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。     

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。     

PIV技术及其在风沙边界层研究中的应用

为了考察粒子图像测速度技术在风沙环境风洞中的测量精度及在风沙边界层研究中的应用潜力,通过筛选适当的示踪颗粒,借助PIV测量系统重新测量了风沙环境风洞中的风廓线,并获得了风沙边界层内跃移沙粒的速度和浓度分布规律。实验结果表明：PIV测得的风速廓线与标准风速廓线仪所测结果相当吻合(R2≥0.99);沙粒跃移的平均水平速度和相对浓度（灰度）沿距离沙面高度分别呈幂函数和负指数分布,沙粒速度随高度和自由风速的增加而增大,相对浓度随高度的增加而快速衰减,风速越小衰减越快,风速越大衰减越慢,这一结果与前人的结论一致。PIV系统为将来能够进行更加精确的风沙运动微观机理研究提供了技术保证。     

野外近地表风沙流脉动特征分析

基于风沙流实时测量系统对流沙地表的输沙强度以及不同高度风速进行实时测量所获得的数据,分析野外实际风场中的风沙流脉动特征。结果表明:沙粒的存在削弱平均流场,但对风速的高阶统计矩和分布形式影响不大,不同高度、不同来流平均风速条件下,风沙流中风速仍近似符合高斯分布;瞬时输沙强度服从指数分布,滞后风速约1 s,脉动强烈;30 min输沙强度平均值和脉动标准差均随平均风速的增大而呈幂函数增加,但随风速脉动强度的变化呈先增大后减小的趋势。这意味着脉动风速对输沙率具有重要影响,需要在输沙率预测公式中予以考虑。     

脉动风场下跃移沙粒对风的响应时间

风沙流是大气边界层的典型气固两相流,输沙率对风速的响应滞后时间,被称为风沙流响应时间。通过野外测量发现风沙流响应时间与风速测量高度成正比,通过对瞬时风速进行信号分解并计算近地表输沙率与不同时间尺度风速信号之间的相关性,发现响应时间随风速频率的减小而增加。为进一步得到定量的规律,采用数值模拟的方法分析了周期来流风场中,风沙流响应时间随各种参数(周期\,湍流强度\,来流摩阻风速和测量高度等)的变化规律。模拟结果显示:响应时间与风速变化周期、来流摩阻风速成正比,与湍流强度和边界层高度成反比,输沙强度对跃移层外风速变化的响应时间沿高度先增加后减小。     

风速正弦变化下的非平稳跃移风沙流模拟

 采用有限体积法模拟了风速正弦变化下的一维非平稳跃移风沙流发展过程。考虑风沙流跃移系统的4个子过程,沙粒的流体起动、沙粒的运动、击溅过程和沙粒对风场的反作用。给出在风速正弦变化时,风速变化频率和振幅对于沙粒输运的影响以及输沙率、风速廓线、床面剪切应力以及起跳沙粒数的变化规律。结果表明,输沙率随着振幅增大而增大,随着周期增大而减小;在初始的overshoot现象之后,床面剪切应力变化很小,但起跳的沙粒数随风速呈现类正弦周期变化。     

风沙运动的理论模拟和风洞实验对比研究

针对输沙率这一风沙运动的重要问题,在沙坡头野外风洞进行了实验观测,同时建立热对流-扩散作用下的风沙运动多场耦合模型,采用大涡模拟方法对其进行数值模拟研究。通过对比实验观测和理论模拟,结果表明:实验和理论模拟得到的风速和输沙率都比较吻合,风速近似可分为两部分,在35 cm高度以上,风场受沙粒的影响较弱,风速服从对数分布,与净风场一致;在0~35 cm高度区间,受大量跃移运动沙粒的阻滞作用使得风速单调减小,随着风速加强,跃移运动沙粒的数量也在增大,风速梯度逐渐减小。在风沙流发展过程中,开始阶段输沙率随时间呈指数增大,而后逐渐减小,直至达到动态稳定;随着风速加强,输沙率变大,风沙运动达到动态稳定的时间变短。风洞实验和理论模拟的输沙率结果在10 cm以上吻合得很好,但在10 cm以下,风洞实验和理论模拟差别较大。同一风速下,采用最小二乘法对风洞实验和理论模拟的输沙率进行拟合并得到拟合公式:输沙率沿高度呈指数规律递减。同时,在不同风速下对同一高度层输沙率对比分析表明,贴近地表处(0~10 cm)高度层内输沙率随风速增大所占的百分率降低;而在10 cm以上高度层内,随着风速的增大,其输沙率所占总输沙率的百分比却有明显增加。     

