树冠流动阻力特性数值模拟与实验研究

摘要

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. The leaf area index (LAI), solid volume fraction (SVF) and the fractal dimension (Df) were used to analyze the complicated morphology of vegetation canopy. The influences of leaf area index, solid volume fraction and fractal dimension on flow resistance were studied. The results show that leaf area index approximately has linear relationship with solid volume fraction, and the fractal dimension of canopy has logarithmic relationship with leaf area index and solid volume fraction, respectively. 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. The results show that the simulation results from 2D simplified model are slightly larger than experimental results, and can qualitatively predict the trend of the relationship between the drag coefficient of the canopy and leaf area index. However, the results obtained by using the proposed 2D simplified canopy model show good agreement with the experimental results after modifying the simulation results with a correction factor.%为深入分析树冠内部结构参数对树冠绕流流动阻力的影响，采用数值模拟和实验测试相结合的方法研究几种典型形态特征树冠的内部流场特征和流动阻力。用树冠叶面积指数(LAI)、稠密度(SVF)和分形维数(Df)表征树冠复杂的形态结构，讨论树冠形态结构参数对树冠流动阻力的影响规律。研究结果表明：模型叶面积指数与稠密度呈近似线性关系，而树冠分形维数分别与树冠稠密度和叶面积指数满足近似对数关系。通过对数值模拟计算结果回归分析，给出树冠流动阻力系数与叶面积指数的关系式。研究结果表明：真实条件下的树冠流动阻力结果要明显高于二维简化模型的数值计算结果，但二者在变化趋势上表现出较好的一致性；在引入树冠阻力系数修正系数后，数值计算结果与实验结果吻合较好。