基于湍流结构的风沙颗粒起动动态演化的概念模式

从驱动沙粒群起动的湍流结构入手,提出了基于气固两相流自调节机制的风沙颗粒起动的概念模式。受风沙边界层湍流相干结构的产生、消亡、湍流的随机脉动和运动沙粒群碰撞过程的动量损失等因素的控制和影响,风沙两相流始终处于不平衡—平衡—不平衡的动态演化之中,在此过程中,存在着普通的空气动力学起动、典型的冲击起动、湍流猝发起动等多种起动方式的循环交替的动态演化。上述多种的沙粒起动方式能够根据风沙边界层边壁区的湍流运动应力和沙粒群碰撞过程的动量损失来界定。该概念模式需要综合利用相位多普勒粒子动态分析测速技术、粒子图像测速技术和高速数字摄影技术等手段来进一步验证。该概念模式的提出,将为深入理解沙粒起动的力学机制提供新的思路。     

非均匀风沙起动规律研究

文章将非均匀沙水流起动研究方法中采用的暴露度的概念引入到天然风沙起动的研究中, 对暴露度的统计规律进行了分析。利用风沙颗粒起动时力矩平衡的条件, 建立了天然风沙起动公式, 用风洞试验资料进行了验证。计算结果表明, 在非均匀沙起动粒径与平均粒径相当的条件下, 混合沙的起动风速较小的结论, 与实际情况相符。说明在天然风沙起动研究中采用暴露度来反映风沙不均匀性对起动的影响是合理的。     

土壤风蚀过程颗粒释放机理研究

针对目前土壤风蚀中土壤颗粒释放过程机理研究存在的不足,建立了颗粒在土壤表面起动、滚动、起跳的土壤颗粒释放力学模型,同时考虑了太阳辐射及地表温度产生的向上垂向风速对颗粒起动风速的影响,并采用前人的实验结果与本文模型数值计算结果进行了对比分析,证明其模型的合理性。数值结果表明：起动风速随颗粒粒径的增大先减小后增大,其中在0.12~0.14 mm粒径范围内颗粒最容易起动,同时床面温度越高对颗粒的起动风速影响也越明显,同一粒径下床面温度越高起动风速越小。颗粒在床面上的运动不是纯滚动,而是滚动中有滑动。颗粒起跳速度主要分布在0.3~0.65 m/s之间;起跳角度30°~35°之间;颗粒起跳的角速度主要分布在600~1 200 rev/s。土壤颗粒释放微观过程的研究将对改进土壤风蚀输送过程中的宏观监测与预报起到重要作用。     

几种常用风沙颗粒测速方法对比

沙粒运动速度是风沙颗粒运动学和风沙流（一种特殊的气固两相流）研究中的一个重要参数,受到风沙物理学研究者们的长期关注。风沙颗粒运动速度研究遇到的关键问题是测量方面的困难,目前已经尝试应用了多种测量技术。对几种常用测试手段——光电管测速计、高速摄影、粒子动态分析仪（PDA）和粒子图像测速仪（PIV）的测量结果进行了对比分析。光电管测速计、高速摄影法、以及粒子动态分析仪在研究颗粒速度的分布方面比较有用,而粒子图像测速仪则是一种全场测速技术,是测量特定区域内平均速度场时空变化的有用工具。上述几种测试方法所得的结果既有相似之处,但也存在较大差别。根据这些测速手段所得到的沙粒速度的概率分布特征是相似的,但测量结果所反映的沙粒平均速度与风速、粒径以及其他可能影响因子间的关系存在明显差异。目前的测量技术均不能直接测量贴近层或沙床表面的沙粒速度,而这些数据在研究地表颗粒的起动机制时是相当重要的。虽然用不同的测量技术所得出的沙粒平均速度随高度的变化关系均为幂函数,但这些幂函数的估算结果差别很大。基于对比分析,我们认为,各种测速技术均有其独特的测量原理,各有其优劣,但对其测量结果的可靠性尚无定论。所以,为了充分运用不同的测速技术,需要做大量的工作对其进行验证标定,研发一种可靠的技术来测量风沙流中的沙粒速度仍然是目前风沙物理研究的重要任务。     

