Sand Saltation Research Articles

Field Observation Evidence for Kink Points in the Vertical Kinetic Energy Flux Profiles of Wind‐Blown Sand Over Gobi and Its Significance

AbstractWind‐blown sand over Gobi can cause severe sand abrasion. The vertical flux profiles, especially that of the kinetic energy (KE), determine the erosion intensity. However, the vertical distribution of the KE flux of wind‐blown sand over Gobi remains poorly understood. Here, we report comprehensive field‐based measurements of the vertical flux profiles of wind‐blown sand over three typical Gobi surfaces during four transport events. The results show that unlike the exponential decrease in the particle count flux with height, the KE flux profile curve was non‐monotonic and could be well fitted by a Pearson VII function. The height where the maximum KE flux density occurred (kink points) ranged from 0.09 to 0.15 m above the ground. In addition, the non‐monotonic KE flux profiles primarily emerged during periods of strong transport and were mainly caused by large saltating sand particles due to their high KE as they rebounded from the grain‐bed collision.

Evaluation of Velocity Impact on Sand Particles in Two-Phase Annular Flow in Horizontal Pipes

Experimental investigations on sand particles in annular flow in horizontal pipes were presented. The aim is to enhance the knowledge related to sand transportation in the oil and gas industry. More so, to add values in facilitating the optimum design of oil and gas production systems where annular flow is often encountered. The experimental investigations were conducted using a closed-loop consisting of a horizontal pipe with an internal diameter of 2-inch (0.0504m). The experimental investigations covered both: water/sand flow and water/air/sand annular flow. The water/sand flow experiments were carried out to establish sand transport regime in liquid such as sand saltation, sand streak, and sand suspension. While, water/air/sand annular flow experiments revealed that the only the only sand transport regime in annular flow are saltation and suspension. Sand concentration profile in the annular flows were also estimated by using the sampling method. It was found that the bigger the sand particle size (500microns) leads to higher concentration of sand being transported at the bottom of the pipe because of gravity effect, while the smaller the sand particle size (212microns), leads to the higher sand concentration. Also, the sand concentration increased at the gas core as the superficial gas velocity increases.

Modified sloped bed splash function in sand saltation

Wind-blown sand induces serious desertification and consequently results in financial loss. Grain-bed interaction processes are primary responsible for sand saltation, which are generally described using a splash function. However, current splash functions are designed for horizontal beds although almost all sand surfaces are sloped (e.g., sand ripples and dunes), leading to the possibility of the predicted sand flux over a slope being several times higher than the observed flux. Herein, a three-dimensional splash function for a sloped bed that reasonably describes grain-bed interactions is proposed. The modified sloped bed splash function is introduced into a fluid-solid-coupled large-eddy simulation model, with which the wind-blown sand moments over various slope beds could be exactly reproduced. Simulation results show that the sediment transport mass flux on the uphill exhibited a nearly cubic dependence with the friction velocity, while that on the downhill showed an intermediate relationship between quadratic- and cubic-dependence. This research provides a useful approach for investigating natural aeolian sand movements.

Large scale structures of turbulent flows in the atmospheric surface layer with and without sand

Field measurements at the Qingtu Lake Observation Array site were performed to explore the features of large scale structures relevant to very large scale motions (VLSMs) and large scale motions (LSMs) in the atmospheric surface layer during dust weather events. The three velocity components, PM10 concentrations (particles with size less than 10 μm), and sand saltation fluxes at different heights were measured synchronously. By comparing with the results in clean-air flows, the influences of sand particles on the VLSMs were investigated. The present results indicate that the streamwise turbulent kinetic energy across all scales is enhanced in the sand-laden flows. However, the energy enhancement of small-scale motions is more pronounced than that of large scale structures, resulting in a decreased energy fraction of the large scale structures in the sand-laden flows. The inclination angles of the large scale structures are found to increase with sand concentration, owing to the decreased velocity gradient. The streamwise length scale of large scale structures and the size of the most energetic turbulent structures are found to be unchanged in the sand-laden flows relative to those in the clean-air flows. This is the study on the particle-laden wall turbulence at the largest Reynolds number to date, and the results demonstrate that the dispersed tiny particles in very dilute particle–gas flows can affect the turbulence even for the large scale structures.

