Wind-blown Sand Flux Research Articles

Aerodynamic Characteristics Over Fine-Grained Gravel Surfaces in a Wind Tunnel

Gravels can protect soil from wind erosion, however, there is little known about the effects of fine-grained gravel on aerodynamic characteristics of the near-surface airflow. Drag coefficient, wind-speed gradient, and turbulent transfer coefficient over different coverages of gravel surfaces were investigated in a compact boundary-layer wind tunnel. The drag coefficient of the fine-grained gravel surface reached the maximum value at 15% coverage and then tended to stabilize at gravel coverage 20% and greater. At a height of 4 cm, near-surface airflow on gravel surfaces can be divided clearly into upper and lower sublayers, defined as the inertial and roughness sublayers, respectively. The coefficient of variation of wind speed over gravel surfaces in the roughness sublayer was 8.6 times that in the inertial sublayer, indicating a greater effect of gravel coverage on wind-speed fluctuations in the lower layer. At a height of 4 cm, wind-speed fluctuations under the observed wind speeds were independent of changes in gravel coverage. In addition, an energy-exchange region, where sand particles can absorb more energy from the surrounding airflow, was found between the roughness and inertial sublayers, enhancing the erosional state of wind-blown sand. This finding can be applied to evaluate the aerodynamic stability of the gravel surface in the Gobi Desert and provide a theoretical basis for elucidation of the vertical distributions of wind-blown sand flux.

Vertical Profiles of Wind-Blown Sand Flux over Fine Gravel Surfaces and Their Implications for Field Observation in Arid Regions

We used a compact boundary layer wind tunnel equipped with a turbulence generator and a piezoelectric blown-sand meter to investigate the effects of the surface coverage of fine gravel on wind-blown sand flux. The vertical profile of wind-blown sand over a flat sand surface showed an exponential distribution at all wind speeds, whereas the profile over gravel surfaces of 20% or greater coverage showed a non-monotonic vertical distribution. At 20% to 30% gravel coverages, a peak of wind-blown sand flux developed between 6 and 10 cm above the ground at all wind speeds because of less energy loss due to grain-bed collisions at that level. To analyze the erosional state of wind-blown sand, we used the Wu–Ling index (λ) of the mass-flux density of sand-bearing wind. Values of λ for all gravel coverages were greater than 1 at all wind speeds, indicating an unsaturated (erosional) state. Moreover, we found that the wind-blown sand flux at 4 cm height accounted for about 20% of the total flux regardless of wind speed and gravel coverage. This finding can simplify future estimations of total near-surface wind-blown sand flux based on field observations because such measurements can be taken at just one height.

Review on charging model of sand particles due to collisions

ABSTRACT In this paper, the models describing the charge transfer between two sand particles due to collisions are reviewed. By comparing the experimental results and the calculated results by the models carried on an individual particle due to a single collision, it indicates the Mosaic model is more reasonable to describe the collision charging mechanism. The Mosaic model cannot only describe the dependence of the collision charges on the relative collision speed and the particle size, but also reveal the relationship between the collision charges with the environmental temperature, the relative humidity and the material parameters, e.g., the absorption energy. Based on the Mosaic model, the model to describe the charges transfer due to multiple collisions is also developed, which can be used to calculate the charges carried by sand particles due to multiple collisions in the wind blown sand flux.

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.

Electrification of wind-blown sand: Recent advances and key issues

In this paper, we summarize the recent advances on the electrification of wind-blown sand. Some of the outstanding questions, such as the interpretation of the electrification of wind-blown sand, measurements on and models of the features of the wind-blown sand E-fields, as well as their effect on the wind-blown sand flux and electromagnetic wave propagation, are reviewed. We end by highlighting the challenges that remain.

RETRACTED: The influence of sand bed temperature on lift-off and falling parameters in windblown sand flux

In deserts the temperature of the sand bed can reach up to more than 50 degrees C, however, existing knowledge on the lift-off and falling velocities and angles of sand particles in windblown sand flux is based mainly on experimental results over a temperature range of 20 degrees C to 30 degrees C. Consequently, existing experimental results cannot reflect the actual interaction between saltating sands and the bed. In this study, the influence of the sand bed temperature on the probability distribution of lift-off and falling velocities and angles of sand particles is investigated through an improved Particle Image Velocimetry (PIV) system. Results demonstrate that the distribution shape of lift-off and falling velocities and angles of sand particles is not greatly influenced by the temperature of sand bed, however it has a certain influence on the center of the probability distributions of horizontal velocities, as well as the average, decay constant and amplitude of the distributions of vertical velocities and angles. We present formulas to describe the probability distributions of lift-off and falling velocities and angles with regard to the influence of sand bed temperature. (C) 2013 Elsevier B.V. All rights reserved.

