二维沙丘迎风坡沙粒跃移运动的数值模拟

为了研究沙丘迎风坡面上沙粒的跃移运动,本文根据风工程和空气动力学的最新理论,给出了沙丘迎风坡面上风场的空间分布规律,在此基础上对沙粒跃移运动进行了数值计算。由于沙丘周围流场情况较为复杂,各处的风速廓线也不同,故选取不同的坡面位置进行跃移计算,其中各处的起沙率由已有的实验结果或拟合公式给出。计算结果表明：从坡脚到坡顶,平均风速加速比和摩阻风速逐渐增加,到沙丘顶部达到最大值;同时沿坡面向上,各截面处单宽输沙率和距离当地地面相同高度处输沙浓度逐渐加强,这与已有文献报道的结果吻合良好。     

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。     

海岸沙丘表面不同部位风沙流中不同粒径沙粒垂向分布的变化

河北昌黎黄金海岸典型沙丘的实地观测表明,风沙流中中沙与细沙输沙量的垂向分布在沙丘表面不同部位的变化并无本质性差别但有一定变化幅度的差异,如绝对输沙量均在迎风坡至丘顶增加、丘顶至背风坡坡脚减小,但中沙输沙量在迎风坡增加的幅度小于细沙、在背风坡减少的幅度大于细沙;相对输沙量在沙丘不同部位的变化趋向类似,但细沙的变化幅度要小于中沙;中沙的垂向分布模式均为幂函数,细沙则表现为指数分布(0~10cm高度内)和幂函数分布(10~30cm高度内)。     

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。     

半干旱区不同类型沙丘风沙流结构特征

采用两种阶梯式集沙仪和小型气象站于2017年4—5月对科尔沁沙地流动沙丘、半固定沙丘和固定沙丘0~75 cm气流层风沙流的总输沙量、输沙率、粒径组成分布和风蚀特征值进行了观测。结果表明：（1）随着高度增加,总输沙量下降,随着风速增加,总输沙量上升;92.20%~95.60%的输沙量发生在0~21 cm高度。（2）总输沙率（Q）流动沙丘>半固定沙丘>固定沙丘,将Q与地上200 cm风速进行函数拟合,流动沙丘幂函数最佳（R²=0.986）,半固定（R²=0.990）和固定沙丘（R²=0.956）指数函数最佳。（3）将各高度的输沙率与地上200 cm风速进行函数拟合,流动沙丘（R²≥0.905）和半固定沙丘（R²≥0.968）拟合度幂函数好于指数函数,固定沙丘（R²≥0.923）指数函数优于幂函数。（4）在一定高度下,3类沙丘输沙率均随着风速的增加而增加;在一定风速下,输沙率随着高度的增加而逐渐递减。（5）3类沙丘的特征值随着风速的增加呈现出逐渐递增的趋势;流动沙丘以λ>1为主,表现出持续侵蚀输送沙粒的能力;半固定沙丘当风速>9.0 m·s^-1时逐渐出现侵蚀状态;固定沙丘以λ<1为主,近地表风沙以堆积状态为主。（6）3类沙丘主要由粒径为0.1~0.25 mm的细沙构成,在0~30 cm高度,细沙占输沙量的50.09%~85.11%,在30~75 cm高度,细沙占输沙量的43.53%~75.53%。     

海岸风沙流中不同粒径组沙粒的垂向分布模式

通过对河北昌黎黄金海岸沙丘风沙流的野外实地观测与室内风洞模拟实验数据的数值模拟,探讨了我国典型海岸沙丘风沙流中不同粒径组沙粒输沙量的垂向分布模式。结果表明,河北昌黎黄金海岸沙丘表面风沙流中不同粒径组沙粒输沙量的垂向分布特征并不一致,其中细沙和中沙符合典型的指数递减分布规律,但粗沙则为负幂函数分布。究其原因,主要与不同粒径组沙粒输沙量的分布高度及运动方式差异有关。在实际非均匀沙床面上,粗沙主要集中分布于沙丘表面4cm高度内湍流发育的近地表层,运动方式以蠕移为主,沙丘表面湍流的主导作用使其输沙量随高度的变化满足负幂律关系,但中沙和细沙则以跃移运动为主,跃移沙粒输沙量的垂向分布呈现指数递减特征。     

