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旋对林冠防风作用的影响。(1)在多翼风洞中制作了均匀振荡流场，作为湍流的一级近似。森林冠层模型被放置在这个振荡流。在下游区域的冠，时间平均速度分布，湍流强度和雷诺应力的测量与CTA风速仪。振荡流的湍流强度在林冠上方和下方较强。(2)在风洞中制作了湍流度约为20%的线性剪切流场.在林冠模型的下游区域测量了主流中切变和湍流的影响。在高层湍流中，湍流增强了动量的垂直扩散，降低了林冠的防风作用。(3)采用FDM-SMAC方法对流场进行了数值模拟。冠层由大量的质点近似，质点之间通过弹簧和阻尼器连接。在时间推进中，气流的运动和树冠中颗粒的运动是联系在一起的。数值计算了各种稀疏和密集树冠的绕流场。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.