An improved particle tracking velocimetry(PTV) technique to evaluate the velocity field of saltating particles

Velocity is a key parameter characterizing the movement of saltating particles. High-speed photography is an efficient method to record the velocity. But, manually determining the relevant information from these photographs is quite laborious. However, particle tracking velocimetry(PTV) can be used to measure the instantaneous velocity in fluids using tracer particles. The tracer particles have three basic features in fluids: similar movement patterns within a small region, a uniform particle distribution, and high particle density. Unfortunately, the saltation of sand particles in air is a stochastic process, and PTV has not yet been able to accurately determine the velocity field in a cloud of blowing sand. The aim of the present study was to develop an improved PTV technique to measure the downwind(horizontal) and vertical velocities of saltating sand. To demonstrate the feasibility of this new technique, we used it to investigate two-dimensional saltation of particles above a loose sand surface in a wind tunnel. We analyzed the properties of the saltating particles, including the probability distribution of particle velocity, variations in the mean velocity as a function of height, and particle turbulence. By automating much of the analysis, the improved PTV method can satisfy the requirement for a large sample size and can measure the velocity field of blowing sand more accurately than previously-used techniques. The results shed new light on the complicated mechanisms involved in sand saltation.     

Separating emitted dust from the total suspension in airflow based on the characteristics of PM10 vertical concentration profiles on a Gobi surface in northwestern China

During aeolian processes, the two most critical factors related to dust emissions are soil particle and aggregate saltation, which greatly affect the vertical profiles of near-surface dust concentrations. In this study, we measured PM10 concentrations at four different heights (0.10, 0.50, 1.00 and 2.00 m) with and without continuous and simultaneous aeolian saltation processes on a Gobi surface in northwestern China from 31 March to 10 April, 2017. We found that the vertical concentration profiles of suspended PM10 matched the log-law model well when there was no aeolian saltation. For the erosion process with saltation, we divided the vertical concentration profiles of PM10 into the saltation-affected layer and the airflow-transport layer according to two different dust sources (i.e., locally emitted PM10 and upwind transported PM10). The transition height between the saltation-affected layer and the airflow-transport layer was not fixed and varied with saltation intensity. From this new perspective, we calculated the airflow-transport layer and the dust emission rate at different times during a wind erosion event occurred on 5 April 2017. We found that dust emissions during wind erosion are primarily controlled by saltation intensity, contributing little to PM10 concentrations above the ground surface compared to PM10 concentrations transported from upwind directions. As erosion progresses, the surface supply of erodible grains is the most crucial factor for saltation intensity. When there was a sufficient amount of erodible grains, there was a significant correlation among the friction velocity, saltation intensity and dust emission rate. However, when supply is limited by factors such as surface renewal or an increase in soil moisture, the friction velocity will not necessarily correlate with the other two factors. Therefore, for the Gobi surface, compared to limiting dust emissions from upwind directions, restricting the transport of suspended dust in its path is by far a more efficient and realistic option for small areas that are often exposed to dust storms. This study provides some theoretical basis for correctly estimating PM10 concentrations in the Gobi areas.     

戈壁、流沙地表风沙流特性研究

通过对戈壁、流沙地表风沙流特性的风洞模拟实验研究,风沙流中的风速廓线分布满足幂函数的形式,其幂指数在0.20左右。对于戈壁地表,在不同风速下,相同高度层含沙量具有很大的相关性,风沙活动层主要集中在距地表20cm范围内;由于沙粒与戈壁地表的砾石发生碰撞,风沙流不再服从对数关系递减,其极值出现的高度随风速的增加而上移,呈现"象鼻效应"。     

Fetch effect on the developmental process of aeolian sand transport in a wind tunnel