Analysis of sand particles’ lift-off and incident velocities in wind-blown sand flux

In the research of windblown sand movement, the lift-off and incident velocities of saltating sand particles play a significant role in bridging the spatial and temporal scales from single sand particle’s motion to windblown sand flux. In this paper, we achieved wind tunnel measurements of the movement of sand particles near sand bed through improving the wind tunnel experimental scheme of particle image velocimetry (PIV) and data processing method. And then the influence of observation height on the probability distributions of lift-off and incident velocities of sand particles was analyzed. The results demonstrate that the observation height has no obvious influence on the distribution pattern of the lift-off and incident velocities of sand particles, i.e., the probability distribution of horizontal and vertical velocities of lift-off and incident sand particles follow a Gaussian distribution and a negative exponential distribution, respectively. However, it influences the center of the Gaussian distribution, the decay constant and the amplitude of the negative exponential distribution.

RETRACTED: The influence of sand diameter and wind velocity on sand particle lift-off and incident angles in the windblown sand flux

The problem of impact entrainment is one of the central issues in understanding the process of saltation. In this paper, through improving the experimental scheme of Particle Image Velocimetry (PIV) and data processing methods using wind tunnel data, the measurement of sand particle lift-off and incident angles near the sand bed was achieved. Based on experimental data, we analyze the probability distribution of sand particle lift-off and incident angles and the influence of wind velocity and sand diameter on the probability distribution, as well as the relation between incident angle and lift-off angle. The results demonstrate that wind velocity and sand diameter have no obvious influence on the distribution shape of sand particle lift-off and incident angles. However, wind velocity will influence the decay constant and amplitude of the probability distribution. (c) 2013 Elsevier B.V. All rights reserved.

A field observational study of electrification within a dust storm in Minqin, China

Previous studies have confirmed the existence of vertical electric fields generated within windblown sand and dust storms. The magnitude of the vertical electric field is generally hundreds of times or even thousand times that of the fair weather value. However, studies of the horizontal electric field and relation between the electric field and other physical quantities, such as wind velocity, wind-blown sand flux, humidity and temperature, are still limited. In this paper, we present simultaneous field observations of the electric field and other physical quantities within a dust storm in Minqin, China. These observations revealed that the horizontal and upward-pointing electric fields coexist in the dust storm, where the direction of horizontal electric field point along with the wind direction and the magnitude is greater than that of the vertical electric field. The average and standard deviation of the horizontal and vertical electric fields are linearly related to the velocity in the increasing and decreasing stage of dust storm. In addition, the magnitude of the horizontal and vertical electric fields may be related to the atmospheric temperature or humidity. The average and standard deviation of the electric field at one height is linearly related to that at another height, and they first increase and then decrease with the height. The probability density function of the electric field follows a Gaussian distribution with a magnitude greater than the average electric field itself.

Numerical investigation on two-grain-bed collisions in windblown sand transport

Grain/bed collisions play a significant role in the transportation of granular materials, especially in windblown sand movements. In this study, grain/bed collisions regarding two sand grains simultaneously impact onto a mix-sized sandy bed are numerically investigated by the discrete element method (DEM). Results demonstrate that even under the same initial conditions, the outcome of two sand grains simultaneously impacting a 2D sand packing (termed as two-grain–bed collisions) is different from the superposition of two grain/bed collision events (termed as twice grain–bed collisions). Analysis show that when the windblown sand concentration is higher than a critical concentration Q cr , the number of ejected sand grains given by twice grain–bed collisions is 23% higher than that by two-grain–bed collisions. Therefore, two-grain–bed collisions considering the interaction of two grains simultaneously impact a sand-bed is a significant factor that cannot be overlooked in numerical schemes of windblown sand flux. The ejected number of two sand grains simultaneously impacting the sand-bed (case A ) is different from the superposition of two grain/bed collision (case B ). In the case B , the L/d has an effect on the ejected number. When the distance L between the two impacting sand grains is long enough, the interaction effect of simultaneous collisions can be ignored. ► We conducted numerical simulations of two-grains-bed collisions. ► Results of two-grains-bed collisions and twice grain-bed collisions are compared. ► The distance of the two simultaneously impacting grains has effect the outcomes. ► We present a formula of the critical distance as a function of impact velocity.

