Simulation Study On Flow Field And Particles Deposition Of Vegetation Canopy

Trees play important roles in keeping soil and water, improving climate and the air quality of our cities. Wind is an important factor for scalar fluxes(heat, water vapor, carbon dioxide, etc.) and movements of spores, pollen and particles within a vegetation canopy. Information about the wind flow within the vegetation canopy is important in meteorological, agricultural and ecological studies. People working and living in urban areas are exposed to pollutants emitted from motor vehicles, poisonous gases from industry, and chemical and biological toxics from unexpected accidents. The dispersion of those hazardous materials is largely determined by meteorological elements such as ambient wind speed and direction and atmospheric stability and by building configurations such as street and building aspect ratios. And the absorption of these toxics gases and suspended particle mainly depend on trees and lawn in the city. However, accurate measurement of wind flow and the capture of particles are difficult due to the complexity in the array of vegetation elements(leaves, branches and so on) and the complex process of air momentum transport within a vegetation canopy. So a wide variety of computational fluid dynamics(CFD) approaches have been employed in the article. This thesis studied the addressed problems and the main academic contributions are as follows:Firstly, a series of simulations of air flow through 2D simplified vegetation canopies and 3D vegetation canopies along with different canopy morphology have been conducted by computational fluid dynamics(CFD). The predictive skills of single- and two-equation(k-ε) models with or without source/sink to compute profiles of mean velocity(u), turbulent kinetic energy(TKE), turbulent kinetic energy dissipation rate(ε) and turbulent intensity(I) were compared against datasets collected from other works and field measurements. In conclusion, it was demonstrated that the adoption turbulence models with source/sink terms for vegetaion canopies, proved to be a physically accurate and numerically robust method, and the models and methods were recommended for future use in simulating turbulent flows in vegetation canopies.Then, computational fluid dynamics(CFD) and field experiments were used to investigate the flow characteristics and flow resistance through vegetation canopies with several typical morphological characteristics. A model expression between the drag coefficient of the canopy and leaf area index was presented by analyzing simulation results. The model expression was validated using experimental results in a wind tunnel with four kinds of tree branches, it was found that: the simulation data from 2D simplified model was slightly larger than experimental results, but simulation data qualitatively predict the trend of the relationship between the drag coefficient of the canopy and leaf area index. However, the results obtained using the proposed 2D simplified canopy model show good agreement with the experimental data after modifying the simulation results with a correction factor k.In order to investigate the particle deposition on the vegetation canopy, computational fluid dynamics(CFD) and UDF were used. The effect of particle deposition velocity were analyzed by the main parameters such as leaf area index(LAI), leaf area density(LAD), particle diameter(dp) and inlet velocity v(z). And the simulations were compared against other works and field measurements so as to increase particle capture efficiency by trees and improve urban environment.