As the sand mass flux increases from zero at the leading edge of a saltating surface to the equilibrium mass flux at the critical fetch length, the wind flow is modified and then the relative contribution of aerodynamic and bombardment entrainment is changed. In the end the velocity, trajectory and mass flux profile will vary simultaneously. But how the transportation of different sand size groups varies with fetch distance is still unclear. Wind tunnel experiments were conducted to investigate the fetch effect on mass flux and its distribution with height of the total sand and each size group in transportation. The mass flux was measured at six fetch length locations(0.5, 1.2, 1.9, 2.6, 3.4 and 4.1 m) and at three free-stream wind velocities(8.8, 12.2 and 14.5 m/s). The results reveal that the total mass flux and the mass flux of each size group with height can be expressed by q=aexp(–bh), where q is the sand mass flux at height h, and a and b are regression coefficients. The coefficient b represents the relative decay rate. Both the relative decay rates of total mass flux and each size group are independent of fetch length after a quick decay over a short fetch. This is much shorter than that of mass flux. The equilibrium of the relative decay rate cannot be regarded as an equilibrium mass flux profile for aeolian sand transport. The mass fluxes of 176.0, 209.3 and 148.0 μm size groups increase more quickly than that of other size groups, which indicates strong size-selection of grains exists along the fetch length. The maximal size group in mass flux(176.0 μm) is smaller than the maximal size group of the bed grains(209.3 μm). The relative contribution of each size group to the total mass flux is not monotonically decreasing with grain size due to the lift-off of some small grains being reduced due to the protection by large grains. The results indicate that there are complex interactions among different size groups in the developmental process of aeolian sand transport and more attention should be focused on the fetch effect because it has different influences on the total mass flux, the mass flux profile and its relative decay rate.     

Contribution of underlying terrain to sand dunes: evidence from the Qaidam Basin,Northwest China

Underlying terrain strongly influences dune formation. However, the impacts of underlying terrain on the dune formation are poorly studied. In the present research, we focused on dunes that formed in the alluvial fans and dry salt flats in the Qaidam Basin, Northwest China. We quantified the dunes' sediment characteristics on different types of underlying terrain and the terrain's effects on the surface quartz grains by analyzing grain-size distribution, soluble salt contents and grain surface micro-textures. Results showed that barchan dunes were dominated by medium sands with a unimodal frequency distribution, whose peak corresponded to the saltation load. Linear dunes were mainly composed of fine sands with a bimodal frequency distribution, whose main peak represented the saltation load, and whose secondary peak represented the modified saltation or suspension load. Sand was transported from source area by running water (inland rivers) over short distances and by wind over relatively longer distances. Thus, quartz grains had poor roundness and were dominated by sub-angular and angular shapes. Surface micro-textures indicated that dune sands were successively transported by exogenic agents (glaciation, fluviation and wind). Soluble salt contents were low in dunes that developed in the alluvial fans, which represented a low-energy chemical environment, so the grain surface micro-textures mainly resulted from mechanical erosion, with weak micro-textures formed by SiO₂ solution and precipitation. However, soluble salt contents were much higher in dunes that developed in the dry salt flats, which indicated a high-energy chemical environment. Therefore, in addition to micro-structures caused by mechanical erosion, micro-textures formed by SiO₂ solution and precipitation also well developed. Our results improve understanding of the sediment characteristics of dune sands and the effects of underlying terrain on dune development in the Qaidam Basin, China.     

Effects of gravel mulch on aeolian transport:a field wind tunnel simulation

The shape,size and coverage of gravels have significant impacts on aeolian sand transport.This study provided an understanding of aeolian transport over the gravel mulching surfaces at different wind velocities by means of a mobile wind tunnel simulation.The tested gravel coverage increased from 5% to 80%,with a progressive increment of 5%.The gravels used in the experiments have three sizes in diameter.Wind velocities were measured using 10 sand-proof pitot-static probes,and mean velocity fields were obtained and discussed.The results showed that mean velocity fields obtained over different gravel mulches were similar.The analysis of wind speed patterns revealed an inherent link between gravel mulches and mean airflow characteristics on the gravel surfaces.The optimal gravel coverage is considered to be the critical level above or below which aeolian transport characteristics differ strongly.According to the present study,the optimal gravel coverage was found to be around 30% or 40%.Threshold velocity linearly increased with gravel coverage.Sand transport rate first increased with height above the wind tunnel floor(Hf),reaching a peak at some midpoint,and then decreased.     

