不同配置乔灌混交林防风效益的风洞试验

Wind tunnel experiment on protection benefits of arbor-shrub mixed forest belts in different configurations

防护林对减轻风沙灾害,改善和恢复干旱区生态环境意义重大。为优化干旱区防护林配置,该研究以旱柳、梭梭和油蒿为研究对象,设计A型(高密度)、B型(中密度)、C型(低密度)3类乔灌混交林模型,每类林带依据林龄(5、10和20 a)设计3种高度,共9种乔灌混交林模型。在12 m/s风速条件下进行风洞试验,分析不同配置乔灌混交林的风速频数、风速流场、有效防护面积。结果表明:不同配置的乔灌混交林背风面风速流场差异显著,等密度林带高度增加,带后相同位置的风速减小、气流越平稳,树龄5 a的中密度和低密度林带风速分别集中在7.2~7.6、7.5~8.5 m/s,树龄20 a的中密度和低密度林带风速分别集中分布在4~5、4.5~5 m/s;树龄10 a的中密度和低密度带后最大风速与高密度林带相似,但最小风速分别为高密度林带的5.88、7.27倍,说明高密度林带降低风速作用明显,但风速随着林带距离的增加恢复迅速;不同密度林带的防风效能最佳区随带高的增加而向后移动,林带高度增加,林带密度优势减弱。因此,中高林龄的中密度乔灌混交林生态效益、经济效益最优。

Shelter forest is an important part of the vegetation ecosystem in arid and semi-arid areas.It can change the airflow velocity and direction,reduce wind speed,and control quicksand.It plays an important role in improving the oasis ecological environment,protecting biodiversity,and promoting social and economic sustainable development.The density,height and tree species of the forest belts are the key factors that determine whether the forest belts can continue to exert the best protection benefits.To optimize configuration of shelter forest in arid areas,Salix matsudana,Artemisia ordosica and Haloxylon ammodendron were selected to design the models of A-type(high density),B-type(medium-density),C-type(low-density)arbor-shrub mixed forest belts according to the density and principle of similarity.The forest belts of different densities were designed to 9 types of arbor-shrub mixed forest belts with three heights,according to the forest age(5,10,20 years).Wind tunnel experiments were carried out under the wind speed of 12 m/s and we observed 99 points in the forest belts with a measurement height of 2 cm to analyze the wind speed frequency,wind speed flow field,probability density and effective windproof area of the arbor-shrub mixed forest belts in different configurations.The results showed that there were significant differences in wind speed and flow field on the leeward side of arbor-shrub mixed forest belts in different configurations.The wind speed at the same position decreased,while the airflow becomes stable as the height of the constant density forest belt increased.The wind speeds of high-density,medium-density and low-density forest belts of five years old were concentrated at 6.5-8.5,7.2-7.6,7.5-8.5 m/s,respectively,whereas the wind speeds of three-density forest belts of 20 years old were concentrated at 2.5-3.5、4-5、4.5-5 m/s,respectively.The forest belt density was an important factor that affects the forest belt's windproof performance when the forest belt height was not high.The average wind speed of the 10-year-old medium-density forest belt was similar to the average wind speed of the 5-year-old high-density forest belt.Windproof effects of forest belts can be improved by increasing the forest belt density.The maximum wind speeds of the 10-year-old medium-density and low-density forest belts at the same height were similar to that of high density,but the minimum wind speeds were 5.88 and 7.27 times of the high density forest belt,respectively,indicating that the high-density forest belt had a significant effect on reducing wind speed,but the wind speed recovered rapidly with the increase of the forest belt distance.The optimum wind protection area of forest belts with different densities moved backwards with the increase of belt height.When the forest belt height increased,the advantage of forest belt density decreased.Therefore,the middle and high forest age and middle density arbor-shrub mixed forest belts can be the optimum ecological and economic benefits.