Wind tunnel experiments on vertical distribution of wind-blown sand flux and change of the quantity of sand erosion and deposition above gravel beds under different sand supplies

Wind tunnel experiments were carried out with respect to the vertical distributions of wind-blown sand flux and the processes of aeolian erosion and deposition under different wind velocities and sand supplies above beds with different gravel coverage. Preliminary results revealed that the vertical distribution of wind-blown sand flux was a way to determine whether the gobi sand stream was the saturated one or not. It had different significances to indicate characteristics of transport and deposition above gobi beds. Whether bed processes are of aeolian erosion or deposition was determined by the sand stream near the surface, especially within 0–6 cm height, while the sand transport was mainly influenced by the sand stream in the saltating layer above the height of 6 cm. The degree of the abundance of sand supply was one of the important factors to determine the saturation level of sand stream, which influenced the characteristic of aeolian erosion and deposition on gravel beds. Given the similar wind condition, the sand transport rates controlled by the saturated flow were between 2 and 8 times of the unsaturated one. Those bed processes controlled by the saturated flow were mainly of deposition, and the amount of sand accumulation increased largely as the wind speed increased. In contrast, the bed processes controlled by the unsaturated flow were mainly of aeolian erosion. Meanwhile, there was an obvious blocking sand ability within the height of 0–2 cm, and the maximal value of sand transport occurred within the surface of 2–5 cm height.

Evolution of windblown sand flux and dune field — Trans-scale modeling and simulation

Windblown sand flux and dune field evolving toward the oasis have been a common ecological and environmental threat confronted by many countries. Meanwhile, it is also a kind of complex dynamical process involving multiple temporal and spatial scales which is still out of accurate description through current field observations. Available models and reliable quantitative simulations are of significant value to predict the spreading rate of desertification and provide an optimal design for sand prevention. This paper presents a “triple-jump” method to realize quantitative simulations to the formation and evolution of an aeolian dune field from an arbitrary initial configuration. Simulated results achieve a satisfactory agreement with observations qualitatively and quantitatively, which also reveal the characteristics and dynamical behaviors of dunes and dune field. Such a paradigm is of a good level of generality, which provides an exploratory probe into the subject of multi-scale physics.

A three‐dimensional analysis on lift‐off velocities of sand grains in wind‐blown sand flux

AbstractThis paper focuses on a key problem in wind‐blown sand transport research, the lift‐off velocities of sand grains. A three‐dimensional grain‐bed collision model comprising an impact sand grain, a creeping sand grain and a sustaining grain is presented. Based on this model, the analytical formulae of sand grains' linear and angular velocities are derived. The rebound/ejected coefficient (ratio of rebound/ejected velocity to impact velocity) and angle are numerically calculated using the analytical formulae and they agree well with the experiment results, which indicates that the three‐dimensional model is credible and necessary. Comparisons of our results with the results gained by a two‐dimensional grain‐bed collision model show that all components of both rebound and ejected velocities gained by the three‐dimensional model are lower than the corresponding ones done by two‐dimensional model apart from the horizontal rebound velocity. It also shows the lateral velocity and the lateral angular velocity, as well as the upward angular velocity, cannot be obtained by a two‐dimensional model, and cannot be ignored when the impact velocity is high. Copyright © 2008 John Wiley & Sons, Ltd.

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 velocity 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 velocity of wind. A comparison of the predicted fraction and the statistical fraction with and without the wind-sand couple effect is made.

DPTM simulation of aeolian sand ripple

Aeolian sand ripple and its time evolution are simulated by the discrete particle tracing method (DPTM) presented in this paper. The difference between this method and the current methods is that the former can consider the three main factors relevant to the formation of natural aeolian sand ripples, which are the wind-blown sand flux above the sand bed formed by lots of sand particles with different diameters, the particle-bed collision and after it the rebound and ejection of sand particles in the sand bed, the saltation of high-speed sand particles and the creep of low-speed sand particles, respectively. The simulated aeolian sand ripple is close to the natural sand ripple not only in basic shape and characteristic, particle size segregation and stratigraphy, but also in formation stages. In addition, three important speeds can be obtained by this method, which are the propagation speed of the saturated aeolian sand ripple and the critical frictional wind speeds of emergence and disappearance of sand ripple.

