小网窄带防护林叠加防风效果风洞实验

小网窄带防护林在干旱区防护林构建中发挥着重要作用,研究小网窄带防护林叠加防风效果对指导防风固沙林空间配置与结构优化具有重要意义。选取两种典型小网窄带防护林网,对连续6个林网叠加的防护林开展基于流场分析防风效果的风洞模拟试验。结果表明:小网窄带防护林叠加后随着林网数量的增加各林网内的风速逐渐减小且在中间林网位置趋于稳定,乔木纯林林网在第3林网基本达到稳定,乔灌混交林网在第2林网达到稳定;两种防护林各林网内风速均符合正态分布特征,风速稳定后多为右偏态高狭峰;16 m·s^-1风速下,乔木纯林网叠加(0~252 cm)的防风效能分布范围为16%~74%,整体在60%防风效能下发挥着良好防风效果,乔灌混交林林网叠加的防风效能分布范围为15%~89%,整体在70%防风效能下发挥着良好防风效果;16 m·s^-1风速下,乔木纯林林网叠加(0~42H,H=6 cm)的风速加速率为0.25~0.94,乔灌混交林林网叠加的风速加速率为0.1~0.94;根据风速加速率的分布特征划分出4个不同的风速分布区,分析还发现灌木对林带枝下高范围的近地层气流影响显著,对削弱近地层风速起到了重要作用。

Windbreak effect of superimposed narrow belt and small net shelterbelts base on wind velocity flow field

Narrow belt and small net shelterbelts play an important role in arid land vegetation construction. The study of windbreak effect of narrow belt and small net shelterbelts is of great significance to the research of spatial allocation and structure optimization of shelterbelts. In this research, we selected two typical types of narrow belt and small net shelterbelts as research objects and conducted wind tunnel simulation experiments on six superimposed shelterbelt nets based on wind velocity flow field. The result showed that: after imposing narrow belt and small net shelterbelts, wind velocity presented a decreasing tendency with the increase of the number of superimposed shelterbelt nets, and also tended to become stable in the middle of the whole shelterbelts. Wind velocity became stable in the third net of the shelterbelts composed of only trees, and became stable in the second net of the shelterbelts composed of bushes and trees. Wind velocity of both types of shelterbelts expressed normal distribution characteristics, and presented right skewness and leptokurtic distributions when wind velocity was stable. Under wind velocity of 16 m·s^-1, distribution range of windbreak efficiency of superimposed shelterbelts composed of only trees (0-252 cm) was from 16% to 74%, and overall, played a good windbreak effect within 60% of efficiency; distribution range of windbreak efficiency of superimposed shelterbelts composed of bushes and trees (0-252 cm) was from 15% to 89%, and overall, played a good windbreak effect within 70% of efficiency. Under wind velocity of 16 m·s^-1, the range of windspeed acceleration rate of superimposed shelterbelts composed of only trees (0-42H, H=6 cm) was from 0.25 to 0.94, and the range of windspeed acceleration rate of superimposed shelterbelts composed of bushes and trees (0-42H, H=6 cm) was from 0.10 to 0.94. Based on the characteristic of windspeed acceleration rate distribution, vertical wind velocity was divided into four different areas. The result also showed that bushes had significant impact on the wind flow nearby surface layer and also played an important role in reducing wind speed under crown height.