The Effect Of The Spatial-distribution-trees On Airflow In Typical Street Canyon

With the acceleration of urbanization,about 66%of the world's population will live in cities by 2050.The air quality in the street canyon is closely related to human health.In recent years,there are some scholars have pointed out that the measure use trees to relieve air pollution in street canyon is not feasible from the aerodynamic point of view.In this paper,according to the best practice guideline of the quality assurance and improvement of microscale meteorological models action plan COST Action 732,AIJ guidelines for practical applications of CFD to pedestrian wind environment around buildings,Design code for residential buildings(GB50096-2011),Code for design of civil buildings(GB50352-2005)and tree spacing in real life,the boundary conditions and related parameter settings of numerical simulation are determined,the tree greening schemes with different planting density in the deep and shallow street canyon are designed,and CFD simulation the effect of the density and uniformity of the spatial layout of trees on the airflow in typical street canyon(indicated by the street height-to-width ratio H/W)is carried out.The impact of different tree greening measures on pedestrian breath height airflow is analyzed.The main results are as follows:(1)The influence trend of trees on the airflow in street canyon shows a weakening effect,as follows:at both ends,trees weaken the airflow that carried by vortex into street valley interior,the sinking from the windward side roof into canyon interior and reflux from the leeward side of the windward building into the canyon interior,at the same time,trees weaken airflow from the windward side to the leeward side in the middle of H/W<2 shallow street canyon and tilt down to the bottom of the tree canopyof H/W?2 deep street canyon.The results of CFD calculation show that the flow velocity at both ends and the middle of the shallow street canyon gradually decrease with the tree spacing decrease.In the deep street canyon,the flow velocity at the bottom of the tree canopy decrease.(2)In the typical street canyon with uniform planted trees,with the increase of H/W the effect of trees on airflow shows a trend from hindrance to promotion.It is recommended to adopt a wide spacing tree planting scheme,such as plant spacing of 20 meters;uniform planting trees makes the airflow average relative intensity of the pedestrian breathing plane with the increase of H/W,from attenuated 5-10%to enhanced 7?18%.(3)Under the scenario of uniform configuring trees,there is a linear relationship between the average relative intensity of the pedestrian breathing plane Sxrhx,the windward/leeward side Syfm/Sbfm and the relative volume of trees occupy the canyon V0.V0 is negatively correlated with Sxrhx,that is,the denser the tree planting,the more obvious the obstruction effect on the airflow of pedestrian breathing plane.(4)In the typical street canyon with non-uniform configuring trees,in different canyons,for trees enhance the spatial distribution difference of turbulence,the spatial configuration of trees becomes very sensitive to the influence of typical plane airflow.The effects of tree planting schemes on airflow need to be further tested in conjunction with specific street canyon patterns and tree planting space configurations.This paper simulate and analyze the effect of uniform and non-uniform tree planting on the airflow in typical street canyons.The results of the study can provide a reference for determining tree planting strategies in urban planning.However,the airflow movement in the street canyon also affected by the thermal convection caused by solar radiation,various layouts of real city buildings,and obstacles such as car parking obstacles.Related research need to be conducted.