沙尘暴过程中5～7000m高度大气电场及其对颗粒带电量影响

利用GPS探空仪和大气电势测量设备,在塔克拉玛干沙漠腹地测量了一次沙尘天气过程中大气电势的梯度分布,基于电场变化曲线分析了空中沙尘颗粒电荷极性及其数目浓度的变化规律,给出了一种新的沙尘颗粒起电机制模型,讨论了环境电场强度对其带电量的影响。结果表明:沙尘天气过程中大气电势强度先增加后减小,在某一高度改变方向,但其强度再次呈现先增加后减小的变化规律。随着环境风沙电场强度的增加,颗粒带电量指数增加,当电场强度超过10 kV·m~(-1)时其对颗粒电荷量的贡献就可能与离子交换机制的贡献相当。研究结果有助于更为准确地认识沙尘天气过程中大气电场分布规律及其对风沙电现象的影响,同时也有助于设计基于风沙电场测量来获得空中颗粒浓度、带电信息的新方法。     

沙面温度对风蚀动力过程的影响

大量关于土壤可蚀性的研究证明,低温的沙面更易被风蚀。但大多研究以中低纬度低海拔地区为背景,建立的模型并不能完全适用于高海拔寒冷地区。我们使用青藏高原错那湖湖岸的地表沙,利用高低温实验箱调节沙面温度,开展了风洞模拟实验,量化了不同粒径、风速条件下风蚀率随沙面温度的变化关系,旨在分析地表温度对风蚀动力过程的影响及作用机制。结果表明：（1）沙面温度与风蚀率之间呈负线性相关,低温沙面更易被风蚀。这与空气温度影响风蚀过程的作用机理类似,即低温沙面可以增加近地表空气密度和气流拖曳力,使沙粒更容易起动,风蚀量也随之增大;另外,沙面与空气间的温差,会破坏大气稳定度,影响湍流强度,进而影响风蚀强度,这可能也是沙面温度影响风蚀过程的一种主要机制。（2）在自然情况下,寒冷沙面对水汽具有一定的冷凝效应,从而对沙粒的起动与风蚀过程起到抑制作用,而且这种冷凝效应,在低风速、粗颗粒沙面上表现得更为明显。（3）高温沙面可以在一定程度上抑制风蚀。作为影响高寒地区风力侵蚀过程的重要自然因素,沙面温度对青藏高原典型风沙区的风蚀过程起到了重要的控制作用。     

气流中跃移颗粒的受力分析

气流中颗粒跃移过程中受力分析及其运动机理的研究在发展风沙物理学理论方面具有重要的意义,长期以来,受实验条件和实验手段的限制,关于此方面的认识还不统一,采用高速频闪摄影技术,在计算跃移颗粒各运动参数的基础上,反推出跃移颗粒的阻力系数和升力系数,并将颗粒所受的阻力,升力和重力进行了比较,结果表明,阻力最重要,重力次之,升力也不可忽略,它占重力的２０％以上,利用数值计算方法,得出了颗粒的跃移运动轨迹,与     

稀性坡面泥石流粗颗粒在不同水力作用下的起动

"5.12"汶川地震产生了大量崩滑堆积体,这些棱角状粗颗粒体在水流激发下易起动形成坡面泥石流。在对汶川地震区坡面泥石流地质模型调研基础上,建立以立方体为稀性坡面泥石流粗颗粒的基本模型,结合坡面泥石流的宽级配特性,提出立方体模型的滑移、翻转(包括绕边翻转、绕支点翻转及嵌固绕边翻转)等起动方式。并以水力学、泥沙动力学为理论基础,推导得出粗颗粒在滑移、翻转起动方式下的起动平均流速公式。参照陈伯奇的实验数据,计算粒径为5~10 mm颗粒在不同坡度条件下的起动流速,并对比显示两者具有较好的吻合性,表明立方体作为稀性坡面泥石流粗颗粒的起动模型是合理的。计算结果也表明,在同一坡度条件下,粗颗粒滑移起动所需的水流激发流速最低,绕边起动次之,颗粒嵌固起动所需的激发流速最大,如坡度为30°条件下,滑移起动的水流激发流速为0.349 m/s,绕边起动为0.430 m/s,嵌固起动为0.473 m/s。据此推断,当水流达到一定流速(如30°坡度下、流速为0.349 m/s)时,粗颗粒的最初起动以滑移为主;随着流速增大,逐渐出现翻转起动;当水流流速急剧增大时,大部分嵌固粗颗粒会以翻转的方式瞬间起动,从而形成稀性坡面泥石流。成果对稀性坡面泥石流起动条件研究具有一定的理论意义,对泥石流灾害预警具有一定的指导作用。     

