柽柳灌丛沙堆三维流场随背景植被变化的风洞实验

Wind Tunnel Simulation of the Three-Dimensional Airflow Patterns around Tamarix ramosissima Nebkhas under the Change of Background Vegetation Coverage

以塔克拉玛干沙漠南缘沙漠-绿洲过渡带广泛分布的柽柳灌丛沙堆及丘间地植物为原型,依据风沙运动相似理论,制作5组背景植被盖度灌丛沙堆模型,在风洞中进行风流场变化规律模拟实验。结果表明:柽柳沙堆三维流场结构可划分为6个区域,即沙堆前减速区、沙堆迎风坡加速区、沙堆两侧急流区、沙堆背风侧强涡流区、沙堆后尾流恢复区和沙堆上空混合加速区。灌丛沙堆对流场结构影响范围为沙堆前10H到沙堆后14H区段,旁侧影响宽度不超过沙堆宽度的3/4,垂直方向上主要影响灌丛沙堆总高度相对应的下方区域,最大不超过灌丛沙堆总高度的2倍。风速变化不构成对流场结构的影响,但背景植被变化对流场结构影响明显。随着植被盖度增大,沙堆前减速区和沙堆后尾流恢复区范围扩大,两侧急流区和背风侧涡流区范围缩小。背景植被的出现明显降低了其所在高度层的风速,柽柳沙堆各分区地表加速率均随背景植被盖度呈指数规律下降,当植被盖度在16%时加速率变化趋于平缓。从维护绿洲安全的角度出发,根据风沙动力学原理可将16%视为灌丛沙堆科学保育中可能的临界背景植被覆盖值。

This paper aimed to find out the law of three-dimensional airflow patterns around Tamarix ramo- sissirna nebkhas under different background vegetation coverages. In order to reveal erosion and deposition process mechanism and provide a theoretical basis for scientific conservation of nebkhas, the Tamarix ramosissima nebkhas and sparsifolia at southern edge of the Taklimakan Desert-oasis ecotone were chosen as experimental model and wind velocity was investigated through wind tunnel simulation. Results showed that the three-dimensional airflow structure of Tamarix ramosissima nebkhas can be divided into six re- gions： the deceleration zone before nebkhas, the acceleration region of nebkhas windward, the high speed zone on both sides of nebkhas, the strong vortex area on leeward side of nebkhas, the recovery zone after nebkhas and the mixed acceleration zone above nebkhas. The influence region of nebkhas on airflow patterns was from --10H to 14H in horizontal direction and the width of side influence zone was less than 3/4 of nebkhas width. While in vertical direction, the airflow patterns were significantly affected by the area below the overall height of nebkhas and the maximum influence height was less than 2H. The background vegeta- tion coverage played an important role in affecting the airflow patterns instead of wind velocity. With the in- crease of background vegetation coverage, the deceleration zone and recovery zone were expanded at front of nebkhas and behind nebkhas, respectively. Meanwhile, the strong vortex zone and high speed zone be- came shrinked on lee side and both sides of nebkhas. The wind velocity was significantly reduced at the height where the background vegetation existed. The airflow acceleration rates at each region of nebkhas showed a nearly exponential decay with background vegetation coverage. When the coverage was more than 16 %, the acceleration rate tended to be steady. From the perspective of maintaining oasis security and ac- cording to the principle of wind-blown sand dynamics, background vegetation coverage should be maintained at least at ]6% in desert-oasis ecotones so as to maintaining security and stability of oases.