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.

The trajectory parameters analysis of saltating sand grains driven by wind

The study of trajectory parameters of saltating sand grains is of great importance to forming the kinematic theory of wind-sand current. By establishing the simplified theoretical pattern and probing into the relationship between lift-off angle (α) and landing angle (β), relationship between the ratio of saltation length ( L) to maximum height (H) and lift-off angle as

Redevelopment of liftoff rates of saltating sand grains based on a simple optimization model

This paper is a redevelopment result of liftoff rates of saltating sand grains based on our previous work.Aeolian sand flow is a complex multi-phase flow because of a special two-phase gas-solid flow near ground surface.Despite extensive research on the movement of blowing sand,no model fully characterizes aeolian sand flow,and large differences often exist between simulations of aeolian sand movement and field observations.One key problem is a few of sufficient research on liftoff rates of saltating sand grains(also called the number of liftoff sand grains per unit time and per unit bed area).It is necessary to re-search in advance liftoff rates of saltating sand grains.We redeveloped liftoff rates of saltating sand grains by establishing an optimization model based on the flux of aeolian sand flow at different heights of the sampler in wind tunnel and the simulated capture of saltating sand grains by different heights of the sampler that are from different liftoff position(distance from the sampler) in order to revise previous inversion condition of liftoff rates of saltating sand grains.Liftoff rates increased rapidly with increasing wind speed.For frictional wind velocities of u=0.67,0.77,0.82,0.83,and 0.87 m s-1,liftoff rates were 3840,954502,5235114,5499407,and 7696291 sand grain s-1 m-2,respectively.These rates could be expressed as the square of the instantaneous frictional wind velocity and a constant(0.663) that differs from the critical(threshold) frictional wind velocity at which saltation begins.Although our results require additional experimental validation and the simple optimization model must be improved,they nonetheless provide a strong basis for future research.

Analysis of the forces acting on the saltating particles in the coupled wind-sand-electricity fields

Based on the theoretical model describing the saltation of sand particles in the coupled wind-sand- electricity fields, the numerical simulations of the forces acting on saltating particles, such as the aerodynamic drag force, Magnus effect, Saffman force and electrostatic force, are analyzed in com- parison to the gravity force of the particles in the steady windblown sand movement. Furthermore, the laws of the above forces vary with the friction velocity, the diameter of the sand particle, the initial an- gular velocity and the lift-off velocity are discussed.

Near-surface sand-dust horizontal flux in Tazhong-the hinterland of the Taklimakan Desert

Tazhong is the hinterland and a sandstorm high-frequency area of the Taklimakan Desert. However, little is known about the detailed time-series of aeolian sand transport in this area. An experiment to study the sand-dust horizontal flux of near-surface was carried out in Tazhong from January to December 2009. By measur- ing the sand-dust horizontal flux throughout sixteen sand-dust weather processes with a 200-cm tall Big Spring Number Eight （BSNE） sampler tower, we quantitatively analyzed the vertical variation of the sand-dust horizontal flux. And the total sand-dust horizontal flux of different time-series that passed through a section of 100 cm in width and 200 cm in height was estimated combining the data of saltation movement continuously recorded by piezo- electric saltation sensors （Sensit）. The results indicated that, in the surface layer ranging from 0-200 cm, the inten- sity of sand-dust horizontal flux decreased with the increase of the height, and the physical quantities obeyed power function well. The total sand-dust horizontal flux of the sixteen sand-dust weather processes that passed through a section of 100 cm in width and 200 cm in height was about 2,144.9 kg, the maximum of one sand-dust weather event was about 396.3 kg, and the annual total sand-dust horizontal flux was about 3,903.2 kg. The high levels of aeolian sand transport occurred during daytime, especially from 13：00 to 16：00 in the afternoon. We try to develop a new method for estimation of the detailed time-series of aeolian sand transport.