Sand particle lift-off velocity measurements and numerical simulation of mass flux distributions in a wind tunnel

Lift-off velocity of saltating sand particles in wind-blown sand located at 1.0 mm above the sand bed surface was measured using a phase Doppler particle analyzer in a wind tunnel. The results show that the probability distribution of lift-off velocity can be expressed as a lognormal function, while that of lift-off angle follows an exponential function. The probability distribution of lift-off angle conditioned for each lift-off velocity also follows an exponential function, with a slope that becomes steeper with increasing lift-off velocity. This implies that the probability distribution of lift-off velocity is strongly dependent on the lift-off angle. However, these lift-off parameters are generally treated as an independent joint probability distribution in the literature. Numerical simulations were carried out to investigate the effects of conditional versus independent joint probability distributions on the vertical sand mass flux distribution. The simulation results derived from the conditional joint probability distribution agree much better with experimental data than those from the independent ones. Thus, it is better to describe the lift-off velocity of saltating sand particles using the conditional joint probability distribution. These results improve our understanding of saltation processes in wind-blown sand.     

Impact of utility-scale solar photovoltaic array on the aeolian sediment transport in Hobq Desert,China

Deserts are ideal places to develop ground-mounted large-scale solar photovoltaic (PV) powerstation. Unfortunately, solar energy production, operation, and maintenance are affected bygeomorphological changes caused by surface erosion that may occur after the construction of the solar PVpower station. In order to avoid damage to a solar PV power station in sandy areas, it is necessary toinvestigate the characteristics of wind-sand movement under the interference of solar PV array. The studywas undertaken by measuring sediment transport of different wind directions above shifting dunes andthree observation sites around the PV panels in the Hobq Desert, China. The results showed that the twoparameterexponential function provides better fit for the measured flux density profiles to the near-surfaceof solar PV array. However, the saltation height of sand particles changes with the intersection anglebetween the solar PV array and wind direction exceed 45°. The sediment transport rate above shifting duneswas always the greatest, while that around the test PV panels varied accordingly to the wind direction.Moreover, the aeolian sediment transport on the solar PV array was significantly affected by wind direction.The value of sand inhibition rate ranged from 35.46% to 88.51% at different wind directions. When theintersection angle exceeds 45°, the mean value of sediment transport rate above the solar PV array reducesto 82.58% compared with the shifting dunes. The results of our study expand our understanding of theformation and evolution of aeolian geomorphology at the solar PV footprint. This will facilitate the designand control engineering plans for solar PV array in sandy areas that operate according to the wind regime.     

塔里木盆地塔中地区野外微梯度风沙流观测

利用2014年7月至8月塔中地区沙尘与扬沙天气下风沙流运动的实测数据,对0~85 mm高度内风沙流运动进行了研究。结果表明:(1)0~85 mm高度层,输沙率(Q)随风速(v)增大呈幂函数规律增加,R2≥0.975 7,且输沙率主要集中在0~35 mm高度内。(2)总输沙率和撞击颗粒数的最佳拟合函数为线性函数,R2≥0.878 2,相关性较好;塔中地区1 min最小临界起沙风速为4.0 m·s-1。(3)跃移运动集中在10:00—20:00,总输沙率最大值出现在12:00—16:00,17:00以后输沙率明显下降。     

Estimation of the quantity of aeolian saltation sediments blown into the Yellow River from the Ulanbuh Desert,China

