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1.
The flux profile of a blowing sand cloud: a wind tunnel investigation 总被引：11，自引：0，他引：11
The flux profile of a blowing sand cloud, or the variation of blown sand flux with height, is the reflection of blown sand particles that move in different trajectories, and also the basis for checking drifting sand. Here we report the wind tunnel results of systematic tests of the flux profiles of different sized sands at different free-stream wind velocities. The results reveal that within the 60-cm near-surface layer, the decay of blown sand flux with height can be expressed by an exponential function: qh=aexp(−h/b), where, qh is the blown sand transport rate at height h, a and b are parameters that vary with wind velocity and sand size. The significance of coefficient a and b in the function is defined: a represents the transport rate in true creep and b implies the relative decay rate with height of the blown sand transport rate. The true creep fraction, the ratio of the sand transported on the surface (h=0) to the total transport varies widely, decreasing with both sand size and wind speed. The flux profiles are converted to straight lines by plotting sand transport rate, qh, on a log-scale. The slope of the straight lines that represents the relative decay rate with height of sand transport rate decreases with an increase in free-stream wind velocity and sand grain size, implying that relatively more of the blown sand is transported to greater heights as grain size and wind speed increase. The average saltating height represented by the height where 50% of the cumulative flux percentage occurs increases with both wind speed and grain size, implying that saltation becomes more intense as grain size and/or wind velocity increase. 相似文献
2.
Velocity profile of a sand cloud blowing over a gravel surface 总被引：2，自引：0，他引：2
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. 相似文献
3.
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 wi 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 wi 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. 相似文献
4.
In a blowing sand system,the wind provides the driving forces for the particle movement while the moving particles exert the opposite forces to the wind by extracting its momentum.The wind-sand interaction that can be characterized by shear stress and force exerted on the wind by moving particles results in the modification of wind profiles.Detailed wind pro-files re-adapted to blown sand movement are measured in a wind tunnel for different grain size populations and at differ-ent free-stream wind velocities.The shear stress with a blowing sand cloud and force exerted on the wind by moving par-ticles are calculated from the measured wind velocity profiles.The results suggest that the wind profiles with presence of blowing sand cloud assume convex-upward curves on the u(z)-ln(z) plot compared with the straight lines characterizing the velocity profiles of clean wind,and they can be better fitted by power function than log-linear function.The exponent of the power function ranging from 0.1 to 0.17 tends to increase with an increase in wind velocity but decrease with an increase in particle size.The force per unit volume exerted on the wind by blown sand drift that is calculated based on the empirical power functions for the wind velocity profiles is found to decrease with height.The particle-induced force makes the total shear stress with blowing sand cloud partitioned into air-borne stress that results from the wind velocity gradient and grain-borne stress that results from the upward or downward movement of particles.The air-borne stress in-creases with an increase in height,while the grain-borne stress decreases with an increase in height.The air-borne shear stress at the top of sand cloud layer increases with both wind velocity and grain size,implying that it increases with sand transport rate for a given grain size.The shear stress with a blowing sand cloud is also closely related to the sand transport rate.Both the total shear stress and grain-borne stress on the grain top is directly proportional to the squ 相似文献
5.
WITSEG集沙仪:风洞用多路集沙仪 总被引：21，自引：9，他引：12
风沙流通量廓线(风沙流结构)是以不同轨迹运动的沙粒在垂直方向上的宏观反映。建立风沙流通量廓线函数需要测定不同高度的输沙率。为此,我们设计了适用于风洞实验的多路集沙仪(WITSEG集沙仪),并通过风洞实验对其进行了检验。WITSEG集沙仪高60 cm,由60个进沙口和集沙盒组成,每个进沙口高1 cm。该集沙仪可以测量风沙流中60个不同高度的输沙率。在设计WITSEG集沙仪时,着重实用性和集沙效率。为了使用方便,带有进沙口的入口段、集沙盒和保护盖板设计为活动式。为了提高集沙效率,入口段设计成楔形,使得进沙口宽0.5 cm,而集沙盒宽度为1.5 cm。每个集沙盒留有两个过滤网排气孔,以减小集沙盒内的静压、提高采集效率。风洞实验检验表明,用WITSEG集沙仪测得的风沙流结构和总输沙率与风速的关系与已广泛接受的结论非常一致。WITSEG集沙仪能观测输沙率随高度的详细变化,是研究风沙流结构的较好工具。 相似文献
6.