A test of the efficacy of sand saltation for silt production: Implications for the interpretation of loess

AbstractProduction of the silt that forms loess is attributed to processes operating in both glacial systems (glacial grinding) and sandy deserts (saltation-induced fracturing). However, the efficacy of saltation for significant silt production is controversial. Understanding the potential for silt production in deserts is essential for determining the paleoclimatic significance of loess. To better assess the significance of eolian abrasion for silt production, experimental abrasion was conducted in a device designed to simulate sand saltation at sustained storm-wind velocities (∼25 m/s). The design differs from previous work in (1) maintaining strong measured velocities for long duration, (2) removing preexisting silt and utilizing control samples, (3) and scaling results to estimate potential for loess accumulation. Scaling experimental rates of production to geologic proportions indicates that eolian abrasion of sand produces insufficient silt to create geologically significant loess deposits.

The Influence of Surface Stress Fluctuation on Saltation Sand Transport Around Threshold

AbstractMultiscale wind velocity fluctuation and complex inertial response of sand saltation to wind variations give rise to intermittent and nonuniform sand transport, which greatly challenges the prediction of transport rate under wind conditions around saltation threshold. In this paper, three‐dimensional sand saltation within simulation domain of wind tunnel scale is modeled based on large eddy simulation of turbulent wind. Special attention is paid to the prediction of surface shear stress and consequently aerodynamic entrainment. When the wall stress model considering the influence of large‐scale turbulent structures was employed in case of relatively small resistance level for calculating particle aerodynamic entrainment, the model results predict sand transport below the impact threshold. The sand transport rate in this regime increases exponentially with mean wall stress, in contrast to the standard power law‐like behavior above the impact threshold. Aerodynamic entrainment and strong transport events lag behind high‐speed fluids sweep on a large scale even in wind tunnel, but the spatiotemporal variability of small‐scale sand transport is still unclear and quite complex due to transport inertia.

The Scale Characteristics and Formation Mechanism of Aeolian Sand Streamers Based on Large Eddy Simulation

AbstractAeolian sand transport near the surface in the field often occurs in the form of sand streamers, which is very important to understand in‐depth the physics of aeolian transport and accurately predict the sand transport rate. In this paper, sand streamers were successfully reproduced by combining the Lagrangian trajectory simulation of particles with large eddy simulation of turbulence. The simulated sand streamers are hill‐like. Their characteristic scales are extracted by the two‐point correlation of sand concentration. It is found that sand streamers corresponding to the correlation threshold of are 2.0–4.5 m long and 0.3–0.4 m wide, while 30–100 m long and 1.2–2.4 m wide for , which are substantially consistent with the existing field observation results and increases with particle size. Conditional averaging method provided an ensemble‐mean visualization of flow structures that are responsible for the formation of streamers. It is found that sand streamers on a flat and erodible surface exist in the near‐surface wake area of large/very large scale turbulent motions (LSMs/VLSMs), indicating that sand streamers are the footprint of LSMs/VLSMs. Moreover, the characteristic scales of sand streamers will be affected by the splashing process of sand saltation. On a nonerodible surface, sand streamers trailing from upstream sandy surface become narrow though it can propagate downstream for a long distance.

Relationship between air temperature and horizontal sand-dust flux observed in the Taklimakan Desert, China

Over the hinterland of the Taklimakan Desert (TD), the diurnal and seasonal cycles of the air temperature (AT) and horizontal sand-dust flux (HSDF) were investigated, based on in situ data including sand saltation counts and meteorological parameters in the period 2008–2010. In particular, the relationship between AT and HSDF was determined. It was found that 74.7% of HSDF was observed between 10:00 and 20:00, and HSDF in spring and summer accounted for 92.3% of the annual flux. Further, the diurnal and annual cycles of HSDF and AT presented an exponential distribution, with correlation coefficients (R) of 0.97 and 0.93, respectively. Additionally, when AT was 0 °C or lower, the frequency of sand saltation was only 4.3%. The maximum sand saltation frequency was 70.1%, which occurred at 33–34 °C, while 67.6% of the total HSDF occurred at 24–34 °C. Finally, a Gauss equation was established over the TD: Q = 250.2 + 1368.5e−2 × [(T−30.8)/7.3]^2, with R = 0.90. Results revealed a strong correlation between HSDF and AT, indicating that AT should be noticeable in HSDF parameterization schemes.