Electric field in windblown sand flux with thermal diffusion

A theoretical model considering thermal diffusion (TD) from a sand bed is suggested in this paper to account for the effect of both TD and the electric field produced by charged sand grains during the evolution of windblown sand flux, where the coupled interaction among wind speeds in the vertical and the horizontal directions, sand movement (saltation, suspension, and creep), and the electric field are taken into consideration. The numerical results show that during the evolution of windblown sand flux the electric field varies along with time and height. Then the profiles of the electric field produced by charged sand grains in windblown sand flux are quantitatively analyzed in detail and are compared with those without considering TD at different moments and different heights. Also, the results indicate that different conditions would produce different electric fields in magnitude: electric fields considering TD are more intense than those without considering thermal diffusion (NTD) at the same time and height. In the meantime, when the windblown sand flux reaches steady state, the mass flux profile and the transport rate of sand grains will increase, while horizontal wind speed will decease because of the effect of TD and the electric field.

Theoretical prediction of liftoff angular velocity distributions of sand particles in windblown sand flux

The liftoff angular velocities, including the rebound ones and the ejected ones, of sand particles in a windblown sand flux have a significant influence on the saltation trajectories and the transport flux and so on. However, they are not easy to determine actually both in field and in a wind tunnel, especially when the sand particles are small and the wind speed is low. A stochastic particle‐bed collision model is developed in this paper to calculate the liftoff angular velocities. Having taken the incident velocity and angle of an incident particle, the impact angle between the incident and the impacted particles in the sand bed and its creep velocity as well as the contact angle between the impacted particle and other particles in the bed as random variables in the collision model, the probability distributions of the liftoff angular velocities for different particle diameters and wind speeds are gained and agree well to the experimental results. The rebound probability distribution function looks like Gumbel distribution with a cusp in the peak, and the ejected one looks like the normal one. The effect of the wind speed and the particle diameter on the probability density functions is discussed in detail.

Experimental analysis of sand particles' lift-off and incident velocities in wind-blown sand flux

The probability distributions of sand particles' lift-off and incident velocities in a wind-blown sand flux play very important roles in the simulation of the wind-blown sand movement. In this paper, the vertical and the horizontal speeds of sand particles located at 1.0 mm above a sand-bed in a wind-blown sand flux are observed with the aid of Phase Doppler Anemometry (PDA) in a wind tunnel. Based on the experimental data, the probability distributions of not only the vertical lift-off speed but also the lift-off velocity as well as its horizontal component and the incident velocity as well as its vertical and horizontal components can be obtained by the equal distance histogram method. It is found, according to the results of the χ 2-test for these probability distributions, that the probability density functions (pdf's) of the sand particles' lift-off and incident velocities as well as their vertical components are described by the Gamma density function with different peak values and shapes and the downwind incident and lift-off horizontal speeds, respectively, can be described by the lognormal and the Gamma density functions. These pdf's depend on not only the sand particle diameter but also the wind speed.

Theoretical model of the electric field produced by charged particles in windblown sand flux

Taking into account the coupled interactions among wind velocity, sand movement, and the electric field, we develop a general theoretical model for calculating the electric fields produced by charged sand particles in the three sand movement types, saltation, suspension and creep, quantifying the electric field of a point charge by Coulomb's law. The numerical results of the electric field are in good agreement with both the field data and the wind tunnel experimental results. The profile of the electric field intensity produced by charged particles in windblown sand flux is quantitatively analyzed in detail and compared with those generated by charged particles only in the saltation layer or in the creep layer. The results demonstrate that the profile of the electric field produced by charged particles in one sand movement type is different from that by those in other types and that the signs of the charge acquired by the particles also alter the features of the profile. Finally, the effects of the wind velocity and the charge of the windblown sand particles on the electric field intensity are discussed.

A laboratory test of the electrification phenomenon in wind-blown sand flux

The measured data in the wind-tunnel tests show that the wind-blown sand particles acquired a negative charge when their diameters are smaller than 250 μm and positive charge when their diameters are larger than 500 μm, which confirms Latham’s assumption that the large particles in wind-blown sand flux acquired positive charge while negative charge developed on small ones. In the meanwhile, the measured data also show that the average charge-to-mass ratio for wind-blown sand particles decreases with the increase of the particle diameter and the wind velocity, and increases with the rise of height. The electric field in wind-blown sand flux is mainly formed by the moving charged sand particles. Its direction is vertical to the Earth’s surface and upward, which is opposite to that of the fair-weather field. The electric field increases with wind velocity and height increasing. These experimental results will lay the foundation for developing the theoretical analysis of the electrification phenomenon in wind-blown sand flux.