稳态风沙流中沙粒体积浓度的模型分析

本文建立了一个简单有效的稳态风沙流中沙粒体积浓度的数学模型,包括3个部分:稳态风沙流的风速廓线描述、跃移沙粒轨迹的计算、床面沙粒起跳速度分布的描述。利用已知实验的风速廓线作为气流速度场输入参数,跃移沙粒轨迹的计算主要考虑重力和拖曳力,基于体积观点的床面起跳沙粒的水平速度和垂直速度概率密度分布函数分别采用正态分布和指数分布函数来描述。根据稳态风沙流中运动沙粒的动态平衡特征可推导计算沙粒体积浓度。计算的沙粒体积浓度与风洞实验结果基本一致,说明本模型有较好的预测能力。     

风沙跃移运动发展过程的离散动力学模拟

采用类似分子动力学的离散方法对二维风沙跃移过程运用高性能并行计算进行理论模拟。在本模拟模型中,考虑了沙粒与床面的碰撞、跃移沙粒与气流的相互作用等基本力学过程组成的复杂系统。通过并行运算技术使计算沙粒数达到72000的巨量计算得以实现。初步结果显示：自然跃移运动的基本特征如风沙流层内输沙率廓线可以较为成功的得以模拟。     

碰撞激起沙粒的起跳初速度分布函数

本文根据已有粒/床碰撞研究得出的基本定性结论以及组合论的基本原理,给出了风沙流中碰撞激起沙粒的起跳初速度分布函数,该函数服从瑞利分布的形式。以这一起跳初速度分布函数为基础建立风沙耦合跃移运动的基本模型,计算了单宽输沙率等风沙运动机理研究中普遍关心的物理量。将计算结果和已有实验结果对比分析表明,本文给出的起跳初速度分布函数是科学的,这对风沙流中沙粒起跳初速度分布函数给出了一个定性的认识。     

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。     

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。     

三维空间中沙粒旋转对其跃移轨迹的影响

风沙流中沙粒旋转通常具有明显的三维旋转特性。本文通过数值建模考察了三维空间中不同沙粒旋转状态所产生的Magnus力对其跃移运动轨迹特征的影响。在三维坐标系中,X、Y、Z轴分别表示水平、垂直和横向方向,气流沿X方向流动,沙粒起跳时位于XY平面内。结果表明,跃移沙粒仅绕Z轴旋转起跳时,初始起跳角速度的增大会导致在XY平面内的跃移高度和跃移距离减小而降落角增大,但Z方向没有运动发生。跃移沙粒仅绕X轴或Y轴旋转起跳时,绕X轴或Y轴的初始起跳角速度的变化对在XY平面内的跃移轨迹特征量没有显著影响,但沙粒偏离XY平面的偏离距离和偏离角的绝对值随起跳角速度绝对值的增加而增大。因此,沙粒起跳旋转状态对其跃移轨迹有重要的影响。     

非均匀沙的跃移计算

 为研究更加接近实际情况的风沙运动,并为实验研究提供一些理论分析和数值资料,本研究用数值计算的方法模拟了几种不同粒径分布方式相应的风沙跃移运动。分别计算了沙粒为单一粒径,以及粒径数学期望为该值时的伪随机分布和均匀分布共7种情况下的风沙跃移运动。计算结果表明,对于相同风速,气流对单一粒径组成的均匀沙的携带能力较弱,而对由多粒径组成的非均匀沙的携带能力较强。同时,因沙粒粒径组成的改变导致当跃移运动达到饱和时,风沙流饱和层的高度和沙粒的跃移长度、高度均有所变化。     

颗粒运动及其数理简析

根据大量的野外观测和沙(雪)颗粒运动动态摄影的资料,分析地球表面上气流带动的各种固体颗粒的主要运动形式——振动、滚动、滑移、跃移和悬移的物理图象中力的作用,给出其主要运动参数的表达式,建立各自简单的数学模型。特别对跃移运动还给出模拟轨迹方程。该模型与实际拍摄的轨迹相对比表明,较之以前风沙物理研究者模拟的跃移轨迹要好。文中首次提出的滑移概念,即振动、滚动和滑移三种颗粒运动形式代替以往分类的蠕移、跃移和悬移运动形式更准确和清晰。文中还明确回答了第一颗砂粒是怎样起跳的这一关键问题。     

