To study the complex terrain of valley cities and its wind fields and pollutants diffusion,computational fluid dynamics(CFD) and complex terrain grid generation technique are used to establish the numerical simulation model, realizing application of CFD in complex terrain air movement and pollutants diffusion.To set up the boundary in CFD,the digital terrain of the study area was obtained by extracting altitudes using Google Earth elevation extraction software and then imported into Gambit software. In Gambit,the computational meshes over complex terrain were established using journal files written in program. In the CFD model, the turbulence was modelled by using the large eddy simulation(LES),and the pollutant dispersion patterns were described by the mixture multiphase model.The validity of the proposed approach was verified by the good agreement between the simulated distributions of air pollutant in Lanzhou City and monitoring results. The result showed that the complex terrain has a great impact on the wind field. For example,wind speed showed strip distribution characteristics due to the role of the blocking effect of mountains. The wind is weak at the back of the mountain. Meanwhile, the wind fields will determine the pollutants diffusion, as the divergence of the wind fields can affect the pollutant diffusion scope and pollution level. Under the given northwest wind conditions that air speed is 5 m∕s above 10 m from the ground, the concentration of industrial air pollution on flat terrain is diluted about 10 times at the distance of 2.2 km from the pollution center. With mountain blocking,the pollution diffusion distance at which pollution concentration diluted 100 times is about one third of that distance in the relatively flat area,indicating that mountain blocking can inhibit the longitudinal diffusion of pollutants. In addition, the different inlet wind directions would change the interaction between the wind and the mountain,which will change the ground wind speed and local wind fields,resulting in the variations of the pollutant diffusion and distributions. Studies have shown that CFD method is feasible to study the influence of terrain on wind fields and pollutants diffusion.%为研究河谷型城市地形及其引起的风场和污染物扩散的复杂问题,利用CFD(计算流体力学)方法和复杂地形网格生成技术,建立河谷型城市风场及大气污染分布的数值仿真模型,实现CFD方法在复杂地形空气运动和污染物扩散方面的应用.分别使用LES(large eddy simulation)模型和mixture模型研究兰州市地面风场特征和污染物扩散形态,计算得到的污染物分布结果与实测结果分布一致.结果表明:复杂地形对空气运动的影响很大,如风速因山体屏障作用会呈现带状分布特征,山体后侧易出现弱风区域;同时,风场会密切影响污染物扩散,决定了污染物扩散形态,如幅散能够影响污染物扩散范围及污染水平.而给定西北风条件下,如地面以上10 m、风速为5 m∕s、不受地形阻挡情况下,工业区污染物浓度被稀释10倍,约扩散2.2 km;山体阻挡会抑制污染物纵向扩散,表现在山体阻挡情况下污染物稀释100倍时的扩散长度约为相对平坦区域的1∕3.此外,不同的入口风向会引起空气运动与山体相互作用发生变化,进而会使得地面风速、局部风场存在差异,造成污染物扩散及分布形态差异.研究显示,CFD方法可行,模型可靠,可以用来研究地形对风场和污染物扩散的影响.
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