Analysis of velocity profile measurements from wind-tunnel experiments with saltation

Investigations of wind-field modification due to the presence of saltating sediments have relied heavily on wind tunnels, which are known to impose geometric constraints on full boundary layer development. There remains great uncertainty as to which portion of the vertical wind-speed profile to analyze when deriving estimates of shear velocity or surface roughness length because the lower sections are modified to varying degree by saltation, whereas the upper segments may be altered by artificially induced wake-like effects. Thus, it is not obvious which of several alternative velocity-profile parameterizations (e.g., Law of the Wall, Velocity Defect Law, Wake Law) should be employed under such circumstances.A series of experimental wind-tunnel runs was conducted across a range of wind speed using fine- and coarse-grained sand to collect high-quality, fine-resolution data within and above the saltation layer using thermal anemometry and ruggedized probes. After each run, the rippled bottom was fixed with fine mist, and the experiment repeated without saltation. The measured wind-speed profiles were analyzed using six different approaches to derive estimates of shear velocity and roughness length. The results were compared to parameter estimates derived directly from sediment transport rate measurements, and on this basis, it is suggested that one of the six approaches is more robust than the others. Specifically, the best estimate of shear velocity during saltation is provided by the logarithmic law applied to the profile data within about 0.05 m of the bottom, despite the fact that this near-surface region is where profile modification by saltating sediments is most pronounced. Uncertainty remains as to whether this conclusion can be generalized to field situations because progressive downwind adjustments in the interrelationship between the saltation layer and the wind field are anticipated in wind tunnels, thereby confounding most analyses based on equilibrium assumptions.     

基于起跳初速度分布的沙颗粒浓度廓线的数值模拟

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。     

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.     

Scales of boundary resistance in coarse-grained channels: turbulent velocity profiles and implications

Gravel-bed surfaces are characterized by morphological features occurring at different roughness scales. The total shear stress generated by the flow above such surfaces is balanced by the sum of friction drag (grain stress) and form drag components (created by bed forms). To facilitate a better understanding of total resistance and bed load transport processes, there is a need to mathematically separate shear stress into its component parts. One way to do so is to examine the properties of vertical velocity profiles above such surfaces. These profiles are characterized by an inner layer that reflects grain resistance and an outer layer that reflects total resistance. A flume-based project was conducted to address these concerns through systematically comparing different roughness scales to ascertain how increased roughness affects the properties of vertical velocity profiles. Great care was taken to create natural roughness features and to obtain flow data at a high spatial and temporal resolution using an Acoustic Doppler Velocimeter.Average vertical velocity profiles above each roughness scale were clearly segmented. The vertical extent of the inner flow region was directly related to the scale of roughness present on the bed (and independent of flow depth), increasing with increased roughness. On a rough but rather uniform “plane” bed made of heterogeneous coarse sediments (with no bed forms), the shape of the velocity profile was clearly dominated by the local variations in grain characteristics. When pebble clusters were superimposed, the average shear stress in the outer flow region increased by 100% from the plane bed conditions. The ratio of inner grain shear stress to outer total shear stress for this pebble cluster experiment was 0.18 under shallow flow conditions and 0.3 under deep flow conditions. The grain stress component that should be used in bed load transport equations therefore appears to vary in these experiments between 15% and 30% of the total channel stress, increasing with decreased resistance. Roughness height (K_s/D₅₀) values at the grain scale for the plane bed and pebble cluster experiments were 0.73 and 0.63, respectively. These are values that should be used in flow resistance equations to predict grain resistance and grain stress for bed load transport modeling.     