Comparisons suggest more efforts are required to parameterize wind flow around shrub vegetation elements for predicting aeolian flux

Upon interacting with the atmosphere, vegetation could alter the wind distribution and consequently the erodibility of nearby region. The parameterization of wind distribution around vegetation is crucial for the prediction of surface aeolian flux. This paper compared the performances of existing empirical distribution models in the estimation of aeolian flux for shrub vegetation, focusing on distribution pattern and vegetation porosity (main parameter of distribution function). Predicted dust fluxes directly entrained by air flow show weak sensitivity to both distribution pattern and porosity in the case of low vegetation density, which suggests some aspects in dust forecast models might be simplified. However, both distribution pattern and porosity show significant effect on sand saltation transport rate in the lee of vegetation element and, consequently, on the formation and evolution of surface aeolian landforms. The contribution of dust fluxes released in wind increase zone to the total emission by using current parameterizations increases with both the decrease of wind speed and the increase of vegetation density. Nevertheless, the parameterization of wind increase zone needs to be validated and improved by further experimental and numerical investigations.

Statistics and spectra of turbulence under different roughness in the near‐neutral atmospheric surface layer

AbstractAccurate knowledge of the contacts between surface roughness and the resultant wind speed are important for climatic models, wind power meteorology, agriculture and erosion hazards especially on sand saltation in arid and semi‐arid environments, where vegetation cover is scarce. In this study, synchronous measurements of three‐dimensional wind speed below 5 m are carried out in three different surface roughness conditions in Minqin, China, and the difference in the turbulence statistics and the structure of the very large‐scale motions (VLSMs) were revealed. The results show that the slope of the mean wind profile (MVP), the turbulent kinetic energy (TKE) and Reynolds stress increase with the surface roughness. The roughness seems to suppress the ejection events and the surface roughness will not only weaken the energy of the VLSMs, but also reduce the scale values of VLSMs near the wall. These influences may cause some changes regarding the dust transportation in streamwise and vertical directions during the sand and dust storm (SDS). That is, the decrease of the mean velocity near the ground will reduce the dust transportation in the streamwise direction and influence of the roughness on the ejection and sweep events will change the dust transportation in the vertical direction. Furthermore, the increase of roughness will weaken the scale and energy of VLSMs, which will lead to the decrease of the capacity of dust transportation. © 2019 John Wiley & Sons, Ltd.

Four-Channel Photoelectric Counter of Saltating Sand Particles

A four-channel photoelectric counter has been designed and manufactured for the measurements of saltating sand concentration fluctuations and size distribution function of particles with diameters >30 μm over desertified areas. The measurement results are given. It is shown that the vertical profile of the saltating sand concentration does not change on average in the altitude range from 3 to 7 cm over a desertified area in the neighborhood of the Volga. The distribution function of saltating sand particles over the desertified area is approximated with satisfactory accuracy by the lognormal distribution near the distribution maximum. Noticeable distinctions between the abovementioned distributions for large and small sand particles are observed.