The Ulanbuh Desert borders the upper reach of the Yellow River.Every year,a mass of aeolian sand is blown into the Yellow River by the prevailing wind and the coarse aeolian sand results in serious silting in the Yellow River.To estimate the quantity of aeolian sediments from the Ulanbuh Desert blown into the Yellow River,we simulated the saltation processes of aeolian sediments in the Ulanbuh Desert.Then we used a saltation submodel of the IWEMS(Integrated Wind-Erosion Modeling System)and its accompanying RS(Remote Sensing)and GIS(Geographic Information System)modules to estimate the quantity of saltation sediments blown into the Yellow River from the Ulanbuh Desert.We calibrated the saltation submodel by the synchronous observation to wind velocity and saltation sediments on several points with different vegetation cover.The vegetation cover,frontal area of vegetation,roughness length,and threshold friction velocity in various regions of the Ulanbuh Desert were obtained using NDVI(Normalized Difference Vegetation Index)data,measured sand-particle sizes,and empirical relationships among vegetation cover,sand-particle diameters,and wind velocity.Using these variables along with the observed wind velocities and saltation sediments for the observed points,the saltation model was validated.The model results were shown to be satisfactory(RMSE less than 0.05 and|Re|less than 17%).In this study,a subdaily wind-velocity program,WINDGEN,was developed using this model to simulate hourly wind velocities around the Ulanbuh Desert.By incorporating simulated hourly wind-velocity and wind-direction data,the quantity of saltation sediments blown into the Yellow River was calculated with the saltation submodel.The annual quantity of aeolian sediments blown into the Yellow River from the Ulanbuh Desert was 5.56×106t from 2001 to 2010,most of which occurred in spring(from March to May);for example,6.54×105tons of aeolian sand were blown into the Yellow River on 25 April,2010.However,in summer and winter,the saltation process occasionally occurred.This research has supplied some references to prevent blown sand hazards and silting in the Yellow River.     

北京平原土壤机械组成和抗风蚀能力的分析

土壤遇风起沙是风沙活动产生的最根本原因,它的基本点有二:一是具有足以使砂粒产生运动的一定强度的起沙风;二是土壤颗粒小、质地干燥疏松,具有遇风起沙产生运动的属性。本文分析了北京土壤的机械组成特征和结构性能等。叙述了在永定河边部一些地块的野外风沙观测和沙风洞试验,证实土壤结构性、干松程度、植被覆盖状况是决定土壤抗风蚀性能的三要素。最后,试用不易蚀因子含量评价北京平原土壤的抗风蚀能力。     

风沙流研究的历史、现状及其趋势

本文扼要回顾了风沙流的研究历史;简明评述了颗粒运动特征、风沙流结构、输沙率模型、起沙风模型、表面气流变化、风沙两相流等研究现状内容;就目前研究中存在的问题、研究趋势等方面作了探讨。     

塔克拉玛干沙漠北缘荒漠过渡带风沙流结构特征分析

利用多种集沙仪,通过野外实时输沙观测,对塔克拉玛干沙漠北缘荒漠过渡带的地表风沙流特征进行了分析,结果表明：① 100 cm高度范围内,总输沙量的47.3%分布在30 cm高度内,这一比例小于前人的研究结果;输沙量随高度的变化比较符合幂函数分布。② 风沙流输沙的粒径以细砂、极细砂与粉砂为主,各高度层所占比例均达99%以上;风沙流输沙平均粒径随高度增加而减小,沙尘的含量随高度增加呈现“象鼻”状分布。③ 风沙流中贴地层风速廓线受风沙相互作用的影响,不再符合对数分布,更加符合幂函数u=az^b分布。     

格状沙障内风速波动特征初步研究

通过对不同孔隙度格状沙障内风沙流中风速波动特征研究发现:风沙流中各高度层风速波动具有很好的相关性,其波动幅度随孔隙度的增加而减小;瞬时风速在时间序列上波动的均一性随不同高度间距离增加呈递减趋势;瞬时风速的波动性一方面受下垫面性质及外在环境条件的影响,另一方面与其所在高度层沙粒数量和运动状态有关.     

