction. In the barrier-free case, a large recirculation vortex was observed. However, the central barriers forced the recirculated airflow in the middle of the canyon and newly formed vortices near the leeward walls. The two counter-direction vortices in the hedge and solid barrier cases transported pollutants toward the center of the canyon and enhanced the vertical pollutant removal at the top of the street canyon. The hedge barrier (solid barrier) reduced spatially-averaged concentration by about 59% (45%) near the leeward wall, 64% (20%) near the windward wall, and 45% (17%) in the whole street canyon compared to the barrier-free case. The effects of leaf area density (LAD) and barrier width were further investigated under the perpendicular wind direction. Increasing the LAD or the width of the hedge barrier decreased concentration near the leeward walls but increased canyon-averaged concentration. Increasing the width of the solid barrier decreased the concentration near the leeward walls and the canyon-averaged concentration. In an oblique wind direction, the hedge and solid barriers reduced by about 30% and 60% the spatially-averaged concentration near the building walls compared to the barrier-free case.