基于起跳初速度分布的沙颗粒浓度廓线的数值模拟 总被引：1，自引：1，他引：0
跃移层内沙颗粒浓度分布是风沙两相流相互作用的结果,准确的沙颗粒浓度分布有助于弄清风沙互馈机制及沙颗粒间相互作用机制。由于沙颗粒浓度分布与沙颗粒起跳初速度分布以及气流运动密切相关,本文基于特定的沙颗粒起跳初速度分布函数,通过构建的沙颗粒在气流中运动的物理模型,并利用四阶精度的Adams-Bashforth-Moulton方法对所构建运动模型进行求解,统计分析稳定状态下两相流中沙颗粒运动轨迹的分布,分析其浓度廓线的垂向分布规律。计算结果表明跃移层内沙颗粒浓度分布廓线与高程呈负指数或伽马分布关系;高度一定时沙颗粒浓度廓线随摩阻风速的增大而减小,随颗粒直径的增大而增大。 相似文献
7.
Height profile of the mean velocity of an aeolian saltating cloud: Wind tunnel measurements by Particle Image Velocimetry 总被引：3，自引：0，他引：3
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. 相似文献
8.
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。 相似文献
9.
对新疆伊犁河谷塔克尔莫乎尔沙漠抛物线形沙丘表层78个风成沙样的粒度分析表明,平均粒径均值为220 μm,平均中值粒径为299 μm,标准偏差平均为1.98,偏度平均为-0.29。纵向分布上,从风蚀洼地到迎风坡下部,风成砂的平均粒径和分选性波动变化明显,沿迎风坡向上平均粒径减小,在脊线附近有较粗粒径的分布带。在沙丘背风坡平均粒径减小、负偏增加、分选性较差。在粒度参数等值线及三维分布方面,风蚀洼地和迎风坡下部呈现沿沙丘轴向交替分布的封闭等值曲线,两侧沙臂粒度参数大小波动变化,沙臂北翼坡脚粒度组成呈现很高的负偏和峰度值,反映在区域风况作用下,抛物线形沙丘本身地形起伏、两侧沙臂灌丛沙堆分布对沙丘表面风沙流运移有影响,风沙流与沙臂斜交在背风坡产生涡流引发较强的沉积作用。 相似文献
10.
海岸风沙流中不同粒径组沙粒的垂向分布模式 总被引：4，自引：1，他引：3
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。 相似文献
11.
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. 相似文献
12.
本文应用流体计算软件FLUENT6.3,采用非结构化网格划分技术模拟了气流特征和砾石几何参数对床面空气动力学粗糙度的影响。结果表明：空气动力学粗糙度（z0）与风速（u）、摩阻速度（u*）之间存在定量关系：z0=a exp（bu/u*）。砾石高度对空气动力学粗糙度的影响显著优于砾石直径,空气动力学粗糙度随砾石密度的变化比较复杂,先增加后减小。FLUENT在模拟风洞砾石床面动力学过程中的成功应用,是我们在研究方法上的一次有益尝试。 相似文献
13.
海岸横向沙脊表面风沙流结构粒度响应的野外观测研究 总被引：11，自引：8，他引：3
风沙流中沙粒粒度组成的垂向变化及不同粒径沙粒的垂向分布特征等是风沙流结构粒度响应的基本研究内容,但一直较缺少野外观测实证证据。在我国海岸沙丘集中分布的河北昌黎黄金海岸,选择规模高大、形态典型的横向沙脊,采用野外梯度风速仪和平口式积沙仪对其顶部风沙流结构的粒度响应进行了野外实地观测。结果表明,海岸沙丘表面风力的分选作用明显,与横向沙脊表面相比风沙流所含沙物质中细沙组分比例增多、粗沙组分比例降低;粒度构成的垂向变化是,在风沙流0—10 cm、10—40 cm和40—60 cm三个高程段内,分别是以中沙为主、中沙与细沙并重和以中沙为主,其中中沙含量在三个高程段内分别呈现出自地表向上逐渐降低、含量比较稳定、向上逐渐升高的垂向变化,细沙则呈现相应的反向变化,随之平均粒径也呈现为自下而上逐步变细、比较稳定到逐渐变粗的变化,但分选系数、偏度和峰态无明显差异;横向沙脊顶部的风沙流结构整体符合指数分布,但不同粒径沙粒的垂向分布特征并不相同,其中粗沙为幂函数分布,细沙遵循指数函数递减,中沙的指数函数和幂函数拟合均有较高相关系数,但在0—40 cm高度内更符合指数函数分布。 相似文献
14.
海岸湿沙表面风沙传输特征的风洞实验研究 总被引：4，自引：2，他引：2
通过湿沙表面风沙传输的风洞实验,研究了湿润海岸风沙流的垂直结构、输沙率随风速与表层湿度的变化规律。输沙测量使用60 cm高直立式积沙仪,湿度（M）为沙面表层1 mm厚的重量湿度值。结果表明,湿沙表面的输沙量和高度呈指数关系。一般,湿度增大,整体输沙率降低,高湿度床面的沙粒有相对更大的比例被传输到更高的位置。比起低湿度（M＜0.587%）沙粒,高湿度（0.587%＜M＜1.448%）沙粒的垂直运动对湿度变化的影响更加敏感,尤其是在跃移层的底部,当M＞1.448%时,输沙率已经很低,小于0.99 g·cm-1·s-1。伴随湿度0.587%和1.448%的过渡,风沙流垂向分布被分为3个不同坡度的区域,曲线坡度反映了沙粒间不同水分存在形式的影响差异。对于跃移沙粒,高湿度表面（M＞1.448%）仅起到了一个传输平台的作用;当表面变干到某种程度（M=0.587%）之前,表面湿度是跃移运动的主要控制因子,然后风速才重新开始影响输沙。 相似文献
15.
