关键词: 树木, 街谷, 气流, 数值模拟, 行人风环境

Abstract:

In order to show the degree of change of the airflow at pedestrian breath height due to trees, trees with four different spatial distribution inside the ideal deep street canyon (H/W = 2) were simulated by CFD from the aerodynamic point of view. Tree canopy was treated as uniform porous media, and an additional source term is integrated to account for additional dissipation due to trees. Our results show that the effect of different spatial configuration of trees on the airflow varies greatly with trees' spatial distribution pattern: (1) Within the street canyon, uniform planted trees hinder the pedestrian airflow while non-uniform planted trees increase its rate. The effects of the on pedestrian airflow are very different under the four spatial distributions. The order of the obstruction effect of trees on airflow from the largest to the smallest is evenly spaced 8m (Spa8m) > evenly spaced 6 m (Spa6m) > evenly spaced 20 m (Spa20m) > not uniformly planted (Non-uniform). The corresponding average airflow enhancement index sequence is$\bar{D}_{spa8}$（-19.31%）<$\bar{D}_{spa6}$（-16.14%）<$\bar{D}_{spa20}$（-10.73%）<$\bar{D}_{non-uniform}$（1.25%）. (2) The pedestrian airflow win the street canyon with uneven-planting was 106.49% higher than that in the control case (uniform tree Planting, Spa8m). Uneven-planting scheme is the case that trees are planted with the sufficient free space at both ends of the street and no trees in the middle. It can allow the corner vortex to infiltrate into the middle of the street valley, promote the vertical vortex inside the street valley and the horizontal vortex movement at both ends, enhance turbulence and vertical exchange, effectively reduce the “tuyere effect” at both ends of the street and the “calm wind effect” in the middle of the street. It improves the wind environment of the entire street valley at the pedestrian breathing plane. (3) Trees with reasonable spatial distribution can improve the street pedestrian wind environment. The airflow at the breath height of pedestrians in the street valley is very sensitive to the local conditions, and the configuration (spatial cluster and density) of the trees will cause a strong spatial change of it. These results point out the importance of trees' spatial distribution in urban greening measures under existing urban building layout with the goal of improving the pedestrian wind environment, alleviating the spread of pollution and the disease by careful landscape design.

Key words: tree, street canyon, airflow, numerical simulation, pedestrian flow