风沙流中沙粒拖曳力系数研究

 拖曳力系数是计算风沙流中沙粒受空气阻力的重要参数。考虑实际风沙流中沙粒浓度及沙粒形状,利用FLUENT软件首先计算了不同风速下距地表不同高度处两沙粒的拖曳力系数,给出了影响沙粒拖曳力系数的间距范围,然后计算了真实风沙流中不同高度沙粒拖曳力系数。结果表明,给定风速下拖曳力系数随距地面高度的增加先减小后增加,并将沙粒拖曳力系数拟合成距床面高度的函数,该函数与风速有关。     

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。     

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。     

Wind tunnel experimental investigation of sand velocity in aeolian sand transport

Sand velocity in aeolian sand transport was measured using the laser Doppler technique of PDPA (Phase Doppler Particle Analyzer) in a wind tunnel. The sand velocity profile, probability distribution of particle velocity, particle velocity fluctuation and particle turbulence were analyzed in detail. The experimental results verified that the sand horizontal velocity profile can be expressed by a logarithmic function above 0.01 m, while a deviation occurs below 0.01 m. The mean vertical velocity of grains generally ranges from − 0.2 m/s to 0.2 m/s, and is downward at the lower height, upward at the higher height. The probability distributions of the horizontal velocity of ascending and descending particles have a typical peak and are right-skewed at a height of 4 mm in the lower part of saltation layer. The vertical profile of the horizontal RMS velocity fluctuation of particles shows a single peak. The horizontal RMS velocity fluctuation of sand particles is generally larger than the vertical RMS velocity fluctuation. The RMS velocity fluctuations of grains in both horizontal and vertical directions increase with wind velocity. The particle turbulence intensity decreases with height. The present investigation is helpful in understanding the sand movement mechanism in windblown sand transport and also provides a reference for the study of blowing sand velocity.     

Grain size and transport characteristics of non-uniform sand in aeolian saltation

Size frequency distributions of sediment particles in a wind tunnel containing a bed of non-uniform sand are investigated by re-interpreting existing experimental data using particle-size analysis. Each particle sample is classified into one of eight groups according to its size grading. The analysis reveals that the modal shape of the particle-size frequency distributions of the saltating sand at different elevations or longitudinal distances is similar to that of the mixed sand in the bed once the boundary layer is fully developed. The standard deviation of the grain-size frequency distribution increases with increasing elevation above the bed then stays constant, whereas its skewness decreases. The mean grain size decays exponentially with elevation. The aeolian sand mass flux is determined for each size grading at different vertical and horizontal measurement locations. The vertical profile of aeolian horizontal mass flux depends on the size grading. The distribution of the sand transport rate according to the mean grain size in each grading fits the normal distribution. A parameter w_i is defined to reflect the likelihood of saltation for sand particles of the i-th size grading, and the mean sand size corresponding to the maximum value of w_i is found to be 0.2 mm. In addition, wind velocity strongly influences the magnitudes of the particle-size distribution and the sand mass flux distribution in both vertical and longitudinal directions.     

