Impacts of tree-planting pattern and trunk height on the airflow and pollutant dispersion inside a street canyon

This study investigates the impacts of tree-planting pattern and tree trunk height on the flow structure and pollutant dispersion inside a street canyon using the numerical model validated by wind tunnel dataset. The isolated canyon of L/H = 10 and W/H = 2 with trees are simulated considering the center tree-planting (CTP) and double rows tree-planting (DRTP) patterns as well as eight trunk heights under perpendicular approaching wind. The results reveal that tree-planting pattern and especially the trunk height affect significantly the flow and pollutant distributions. For each tree-planting pattern, the biggest characteristic of flow structure is, a strong stream of air passes through the canyon, resulting in the disappearance of canyon vortex and corner eddies at the canyon ends when h >= H. When compared to treeless case: trees lead to reduced ACH (air exchange rate) ranging from H/3 to 8H/9 and enhanced ACH at h = H and 10H/9; averaged/maximum concentration increases on wall A and slight decreases on wall B are found at h ranging from H/3 to 7H/9, while concentration dramatic decreases on wall A and significant increases on wall B are observed at h = H and 10H/9. Compared to the CTP counterpart, the DRTP present the similar flow structures, ACH and concentration distributions, which indicates that when the total volume and trunk height of trees are the same, the tree-planting patterns have little impacts on the flow structure and concentration distribution. This study will provide technical support for urban road planning to control air pollution.