腾格里沙漠东南缘横向沙丘粒度变化及其与坡面形态的关系

腾格里沙漠东南缘的横向沙丘因断面形态不同可分为凸形和凹形两种类型。横向沙丘丘顶至落沙坡为风沙堆积区, 沙粒沿滑落面作重力下滑时, 较粗颗粒先到坡脚, 背风坡的沉积物下粗上细。沙丘迎风坡则因断面形态不同而影响沙丘表面过程。其中, 凸形沙丘迎风坡为高饱和风沙流环境, 下部有大量蚀余粗颗粒, 上部发生加积, 故下粗上细; 而凹形沙丘的迎风坡则为低饱和风沙流环境, 表面普遍风蚀, 前积纹层出露表面, 沉积物下粗上细, 实际上是背风坡沉积物重力分异结果的重现。沙丘高度与平均粒径和分选参数之间具有良好的一致性。风成床面作为一种地形, 对风沙地貌表面过程产生了重要影响, 这是风沙地貌尚须努力开拓的领域之一。     

塔克拉玛干沙漠腹地沙粒胶结体的粒度特征

对塔克拉玛干沙漠腹地沙粒胶结体进行溶蚀分散获取组成沙粒,采用激光粒度仪分析粒度成分,并与区域内沙丘沙、垄间平地沙和风沙流输沙的粒度特征进行对比,分析沙粒胶结体的粒度特征。结果表明：（1）沙粒胶结体中沙粒粒径呈多峰态分布,垄间平地沙呈双峰态分布,而沙丘沙和风沙流输沙呈单峰态分布;（2）沙粒胶结体内沙粒分选性较差,偏度属正偏,平均粒径（3.17 Φ）介于沙丘沙（3.10 Φ）和垄间地沙（3.28 Φ）、风沙流输沙（3.67 Φ）之间;（3）沙粒胶结体中沙物质主要组分为细沙和极细沙,与沙丘沙、垄间平沙地沙一致,而与风沙流输沙（极细沙和粉沙）不一致。与沙丘沙、垄间平地沙以及风沙流输沙相比,沙粒胶结体中粉沙、黏土和中沙相对富集,是现代地表物质的混合物;（4）与沙丘沙、垄间平地沙和风沙流输沙相比,沙粒胶结体内沙粒的蠕移-跃移、跃移-悬移截点粒径均偏细,蠕移组分所占比重很高。从组成颗粒的粒度组成来看,沙粒胶结体的形成受局地沙源和风动力的共同影响,是特殊环境条件下现代地表过程的产物。     

风沙流起动阶段沙粒输运特征

为了探究风沙流起动过程中沙粒输运特征,利用PTV测量技术在风洞中对风沙流起动过程进行了测量,分析了沙粒空间分布、沙粒平均水平速度、输沙率、沙粒数密度和输沙通量随时间的变化规律。结果表明：风沙流起动时间大约为1.5 s。起动过程中,输沙率随时间迅速增加,气流中沙粒总数目随时间的变化可表示为指数函数,沙粒数密度和输沙通量随高度的变化均可近似表示为负指数衰减函数。在t=1.0 s时刻的沙粒平均水平速度大于相同高度处以后时刻的沙粒平均水平速度,同一高度处t=1.5 s以后的沙粒数密度大于t=0.5 s、1.0 s时刻的沙粒数密度,同一高度处t=1.5 s以后的输沙通量大于t=1.0 s时刻的输沙通量。沙粒数密度随高度的衰减率一般随时间的增加而减小,并在t=1.5 s后逐渐接近稳定值。     

偏西风作用下敦煌月牙泉金字塔沙山顶部风沙流初步观测研究

金字塔沙山丘体高大,形态复杂,坡面风沙动力过程实地观测困难,至今尚无坡面风沙流野外观测数据。通过新研制自动集沙仪,对偏西风作用下敦煌月牙泉北侧金字塔沙山顶部风沙流进行了实时观测,共观测8组近地表51.4 cm高度内输沙数据,各观测时段中2 m高度平均风速7.46～14.15 m·s^-1,各组观测时长18~95 min。结果表明：偏西风纵向气流作用下金字塔沙山风沙流结构与平坦地表一致,即输沙量随高度增加呈单一的指数规律递减,且风沙流主要在近地表30 cm高度内输移。风沙流结构特征值λ以大于1为主,且随风速增大而增大,表明金字塔沙山坡面过程主要表现为风蚀过程。偏西风作用下金字塔沙山风沙流中沙粒以细沙为主,平均粒径随高度增加而减小;沙山迎风坡沙粒从坡脚到沙脊线逐渐粗化,风蚀作用增强,细沙从而随风沙流搬运至沙山顶,对金字塔沙山多以细沙为主的格局形成起重要作用。     