Observation of saltation activity at Tazhong area in Taklimakan Desert,China

A two-year field observation of saltation activity was carried out at Tazhong area, the hinterland area of the Taklimakan Desert with highly frequent dust storms. From 1 September 2008 to 31 August 2010, a piezoelectric saltation sensor （Sensit） was used to continuously collect the data on saltation activity at a level sand surface. Analysis on the data suggests that saltation activity can occur at any time of the year when conditions are favorable; however, the necessary conditions are rarely satisfied in most time. In the daytime of spring or summer, saltation activity can persist even over a continuous one-hour-or-so period. It is found that, from 1 September 2008 to 31 August 2010, saltation activity accounts for more than 3% of the total yearly time, and it tends to peak in spring and summer months with strong winds. During winter months when winds are weak, however, it is often at a minimum. It seems that precipitation does not appear to be significant in reducing saltation activity in arid regions like Tazhong.

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.

Numerical simulation of sand load applied on high-speed train in sand environment

High-speed train running in the sand environment is different from the general environment. In the former situation, there will be sand load applied on high-speed train(SLAHT) caused by sand particles hitting train surface. This will have a great impact on the train stability, running drag and surface corrosion. Numerical simulation method of SLAHT in sand environment is studied. The velocity and mass flow rate models of saltation and suspension sand particles and the calculation model of SLAHT caused by sand particles hitting train surface are established. The discrete phase method is adopted for numerical simulating the process of saltation and suspension sand particles moving to train surface and generating sand load. By comparison with the field tests, the numerical simulation reliability is analysed. The theoretical formula of SLAHT changing with cross-wind and train speed is proposed. SLAHT changing law is analyzed. Research results indicate that SLAHT changing with cross-wind and train speed is a quadratic relationship. When train speed is constant, SLAHT increases quadratically with cross-wind speed improvement. When cross-wind speed is constant, SLAHT increases quadratically with train speed improvement.

Large scale sand saltation over hard surface:a controlled experiment in still air

Saltation is the major particle movement type in wind erosion process.Saltating sand grains can rebound up to tens of times larger in length and height over hard surface(such as gravel surface)than over loose sand surface.Gravels usually have different faces,causing distinct response of the impacting grains,but the effects of the grain and gravel-surface contact angle on grain rebound are not yet well quantified.We performed full-range controlled experiments of grain saltation using different contact angles,grain sizes and impact speeds in still air,to show that contact angle increases the height of representative saltation path but decreases particle travel length.The results were compared with outputs from the COMprehensive numerical model of SALTation(COMSALT).Large saltation height of 4.8 m and length of 9.0 m were recorded.The maximum and representative saltation height over the gravel surface were found to be about 4.9 times and 12.8 times those over the loose sandy surface,respectively.The maximum saltation length may be reduced by 58%and the representative saltation height may be increased by 77%as contact angle increases from 20°to 40°.We further showed that the collision inertia contributes 60%of the saltation length,and wind contributes to the other 40%.These quantitative findings have important implications for modeling saltation trajectory over gravel surface.

Effect of stones on the sand saltation threshold during natural sand and dust storms in a stony desert in Tsogt-Ovoo in the Gobi Desert,Mongolia

Non-erodible elements such as stones and vegetation are key to controlling wind erosion and dust emission in drylands.Stony deserts are widely distributed in the Gobi Desert,but the effect of stones on wind erosion and dust emission have not been well studied,except under artificial conditions.In this study,we evaluated the effect of stones on wind erosion and dust emission by measuring the sand saltation threshold in a stony desert in Tsogt-Ovoo in the Gobi Desert,Mongolia,under natural surface conditions during sand and dust storms.We quantified the amount of stones by measuring the roughness density,and determined the threshold friction velocity for sand saltation by measuring wind speed and sand saltation count.Our results showed that the threshold friction velocity increased with the roughness density of stones.In the northern part of the study area,where neither a surface crust nor vegetation was observed,the roughness density of stones was 0.000 in a topographic depression(TD),0.050 on a northern slope(N.SL),and 0.160 on the northern mountain(N.MT).The mean threshold friction velocity values were 0.23,0.41,and 0.57 m/s at the TD,N.SL,and N.MT sites,respectively.In the southern part of the study area,the roughness density values of stones were 0.000 and 0.070-0.320 at the TD and southern slope sites,respectively,and the mean threshold friction velocities were 0.23 and 0.45-0.71 m/s,respectively.We further compared the observed threshold friction velocities with simulated threshold friction velocities using Raupach's theoretical roughness correction and the measured roughness density values,and found that Raupach's roughness correction worked very well in the simulation of threshold friction velocity in the stony desert.This means that the results of our stone measurement can be applied to a numerical dust model.

