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柽柳灌丛沙堆风沙动力学过程的风洞实验
谭凤翥
学位类型硕士
导师王雪芹
2017-05-01
学位授予单位中国科学院大学
学位授予地点新疆乌鲁木齐
学位专业工程硕士
关键词柽柳灌丛沙堆 三维流场 蚀积强度 风沙流结构 临界植被盖度 风洞实验 Tamarix ramosissima nebkhas three-dimensional airflow patterns erosion intensity wind-sand flow structure critical vegetation coverage wind tunnel simulation
摘要The study took the Tamarix nebkhas and interdune plants, which are widelydistributed in oasis-desert ecotone at the southern rim of the Taklimakan desert, as theresearch prototype to make five sets of nebkhas model with background vegetationcoverage of 0%, 4%, 10%, 16% and 26% respectively to conduct the simulationexperiment of airflow patterns and erosion and accumulation change rule in the windtunnel. The wind flow field, surface alteration, surface aerodynamics, sedimenttransport rate and sand flow structure of Tamarix nebkhas had been systematicallystudied so as to further understand land degradation causes and processes of oasis-deserttransition zone, and provide theoretical basis for oasis security and nebkhasconservation. The results showed that:(1) The three-dimensional airflow structure of Tamarix ramosissima nebkhas canbe divided into six regions: the deceleration zone before nebkhas, the accelerationregion of nebkhas windward, the high speed zone on both sides of nebkhas, the strongvortex area on leeward side of nebkhas, the recovery zone after nebkhas and the mixedacceleration zone above nebkhas. The influence region of nebkhas on airflow patternswas -10H~14H in horizontal direction and width of side influence zone was less than3/4 of nebkhas width. In vertical direction, the airflow patterns were significantlyaffected by nebkhas height, the maximum influence height was less than 2H. Thebackground vegetation coverage played an important role in affecting the airflowpatterns. With increase of background vegetation coverage, the deceleration zone atfront of nebkhas and recovery zone behind nebkhas were expanded, meanwhile thestrong vortex zone on lee side and high speed zone at both sides of nebkhas shrinked.The wind velocity was significantly reduced at the layer where the backgroundvegetation existed. The airflow acceleration rate at each region of nebkhas showed anearly exponential decay with background vegetation coverage. When the coverage ismore than 16% the acceleration rate tended to be steady.(2) With increase of background vegetation coverage from 0%, to 4%, to 10% , to16% and to 26%, the threshold wind velocity of sand-driving on interdune was 4.4 m·s-1, 5.4 m·s-1, 5.8 m·s-16.2 m·s-1and 7.2 m·s-1respectively, while on nebkhas they were3.2 m / s, 4.4 m·s-1, 5.0 m·s-1, 5.2 m·s-1, 5.4 m·s-1respectively. The threshold wind speedat interdune is about 1 m·s-1higher than that on nebkhas surface, which suggest thatnebkhas surface is more prone to wind erosion than the interdune. Tamarix ramosissimanebkhas and interdune can be divided into six regions. They are named theaccumulation area before nebkhas, the erosion area on windward slope of nebkhas, theerosion area on both sides of the nebkhas, the vortex flow sediment area on leewardside of nebkhas, the wake accumulation area after nebkhas and the no affected area,respectively. Both wind velocity and background vegetation coverage played animportant role in affecting the erosion distribution. With increase of wind velocity,the accumulation area before nebkhas and the wake accumulation area after nebkhasare narrowed, while the erosion area on both sides of the nebkhas are expandedsignificantly. The erosion area ratio, erosion intensity and wind erosion rate at nebkhasand interdune decreased exponentially with increase of background vegetationcoverage. When the vegetation coverage is low the influence of wind velocity on theerosion intensity is obvious. The nebkhas on the bare sand can aggravate wind erosion,but nebkhas can block sand flow even sparse vegetation existing on the interdune.(3) The surface aerodynamic parameters were significantly correlated with thebackground vegetation coverage. With the increase of vegetation coverage, theaerodynamic roughness length (z0) and the drag coefficient (cd) increased exponentially,the friction wind speed (u*) increased logarithmically, while the sediment transport fluxdecreases exponentially. There was a significant correlation between surfaceaerodynamic parameters and the sediment transport flux. The sediment transport fluxdecreased exponentially with the increasing of roughness length (z0), friction windspeed (u*) and drag coefficient (cd).(4) Based on the analysis of the correlation of the acceleration rate, the winderosion rate, the sediment transport flux and the background vegetation coverage, it was found that there was a good correlation between the acceleration and erosion intensityof nebkhas. The total wind erosion rate and sediment transport flux on Tamarix shruband its interdune showed exponentially law with background vegetation coverage, thechange was not obvious when the vegetation coverage was>16%. It is inferred that themaintenance of no less than 16% of the background vegetation cover should be theprerequisite for scientific conservation of nebkhas in oasis-desert ecotone.
