柽柳灌丛沙堆三维流场随背景植被变化的风洞实验
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摘要
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 ramosissima 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 regions: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 -10 H to 14 H 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 2 H.The background vegetation coverage played an important role in affecting the airflow patterns instead of wind velocity.With the increase 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 became 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 according to the principle of wind-blown sand dynamics,background vegetation coverage should be maintained at least at 16% in desert-oasis ecotones so as to maintaining security and stability of oases.
This paper aimed to find out the law of three-dimensional airflow patterns around Tamarix ramosissima 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 regions: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 -10 H to 14 H 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 2 H.The background vegetation coverage played an important role in affecting the airflow patterns instead of wind velocity.With the increase 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 became 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 according to the principle of wind-blown sand dynamics,background vegetation coverage should be maintained at least at 16% in desert-oasis ecotones so as to maintaining security and stability of oases.
引文
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[10]Dong Z B,Luo W Y,Qian G Q,et al.Wind tunnel simulation of the three-dimensional airflow patterns around shrubs[J].Journal of Geophysical Research-Earth Surface,2008,113:F02016.
[11]李志忠,武胜利,肖晨曦,等.新疆和田河流域灌丛沙堆风洞流场的实验研究(I)[J].中国沙漠,2007,27(1):9-14.
[12]安晶,哈斯,杜会石,等.内蒙古高原小叶锦鸡儿灌丛沙堆对气流结构与风蚀的影响[J].干旱区研究,2015,32(2):304-312.
[13]White B R.Laboratory simulation of aeolian sand transport and physical modeling of flow around dunes[J].Annals of Arid Zone,1996,35:187-213.
[14]Zhang W,Kang J-H,Lee S-J.Tracking of saltating sand trajectories over a flat surface embedded in an atmospheric boundary layer[J].Geomorphology,2007,86:320-331.
[15]Owen P R,Gillette D.Wind tunnel constraint on saltation[C]//Proceedings of International Workshop on the Physics of Blown Sand,2.Aarhus,Denmark:University of Aarhus,1985:253-269.
[16]唐玉龙,安志山,张克存.不同结构单排林带防风效应的风洞模拟[J].中国沙漠,2012,32(3):647-654.
[17]Wasson R J,Nanninga P M.Estimating wind transport of sand on vegetated surface[J].Earth Surface Progress and Landforms,1986,11:505-514.
[18]张春来,邹学勇,董光荣,等.植被对土壤风蚀影响的风洞实验研究[J].水土保持学报,2003,17(3):31-33.
[19]杨东亮.策勒绿洲-沙漠过渡带典型下垫面风沙活动研究[D].乌鲁木齐:中国科学院新疆生态与地理研究所,2011.
[20]杨帆.差异沙源供给条件下柽柳灌丛与沙堆形态间的互馈关系[D].乌鲁木齐:中国科学院新疆生态与地理研究所,2012.
[21]郭忠升.水土保持植被的有效盖度、临界盖度和潜势盖度[J].水土保持通报,2000,20(2):60-62.
[2]Bagnold R A.The Physics of Blown Sand and Desert Dunes[J].London,UK:Methuen,1941.
[3]武胜利,李志忠,肖晨曦,等.灌丛沙堆的研究进展与意义[J].中国沙漠,2006,26(5):734-738.
[4]杜建会.绿洲荒漠交错带灌丛沙堆形成演变过程及其影响因素[D].北京:北京师范大学,2009.
[5]Hesp P A.The formation of shadow dunes[J].Journal of sedimentary Petrology,1981,51:101-112.
[6]Wasson R J,Hyde R.Factors determining desert dune type[J].Nature,1983,304(28):337-339.
[7]张萍,哈斯,吴霞,等.单个油蒿灌丛沙堆气流结构的野外观测研究[J].应用基础与工程科学学报,2013,21(5):881-889.
[8]Leenders J K,van Boxel J H,Sterk G.The effect of single vegetation elements on wind speed and sediment transport in the Sahelian zone of Burkina Faso[J].Earth Surface Processes and Landforms,2007,32(10):1454-1474.
[9]武胜利,李志忠,海鹰,等.新疆和田河流域单株柽柳灌丛流场的实验研究[J].干旱区研究,2006,23(4):539-543.
[10]Dong Z B,Luo W Y,Qian G Q,et al.Wind tunnel simulation of the three-dimensional airflow patterns around shrubs[J].Journal of Geophysical Research-Earth Surface,2008,113:F02016.
[11]李志忠,武胜利,肖晨曦,等.新疆和田河流域灌丛沙堆风洞流场的实验研究(I)[J].中国沙漠,2007,27(1):9-14.
[12]安晶,哈斯,杜会石,等.内蒙古高原小叶锦鸡儿灌丛沙堆对气流结构与风蚀的影响[J].干旱区研究,2015,32(2):304-312.
[13]White B R.Laboratory simulation of aeolian sand transport and physical modeling of flow around dunes[J].Annals of Arid Zone,1996,35:187-213.
[14]Zhang W,Kang J-H,Lee S-J.Tracking of saltating sand trajectories over a flat surface embedded in an atmospheric boundary layer[J].Geomorphology,2007,86:320-331.
[15]Owen P R,Gillette D.Wind tunnel constraint on saltation[C]//Proceedings of International Workshop on the Physics of Blown Sand,2.Aarhus,Denmark:University of Aarhus,1985:253-269.
[16]唐玉龙,安志山,张克存.不同结构单排林带防风效应的风洞模拟[J].中国沙漠,2012,32(3):647-654.
[17]Wasson R J,Nanninga P M.Estimating wind transport of sand on vegetated surface[J].Earth Surface Progress and Landforms,1986,11:505-514.
[18]张春来,邹学勇,董光荣,等.植被对土壤风蚀影响的风洞实验研究[J].水土保持学报,2003,17(3):31-33.
[19]杨东亮.策勒绿洲-沙漠过渡带典型下垫面风沙活动研究[D].乌鲁木齐:中国科学院新疆生态与地理研究所,2011.
[20]杨帆.差异沙源供给条件下柽柳灌丛与沙堆形态间的互馈关系[D].乌鲁木齐:中国科学院新疆生态与地理研究所,2012.
[21]郭忠升.水土保持植被的有效盖度、临界盖度和潜势盖度[J].水土保持通报,2000,20(2):60-62.