Aerodynamic grain‐size distribution of blown sand

AbstractAeolian sand entrainment, saltation and deposition are important and closely related near surface processes. Determining how grains are sorted by wind requires a detailed understanding of how aerodynamic sand transport processes vary within the saltating layer with height above the bed. Grain‐size distribution of sand throughout the saltation layer and, in particular, how the associated flux of different grain size changes with variation in wind velocity, remain unclear. In the present study, a blowdown wind tunnel with a 50 cm thick boundary layer was used to investigate saltating sand grains by analyzing the weight percentage and transport flux of different grain‐size fractions and the mean grain size at different wind velocities. It was found that mean grain size decreases with height above the sand bed before undergoing a reversal. The height of the reversal point ranges from 4 to 40 cm, and increases with wind velocity following a non‐linear relationship. The content of the finer fractions (very fine and fine sand) initially increases above the sand bed and then decreases slightly with height, whereas that of the coarser fractions (medium and coarse sand) exhibits the opposite trend. The content of coarser grains and the mean grain size of sand in the saltation layer increase with wind velocity, indicating erosional selectivity with respect to grains in multi‐sized sand beds; but this size selectivity decreases with increasing wind velocity. The vertical mass flux structure of fine sand and very fine sand does not obey a general exponential decay pattern under strong wind conditions; and the coarser the sand grain, the greater the decrease rate of their transport mass with height. The results of these experiments suggest that the grain‐size distribution of a saltating sand cloud is governed by both wind velocity and height within the near‐surface boundary layer.

Comparison of approaches for calculating the threshold velocity for sand movement based on field experiments in Xinjiang, China

An accurate threshold velocity ut for sand movement is essential for predicting wind erosion events and calculating the magnitude of horizontal sand flux. We compared various approaches for determining ut using data observed during 4–31 July 2009 in the hinterland of Taklimakan Desert. We used the Gaussian time fraction equivalence method developed by Stout with an interval of 1 day to obtain ut values of 3.03–5.62 m/s (the approach of observations through field experiments). The Kurosak (ut50%) method yielded values of 3.71–5.74 m/s (the approach of statistical calculations). The Marticorena and Shao models gave ut values of 4.87–4.90 and 5.82–6.78 m/s, respectively (the approach of model parameterizations). To test the accuracy of these threshold velocities above, we estimated the total horizontal sand flux and the duration of sand saltation. Estimates of the total horizontal sand flux were 1311.9, 1166.4, 1279.9, and 661.6 kg/m for the Stout, Kurosak, Marticorena, and Shao methods, respectively, while the observed value was 732.9 kg/m. The correlation coefficient between observed and estimated values based on the Stout, Kurosak, Marticorena, and Shao methods was 0.75, 0.79, 0.77, and 0.83, respectively. The estimated duration of sand saltation using the ut been as Stout, Kurosak, Marticorena, and Shao was 8211, 6575, 7567, and 3463 min, of which 6208, 5646, 5986, and 3346 min were correct reports, respectively, and the observed value was 7663 min. We discuss the advantages and disadvantages of these methods combining those results above, which can be used to advise researchers to improve the threshold velocity used in future studies.

A study on velocity of aeolian sand particles over leeward slope by particle image velocimetry

Current studies on the movement of saltation sand grains in aeolian flow over a complex surface are not satisfactory. This is mainly because the landform conditions of most aeolian sand transport models are based on flat sand beds without considering the effect of complex surface and slope on the structure of aeolian sand flow. In this paper, we used particle image velocimetry (PIV) to measure the state of motion of aeolian sand particles over the leeward slope area, analyzed the characteristics of these sands particles with height H above the surface on the leeward slope, including the relationship of their mean speed to the slope height and the incoming wind speed, and the relationship of particle size with height. Finally, we proposed a formula to describe the linear relationship of the dust emission rate to the incoming wind speed on the bed surface in the leeward slope area. Our conclusions are of significance for the study of wind-driven sand flow in the leeward slope.