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。     

粗糙床面风廓线的转折特征及其物理意义

为了进一步认识粗糙床面空气动力学粗糙度的物理意义,理解空气动力学粗糙度对动量传递和跃移起动的影响机制,从风洞试验测定的粗糙床面风廓线转折特征入手,分析了粗糙床面的空气动力学性质并提出了内边界层动量传递及近壁区沙粒起动的可能机制。结果表明,细高粗糙元（方向比率在4~20之间）和孔隙粗糙元（孔隙度在0.15~0.75）风廓线呈现4个转折段,对应的湍流垂直分层为近壁区-尾涡层（Z≤0.15H~0.5H）、内边界层-尾涡层过渡层（0.15H≤Z≤0.75H）、内边界层（0.3H~0.75H≤Z≤1.2H~6H）和内边界层与外边界层过渡层等。粗矮粗糙元（粗糙元的方向比率在0.4~1.25之间）风廓线存在几个转折段,对应气流垂直分层为近壁区-尾涡层（Z≤1H~1.5H）和内边界层（1H~1.5H≤Z≤7H~35H）等。细高粗糙元和孔隙粗糙元覆盖21组床面（侧影盖度在0.007~0.50,粗糙元的高度在10~100 mm）的内边界层内空气动力学粗糙度在0.07~30.74 mm之间,比内边界层以上或以下过渡层的空气动力学粗糙度高几个数量级到数倍;内边界层摩阻风速的0.50~1.66 m\5s-1之间,是内边界层以下过渡层的1.5~10倍、内边界层以上过渡层的摩阻风速的1.1~2.8倍。内边界层的空气动力学粗糙度和摩阻风速分别代表了粗糙床面对气流阻力特征和湍流切应力。内边界层以下过渡层湍流切应力与粗糙元之间光滑地表所受切应力关系不大,而近壁层切应力与光滑地表所受切应力直接相关。近壁层猝发过程上抛运动和内边界层湍流猝发过程下扫运动耦合关系是内边界层的一部分动量传递到近壁区并导致沙粒起动的可能机制。     

风沙二相流运动特点的分析

本文以平坦沙漠上定常，充分发展的风沙二相流为例分析了颗粒的跃移运动，在这种运动中颗粒吸收气流的机械能和水平动量向下输送，并在落地时传递给床面。本文还用拟流体观点分析了固相运动的特点：在床面上有较大的滑移速度，有较大的垂向速度脉动，例题垂向的平均速度却为零，等等。     

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。     

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。     

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.     

海岸湿沙表面风沙传输特征的风洞实验研究

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。     

粗糙床面风廓线统一对数区的空气动力学粗糙度及其空间变异特征

为了进一步理解粗糙床面阻力效应,减小空气动力学粗糙度测试中的不确定性,依据风沙风洞测试的3类粗糙元（细高粗糙元、孔隙粗糙元和粗矮粗糙元）覆盖的39个粗糙床面在不同自由风速下的风廓线数据,提出了风廓线统一对数区的概念并得出以下结论：粗糙床面风廓线统一对数区范围约在0.1～0.3 h至边界层顶部,空气动力学粗糙度是变应力层内床面对气流阻力效应的垂向平均;在统一对数区内拟合的空气动力学粗糙度的垂向变异分为先增后减型（概率为71%）、减小型（20%）和增加型（9%）等类型,而采用统一对数区的空气动力学粗糙度可以避免垂向变异带来的不确定性;统一对数区的无量纲空气动力学粗糙度随粗糙元密度以幂函数形式增加的特征,进一步表明该指标能更好地表征粗糙床面对气流阻力效应;尾涡流风廓线统一对数区的空气动力学粗糙度约为街流区1~5倍,表明街流区风廓线统一对数区的空气动力学粗糙度是模拟跃移起动更合适的参数。     

The wind-saltation interaction in a saltation boundary layer with a downwind air pressure gradient

Studies of interactions between wind and saltating particles （i.e., the wind-saltation interaction） are usually conducted without consideration of the downwind air pressure gradient. However, in a wind tunnel with limited size, this gradient is required to maintain the movement of the saltation cloud. Attempts are made to investigate the effects of the downwind air pressure gradient on the wind-saltation interaction in a saltation boundary layer based on the experimental results from a wind tunnel with a relatively small cross-sectional area. The wind-saltation interaction is characterized by airborne stress, grain-borne stress, and the force exerted on the wind by the saltation cloud. Basic equations were developed for wind-saltation interactions without and with a downwind air pressure gradient. The results reveal that unacceptable values of negative grain-borne stress and negative force exerted on the wind by the saltation cloud are obtained if the downwind air pressure gradient is ignored. When this air pressure gradient is defined using the measured wind velocity profiles in the presence of saltation and the downwind air pressure gradient is taken into account, reasonable values for grain-borne stress and the force exerted on the wind by the saltation cloud are obtained. These results suggest that attention must be paid to the effects of downwind air pressure gradients when studying the wind-saltation interaction in a wind tunnel. Consideration of the downwind air pressure gradient, inertial forces, and other unidentified variables will provide a more thorough understanding of the interactions within a saltation boundary layer.     