Grain‐Size Characteristics of Dune Networks in China's Tengger Desert

Grain‐size characteristics of dune networks in the Tengger Desert of northwestern China were investigated. Grain‐size parameters (mean, standard deviation, skew, and kurtosis) were determined on dune surfaces at windward toe, stoss, crest and leeward toe locations. Multiple discriminant analyses were applied to distinguish deposition environments. Results indicated that the aeolian sediment is mainly composed of very fine and fine sand in the dune networks of the Tengger Desert. Sorting improves as grain size becomes finer. However, the relationships between mean grain size, skew, and kurtosis vary in space. There is a negative relationship between skew and sorting; similarly, the relationships between sorting and skew, and skew and kurtosis change in space. The sediment deposition environment includes aeolian sediment and lacustrine sediment. The lacustrine sediment provides the source material for dune windward toe and stoss formation and development, but the aeolian sediment provides the source material for dune crest and leeward toe formation and development. According to the log‐probability of grain size distributions, aeolian sediments in dune networks are composed of two distinct saltation populations.     

Sonic anemometers in aeolian sediment transport research

Fast-response wind and turbulence instruments, including sonic anemometers, are used more and more in aeolian sediment transport research. These instruments give information on mean wind, but also on fluctuations and turbulent statistics, such as the uw covariance, which is a direct measure of Reynolds' stress (RS) and friction velocity. This paper discusses the interpretation of sonic anemometer data, the transformations needed to get proper results and turbulence spectra, and how they are influenced by instrument size, sampling frequency, and measurement height.Turbulence spectra characterize how much the different frequencies in the turbulent signals contribute to the variance of wind speed, or to the covariance of horizontal and vertical wind speed. They are important in determining the measurement strategy when working with fast-response instruments, such as sonic anemometers, and are useful for interpreting the measurement results. Choices on the type of sonic anemometer, observation height, sampling period, sampling frequency, and filtering can be made on the basis of expected high and low-frequency losses in turbulent signals, which are affected by those variables, as well as wind speed and atmospheric stability.Friction velocity and RS, important variables in aeolian sediment transport research, are very sensitive to tilt or slope errors. During a field experiment, the slope sensitivity of the RS was established as 9% per degree of slope, which is 1.5 times the value reported in literature on the basis of theoretical considerations. An important reason for the difference probably is the large influence of streamline curvature on turbulence statistics and thereby on the slope sensitivity of the RS. An error of 9% per degree of slope in the RS will translate into an error of approximately 4% per degree of slope in the calculated friction velocity.Space–time correlation of the horizontal wind speed is much larger than that of the vertical wind speed and the instantaneous RS. This largely explains why, in previous studies, a poor correlation was found between instantaneous RS measured at 3 m height and saltation flux near the surface, whereas the correlation between wind speed at some height and saltation flux was much better. Therefore, the poor correlation between RS away from the surface and saltation flux does not contradict that saltation flux is caused by RS.     

基于高速摄影技术的风沙流输沙通量剖面的风洞实验测量

风沙研究者非常重视对输沙通量随高度变化特征的研究,并为寻找可靠的测量手段付出了不懈的努力。基于高速摄影技术获得的沙粒平均水平速度与沙粒数的垂直剖面,推导了较低风速下环境风洞内输沙通量的垂直剖面。结果表明：沙粒平均水平速度随高度呈幂函数增加,颗粒浓度随高度的算数平方根呈指数衰减。由颗粒平均水平速度剖面与浓度剖面的乘积可获得输沙通量剖面。所获得的输沙通量随高度变化曲线在距床面1~3 mm处均有一个明显的拐点,拐点上方输沙通量随高度呈指数衰减。在床面与拐点之间输沙通量没有明显的变化趋势,这可能是由于气流中颗粒间的碰撞以及颗粒与床面碰撞的影响。平均跃移高度和相对衰减系数是描述输沙通量随高度变化的两个重要参数,两者有着很好的相关性,表明了随着风速增加和沙粒粒径减小跃移颗粒可以达到更大的高度,随着风速减小与粒径增大,输沙通量迅速衰减。     

腾格里沙漠东南缘沙丘迎风坡风速变化的初步研究

对腾格里沙漠东南缘格状沙丘表面气流的观测结果表明,迎风坡风速的增加程度因坡成形态和原始气流的方向,强度而不同。迎风坡风速放大率介于１．０６～１．９１之间,且在格状沙丘主梁,主要与坡形系数和高度有关;在副梁随原始风入射角,坡形系数而变化,风速放大率对沙丘形态的影响主一要增加丘顶区域的输沙率和活动性,且因区域气流的方向和强度而不同,在横向气流条件下,其迎风坡随区域气流强度和增加而变缓,变长,在双向（斜     

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.     

