农田近地表风沙流风程效应变化特征研究

风沙流的风程效应研究是定量获取风沙流沿程变化的核心和难点,风程效应是指输沙率随沙床表面或地块长度的增加而不断增大,而后趋于稳定的变化特征,饱和输沙率（f_max）和饱和路径长度（L_sat）是风程效应的重要参数。采用自动连续称重式集沙仪,以河北坝上地区康保县境内典型旱作农田为研究对象,观测了2017、2018年和2021年内4次典型风蚀事件,分析近地表5 cm高度风沙流的风程效应在5 min时间尺度下的变化特征。结果表明：（1） 近地表输沙通量随风程距离的增大而增大。（2） 4次风蚀事件中L_sat的变化范围在11~280 m之间,并存在明显差异,其变化与风速无关。（3） 近地表风沙流的f_max与风速（U）呈幂函数关系。（4） 风程效应的变化特征与地表可蚀性因子、地表微地貌变化有着紧密联系,未来应对不同的土壤类型和质地农田的风程效应进行深入研究。     

Aeolian sand transport along beaches

Storm‐generated ephemeral transverse sand ridges were observed developing along the beach fronting Sir Richard Peninsula, South Australia during 25 March 1984. The ridges displayed a mean height of 10 cm and a wavelength of 12 m; their breadth was approximately 2 m, and length varied with beach width but ranged up to 40 m over 10 km of coastline. The steeper sides of the ridges faced upwind due to erosion after initial ridge development. Damp sand in the swales inhibited wind scour and restricted sand supply, but provided a firm substrate over which the sand could saltate. Approximately 5000 m³ of sand were incorporated into the ridges by westerly winds blowing at velocities between 45 and 69 km/hr. This observation emphasises the role of alongshore winds in transporting beach sediments and developing essentially ephemeral forms, which might, nevertheless, be preserved in the geological record. The significance of these forms varies with coastal orientation and local wind regimes.     

On the rate of aeolian sand transport

A simple explicit formula for the transport rate of windblown sand is derived based on physical reasoning. The curve relating the dimensionless transport rate to the dimensionless friction speed exhibits the empirically well-established peak. In fact, the theory developed in the paper can explain the peaked shape of this curve. Three empirically determined coefficients in the formula can be given a physical interpretation. The formula is shown to fit transport rates measured in wind-tunnel experiments well, except for very coarse sand. Formulae for the wind profile in the saltation layer and for the grain dislodgement rate in dependence on the friction speed are obtained as part of the theory.     

稳态风沙流中瞬态输沙特征

风沙流中沙粒运动在来流风速不变时也会表现出非稳态特征。在风洞内利用粒子图像测速系统（PIV）测量了风沙运动的时间序列,并基于PIV测量技术提出风沙流中沙粒平均直径、数密度、平均水平速度和输沙通量等参数在某一时刻的计算方法,其中输沙通量的计算考虑沙粒大小垂向分布的影响。结果表明：来流风速不变时,沙粒平均直径、数密度、平均水平速度和输沙通量随时间具有明显的波动性;沙粒平均直径和平均水平速度的标准偏差一般随高度增加而增加,沙粒数密度和输沙通量标准偏差随高度增大而减小;这些参数的相对标准偏差均随高度增加而增大。     

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.     

The harmonious character in equilibrium aeolian transport on mixed sand bed

Wind tunnel experiments were carried out to measure the spatial distribution in equilibrium transport of four types of mixed sand. The flux profiles of each grain size group were calculated. It is found that the vertical distribution of mean grain size has a close relation with the grain size distribution in the sand bed. In a log-linear plot, the flux profiles of main grain size groups are all straight lines and their slopes are nearly equal. It is also found that the ratio of transport rate of each size group to the whole transport rate is directly proportional to the mass ratio of each size group in the sand bed and the proportion value is only dependent on the grain size. This harmonious law is applicable to all four types of mixed sand used in the experiment.     

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.     

An aeolian transport model for flat shifting sand fields under dynamic-limiting conditions

Estimating aeolian transport rates in the field is a difficult task even though various transport equations have been proposed. In this study, we used 5 years of field data on aeolian transport from an experimental plot for aeolian research located in the southeastern Tengger Desert to develop a practical aeolian transport equation capable of estimating transport rates in flat shifting sand fields. The resulting equation relates transport to the average wind velocity during a given period through a modified power function. The transport rate increases linearly with the cube of the average wind velocity. The equation can be generalized rate to a form that contains a proportionality coefficient and a threshold average wind velocity. The proposed model has an advantage over previous models in that it is based on a much larger sample size and corrected to account for variability in wind direction, sampling efficiency, and the height of the wind measurements.     