风沙物理学中两个焦点问题研究现状与未来研究思路刍议

风沙物理学经过数十年的发展与完善,可以说基本上自成体系。但由于学科的起步较晚,加之现象的复杂性,目前仍有很多重要的理论问题没能彻底解决,至少还未达成共识,成为学术界长期以来争论的焦点,从而影响了学科的继续发展。对“沙粒起动机制”和“风成沙纹的形成机理”两个焦点问题在进行研究现状剖析的基础上,浅析未来研究思路。提出沙粒起动主要驱动力是驻点升力的观点,以空气动力学理论为基础的沙粒起动机制研究将是未来该领域的发展趋势;关于沙纹的形成,认为沙粒粒径的不均匀性以及所导致的非平衡运动是前提和基础,风沙流运动过程中类似左右沙丘运动的蚀积转换规律是使沙纹能够维持其稳定形态的必要条件,而沙纹有规律、整齐划一的分布与排列可能与背风面气流小尺度的涡流有关。     

风沙运动的动态摄影实验

风力作用下，地表沙粒运动，特别是沙粒的跃移运动是造成各种风沙危害的重要原因，而风沙运动又是从固体颗粒—气体，这种二相流的一个重要的研究分支。因此，在国内外曾引起不同学科的众多学者的极大关注，进行了大量的室内外实验研究和理论上的探讨。尽管如此，风沙运动过程在许多方面至今还没有搞清楚。然而，要想深入了解     

沙粒起动风速的影响因素研究

通过对风场中沙床表面沙粒的受力分析,建立了坡面沙粒起动风速的理论公式,并将计算结果与已有实验研究结果进行了比较,结果表明,给出的理论计算公式是正确的。在此基础上,详细讨论了坡角、风向角、荷质比、电场力以及空气上升力对坡面沙粒起动风速的影响。其结果是：坡角一定时,沙粒的起动风速随风向角的增大而增大;风向一定时,背风坡沙粒的起动风速随坡角的增大而变小;弱电场中电场力对沙粒起动风速几乎没有影响,强电场中随着沙粒荷质比的增大,起动风速明显减小;空气上升力明显使沙粒起动风速减小。     

风沙流中颗粒跃移研究的某些进展与问题

风沙流研究是风沙物理学的主要内容,研究方法上大致分宏观和微观研究两个方面。宏观研究着眼于风沙流的整体结构和动力学过程,微观研究侧重于单一颗粒的运动状态。宏观和微观研究相结合是解决风沙物理学中一系列问题的一条合理路线。但长期以来,由于在颗粒运动机理研究方面进展缓慢且不成熟,对联系宏观和微观研究的纽带——起跳粒子的运动状态分布的重要性认识不够,造成风沙流宏观研究和微观研究的严重脱节。本文综述国内外在这两方面的研究历史和现状,指出了现存问题和解决途径,并对风沙物理学的发展方向提出了一些建议。     

沙土含水率对起沙临界风速影响

根据重力、凝聚力与拖曳力的力矩平衡关系，推导出起沙临界风速与粒径和沙土含水率的关系，并分析了含水率对起沙临界风速的定量作用     

A parameterisation for the threshold shear velocity to initiate deflation of dry and wet sediment

A new parameterisation for the threshold shear velocity to initiate deflation of dry and wet particles is presented. It is based on the balance of moments acting on particles at the instant of particle motion. The model hence includes a term for the aerodynamic forces, including the drag force, the lift force and the aerodynamic-moment force, and a term for the interparticle forces. The effect of gravitation is incorporated in both terms. Rather than using an implicit function for the effect of the aerodynamic forces as reported earlier in literature, a constant aerodynamic coefficient was introduced. From consideration of the van der Waals force between two particles, it was further shown that the effect of the interparticle cohesion force between two dry particles on the deflation threshold should be inversely proportional to the particle diameter squared. The interparticle force was further extended to include wet bonding forces. The latter were considered as the sum of capillary forces and adhesive forces. A model that expresses the capillary force as a function of particle diameter squared and the inverse of capillary potential was deduced from consideration of the well-known model of Fisher and the Young–Laplace equation. The adhesive force was assumed to be equal to tensile strength, and a function which is proportional to particle diameter squared and the inverse of the potential due to adhesive forces was derived. By combining the capillary-force model and the adhesive force model, the interparticle force due to wet bonding was simplified and written as a function of particle diameter squared and the inverse of matric potential. The latter was loglinearly related to the gravimetric moisture content, a relationship that is valid in the low-moisture content range that is important in the light of deflation of sediment by wind. By introducing a correction to force the relationship to converge to zero moisture content at oven dryness, the matric potential–moisture content relationship contained only one unknown model parameter, viz. moisture content at −1.5 MPa. Working out the model led to a rather simple parameterisation containing only three coefficients. Two parameters were incorporated in the term that applies to dry sediment and were determined by using experimental data as reported by Iversen and White [Sedimentology 29 (1982) 111]. The third parameter for the wet-sediment part of the model was determined from wind-tunnel experiments on prewetted sand and sandy loam aggregates. The model was validated using data from wind-tunnel experiments on the same but dry sediment, and on data obtained from simulations with the model of Chepil [Soil Sci. Soc. Am. Proc. 20 (1956) 288]. The experiments showed that soil aggregates should be treated as individual particles with a density equal to their bulk density. Furthermore, it was shown that the surface had to dry to a moisture content of about 75% of the moisture content at −1.5 MPa before deflation became sustained. The threshold shear velocities simulated with our model were found to be in good agreement with own observations and with simulations using Chepil's model.     

