横向沙丘背风侧沙粒风蚀起动的风洞模拟

通过风洞实验,探讨了横向沙丘背风侧“二次流”的沉积学和形态-动力学意义。在不同迎风坡坡度的横向沙丘模型背风侧,我们观测了不同位置沙粒的起动风速以及在临界状态下沙粒的运动特征。结果表明,沙丘背风侧的颗粒起动风速不仅与其距沙丘顶部的距离有关,也与沙丘迎风坡坡度有密切关系。根据沙丘背风侧颗粒运动特征,可以将其划分为向后运动区域、晃动或摆动区域以及向前运动区域,产生这一现象的原因是在沙丘背风坡气流分离、反向涡和气流重新辐合共同作用的结果。在所有的观测结果中,迎风坡坡度为15°的沙丘具有最大的沙粒起动风速和最远的气流重新辐合距离,其原因尚需进一步研究。     

横向沙丘气流平均速度变化规律的风洞模拟

在沙丘动力系统中,存在沙丘形态、气流、沙粒运移三者之间复杂的相互作用。通过风洞实验的方法,针对不同形态的6组横向沙丘模型,采用粒子图像测速系统,测量了模型沙丘周围气流水平速度和垂直速度的变化规律。实验结果表明,横向沙丘迎风坡水平气流存在1.28～1.89之间的加速率,垂直气流存在上扬趋势,这二者均有随沙丘迎风坡坡度增大而增大的趋势。在横向沙丘背风坡,由于气流的分离,水平气流速度减小并出现反向,其大小约为自由风速的17％;垂直气流速度存在下沉趋势,其最大沉速出现在气流重附点附近;背风坡气流速度的变化受沙丘迎风坡坡度影响较小,受自由风速的影响较大。沙丘对气流速度的改变在近地层较为显著,随着高度的增加地形影响逐渐减小。     

腾格里沙漠东南缘沙丘表面气流与坡面形态的关系

对腾格里沙漠东南缘格状沙丘主、副梁和新月形沙丘断面风速的野外观测结果表明,迎风坡风速放大率、背风坡气流的方向和强度随区域气流、坡面形态而不同。迎风坡风速放大率对沙丘形态的作用在于增加丘顶区域的输沙率和活动性;背风坡气流不仅影响沙丘形态而且控制沙丘动力学过程。在横向气流条件下,风速放大率使沙丘迎风坡随区域气流强度的增加变长变缓,背风坡受分离流控制使沙丘迁移;在双向－斜向气流条件下,沙丘丘顶处于侵蚀亚环境而两坡变陡变短,背风坡受附着偏向流控制使沙丘纵向延伸。     

包兰铁路沙坡头段防护体系前沿栅栏沙丘形态与近地面流场

防护体系前沿阻沙栅栏不仅是维持栅栏沙丘形态、阻挡流沙进入防护带内的重要屏障,还是栅栏沙丘这一独特沙丘类型形成发育的首要条件。形态测定与流场观测表明,在沙坡头铁路防护体系前沿地带,栅栏沙丘迎风坡平均坡度大于天然沙丘,背风坡坡度小于天然沙丘,背风坡水平长度与迎风坡水平长度之比约为0.7,远大于天然沙丘（0.3）。阻沙栅栏显著影响沙丘表面流场,但栅栏缺失或沙埋后栅栏沙丘与天然沙丘表面流场结构具有很大的相似性。沙丘迎风坡剪切风速均沿坡面至丘顶呈先增大、后减小的趋势,但栅栏沙丘迎风坡脚至中上部剪切风速增长速度较快,中上部至丘顶剪切风速迅速降低而且幅度较大,这种变化对栅栏沙丘的成长具有重要意义。     

腾格里沙漠东南缘格状沙丘表面气流及其地貌学意义

对腾格里沙漠东南缘沙丘形态与区域气流的分析表明,格状沙丘发育在双向低风能环境。沙丘表面气流及沉积过程的观测结果,主梁长期处在横向气流环境,其形态主要受背风坡分离气流控制,并以迎风坡侵蚀、背风坡净堆积形式顺主风向迁移。副梁处在双向-斜向气流环境,背风坡的附体偏向气流控制其形态和动力学过程,而且其强度是原始风入射角余弦的对数函数。在斜向风作用下,副梁迎风坡侵蚀的同时,背风坡中下部也发生沙粒的纵向输移且堆积于主梁背风坡,从而导致副梁沿年合成输沙方向延伸。由此认为,格状沙丘的主梁形成于主风-西北风的作用,而副梁是在主梁基础上并在主风和次风-东北风的交替作用下形成的;其形态动力学类型应属纵向沙丘置于横向沙丘之上一种复杂型沙丘。     

