乌兰布和沙漠绿洲区典型农田防护林带风速流场数值模拟

Numerical simulation of wind speed field in the typical farmland shelterbelts in the oases of the Ulan Buh Desert

研究不同结构配置林带前后流场分布和防护效应的差异,能够揭示林带的防风机理,为林带配置优化提供科学依据。以乌兰布和沙漠绿洲区4种配置林带(0.8 H宽度林带、0.2 H宽度林带、乔灌混交林带和无灌木对比林带,H代表林带高度)。对研究对象进行数值模拟,分析不同林带结构配置对林带风速流场特征、不同高度处的水平风速以及防护距离的影响。结果表明:气流在经过4条林带时均产生不同的速度分区,其中下部气流加速区和带后回流区受林带配置结构影响较大;在近地面0.1 H高度处,0.8 H宽度林带相较于0.2 H宽度林带在带后1 H范围内风速降低90.8%,带后回流强度增加55.2%,林带总体有效防护距离分别为17.8 H和15.5 H;乔灌混交林带相较于无灌木林带,带后1 H范围内风速降低122.4%,回流强度增加15.9%,林带有效防护距离分别为21.5 H和23.7 H,对比无灌木林带减少2.2 H。总之,较宽林带和乔灌混交林带能很好地改善林带后缘风速,抑制近地面气流加速作用,但也增强带后回流,缩短了林带有效防护距离,因此在林带配置中应根据不同位置合理规划。

[Background] The structural configuration of shelterbelt is crucial to the protective effect of shelterbelt.In the current research on the windbreak effect of farmland shelterbelt,field wind speed observations are mostly used,which is complicated and laborious to measure,and the differences in the windbreak effects of different configurations of shelterbelts are not studied enough.Based on the numerical simulation of the wind speed field of the shelterbelt,this study analyzed the windbreak effect of different configurations of shelterbelts and the distribution characteristics of the wind speed field around the shelterbelt.[Methods]Four different configurations of shelterbelts(0.8 H width shelterbelt,0.2 H width shelterbelt,the arbor-shrub mixed shelterbelt and shrub-free shelterbelt,H represents shelterbelt height)in the Ulan Buh Desert were used as study objects.The two-dimensional numerical simulation method was used to simulate the wind speed field around the shelterbelt,and verified with the wind speed measured in the field.The influence of different shelterbelt structures on the characteristics of the wind speed field around the shelterbelt,the horizontal wind speed at different heights,and the efficient protection distance of the shelterbelt was explored.[Results](1)When the airflow passed through four shelterbelts with different structures and configurations,different speed zones were produced around them,which were respectively the speed lifting zone above the shelterbelt,the weak wind zone behind the shelterbelt,the lower airflow acceleration zone,the reflux zone and the airflow recovery zone behind the shelterbelt.The airflow acceleration zone and the reflux zone were greatly affected by the structure of the shelterbelt.(2)The impact on airflow under different shelterbelt widths mainly occured behind the shelterbelt.At a height of 0.1 H near the ground,compared with the 0.2 H width shelterbelt,the wind speed decreased by 90.8%within 1 H behind the shelterbelt and the reflux intensity increased by 55.2%behind the shelterbelt for the 0.8 H width shelterbelt,and the efficient protection distance was 17.8 H and 15.5 H for the two shelterbelts,respectively,which decreased by 2.3 H compared with the efficientprotection distance for the 0.2 H width shelterbelt.(3)At the height of 0.1 H,the wind speed in the arbor-shrub mixed shelterbelt decreased to 0 within 0.7 H after the shelterbelt,while the shrub-free shelterbelt showed a significant acceleration of airflow and reached the maximum wind speed at 1 H after the shelterbelt,which was about 70.5% of the initial wind speed.Compared with the shrub-free shelterbelt,the wind speed decreased by 122.4%and the reflux intensity increased by 15.9%in the post-shelterbelt 1 H range in the arbor-shrub mixed shelterbelt,and the efficient protection distance of the shelterbelt was 21.5 H and 23.7 H,respectively,which was reduced by 2.2 H compared with the shrub-free shelterbelt.[Conclusions]The wider shelterbelt and mixed shelterbelt of trees and shrubs can well improve the wind speed at the back edge of the shelterbelt and effectively suppress the acceleration of airflow near the ground,but also enhance the reflux behind the shelterbelt and shorten the efficient protection distance,so the shelterbelt configuration should be planned reasonably according to different locations.