风沙流中不同粒径组沙粒的 输沙量垂向分布实验研究

在非均匀沙床面上, 风沙流中不同粒径组沙粒的输沙量垂向分布, 是非均匀风沙运动研究的重点。研究首先通过风洞实验, 收集了风洞中垂线垂向输沙量分布沙样, 然后对集沙沙样进行了沙粒粒度分析实验, 实验分析结果得出了不同粒径组沙粒的输沙量垂向分布规律, 基于稳定平衡风沙跃移运动模型和本文实验结果, 最后数值模拟研究了不同粒径组沙粒输沙量垂向分布, 与沙粒起跳速度和角度之间的关系。本文研究结果得出, 在非均匀风沙流中, 粗粒径组沙粒垂向输沙量上部符合指数递减分布但近床面区偏离指数分布, 呈现为偏大型分布, 粗粒径组对应的沙粒起跳速度和角度分布均为指数函数; 细粒径组沙粒垂向输沙量在整 个高度上均符合指数递减规律, 细粒径组沙粒对应的起跳速度分布为指数函数, 起跳角度分布为高斯函数。沙粒的平均起跳速度, 在0.4u*~2.2u* 之间变化, 随着气流风速(u*) 和沙粒粒径的增加而减小。     

以计算流体动力学模型（CFD）模拟的戈壁地表风沙两相流运动特征

尘暴过程中沙砾质戈壁地表的风沙两相流运动,一直是风沙物理学关注的问题。但由于技术限制,野外观测及风洞实验难以捕捉大规模颗粒运动。为此,本文通过构建流体-颗粒碰撞互馈模型进行三维场域数值模拟,分析了不同砾石盖度下尘暴来流中的风沙两相流运动规律。结果表明：基于三维场流体-颗粒碰撞互馈模型修正后的求解结果能够较为准确地模拟戈壁地表风沙两相流运动过程。(1)不同来流尘暴过程,风相速度均会随砾石盖度的增大而减小,并在砾石盖度大于40%的戈壁近地表形成拜格诺焦点。相较于风相,沙相降速不明显。此外,距砾石床面4 cm以上,风沙两相速度基本保持一致。(2)当尘暴速度大于10 m·s^-1时,在距地1.5～2.1 m高度形成风沙两相流速度交点,交点上下表现为“风快沙慢”及“风慢沙快”现象。(3)尘暴过程中近地表沙尘浓度沿均值线呈“脉动”规律,表现为地表粉尘沉降与再释放循环过程,同时砾石盖度大小会直接影响沙尘沉积分布,当盖度大于40%时能够有效抑制沙尘释放。     

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     

新月形沙丘表面100cm高度内风沙流输沙量垂直分布函数分段拟合

为研究新月形沙丘表面不同层位风沙流输沙量的垂直分布函数,实测了塔克拉玛干沙漠腹地典型新月形沙丘表面100 cm高度内(以1 cm分隔)的输沙量。分段拟合分析表明:新月形沙丘迎风坡脚输沙量垂直分布规律不完全服从指数函数,出现与戈壁风沙流结构特征相似的"象鼻效应",在0~3 cm区间内输沙量逐渐增大,3cm以上输沙量随高度呈指数函数衰减;沙丘顶部0~10 cm区间输沙量随高度呈指数函数衰减,10 cm以上呈二次函数衰减;沙丘左翼端输沙量随高度呈幂函数分布,沙丘右翼端0~20 cm内以指数函数衰减,20 cm以上呈三次函数衰减;沙丘背风坡脚风沙流输沙量在0~60 cm和60 cm以上分别呈不同形式的三次函数分布。     

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。     

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.     

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.     

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.     

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.     

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。