新月形沙丘迎风坡气流加速模拟

迎风坡气流加速是决定新月形沙丘发育和形态的动力因子，研究这一加速过程的定量关系有利于认识沙丘的发育、沙丘的形态、沙丘的运动等。通过大量的野外实测数据和前人的理论分析，得出沙丘迎风坡气流加速量主要和沙丘坡度、迎风坡前来流风速以及沙丘高度有关，并建立了新月形沙丘迎风坡气流加速和沙丘形态特征的定量关系。根据这一关系得了在风力较强的风沙环境中，沙丘坡度较小；在风力较弱的风沙环境中，沙丘坡度较陡。地表粗糙度越大，迎风坡坡度越陡。沙丘地貌发育高度存在一上限，这一上限与边界层厚度有关。     

横向沙丘表面气流湍流特征野外观测

沙丘形态动力学特征是风沙地貌的重要研究内容之一,受观测技术等的限制,现有的关于沙丘形态动力学特征的研究成果还不能满足沙丘形成与演化过程研究的需要。利用三维超声风速仪对腾格里沙漠的横向沙丘表面气流特征进行野外观测,结果表明,沙丘脊线和迎风坡中部的气流均为未偏向流,而沙丘背风坡不同高度的气流包括反向流,偏向流和未偏向流。湍流强度、雷诺应力和气流丰富度随气流方向变化而变化,其在沙丘脊线和中部的变化趋势比较简单且有大致相似的规律,而其在沙丘背风坡不同高度比较复杂。     

新月形沙丘表面风速廓线与风沙流结构变异研究

在风沙地貌学中,跃移沙粒与风场的耦合作用下,其最直观的表现形式为风速廓线和风沙流结构的变异。通过对塔克拉玛干沙漠腹地新月形沙丘迎风坡坡脚、沙丘顶部、沙丘的两个兽角前端、沙丘背风坡坡脚5个典型部位的风速廓线和风沙流结构进行了实地观测,并与参照点（丘间粗沙地）的风速廓线和风沙流结构进行了对比。发现受地形扰动作用,沙丘背风坡坡脚处和沙丘的两个兽角前端的风速廓线形式均呈现非对数形式分布。除迎风坡坡脚处风沙流结构与参照点处相似之外,其余各个部位风沙流均表现出不同于参照点的结构形式。直接采用曲线拟合方法对风沙流结构函数进行拟合,并针对有明显分段现象的风沙流结构形式,采取分段拟合,并对造成风速廓线和风沙流结构变异的原因进行了讨论。     

库布齐沙漠南缘抛物线形沙丘表面风沙流结构变异

对库布齐沙漠南缘抛物线形沙丘表面气流和输沙率的野外观测和分析结果表明,沙丘表面约90%的风沙输移集中在距沙面0.10 m高度范围内,输沙率随高度递减的形式在沙丘各部位因风速、下垫面状况和坡面形态不同而发生变异。沙丘迎风坡坡脚因出露坚硬、含砾石地表,颗粒跃移高度大,风沙流上层相对输沙率大;迎风坡沙粒沿坡向上运动,颗粒跃移高度减小,风沙流中近地表相对输沙率大;沙丘背风坡沙粒沿坡向下运动,加之来自丘顶变型跃移物质的影响,风沙流上层相对输沙率较大;脊线受迎风坡各个断面地形差异的影响,各观测点间风沙流结构差异显著。风沙流结构在迎风坡和丘顶均遵循指数递减规律(Q=aexp(-z/b)),其中,指数函数拟合中系数a与输沙率具有良好的幂函数关系,随风速增加而增加,但二者关系较弱;b与二者无相关性。背风坡风沙流结构具有明显的分段现象,以0.10 m高度为界,下层符合指数函数,上层符合幂函数。     

塔克拉玛干沙漠腹地复合沙垄的流场和上覆沙丘组合及动态特征

复合沙垄是最大的沙丘地貌类型之一，表现为简单沙丘类型的垂直叠加和水平连接，在沙垄的高大地形影响下，沙垄不同部位形成不同的风沙环境特征。以塔克拉玛干沙漠腹地/复合沙垄为例，通过野外实地观测研究其流场和上覆沙丘组合特征。研究结果表明，复合沙垄的流场可分为沙垄迎风坡气流加速区、沙垄背风坡气流转向减速区、垄间粗沙平地气流风速风向恢复区和气流风速稳定区。受沙垄流场和地形的控制，沙垄上覆新月形沙丘链，垄间地分有简单新月形沙丘、沙堆、沙片、侵蚀地、简单线形沙丘。上覆沙丘分布特征表现为：垄间地上覆新月形沙丘疏密度较小，沙垄迎风坡和沙垄背风坡上覆新月形沙丘疏密度较大，而且超过1，说明沙垄迎背风坡上覆沙丘有叠置现象。沙垄上覆沙丘高度自沙垄迎风坡底部到顶部增高，背风坡上覆沙丘高度自沙垄顶部到背风坡底部减小，上覆沙丘在自身形态、地形和流场的共同影响下，存在相对运动，表现为分布进一步集中，因此沙垄有增高的趋势。     

