The Effect of Large-Scaled Eddies in the Wind Flows on the Windbreak of Forest Canopies

采用风洞试验和数值模拟两种方法，分析了风流中的大尺度涡对林冠防风功能的影响。(1)在多通道风洞中，采用均匀振荡流动作为湍流的第一近似。森林树冠模型被放置在这种振荡流中。在冠层下游区域，用CTA风速仪测量了时间平均速度分布、湍流强度和雷诺应力。振荡流中的湍流强度在林冠上方和下方较强。(2)在风洞中形成了湍流强度约为20%的线性切变流。测量了主流区切变和湍流对林冠模型下游区域的影响。在高空湍流中，湍流增强了动量的垂直扩散，降低了林冠的防风效果。(3)采用有限差分-SMAC方法对流场进行了数值模拟。通过弹簧和阻尼器将多个颗粒与相邻的颗粒连接起来，近似地表示冠层的顶端。气流的运动和冠内颗粒的运动在时间行进中联系在一起。对不同稀疏型和密型树冠冠层的流场进行了数值计算。这些结果与风洞试验结果吻合较好。

The effect of the large-scaled eddies in the wind flows on the windbreak function of forest canopies is analyzed with two methods, wind tunnel testing and numerical simulation.(1) As the first approximation of turbulent flows, uniformly oscillating flows are made in the multi-fun wind tunnel. Forest canopy models are placed in this oscillating flow. At the downstream region of the canopies, time-mean velocity profile, intensity of turbulence and Reynolds stress are measured with the CTA anemometer. Intensity of turbulence in the oscillating flows is stronger at above and below crowns of forest canopies.(2) The linear shear flows with about 20 % intensity of turbulence are made in the wind tunnel. Effects of shear and turbulence in the main flows are measured on the downstream region of the forest canopy models. In the high-leveled turbulent flows the windbreak effect of forest canopy is decreased because vertical diffusion of momentum is enhanced by turbulence.(3) Flow fields are simulated with the FDM-SMAC method. The crowns of canopy are approximated with a lot of particles that are connected with neighbor particles by spring and damper. The motion of air flows and the motion of particles in the crown are linked in the time-marching. Flow fields around the canopies of various sparse and dense crowns are calculated numerically. These results are good agreement with the results of the experiments with the wind tunnel testing.