不同砾石盖度戈壁床面动力学特征研究

利用野外车载移动式风洞,对莫高窟顶不同砾石盖度戈壁床面的动力学特征进行了实地模拟实验。结果表明：砾石盖度直接决定戈壁床面的粗糙元数量和分布状况,进而影响近地表风速廓线、摩阻速度、床面粗糙度和剪切力;随着风洞进口指示风速的增加,摩阻速度呈线性递增,而动力学粗糙度在波动中呈下降趋势;相同高度,随着砾石盖度的增加,近地表风速逐渐降低,而摩阻速度、动力学粗糙度和剪切力呈线性增加;当床面盖度增加至35%时,动力学粗糙度达到0.30 cm,摩阻速度相应提高到0.93 cm/s,床面剪切力增加至1.11 N/cm。     

Wind tunnel test on the effect of metal net fences on sand flux in a Gobi Desert,China

The Lanzhou-Xinjiang High-speed Railway runs through an expansive windy area in a Gobi Desert, and sand-blocking fences were built to protect the railway from destruction by wind-blown sand. However, the shielding effect of the sand-blocking fence is below the expectation. In this study, effects of metal net fences with porosities of 0.5 and 0.7 were tested in a wind tunnel to determine the effectiveness of the employed two kinds of fences in reducing wind velocity and restraining wind-blown sand. Specifically, the horizontal wind velocities and sediment flux densities above the gravel surface were measured under different free-stream wind velocities for the following conditions: no fence at all, single fence with a porosity of 0.5, single fence with a porosity of 0.7, double fences with a porosity of 0.5, and double fences with a porosity of 0.7. Experimental results showed that the horizontal wind velocity was more significantly decreased by the fence with a porosity of 0.5, especially for the double fences. The horizontal wind velocity decreased approximately 65% at a distance of 3.25 m(i.e., 13 H, where H denotes the fence height) downwind the double fences, and no reverse flow or vortex was observed on the leeward side. The sediment flux density decreased exponentially with height above the gravel surface downwind in all tested fences. The reduction percentage of total sediment flux density was higher for the fence with a porosity of 0.5 than for the fence with a porosity of 0.7, especially for the double fences. Furthermore, the decreasing percentage of total sediment flux density decreased with increasing free-stream wind velocity. The results suggest that compared with metal net fence with a porosity of 0.7, the metal net fence with a porosity of 0.5 is more effective for controlling wind-blown sand in the expansive windy area where the Lanzhou-Xinjiang High-speed Railway runs through.     

土壤风蚀对表层土壤粒度特征的影响

土壤风蚀是一种严重的环境问题。文中选择我国西北干旱及半干旱区,风蚀严重的盐池县风沙土地为研究对象,利用Marven2000粒度分析仪对不同风蚀强度的表层土壤粒度进行分析,探讨土壤风蚀过程对地表粒度的影响,研究表明:随着风蚀程度的增加,表层土壤粒度逐渐变粗,由灌丛-推平耕地-草地-吹蚀地,地表中的细沙,粉沙和粘土的含量逐渐降低,而中沙和粗沙的含量逐渐增加。灌丛表面以细沙为主,占34.9%;推平耕地和草地以中沙为主,占32.7和39.1%;吹蚀地以粗沙为主,占44.2%;土壤平均粒径逐渐变粗,分别为2.48,2.08,1.91,1.12φ。土壤风蚀过程,地表的细沙,粉沙和粘土大量损失,灌丛的细沙,粉沙和粘土分别占34.86,20.01和10.98%,但在严重风蚀的吹蚀地,细沙,粉沙和粘土仅占6.09,3.72和2.52%。由此可见,土壤风蚀造成地表细颗粒物质损失,形成沙漠化土地。不同地表沙粒的分维数不同,随平均粒径增加,分维数总体降低。     

尼龙网方格沙障防风效应复变规律

方格沙障的布设参数直接影响防风效应的复变作用,定量表达其复变规律,对于沙障配置模式的确定具有重要意义。在乌兰布和沙漠机械整平的风沙观测场,铺设9种不同高度、规格的尼龙网方格沙障,观测其在不同风速背景下的风速流场特征,揭示方格沙障防风效应的复变规律。结果表明:尼龙网方格沙障防风效应复合变化受不同指示风速下,沙障高度与规格共同的影响。沙障内部0. 1 m高度的风速随防护宽度的增加呈对数函数递减,指示风速增大1 m·s~(-1),风速衰减率增加0. 07倍;沙障高度增大0. 1 m,风速衰减率增加0. 20倍;方格边长增大1 m,风速衰减率减小0. 07倍。观测的9种规格方格沙障,30 cm高度1 m×1 m规格沙障复变作用最强,15 cm高度2 m×2 m规格沙障复变作用最弱。该结果可为确定沙障合理防护宽度、节约沙障铺设成本、优化沙障布设技术提供基础数据和理论支撑。     