Threshold wind velocity in the lee of two-dimensional transverse dunes

Secondary flow plays an important role in the development of dunes. To investigate this phenomenon, we simulated models of transverse dunes with different stoss slopes in a wind tunnel. We measured the threshold wind velocity at different locations in the lee of the dunes, and observed the corresponding particle movement characteristics under threshold conditions to assess the sedi-mentological significance of secondary airflow. Threshold wind velocity changed with both horizontal distance and stoss slope. Its variation with horizontal distance is characterized by a region in which a rapid reduction occurs, a region with a minimum value, and a region with high value. We confirmed the presence of reversal cells, irrespective of the stoss slope. Neither the variation in threshold wind velocity nor the variation in reattachment distance as a function of stoss slope followed a monotonic function. The 15° stoss slope had the greatest threshold wind velocity and reattachment distance.     

塔克拉玛干沙漠简单线形沙丘形态动力学过程研究

采样分析、风沙观测以及计算结果表明,在不同风向作用以及沙丘形态参数改变等因素的控制下,简单线形沙丘表面起动剪切速度有明显的差异。表面的气流强度随风向的改变而有所不同。气流与沙丘正交时迎风一侧的输沙率远大于背风一侧同地貌部位的输沙率,但当气流与沙丘走向斜交时,沙丘两侧同地貌部位的输沙率差异不是太大,这是线形沙丘在动力学过程中能保持其形态的最主要原因之一。风向的多变以及沙丘形态的变化使风沙流内边界层内随高度的风速与所观测的输沙率虽也表现为幂函数关系,但相关性较低。沙丘表面被输移的沙物质的粒度分布与下垫面物质组成有一定的差异,在中低强度的起沙风范围内,被输移的沙物质的平均粒径小于下垫面沙物质的平均粒径,且随风速或输沙率的增大,粒度组成也趋于统一。总体上,地表沙物质的输移仍是一随机过程。     

Variations of horizontal and vertical velocities over two-dimensional transverse dunes: A wind tunnel simulation of the effect of windward slope

The changes in wind velocity induced by dune topography have an important significance in dune dynamics. In this paper, the horizontal and vertical velocities over six transverse dune models were measured non-intrusively by means of Particle Image Velocimetry in a wind tunnel. The windward slope angle and the free-stream wind velocity both affected the horizontal and vertical velocity components. On the windward side, the acceleration of horizontal velocity depended mainly on the windward slope angle and the height above the dune surface, but was also affected by the free-stream wind velocity. The speed-up ratio increased with increasing slope angle but decreased with increasing height. The ascending vertical velocities also increased with increasing slope angle and free-stream wind velocity. The maximum values moved upper along the dune when the windward angle became steeper. In the leeward sides, the horizontal velocity decreased and reversed because of airflow separation; the maximum reverse velocity in the separation cell was about 17% of the free-stream wind velocity. Behind the dune crest, the airflow moves downwards, and its maximum downward velocity is found near the flow reattachment point. Finally, we discussed the significance of these velocity variations for sediment transport and dune dynamics.     

回涡沙丘的形态特征与表面物质组成

在沙丘分类系统中,回涡沙丘是一种重要的有障沙丘形态,系运移风沙流在障碍物前遇阻堆积而成。野外测量表明,回涡沙丘具有与新月形沙丘相似的外观,其迎风坡坡度一般在2°～18°之间,背风坡坡度较陡为21°～31°。回涡沙丘高度及其与障碍物距离受障碍物高度的控制,障碍物越高则回涡沙丘也越高,沙丘离障碍物也越远,沙丘高度、沙丘与障碍物距离分别与障碍物高度线性相关。粒度特征反映了局地沙源特征,粗细颗粒均可在风力作用下形成障前堆积形态。同一地区回涡沙丘与新月形沙丘表面物质的粒度参数对比表明,由于障碍物前回旋涡流的影响,回涡沙丘表面物质经历了更为强烈的风力分选过程,物质组成更细,分选更好,更趋于正偏和窄峰态。     

Numerical modelling of flow structures over idealized transverse aeolian dunes of varying geometry