The blown sand flux over a sandy surface: a wind tunnel investigation on the fetch effect

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−U_t/U)²U³(ρ/g), where Q is the total sand transport rate, U and U_t 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.     

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.     

Modeling the aeolian sand transport for the desert of Kuwait: Constraints by field observations

The Kuwaiti desert is swept by strong winds, particularly by northwesterly wind, during the summer months (April through September), causing mass-scale sand transport that poses a serious environmental hazard. Understanding that accurate transport rate data are very difficult to obtain either by field measurements or by using any existing sand transport formulae, this paper presents a mathematical model to predict the sand-transport rate for Kuwaiti desert by following a well-known theoretical approach of Lettau and Lettau (1978) and by taking into account the effect of seasonal wind variability. The validity of the model is next examined using the field data from the study of Al-Awadhi and Al-Sudairawi (2002). The examination showed that the model underestimates the rate of sand transport by an average ratio of 0.15. The application of the model, at some seven locations that are evenly distributed across the Kuwaiti desert, indicated that about 40% of the amount of sand transport in Kuwait prevails from the NW during the peak summer months (June and July). The resulting map of sand transport rate indicated that the zone of higher sand transport rate is located within the areas covered by most extensive mobile sand sheets and sand dunes.     

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.     

新疆S214省道防沙体系对近地表风沙流的影响

对新疆S214省道台特玛湖干涸湖盆段防沙体系内外的风沙流输沙和风速进行了同步观测,数据分析表明：观测时防沙体系中阻固沙带已拦截了大量风沙,虽近地表风速被削弱程度不大,但风沙流输沙的43.26%仍可被防沙体系所拦截和固定,而剩余部分则可借助路侧输沙带的较大风力输移到公路下风侧,且不产生路面沙害,表明阻-固-输结合型防沙体系非常适宜单风向强风沙环境。S214省道防沙实践可为其他强风沙环境公路防沙提供重要借鉴经验。     

Evaluation of saltation flux impact responders (Safires) for measuring instantaneous aeolian sand transport intensity

The measurement of aeolian sand transport rates on small scales is of interest to the development and testing of detailed models of sand movement by wind. This paper reports on laboratory evaluations and preliminary field tests of a new design of a piëzo-electric impact responder, called a ‘Safire’, capable of measuring saltation impacts at a frequency of 20 Hz. The advantages of the Safire are: (1) that it provides high-frequency measurements, (2) that it presents a minimal obstruction to the wind flow (no scour observed in the field), and (3) that it is of a (relatively) low-cost.Laboratory calibrations were performed with a vertical gravity flume generating known sand grain fluxes using both mixed sand and specific size fractions. Initial tests investigated three fundamental characteristics: correspondence between digital and analogue signals generated by the instrument, directional response of the probe, and linearity of instrument response to mass flux.Instrument calibration included determination of the momentum threshold required for the sensor to register a grain impact. Based on this lower limit and the known distribution of grain size and speed at different fall heights, a prediction is made as to the sand grain flux the Safire ought to measure, which is then compared with the signal response. The result of this comparison is an assessment of the instrument's efficiency in counting saltating grains. These Safires were also deployed in the field as part of a larger investigation of spatio-temporal transport variability. This experiment provided the opportunity to compare the instrument's performance with traditional sand traps, and this paper develops methods and assumptions required to convert measurements from impact responders to traditional mass transport rates.The evaluations indicate that improvements to the instrument production process are required to ensure a standard momentum threshold among individual instruments. Furthermore, the sensor design needs to be reconsidered in order to eliminate the variation in response depending on azimuth direction, so that the sensor is uniformly omni-directional.     

Sampling efficiency of vertical array aeolian sand traps

Previous investigations have indicated that the sampling efficiency of aeolian sand traps is low and varies greatly in the near-bed region. Outside this region, the efficiency tends to be consistently higher for all types of trap. An evaluation was carried out to compare the sampling efficiency of different types of aeolian sand trap based on the comparison of the “actual” and the measured sand mass flux profiles, with emphasis on the single-tube vertical array trap, conventional array trap, and step-like array trap. A simple formula is proposed to express the actual vertical profile of sand mass flux, which has been validated with the unique data obtained with an isokinetic trap by [Sedimentology 45 (1998) 789]. Using the experimental data collected by the present authors and those by other investigators, sampling efficiencies of three types of trap are examined in terms of the frequency distribution of all the samples. For the single-tube traps, the sampling efficiency varies from 65% to 95%, with a mode at 75%. For both the conventional array and step-like array traps, sampling efficiencies range from 15% to 85%, with the modal frequencies at 35% and 75%, respectively. This review seems to suggest that the peak frequency with higher sampling efficiency coincides with the maximum sand-grain Reynolds number.     