兰新高铁烟墩风区戈壁近地表风沙流跃移质垂直分布特性

跃移质作为风沙流的主体,其近地表垂直分布规律是风沙物理学的重要研究内容,对防沙工程具有重要的指导意义。受研究条件与观测仪器限制,戈壁特别是极端大风区近地表风沙流结构特性研究较为薄弱。利用多梯度风蚀传感器与阶梯式集沙仪对兰新高铁烟墩风区戈壁近地表风沙流跃移质的垂直分布特性进行了观测研究。结果表明：兰新高铁烟墩风区戈壁沙粒发生跃移运动的2 m高临界风速达12 m·s^-1;戈壁近地表风沙流具有明显的阵性特征,沙粒跃移发生的时间比例在50%以下,与平均风速成正相关关系,与风速脉动强度无显著相关关系;2 m高阵风7级风速下,戈壁跃移沙粒主要集中于地表50 cm范围内,近地表风沙流结构呈"象鼻效应",跃移质最大质量通量出现在地表2.5~5 cm高度处,沙粒最大跃移高度可达2 m,且沙粒跃移高度随2 m高风速的增加呈指数规律递增。因此,兰新高铁烟墩风区2 m高阻沙栅栏不足以完全阻截戈壁风沙流,是造成烟墩风区兰新高铁轨道积沙的重要原因之一。     

库布齐沙漠南缘抛物线形沙丘表面风速与输沙率的变异

以库布齐沙漠南缘典型抛物线形沙丘为研究对象,选取3个沙丘前端活动区的纵断面和一个贯穿两翼及翼间平地的横断面,同步观测各个断面的风速与输沙率,讨论抛物线形沙丘表面形态-动力学过程。结果显示,沙丘前端活动区3个纵断面的风速和输沙率遵循一般变化规律,即沿迎风坡上升逐渐增加,至丘顶及附近达到最大,并在背风侧降低;二者在3个纵断面上的变化趋势一致,但具体变化形式不同,沙丘表面动力与输沙过程存在较大的空间异质性。沙丘两翼具有纵向沙丘的动力学性质,翼体两侧风速和输沙率具有对称性。该沙丘沙源匮缺,前端活动区主要依靠两翼内侧侵蚀提供沙源。受沙丘形状和植被等因素影响,坡脚及坡下部风速与输沙率之间没有相关性,因此简单的风速放大率和输沙率变化程度无法准确揭示该类型沙丘表面复杂的形态-动力学关系。     

VERTICAL DISTRIBUTION OF WIND-BLOWN SAND FLUX IN THE SURFACE LAYER,TAKLAMAKAN DESERT,CENTRAL ASIA

The vertical distribution of the wind-blown sand flux in a 40-cm flow layer above the ground surface was investigated through laboratory wind-tunnel tests and field measurements on the mobile dune surface during sand storms in the Taklamakan Desert of China. Results show that vertical distribution of the horizontal mass flux of drifting sand is a discontinuous function of height. More than 90% of the total material is transported in the flow layer from the surface to 14 cm. From 2 to 4 cm above the surface, a distinct transition zone occurs wherein mixed transport by creep, saltation, and suspension becomes saltation and suspension. The flow layer from 14 to 15 cm represents a further transition from saltation to suspension, where the distribution curves of the transport rate against height converge. The basic natural exponential function cannot describe well the vertical distribution of the saltation mass flux in the Taklamakan Desert. As a function of height, saltation mass flux follows a function q_salt = a'Z^-bZ, and the distribution of suspension mass flux fits the power function very well. A total transport rate from surface creep to saltation and suspension in the measured flow layer, which is directly proportional to the effective wind speed squared (V - V_t)², can be predicted by integrating Q = a'Z^-bZ + cZ^-d. The height distribution of the average quantities of transported materials varies as an exponential function of wind speed, and deceases with the increase in total transport quantity. Higher wind speed results in a higher transport rate and a higher vertical gradient for the particle concentration. The increment of relative transport quantity in the higher flux layer increases as wind speed increases, which generates a higher concentration of drifting particles in the upper flow layer. [Key words: aeolian geomorphology, aeolian transport, horizontal sand flux, sand dune, vertical sediment distribution, Taklamakan Desert.]