Issues in Eulerian–Lagrangian modeling of sediment transport under saltation regime

The saltation regime is very important for understanding the sediment transport mechanism. However, there is no consensus on a model for the saltation regime. This study answers several questions raised with respect to the Eulerian–Lagrangian modeling of sediment transport. The first question is why the previous saltation models that use different combinations of hydrodynamic forces yielded acceptable results? The second question is which shear lift model (i.e. a shear lift expression and its coefficient) is more appropriate? Another important question is which hydrodynamic forces have greater contributions to the saltation characteristics of a sediment particle? The last question is what are the contributions of the turbulence fluctuations as well as effects of using two- and three-dimensional (2D and 3D) models on the simulation results? In order to fairly answer these questions, a systematic study was done by considering different scenarios. The current study is the first attempt to clearly discuss these issues. A comprehensive 3D saltation model for non-cohesive sediment was developed that includes all the hydrodynamic forces acting on the particle. The random nature of sediment transport was included using turbulent flow and bed-particle collision models. The eddy interaction model was applied to generate a 3D turbulent flow field. Bed-particle collisions were considered using the concept of a contact zone and a corresponding contact point. The validation of the model was done using the available experimental data for a wide range of sediment size (0.03 to 4.8 cm). For the first question, the results indicated that some of the hydrodynamic effects show opposing trends and some have negligible effects. With these opposing effects it is possible to adjust the coefficients of different models to achieve acceptable agreement with the same experimental data while omitting some aspects of the physics of the process. A suitable model for the shear lift force was developed by linking the lift coefficient to the drag coefficient and the contributions of the hydrodynamic forces and turbulence fluctuations as well as the consequences of using of 2D and 3D models were studied. The results indicate that the shear lift force and turbulent flow fluctuations are important factors for the saltation of both sand and gravel, and they cannot be ignored.

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.

Analysis of variations in the saltating sand grain transport velocity

A technique is developed for retrieving the vertical profile of the mass saltation flux from measurements of the saltating sand mass concentration and wind velocity in the surface air layer. The technique is based on the solution of direct and inverse problems of the saltation sand dynamics and aimed at determining the sliding coefficient of sand grains. The calculation results for the sliding coefficient of sand grains are compared with data of measurements in wind channels. The conditions under which saltating sand trajectories fall in the height ranges from 5 to 10 mm and from 10 to 20 mm are revealed. The effect of variations in the diameter, lift-off velocity and angle, and friction velocity on the sliding coefficient of sand grains is analyzed. The sliding coefficient for 100-μm diameter sand particles is estimated for the first time.

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.

Sand transportation and reverse patterns over leeward face of sand dune

Sand saltation has complex interactions with turbulent flow and dune form. Most models of wind-blown sand consider ideal circumstances such as steady wind velocity and a flat surface, and the bulk of data on wind flow and sand transport over an individual dune has focused mostly on the influence of dune shape or inter-dune space on the wind flow, neglecting the effect of morphology on sand saltation, particularly airflow and sand transportation over the leeward slope. Wind flow structures over the leeward slope of sand dunes have a fundamental influence on the organization of sand dunes. In order to understand sand dune dynamics, lee face airflow and sediment transportation should be paid more attention. Previous field observations could not measure turbulent flow structure well because of the limited observation points and the influence of experiment structure on wind field. In addition, the reverse sand particles over leeward face could not be collected by sand trap in field. Numerous field observations could not measure turbulent flow structure because of the limited observation points and the influence of experimental structures on the wind field. In addition, the reverse transport of sand particles over leeward face could not be collected by sand traps in field. Therefore, this paper aims to investigate the turbulent flow structure and sand transport pattern over the leeward slope. A numerical model of sand saltation over slope terrain is constructed, which also considers the coupling effects between air flow and sand particles. The large eddy simulation method is used to model turbulent flow. Sand transport is simulated by tracking the trajectory of each sand particle. The results show that terrain significantly alters the turbulent air flow structure and wind-blown sand movement, especially over the leeward slope. Here, mass flux increases initially and then decreases with height in the reversed flow region in the direction of wind flow, and the mass flux decreases with height in the reversed direction. The height of 0.5H is the height of vortex core in the reversed flow region. The vortex core is a critical point in the flow region where few particles are transited. In the reversed region, the reversed mass flux of sand particles is 25% of the mass flux in the flow direction. This research may contribute to scientific understanding of the mechanisms of sand motion and wind flow over leeward of dune and it is likely to be significant in desertification control.

The analysis of variations in the saltating sand transfer velocity

The analysis of variations in the saltating sand transfer velocity