其他摘要本研究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,制作背景(丘间地)植被盖度依次为 0%、4%、10%、16%和 26%五组柽柳灌丛沙堆模型,在风洞中进行流场与吹沙模拟实验,系统研究柽柳灌丛沙堆及丘间地的风流场、地表蚀积变化、地表空气动力学性质、以及输沙率与风沙流结构,揭示过渡带沙漠化进程中柽柳灌丛沙堆—丘间地系统蚀积变化的风沙动力学过程,深入理解绿洲-沙漠过渡带土地退化成因和过程,为绿洲外围生态安全和灌丛沙堆的科学保育提供理论依据。主要研究结论如下:(1)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,沙堆两侧急流区和背风侧涡流区随之缩小,沙堆前弱涡流中心向上风向迁移,而沙堆后强涡流中心向灌丛沙堆逐步靠近。背景植被的出现明显降低了其所在高度层的风速,其中以沙堆前减速区和沙堆后尾流恢复区降幅最大。灌丛沙堆各分区平均加速率均随背景植被盖度呈指数规律下降,当植被盖度>16%时各分区加速率变化不大。(2)随着背景植被盖度从 0%、4%、10%、16、26%增大,丘间平沙地临界起动风速依次为 4.4 m·s-1、5.4 m·s-1、5.8 m·s-1、6.2 m·s-1、7.2 m·s-1,灌丛沙堆表面的临界起动风速依次为 3.2 m·s-1、4.4 m·s-1、5.0 m·s-1、5.2 m·s-1、5.4 m·s-1,临界起动风速均随背景植被盖度呈对数规律增大。其中平沙地起动风速高于灌丛沙堆约 1 m·s-1,可见灌丛沙堆相对于平沙地更易发生风蚀。与流场分区相对应,柽柳灌丛沙堆及丘间地蚀积分布亦可划分为 6 个区域,依次为沙堆前积沙区、沙堆迎风坡风蚀区、沙堆两侧风蚀区、沙堆背风侧涡流积沙区、沙堆后尾流积沙区和非灌丛沙堆影响区,各分区蚀积强度和风蚀率均与背景植被盖度呈指数依赖关系。随着指示风速增大,沙堆前积沙区和尾流积沙区范围缩小而沙堆两侧风蚀区面积明显增大;随着背景植被盖度的增大,沙堆前积沙区和尾流积沙区范围增大而沙堆两侧风蚀区范围明显减小。灌丛沙堆及其丘间地整体风蚀面积比、蚀积强度和风蚀率均随背景植被盖度呈指数规律递减。低植被覆盖条件下,风速对蚀积强度和风蚀率影响明显。裸露地表灌丛沙堆的存在会加剧地表风蚀,当背景植被盖度为 4%~16%时,灌丛沙堆即可发挥良好的防风阻沙作用。特别当植被盖度>16%时,柽柳灌丛沙堆及丘间地整体蚀积强度和风蚀率变化不大。(3)灌丛沙堆地表空气动力学参数与背景植被盖度显著相关。随着背景植被盖度增大,地表粗糙度(z0)和阻力系数(cd)成指数规律增大,摩阻风速(u*)呈对数规律增长。随着背景植被盖度增大,输沙通量呈指数规律减小。在背景植被盖度≤16%时,灌丛沙堆有效降低了近地层的输沙率。地表空气动力学参数与输沙通量显著相关,输沙通量随粗糙度(z0)和阻力系数(cd)的增大呈指数规律减小,当盖度>16%时趋于稳定。(4)通过对灌丛沙堆流场加速率、蚀积强度、风蚀率、输沙通量与背景植被盖度的相关关系进行分析,发现柽柳灌丛沙堆各分区加速率和蚀积强度均与背景植被盖度存在良好的相关性;柽柳灌丛沙堆及其丘间地整体风蚀率和输沙通量均与背景植被盖度呈指数分布规律,并在背景植被盖度≥16%时变化不明显。据此推断维持不低于 16%的背景植被覆盖,应该是灌丛沙堆科学保育的前提。
学科领域环境工程
语种中文
文献类型学位论文
条目标识符http://ir.xjlas.org/handle/365004/14897
专题研究系统_荒漠环境研究室
作者单位中国科学院新疆生态与地理研究所
第一作者单位中国科学院新疆生态与地理研究所
推荐引用方式
GB/T 7714
谭凤翥. 柽柳灌丛沙堆风沙动力学过程的风洞实验[D]. 新疆乌鲁木齐. 中国科学院大学,2017.
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