Study On Resistance Characteristics And Particles Deposition Efficiency Of 3D Tree

Trees have the function of breaking wind and fixing sands, conserving water and soil resources, purifying air and so on. When the wind blows through isolated shrubs, trees or forests, a proportion of the wind’s momentum is absorbed by the vegetation. Therefore, trees or forests have a significant influence on reducing wind erosion of the earth’s surface and can provide proper microclimate for livestock and crops. Also, trees are the natural filters of the earth which can affect the transport of atmospheric particulates and the absorption of gaseous pollution.For all the above reasons, a lot of work has been done to study on the characteristic of wind flow around a tree and particle concentration in the environment. The main research conclusions are as follows:(1) A approach combining computational fluid dynamics(CFD) simulation and field experiments is proposed to investigate the flow characteristics and flow resistance through twodimensional(2D) imaginary canopy model. The drag resistance of 2D canopy is relevant to the leaf number, leaf angle and the length-width ratio of one leaf. The various velocity of wind flow has little influence on the drag resistance. A prediction model of the drag coefficient for 2D canopy is also established. Optical porosity(Op) and leaf angle are the independent variable of the function.(2) To obtaion the characteristic of a real tree, one modeling approach that combines Visual Basic for Applications(VBA) and CAD technology is proposed to design three-dimensional(3D) virtual plant models. These tree models can be used for numerical simulation. Regard plants’ organs as basic elements and assemble all geometrical solids into a particular structure to rebuild a 3D virtual plant. It will make the 3D tree more lifelike to control leaves and branches to grow at various angles. To rebuild different model trees, leaf number value is set as a inputting variable. This paper constructs 20 trees and acquires the values of optical porosity(Op), volumetric porosity(Vp), leaf are index(LAI) and leaf area density(LAD) by programming and Photoshop.The relationship between various parameters are discussed and analyzed.(3)The LAI values of model tress are 0.495, 1.499, 2.454 and 3.063, respectively. CFD simulation is conducted at different velocity of inlet. The approach is reliable by comparing the results of numerical simulation and data in existing literatures. Results show that two low-velocity regions are located at the windward and leeward of a tree. The effect of velocity reduction in the leeward of a tree is more sensitive to the change of LAI values. Moreover, a function of the drag coefficient for 3D tree is also established. LAI is the independent variable of the prediction model.(4) The model trees mentioned above are used in discrete phase model simulation at different velocity of wind conditions to investigate the concentration field around a tree. Based on the equation in existing literatures, deposition velocity of particles is the product of inlet velocity and collection efficiency. The dust-blocking ability of a tree is simply analyzed. The relationships among the wind velocity, LAI, particle size distribution, collection efficiency and deposition velocity are discussed in depth. Compare the simulation results and reference data, the mechanism of particle deposition is confirmed.