Height profile of the mean velocity of an aeolian saltating cloud: Wind tunnel measurements by Particle Image Velocimetry

The velocity of saltating particles is an important parameter in studying the aeolian sand movement. We used Particle Image Velocimetry to measure the variation with height of the mean particle velocity of a saltating cloud over a loose sand surface in a wind tunnel. The results suggest that both the horizontal and vertical particle velocities fit the Gaussian distribution well, and that the mean particle velocity of a saltating cloud varies with wind velocity, particle size and the height above bed. The mean horizontal velocity is mainly the result of acceleration by the wind and increases with an increase in friction wind velocity but decreases with an increase in grain size because greater wind velocity causes more acceleration and finer particles are more easily accelerated at a given wind velocity. It also increases with an increase in height by a power function, in agreement with previous results obtained by other methods such as the high-speed multi-flash photographic method and Particle Dynamics Analyzer (PDA), reflecting, first, the increase in wind velocity with height through the boundary layer, and second, the longer trajectory-particle path length increases with height and affords a longer time for acceleration by the wind. An empirical model relating the mean horizontal particle velocity and height, friction wind velocity as well as particle size is developed. The ratio of the mean horizontal particle velocity to the clean wind velocity at the same height increases with height but decreases with grain size. The magnitude of mean vertical velocity is much less (one or two orders less) compared with the mean horizontal velocity. The average movement in the vertical direction of a saltating cloud is upward (the mean vertical velocity is positive). Although the upward velocity of a saltating particle should decrease with height due to gravity the mean vertical (upward) velocity (the average of both ascending and descending particles) generally shows a tendency to increase with height. It seems that at higher elevations the data are more and more dominated by the ‘high-flyers’. The underlying mechanism for the mean vertical velocity distribution patterns needs to be clarified by further study.     

风力作用下沙粒蠕移概率的转化特征

建立了沙粒从蠕移状态向静止、跳跃转化以及继续滚动的概率模型.根据滚动沙粒简化的运动方程,结合两个关键随机变量-沙床表面风速和床面位置参数,求出了静止概率、滚动概率、跳跃概率随时间变化的表达式.在此基础上通过对时间求极限得到稳定情况下沙粒处于三种运动状态的概率.利用沙粒蠕移概率除蠕移概率和跃移概率之和,得到沙粒相对蠕移概率,并以此反映蠕移输沙量占总输沙量的比例.计算结果表明,三种转移概率是时间、沙床表面风速和沙粒直径的函数.静止概率随时间增大而增大,但随粒径增大而减小;滚动概率随时间的变化根据粒径的不同表现出不同的特点.粒径较小,滚动概率随时间的增大先增大后减小,存在极大值;粒径较大时,滚动概率随时间的增大逐渐增大,不存在峰值.跳跃概率随时间的增大而增大.三种概率达到稳定状态所需要的时间随粒径的增大而减少.同粒径的沙粒在相同的时间内,三种转移概率会趋于定值.稳定概率和相对蠕移概率由沙床表面风速分布和粒径大小来确定,具有很大的取值范围;静止稳定概率和跳跃稳定概率随粒径的增大分别增大和减小;而滚动稳定概率随粒径的增大逐步先增大后减小.在一定风速条件下,相对蠕移概率随风速和方差的变化都不大,沙粒粒径是最主要的影响因素.     

Measuring the velocity of sand particles in an air/particle two-phase flow: A comparison of several commonly used methods

The velocity of blown sand particles is an important parameter in aeolian movement （a special case of gas particle two-phase flow） and has ever been a topic of interest. At present, several techniques have been applied in measuring velocity of the blown sand particles. This paper reviews the measurement results of several commonly used methods： photoelectric cell method, high-speed photographic method, Particle Dynamics Analyzer （PDA） method and Particle Image Velocimetry （PIV） method. Photoelectric cell method, high-speed photograph method and PDA method are useful in studying the velocity distribution of particles. PIV is a whole-flow-field technique and a useful tool to study the average velocity field in a target area. These methods got some similar results but considerable differences also exist. They have come to similar conclusions on the velocity distributions at a single height but direct measurement results with respect to the velocity distribution very close to the surface are still scarce except some PDA results. The magnitude of measured mean particle velocity differs greatly. The relationship obtained by different methods between mean particle velocity and wind velocity, particle size and possibly other influencing factors also differs considerably. Although several authors have proposed similar power functions to describe the variation with height of the mean particle velocity, the predicted results have wide differences. Each technique is based on some unique principles, and has its advantages and disad- vantages. To make full use of different techniques, a lot of work needs be done to validate them. Developing a reliable technique to measure the velocity of blown particles is still a necessary task in aeolian research.     