直压立式纱网沙障对近地表输沙量及风速的影响

评价直压立式纱网沙障对流动沙地的固沙效果是其应用的前提。采用阶梯式集沙仪和HOBO风速仪野外测定了纱网沙障的输沙量及风速变化。结果表明:与对照相比,设置纱网沙障后,0~10 cm或0~20 cm高度范围内,带宽2、3、4、5 m带状沙障输沙量分别降低91.4%或89.7%、97.0%或96.4%、98.1%或96.7%、85.9%或84.2%;而2 m×2 m、3 m×3 m、 4 m×4 m、5 m×5 m网格沙障输沙量分别降低92.1%或91.2%、70.4%或65.6%、65.3%或65.2%、39.9%或30.0%。与对照相比,10、30、50 cm高度,带宽2、3、4 m带状沙障平均降低风速分别为26.7%、17.3%、11.8%,2 m×2 m、3 m×3 m、4 m×4 m、5 m×5 m网格沙障平均降低风速分别为55.3%、30.4%、23.0%。综合考虑,设置带宽3~4 m带状或网格纱网沙障能够满足工程固沙减少风沙危害的需求。     

海岸风沙流中不同粒径组沙粒的垂向分布模式

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。     

Numerical simulation of aeolian dusty sand transport in a marginal desert region at the early entrainment stage

Aeolian dusty sand transport in the marginal region of a desert is described numerically from first suspension motion at the early entrainment stage to the unsteady state within a moderate range (1000 m long × 500 m high). A two-dimensional model is built for wind-blown dust flow, and the calculations are carried out using Fluent software. The simulation results describe an integrated picture of aeolian dusty sand transport including uplift, suspension, diffusion, deposition and its space–time concentration. According to the features of a sand–dust storm near the surface, a volume concentration expression of ejection grains is developed as a boundary condition in the simulation approach. The model is verified by comparing results with both experimental data from a wind tunnel and an analytic solution. Uniform dust sizes and R–R distributions are used in the simulation. Gas–solid two-phase flow patterns with these grains are obtained in the downwind space, including the turbulence intensity, gas phase stream functions and solid volume concentration distributions. The influence of wind velocities and grain sizes is analyzed. From the simulation results, spatial distributions of dust volume concentration in the early entrainment stage are described clearly. Different from coarse sands, there is a clear band of uniformly saturated dust concentration in the region directly above the surface.     

A wind tunnel investigation on the transverse motion of aeolian sand

A wind tunnel experiment was performed to investigate aeolian grain motions in the transverse direction, which is perpendicular to the incoming flow and parallels the sand bed. The trajectories in the horizontal plane were recorded by high-speed camera. Statistical analysis of 630 trajectories shows that both the motion orientation and the time-averaged speed follow Gaussian distributions. An exclusive method was used to analyze the driving mechanism. It was concluded that the three-dimensional turbulent air flow, rather than the spin of grain or grain–bed collisions, controls the transverse motion.     

Wind tunnel and field calibration of five aeolian sand traps

The efficiency of five aeolian sand samplers was tested via wind tunnel experiments and field measurements. The samplers were: the Big Spring Number Eight (BSNE) sampler, the Modified Wilson and Cooke (MWAC) sampler, the Suspended Sediment Trap (SUSTRA), the Pollet catcher (POLCA), and the saltiphone. In the wind tunnel, the samplers were calibrated against an isokinetic sampler (a modified Sartorius SM 16711 sampler with adjustable flow rate), and this for three sand types (median diameter: 132, 194 and 287 μm) and five wind speeds (ranging from 6.6 to 14.4 m s⁻¹). In the field, seven calibration tests of two weeks each were conducted. The absolute efficiencies of the BSNE, MWAC and POLCA are more or less comparable and vary between 70% and 120%, depending on sediment size and wind speed. For the SUSTRA, the efficiency is somewhat lower for fine sands and for wind speeds above 11 m s⁻¹. Finally, the saltiphone can accurately detect the periods of saltation transport, but in its current version, the instrument is not accurate when measuring the absolute saltation flux. The most recommendable sampler in the test is the MWAC, not only because of its high efficiency, but also because its efficiency is independent of wind speed.