Study On The Characteristics Of Outdoor Particle Concentration And Its Surrounding Temperature Distribution With Tree Canopy

Particulate matter is one of the important factors affecting indoor and outdoor air quality.The tree canopy plays an important role in the purification of particulates for urban air and in the alleviation of urban heat island effects.How to properly plant the plants,effectively inhibit particle pollution in the atmosphere and reduce the temperature of the outdoor thermal environment in order to protect the environment and alleviate the urban heat island effect are hot issues in the study of atmospheric environment.Nowadays,most of the wind resistance models of plants are obtained thro?gh experimental tests.However,it is difficult to obtain the morphological and structural parameters of plant crown and the wind resistance model with wide adaptability.Most of the dry deposition rate models of plants are studied by considering plants as macroscopic simplified models.Because the research method is based on certain simplified and hypothetical conditions,it is difficult to study the effects of tree canopy structure parameters and leaf distribution characteristics on the capture mechanism of plants,and it is also difficult to obtain a more applicable dry deposition model.In this paper,the micro-scale simulation study on the process of collecting particulate matter from tree canopy was carried out.The momentum equation of virtual tree canopy with different morphological structure was solved by computational fluid dynamics software and 3D virtual plant modeling technology and then get the flow field distribution around the virtual canopy.Euler-Lagrangian model was used to simulate and analyze the deposition behavior of particles on a single tree canopy,and the concentration distribution of the particles inside and around the canopy was obtained.The influence of the tree canopy structure parameters,which are leaf area index(Leaf Area Index,LAI),particle diameter(Particle Diameter,d_p)and inlet wind velocity(u),on dry deposition rate of fine particles of tree canopy were mainly analyzed.In addition,wind tunnel experiments are used to study the effects of the different forms branches on the capture efficiency of aerosols in order to verify the accuracy of the simulation results.The experiment selected several common evergreen plants in the campus as test samples and put the collected branches into the wind tunnel test section.The effect of branches of different forms on aerosol capture efficiency is obtained thro?gh aerosol generator and Y09-6H6 laser dust particle counter.The dry deposition velocity is obtained to verify the feasibility of the simulation results.The heat island effect is one of the characteristics of urban climate.The crown has the function of cooling and humidifying the urban air,which can effectively alleviate the urban heat island effect.In order to study the influence of the canopy on the surrounding temperature,the virtual branches with different morphological structures were simulated and analyzed by computational fluid dynamics software and three-dimensional virtual plant modeling techniques to obtain the temperature distribution around the branches.The effects of tree canopy structure parameter,that is LAI and the operation parameters,that is heat flow velocity on the temperature around the branches were analyzed.The results show that the influence of heat flow velocity and LAI on the temperature field around the tree canopy is significant.The relationship between leaf absorption heat flux,leaf area index and wind speed was put forward to provide references for the simulation of the second boundary conditions of the canopy macro temperature field.