A Computational Fluid Dynamics (CFD) model (PHOENICS™ 3.5) previously validated for wind tunnel measurements is used to simulate the streamwise and vertical velocity flow fields over idealized transverse dunes of varying height (h) and stoss slope basal length (L). The model accurately reproduced patterns of: flow deceleration at the dune toe; stoss flow acceleration; vertical lift in the crest region; lee-side flow separation, re-attachment and reversal; and flow recovery distance. Results indicate that the flow field over transverse dunes is particularly sensitive to changes in dune height, with an increase in height resulting in flow deceleration at the toe, streamwise acceleration and vertical lift at the crest, and an increase in the extent of, and strength of reversed flows within, the lee-side separation cell. In general, the length of the separation zone varied from 3 to 15 h from the crest and increased over taller, steeper dunes. Similarly, the flow recovery distance ranged from 45 to >75 h and was more sensitive to changes in dune height. For the range of dune shapes investigated in this study, the differing effects of height and stoss slope length raise questions regarding the applicability of dune aspect ratio as a parameter for explaining airflow over transverse dunes. Evidence is also provided to support existing research on: streamline curvature and the maintenance of sand transport in the toe region; vertical lift in the crest region and its effect on grainfall delivery; relations between the turbulent shear layer and downward forcing of flow re-attachment; and extended flow recovery distances beyond the separation cell. Field validation is required to test these findings in natural settings. Future applications of the model will characterize turbulence and shear stress fields, examine the effects of more complex isolated dune forms and investigate flow over multiple dunes.     

Field measurement and scaled-down wind-tunnel model measurement of airflow field over a barchan dune

Airflow is measured over a barchan dune in the field and over a scaled-down model in a wind tunnel. The change of the flow speed over the stoss side is represented by the change of speed-up ratio. According to the field measurement, the wind profiles within 0-3m above the stoss can be divided into two segments. The lower segment, about 0.66 m thick, is the inner-boundary layer, within which the friction velocities derived from the wind profiles increase from the upwind inter-dune region to the upper stoss, and then decrease near the dune top. This change, together with the changes of airflow field, speed-up ratio and sand flux, is related to the morphological change and contributes to the stable shape and height of a barchan dune. In the wind tunnel, airflow varies in a similar way as in the field, with the speed-up ratios constantly higher than 1.0 and increasing along the stoss slope. While the segmentation of wind profiles also occurs in the wind tunnel, friction velocities derived from the wind profiles decrease along the stoss, indicating a very thin inner-boundary layer above the wind tunnel model where the detailed wind-speed change becomes difficult to measure using the present instruments.     

风向对半固定沙垄表面风速影响——以古尔班通古特沙漠为例

利用DETI可移动测风系统对古尔班通古特沙漠半固定沙垄表面风速进行实地观测,获取了主要风季不同风向条件下沙垄表面7个典型部位距离地表20 cm、40 cm、60 cm、100 cm和200 cm 5个高度的风速系列数据,系统研究风向对半固定沙垄表面坡面风速及风速廓线的影响。结果表明,迎风坡的气流加速和背风坡的风速降低现象在实际观测中得到证实,但受风向的影响甚大。垄顶风速放大率随入射角的增大呈指数关系递增,而背风坡风速占垄顶风速的比率则随入射角的增大呈线性关系递减。大角度入射气流速度变化主要受控于沙垄形态,小角度入射气流速度变化主要受控于地表植被状况。无论气流以何种角度入射,距沙垄表面20 cm高度气流的加速均较其他高度缓和。沙垄底部和坡中下部的风速廓线变化趋势基本一致,且呈较好的对数拟合关系;两坡中上部和垄顶部的对数拟合方程相关系数偏小;受回旋涡流的影响,风速廓线在背风坡上部有明显偏折,初步断定涡流中心在距地表40 cm左右的高度。     

古尔班通古特沙漠半固定沙垄表面风的脉动特征

利用DETI可移动测风系统,对古尔班通古特沙漠半固定沙垄进行实地观测,获取了主要风季沙垄表面7个典型部位200 cm高度的风向及距离地表20 cm、40 cm、60 cm、100 cm和200 cm 5个高度的风速系列数据。对风速和风向的脉动特征进行分析研究,发现凸起的沙垄不仅影响平均风速在沙垄表面的变化,亦对风速及风向脉动产生显著影响。结果表明,平均风速沿迎风坡逐渐增大,至垄顶达到最大,在背风坡中上部剧烈降低,随后又缓慢恢复。随着入射角的增大,平均风速在沙垄表面的变化幅度也在增大;绝对风速脉动具有与平均风速相同的变化趋势,且随距地表高度的增加绝对风速脉动值亦在增大;就相对风速脉动（阵性度值）而言,背风坡阵性度值要高于迎风坡,其中背风坡中上部是整个沙垄表面阵性度最高的部位,这与背风侧涡流的存在密切相关;沙垄背风坡风向脉动强度远比迎风坡大,随着入射角的减小,背风坡风向的偏转作用也在加强,而涡旋作用相对减弱。