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。     

A Wind Tunnel Investigation of the Shear Stress with A Blowing Sand Cloud

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     

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.]     

VERTICAL DISTRIBUTION OF GRAIN-SIZE PARAMETERS OF DRIFTING PARTICLES DURING SAND STORMS IN THE TAKLAMAKAN DESERT,CENTRAL ASIA

The vertical distributions of grain-size parameters of drifting sand flux during sand storms in the Taklamakan Desert, Central Asia, were investigated through field observation and laboratory wind-tunnel tests. The results show that grain-size parameters of the drifting sands near the ground surface are similar to those of the source deposits. Fine and very fine sands in the flux layer near the ground surface (from zero to 20 cm high) comprise more than 85% of the sample's weight, with a mean diameter of 0.09 mm. The distributions of transported particles are mainly negatively skewed, with good sorting. The creep population consists of poorly sorted coarser particles. In the saltation layer, particle size becomes finer as a power function of height from 4 to 14 cm. Sorting in the saltation layer is good and improves as a logarithmic function of height. The higher the transport layer, the more negatively skewed the particle distribution becomes as a logarithmic function of height. It becomes more leptokurtic as a power function of height in the transport layer from zero to 12 cm high. Grain-size parameters change irregularly at heights above 14 cm. Regional differences in the vertical distribution of grain-size compositions are affected by source deposits, wind speeds, and transport rates, but source has more influence on the sorting of the drifting material than wind velocity.     

Wind velocity and sand transport on a barchan dune

We present measurements of wind velocity and sand flux performed on the windward side of a large barchan dune in Jericoacoara, northeastern Brazil. From the measured profile, we calculate the air shear stress using an analytical approximation and treat the problem of flow separation by an heuristic model. We find that the results from this approach agree well with our field data. Moreover, using the calculated shear velocity, we predict the sand flux according to well-known equilibrium relations and with a phenomenological continuum saltation model that includes saturation transients and thus allows for nonequilibrium conditions. Based on the field data and theoretical predicted results, we indicate the principal differences between saturated and nonsaturated sand flux models. Finally, we show that the measured dune moves with invariant shape and predict its velocity from our data and calculations.     

Vertical profiles of aeolian sand mass flux

Vertical profiles of the horizontal mass flux of blown sand are investigated experimentally using a passive vertical array in a wind tunnel. Considering lower sampling efficiency of the sand trap in the near-bed region, this investigation is complemented by the measurements of the longitudinal profiles of mass flux made using a horizontal sand trap. The experiments were conducted with two test sands and five different stream velocities.In the upper part of the vertical profile, the measured data exhibit an exponential decay distribution with a positive deviation occurring in the near-bed region. The measured longitudinal profiles are similar to the measured vertical profiles. Linking both profiles and the modes of sand transport, it is possible that saltating sand grains give rise to the well-known exponential decay distribution of mass flux, and that creeping and reptating grains force a deviation from it. A simple equation applicable for both the vertical and the longitudinal sand mass flux variations is introduced and the parameters are estimated from experimental data.     

北冰洋浮冰站大气边界层结构的观测研究

利用2003年8月23日-9月3日我国第二次北极科学考察队在北冰洋浮冰站探测的50次大气廓线及相关资料,对北冰洋的大气边界层垂直结构进行了研究。结果显示,观测期间北冰洋(78°N附近,143°-148°W)浮冰区白天的对流边界层高度大于夜间的稳定边界层高度。大气边界层可分为稳定型、不稳定型和多层结构等几种类型。个例分析表明来自高空较强的暖湿气流与冰面近地层冷空气强烈相互作用,会形成强风切变和逆温、逆湿过程,有时100m高度内的风切变达10m/s,逆温达8℃。此种过程会导致北冰洋高纬度地区的大块海冰破裂,形成新的无冰海域,加强了海/冰/气的相互作用。该观测事实将有助于进一步提高对北冰洋高纬度边界层特征及其影响的认识。