Theoretical expressions for soil particle detachment rate due to saltation bombardment in wind erosion

Saltation bombardment is a dominate dust emission mechanism in wind erosion.For loose surfaces,splash entrainment has been well understood theoretically.However,the mass loss predictions of cohesive soils are generally empirical in most wind erosion models.In this study,the soil particle detachment of a bare,smooth,dry,and uncrusted soil surface caused by saltation bombardment is modeled by means of classical mechanics.It is shown that detachment rate can be analytically expressed in terms of the kinetic energy or mass flux of saltating grains and several common mechanical parameters of soils,including Poisson's ratio,Young's modulus,cohesion and friction angle.The novel expressions can describe dust emission rate from cohesive surfaces and are helpful to quantify the anti-erodibility of soil.It is proposed that the mechanical properties of soils should be appropriately included in physically-based wind erosion models.

STUDY ON THE PERFORMANCE OF VERTICAL TUBULAR LEACHING REACTOR

Hydrodynamics in vertical tubular leaching reactor was studied in this work. Based on the observation of particle saltation in curved and square return bends, the inclined return bend was developed and examined. For the narrow and wide size distribution particle system, the method for calculating the particle holdup in upflow and downflow tubes was derived on the basis of generalized fluidization equation. The model of unit pressure drop was given and examined by experiments.

ANALYSIS OF SALTATION CHARACTERISTICS OF BED-LOAD TRANSPORT IN FLOWING WATER

The study of bed-load transport is of great significance both in theory and in practice.This paper discusses the saltation of bed-load solid grains in flowing water.Experiments and theoretic analysis have been made by means of high-speed photographing and advanced data processing technique with a combined method based on mechanical and statistical theories.It indicates that the saltation is the main form of the bed-load transport under ordinary flowing conditions.In the meantime,taking suecessive saltation as the mod- el of bed-load transport,systematic analysis has been made with regard to the kinematic properties and mechanism of saltation.The statistical analysis shows that the probability density functions of the relative height and length of saltation are in conformity with Γ-type distribution,while the probability density func- tions of the relative velocities of saltation are in conformity with the Gaussian distribution.

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.

Momentum profile of aeolian saltation cloud

The momentum profile of an aeolian saltation cloud is poorly understood. In this paper, height profiles for saltation momentum are reconstructed for three particle-size populations at four wind velocities based on profiles for mean particle velocity and relative particle concentration of saltation cloud obtained using particle image velocimetry in a wind tunnel. The results suggest that the saltation momentum profiles are characterized by peak curves with a maximum at some height above the surface. The height of this maximum increases with increasing wind velocity, but decreases with increasing particle size. It is linearly correlated with average saltation height and is comparable with the results of numerical simulations in a previous study. Our results confirm that Bagnold’s kink is an important feature of wind velocity profiles modified by the presence of saltating particles and that the height of the kink is closely related to the average trajectories of the saltating particles.

Stream-Flow Response to Climate Change and Human Activities in an Upstream Catchment of Huai River

Climate change and human activities have great implications for hydrological process and water projects planning. In order to evaluate the impacts on stream-flow, statistical methods and SWAT model are applied to this research. The results indicate that the abrupt change year (1965) of annual stream-flow is chosen as the split point of natural and human influenced (particularly reservoirs) periods. The calibrated SWAT model is proved to be applicable in this catchment and is used to simulate the monthly runoff which can be regarded as the natural runoff induced by climate change. A major finding of this study is that the reservoir regulations have apparently altered the monthly and seasonal stream-flow regimes. By quantifying the impacts of climate variation and human activities, the decreasing trend of annual stream-flow is found, and human activities are proved to be the dominant role in the catchment. This research improves our knowledge of hydrological responses to natural and artificial factors, and provides a better understanding for the future reservoir regulations.