Threshold wind velocity in the lee of two-dimensional transverse dunes

Secondary flow plays an important role in the development of dunes. To investigate this phenomenon, we simulated models of transverse dunes with different stoss slopes in a wind tunnel. We measured the threshold wind velocity at different locations in the lee of the dunes, and observed the corresponding particle movement characteristics under threshold conditions to assess the sedi-mentological significance of secondary airflow. Threshold wind velocity changed with both horizontal distance and stoss slope. Its variation with horizontal distance is characterized by a region in which a rapid reduction occurs, a region with a minimum value, and a region with high value. We confirmed the presence of reversal cells, irrespective of the stoss slope. Neither the variation in threshold wind velocity nor the variation in reattachment distance as a function of stoss slope followed a monotonic function. The 15° stoss slope had the greatest threshold wind velocity and reattachment distance.     

新月形沙丘与线形沙丘共生现象探讨——以撒哈拉沙漠为例

沙丘共生问题是风沙地貌研究的热点,目前尚缺乏较为深入的探讨。以共生沙丘分布最为广泛的撒哈拉沙漠为例,基于Google Earth高清卫星影像对新月形沙丘和简单线形沙丘共生区域进行参数识别,并对其区域风况特征和形成发育环境进行分析,以探讨共生沙丘形成的可能原因。结果表明：撒哈拉沙漠共有17处典型的共生沙丘区域,是世界上最大的共生沙丘分布区。区域及单个共生沙丘形态参数之间均存在一定相关性,但并不具有理论上应具有的良好相关性,表明影响沙丘共生的因素较为复杂。共生沙丘在低、中、高风能环境以及不同风向变率下均可发育,但主要在低风能环境、中等风向变率,锐双峰风况下形成。单一的起沙风风向是共生沙丘形成的重要条件,合成输沙势方向与沙丘走向基本一致。除风况外,障碍物如山脉、河流等因素同样对沙丘共生发挥了重要作用。     

Effect of averaging time on the apparent threshold for aeolian transport

A fundamental feature of any wind-eroding surface is its threshold — the lowest wind speed (or friction velocity) at which soil movement is initiated. Many theoretical equations and numerical models of the saltation process include threshold as an important basic parameter. The question addressed here is whether or not the averaging time of a wind speed measurement affects the observed wind speed threshold. Using wind speed data taken at 1 Hz and simultaneously measuring saltation activity with a piezoelectric saltation sensor it was possible to calculate threshold. Threshold was then recalculated using the same data set averaged over periods of 2, 5, 10, 20, 30, and 60 s. The results reveal that, under typical field conditions with gusty turbulent winds, long averaging times produce an apparent threshold that is considerably lower than the true wind speed at which saltation is initiated. This result suggests that high-frequency sampling of wind speed and saltation activity is critical to the accurate determination of the true threshold of a wind-eroding surface.     

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     

若干风蚀粉尘释放模型述评

粉尘气溶胶对大气、陆地和海洋多方面的影响促进了粉尘释放的模拟研究。Gillette和美国FPA的粉尘释放模型相对地比较简单，主要考虑了摩阻速度和土壤质地对粉尘释放的影响，没有考虑粉尘释放过程的微观机制；邵亚平和Alfaro等人的DPM粉尘释放模型都通过土壤粒度分布、粗糙度和起动摩阻速度等参数表达了地表特征对粉尘释放的影响，且都描述了粉尘释放的微观机制。20多年来粉尘释放的模拟研究取得了重要进展，主要表现在通过室内和野外实验证实了粉尘释放的主要机制为跃移颗粒的冲击，并分别从粉尘的结合能与跃移颗粒动能及跃移颗粒产生的弹坑体积角度来描述这个微观机制。然而所有的模拟工作只是从某一方面反映了粉尘释放过程，对粉尘释放机制还没有完全了解清楚，特别的是目前关于粉尘结合能研究还有很大的不确定性，这也是粉尘释放模型需要改进的地方。