Aeolian transport over a developing transverse dune

The spatial and temporal changes in aeolian transport over a dune are fundamental factors that control the morphology of the dune.In the present study,we obtained direct field observations of aeolian transport over a developing transverse dune at the Shapotou Aeolian Experiment Site in the southeastern part of China’s Tengger Desert.The transport rate versus wind speed relationship is complicated over a developing dune compared with the relationships over flat surfaces and over dunes that are in equilibrium with the wind.We obtained trend lines for transport rate over the transverse dune versus distance.The transport rate generally increased from the toe to the crest above the stoss slope,but the difference in transport rate between the crest and the toe was smaller than those that have been proposed for taller dunes.The crest/toe ratio for transport rates therefore seems to depend greatly on dune height.Flux density profiles for different points above the dune at different wind speeds were well described by the exponential decay law,as has been proposed for saltation flux density profiles.Coefficients in the flux density profile function can be defined in terms of the transport rate and wind speed.However,the dependence of relative decay rate with height and average saltation height on wind speed was weaker than that observed in a wind tunnel and above a flat surface.The preliminary results obtained in this study require more evidence from field observations to fully describe aeolian transport above developing dunes.     

Effect of the W-beam central guardrails on wind-blown sand deposition on desert expressways in sandy regions

Many desert expressways are affected by the deposition of the wind-blown sand,which might block the movement of vehicles or cause accidents.W-beam central guardrails,which are used to improve the safety of desert expressways,are thought to influence the deposition of the wind-blown sand,but this has yet not to be studied adequately.To address this issue,we conducted a wind tunnel test to simulate and explore how the W-beam central guardrails affect the airflow,the wind-blown sand flux and the deposition of the wind-blown sand on desert expressways in sandy regions.The subgrade model is 3.5 cm high and 80.0 cm wide,with a bank slope ratio of 1:3.The W-beam central guardrails model is 3.7 cm high,which included a 1.4-cm-high W-beam and a 2.3-cm-high stand column.The wind velocity was measured by using pitot-static tubes placed at nine different heights(1,2,3,5,7,10,15,30 and 50 cm)above the floor of the chamber.The vertical distribution of the wind-blown sand flux in the wind tunnel was measured by using the sand sampler,which was sectioned into 20 intervals.In addition,we measured the wind-blown sand flux in the field at K50 of the Bachu-Shache desert expressway in the Taklimakan Desert on 11 May 2016,by using a customized 78-cm-high gradient sand sampler for the sand flux structure test.Obstruction by the subgrade leads to the formation of two weak wind zones located at the foot of the windward slope and at the leeward slope of the subgrade,and the wind velocity on the leeward side weakens significantly.The W-beam central guardrails decrease the leeward wind velocity,whereas the velocity increases through the bottom gaps and over the top of the W-beam central guardrails.The vertical distribution of the wind-blown sand flux measured by wind tunnel follows neither a power-law nor an exponential function when affected by either the subgrade or the W-beam central guardrails.At 0.0H and 0.5H(where H=3.5 cm,which is the height of the subgrade),the sand transport is less at the 3 cm height from the subgrade surface than at the 1 and 5 cm heights as a result of obstruction by the W-beam central guardrails,and the maximum sand transportation occurs at the 5 cm height affected by the subgrade surface.The average saltation height in the presence of the W-beam central guardrails is greater than the subgrade height.The field test shows that the sand deposits on the overtaking lane leeward of the W-beam central guardrails and that the thickness of the deposited sand is determined by the difference in the sand mass transported between the inlet and outlet points,which is consistent with the position of the minimum wind velocity in the wind tunnel test.The results of this study could help us to understand the hazards of the wind-blown sand onto subgrade with the W-beam central guardrails.