Mapping of the Sedimentary Facies of the Bottom of the Ouladine Lagoon in Grand-Bassam (Ivory Coast_West Africa)

The Ouladine lagoon has an irregular bottom lined with different substrates. The lithological characteristics make it possible to observe sediments ranging from silts to sands and mixed sediments of variable color. These different fractions have varying proportions. The coarse fraction (>63 μm) has percentages ranging from 0 to 77% and the fine fraction ( μm) from 22.46 to 100% in the different sediments collected. The minority fraction of fine sands occupies the northern banks where the greatest depths of the section parallel to the coastal strip are observed. On the other hand, the sandy fractions occupy the southern shore up to the vicinity of the Azuretti mangrove island. This sandy fraction is also present on almost the entire section from the Comoé-Ebrié lagoon confluence to the closed mouth. The grain size characters of the sediments, such as the diagram of the classification coefficient So and the mean Mz, show well-classified sands. These sediments are deposited in a slightly agitated environment coming mainly from dunes and rivers. The sands of the estuarine complex were emplaced by saltation for most samples and by rotation.

Aeolian sediment transport over sandy gobi:Field studies in the Nanhu gobi along the Hami-Lop Nor Railway

Wind-blown sand over sandy gobi with an abundant sediment supply can cause severe sand hazards.However,compared with the study of aeolian transport over gravel gobi with a limited sediment supply,less attention has been devoted to sandy gobi,and thus,our understanding of wind-blown sand movement on sandy gobi is still poor.Here,we report the results of observations of three transport events on a sandy gobi along the Hami-Lop Nor Railway based on high-frequency saltation particle count and horizontal sediment flux measurements coupled with instantaneous wind velocity measurements.The results reveal that,unlike the notably intermittent aeolian saltation over gravel gobi,continuous transport occurred on the sandy gobi.The mean saltation layer height was 0.23±0.07 m,and it was positively related to both the grain size of surface particles and the wind velocity regardless of the gobi type.The sediment transport rates could be expressed as the power function Q=ap/g[u^(*)(u^(*2)-u^(*)t^(2))]b,and the scaling parameter(b)reached to 2.5,which is much larger than that of other gobi areas(b=1).Our findings suggest that the wind-blown sand over sandy gobi is much more severe than that over gravel gobi,and the Nanhu sandy gobi is the major sand source for sand hazards of the Hami-Lop Nor Railway.Sand-fixation measures such as checkerboard sand barriers with enhanced checkerboard size and barrier height should be the main subject of sand control systems for the Hami-Lop Nor Railway in sandy gobi.

Saltation and suspension of wind-blown particle movement

Several factors that affect the trajectories of sand particles in a wind-blown sand flux are analyzed by considering the stochastic movement of sand particles transported within turbulent flow in the atmospheric boundary layer. The results show that there are remarkably different trajectories for particles with the same diameter, the same vertical liftoff velocity from sand bed and the same friction ve-locity of wind because of the presence of turbulence flow. The vertical fluctuating velocity of sand particles can be regarded as a reflection of the transport mode, which is related to not only the diameters and liftoff velocity of sand particles but also the shear stress velocity of wind. The critical liftoff velocity and the fraction of each transport mode are calculated for the given particle diameter and friction ve-locity of wind. A comparison of the predicted fraction and the statistical fraction with and without the wind-sand couple effect is made.

Soil Moisture Effects on Sand Saltation and Dust Emission Observed over the Horqin Sandy Land Area in China

In this study, the eff ects of soil moisture on sand saltation and dust emission over the Horqin Sandy Land area are investigated, based on observations of three dust events in 2010. The minimum friction velocity initiating the motion of surface particles, namely, the threshold friction velocity, is estimated to be 0.34, 0.40, and 0.50 m s？1 under the very dry, dry, and wet soil conditions, respectively. In comparison with the observations during the dust events under the very dry and dry soil conditions, the dust emission fl ux during the wet event is smaller, but the saltation activities of sand particles （d≧50 μm） are stronger. The size distributions of airborne dust particles （0.1≦d≦20 μm） show that concentrations of the fi ner dust particles （0.1≦d≦0.3 μm） have a secondary peak under dry soil conditions, while they are absent under wet soil conditions. This suggests that the surface soil particle size distribution can be changed by soil moisture. Under wet soil conditions, the particles appear to have a larger size, and hence more potential saltating particles are available. This explains the occurrence of stronger saltation processes observed under wet soil conditions.