Velocity profile of a sand cloud blowing over a gravel surface

Particle dynamic analyzer (PDA) measurement technology was used to study the turbulent characteristics and the variation with height of the mean horizontal (in the downwind direction) and vertical (in the upward direction) particle velocity of a sand cloud blowing over a gravel surface. The results show that the mean horizontal particle velocity of the cloud increases with height, while the mean vertical velocity decreases with height. The variation of the mean horizontal velocity with height is, to some extent, similar to the wind profile that increases logarithmically with height in the turbulent boundary layer. The variation of the mean vertical velocity with height is much more complex than that of the mean horizontal velocity. The increase of the resultant mean velocity with height can be expressed by a modified power function. Particle turbulence in the downwind direction decreases with height, while that in the vertical direction is complex. For fine sands (0.2–0.3 mm and 0.3–0.4 mm), there is a tendency for the particle turbulence to increase with height. In the very near-surface layer (<4 mm), the movement of blown sand particles is very complex due to the rebound of particles on the bed and the interparticle collisions in the air. Wind starts to accelerate particle movement about 4 mm from the surface. The initial rebound on the bed and the interparticle collisions in the air have a profound effect on particle movement below that height, where particle concentration is very high and wind velocity is very low.     

吉兰泰盐湖防护体系阻沙效应及输沙粒度特征

风沙危害是制约荒漠盐碱湖区资源开采的重要因素,研究盐湖防护体系风沙运移规律及其颗粒运动方式对实现盐湖资源可持续开采具有重要意义。以吉兰泰盐湖防护体系为研究对象,通过野外定位监测结合室内分析的方法,逐月对防护体系不同部位输沙通量进行观测,探究吉兰泰盐湖防护体系沙粒运动方式及各月输沙通量变化规律,以期为吉兰泰盐湖防护体系效果评估提供数据支撑。结果表明：（1）从流动沙垄至盐湖湖心风速表现为先减小后增加的趋势,除白刺灌丛外,其余样地风速基本不受风向影响。（2）除防护林带外,季节变化对防护体系50 cm高度以上输沙通量影响较大。（3）防护体系各样地0—40 cm高度的输沙通量占总输沙通量的53.43%—96.63%,区域内沙粒运动方式以跃移为主。（4）从流动沙垄至盐湖湖心,沙物质粒径分布曲线整体呈双峰型,且沙物质颗粒粒径表现为先增大,至盐碱滩地时达到最大,后略减小的趋势。吉兰泰盐湖防护体系50 cm高度以上输沙通量受季节影响较大,但整体仍可拦截95.79%—99.93%的沙粒,从而有效避免盐湖湖面积沙。     

Experimental investigation of the concentration profile of a blowing sand cloud

Detailed wind tunnel tests were carried out to establish the mean downwind velocity and transport rate of different-sized loose dry sand at different free-stream wind velocities and heights, as well as to investigate the vertical variation in the concentration of blowing sand in a cloud. Particle dynamic analyzer (PDA) technology was used to measure the vertical variation in mean downwind velocity of a sand cloud in a wind tunnel. The results reveal that within the near-surface layer, the decay of blown sand flux with height can be expressed using an exponential function. In general, the mean downwind velocity increases with height and free-stream wind velocity, but decreases with grain size. The vertical variation in mean downwind velocity can be expressed by a power function. The concentration profile of sand within the saltation layer, calculated according to its flux profile and mean downwind profile, can be expressed using the exponential function: c_z=ae^−bz, where c_z is the blown sand concentration at height z, and a and bare parameters changing regularly with wind velocity and sand size. The concentration profiles are converted to rays of straight lines by plotting logarithmic concentration values against height. The slope of the straight lines, representing the relative decay rate of concentration with height, decreases with an increase in free-stream wind velocity and grain size, implying that more blown sand is transported to greater heights as grain size and wind speed increase.