The vertical wind-velocity profile within the saltation layer is investigated theoretically. New equations for the fluid shear stress distribution in the saltation layer and the velocity profile affected by saltation are derived. The dependence of the velocity profile on the threshold shear velocity, the vertical variations of sand grain velocity, the vertical sand-mass-flux distribution, and the fluid shear stress within the saltation are demonstrated. Velocity profiles with or without Bagnold's focus are predicted. These velocity profiles are not very sensitive to the vertical distribution of sand-mass flux. Predicted velocity profiles are compared with profiles measured in the wind tunnel, and satisfactory agreement has been achieved. 相似文献
16.
库布齐沙漠北缘不同作物秸秆平铺式沙障的防风效能 总被引：8，自引：0，他引：8
本研究以七星湖沙漠风景区沙丘上的芦苇秸秆、向日葵秸秆和玉米秸秆平铺式沙障为研究对象,通过测定各沙障障格内以及裸沙丘近地表不同高度处的风速,对比研究了各沙障的防风效益。结果表明,①3种沙障在10 cm高度处均具有明显的防风效能,且随着高度的增加,防风效能减小,从10 cm到100 cm高度,3种沙障的防风效能平均降幅达63%。②在沙丘迎风坡,芦苇秸秆沙障的防风效能和地表粗糙度分别是向日葵秸秆沙障的1.36倍和2.34倍,是玉米秸秆沙障的1.76倍和1.88倍;在沙丘坡顶和背风坡,玉米秸秆沙障的防风效能及地表粗糙度均显著高于另外两者（P<0.01）。③3种沙障的风速廓线均呈现出一种类似于“S”型的变化特征,而作为对照的裸沙丘风速廓线则服从典型指数函数分布。 相似文献
17.
《Geomorphology》2004,57(1-2):117-127
Detailed wind tunnel tests were conducted to examine the fetch effect of a sandy surface on a sand cloud blowing over it. The results suggest that the fetch length of a sandy surface has a significant effect on both the vertical flux profile and total horizontal flux. The sand flux over a sandy surface increases with height in the very near surface layer, but then decays exponentially. In agreement with the widely accepted conclusion, the decay function can be expressed by q=aexp(−h/b), where q is the sand flux at height h. Coefficient a that tends to increase with wind speed implies the influence of wind, while coefficient b that defines the relative decay rate shows the influence of both the fetch and wind. The relative decay rate increases with fetch when the fetch length is short, then becomes constant when the fetch reaches a certain length. The threshold fetch length over which the relative decay rate keeps constant increases with wind speed. The average saltation height generally increases with fetch. Both the relative decay rate and average saltation height show that the fetch effect on the flux profile becomes more significant when the wind speed increases. The total sand transport equation for the total fetch can be expressed by Q=C(1−Ut/U)2U3(ρ/g), where Q is the total sand transport rate, U and Ut are the wind velocity and threshold wind velocity at the centerline height of the wind tunnel, respectively, g is gravitational acceleration, ρ is the density of air, and C is a proportionality coefficient that increases with the fetch length, implying that the total sand flux increases with the fetch length. 相似文献
18.
PIV技术及其在风沙边界层研究中的应用 总被引：2，自引：2，他引：0
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。 相似文献
19.
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。 相似文献
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分别构建广州主建成区垂直比例尺为1﹕2 000、1﹕1 000和1﹕500的3个建筑物模型,利用大型边界层风洞,在西北和东南两风向下,基于中性流模拟分析了复杂城市地形下湍流度随高度的变化及其对宏观地形的依赖。结果表明：风廓线指数α与不同高度的湍流度之间的关系密切,利用现有模型,根据4类粗糙度边界层和不同垂直比例尺,可确定相应的湍流度随高度变化模型的主要系数,预测精度高。城市地形下最大湍流度面发育在0~0.2 h之间狭窄的范围内。用湍流度形态指数β来表征湍流度随高度的变化,无论城市屋脊还是平坦地形,随着风程区的延伸,廓线的指数α升高,湍流度形态指数β降低。表明同一高度湍流度值具有由迎风区、丘顶区向背风区增高,沿风程逐渐增大的规律,对地形部位和风程的依赖性强,与来流翻越简单地形时的特征一致。 相似文献