Sand Saltation Research Articles

Laboratory measurement of saltating sand particles' angular velocities and simulation of its effect on saltation trajectory

This paper reports a laboratory observation of spin attitudes and angular velocities of saltating sand particles in wind‐blown sand flux by using the high‐speed and dynamic cinecamera and presents a numerical simulation to show the effect of spin on the saltating trajectories of sand particles. Experiment results show that a saltating sand particle has two basic spin attitudes, rolling spin (Ωx, which is perpendicularity to wind direction and parallel with sand surface) and left/right spin. The percentages of the former and the latter attitude are 5% and 95%, respectively. The left/right spin angular velocities range from 0 revolutions per second (“rev/s” henceforth) to 800 rev/s and obey a single‐peaked distribution, the peak value of which lies in (150 rev/s, 250 rev/s). The rolling spin angular velocity of a saltation sand particle is variational along its entire saltating trajectory. The left/right spin vector is composed of two spin components, Ωy (called lateral spin component, rotating around the wind direction) and Ωz (called up spin component, rotating around the axis perpendicular to the sand bed). The theoretical simulation indicates that lateral and up spin components not only have effects on the trajectories' scales (i.e., heights and lengths) but also have effects on the trajectories' dimensions, especially when they are higher than 2000 rev/s and 200 rev/s, respectively. While the rolling spin angular velocities only change the trajectories' heights and lengths, especially for the rolling angular velocity higher than 300 rev/s.

Visualization of saltating sand particle movement near a flat ground surface

Movement of natural sand particles (d=200-300 μm) in a simulated atmospheric boundary layer was visualized using a digital high-speed camera. The consecutive particle images recorded at 2000 fps (frame per second) enabled us to observe the particle transport in detail, especially near the flat sand surface. Various modes of sand saltation were identified. The transverse motion of particles, often ignored in previous studies, was also visualized. In addition, instantaneous velocity fields of saltating particles were obtained using a particle tracking velocimetry (PTV), and statistical analysis of saltating particle trajectories was performed. The qualitative and quantitative results of the present study will be useful for understanding the basic physics of transport of saltating sand particles.

Effects of the Magnus and Saffman forces on the saltation trajectories of sand grain

Saltating sand grains are the primary component of airborne sand and account for 75% of all sand transport flux. The saltation height and horizontal distance traveled by sand grains are key factors in sand-control engineering. In addition to gravity and aerodynamic drag, the Magnus and Saffman forces also play important roles in saltation. To quantify the magnitudes of these forces in saltation we used high-speed multi-flash photography to observe the movement of saltating sand grains in a wind tunnel; this proved to be an efficient technique for determining the movement and rotational velocities of grains of natural sand with grain sizes ranging from 0.2 to 0.3 mm and shear velocities ( u ⁎) of 0.67, 0.83, and 0.87 m s − 1 . The rotational speed of saltating sand grains varied between 200 and 800 rev s − 1 ; mean clockwise and anticlockwise rotational speeds were nearly identical, and both increased with increasing saltation height. With saltation heights divided into 1 cm intervals, the rotational speeds followed a Lorentzian distribution. Calculations based on a saltation model showed that the maximum increases in saltation height and in horizontal distance due to the Magnus force were 10.2 and 24.9%, respectively. The rate of increase of both parameters increased with increasing lift-off angle. The maximum increases in saltation height and horizontal distance of sand grains caused by the Saffman force were only 4.6% and 3.7%, respectively.

Two-phase measurements of wind and saltating sand in an atmospheric boundary layer

Wind-blown sand movement is a particle-laden two-phase flow related to wind erosion in which the velocity distributions of both wind and sand are of particular interest. In the present study, two types of natural sand, one collected from the Pohang beach (diameter d = 200–300 μm) in South Korea and the other from the Taklimakan desert ( d = 100–125 μm) in China, were tested in a simulated atmospheric boundary layer. A high-speed digital camera system was used to capture images of the saltating sand particles at 2000 fps with an exposure time of 1/3000 s. Instantaneous velocity fields of the saltating sand particles were obtained using a particle tracking velocimetry (PTV) method. From these data, the particle resultant velocity, volume concentration, and streamwise mass flux were estimated as a function of height. The results reveal that the resultant particle velocity has an approximate log-linear profile with vertical height. Both the particle concentration and streamwise mass flux decay dramatically in the near surface region ( z < 20 mm for the beach sand, and z < 15 mm for the desert sand), then decline mildly beyond this region. To investigate the modification of the surrounding wind by the saltating sand particles, a hot-wire anemometry with a robust hot-film probe was employed to measure the wind velocity profiles with and without saltation. The present experimental data on both the saltating sand and wind provide useful information that enhances our understanding of saltation transport and further development of control techniques of wind erosion.

Probability distribution functions for the initial liftoff velocities of saltating sand grains in air

Saltating sand grains are the primary component of airborne sand and account for 75% of all transport flux of sand grains. Although they have been widely studied, the microscopic and macroscopic aspects of blown sand physics have not been united, and this has slowed development of this field. The main reason for this is that the bridge (probability distribution functions for initial liftoff velocities of saltating sand grains) between the macroscopic and microscopic research has not been satisfactorily solved because it is difficult to measure the initial liftoff parameters of saltating sand grains and because the underlying theory is lacking. In this paper, we combined theoretical analyses with wind tunnel experiment data to describe the liftoff parameters of saltating sand grains (the horizontal, vertical, and resultant liftoff velocities and angles). On the basis of these data, the liftoff angles follow a LogNorm4 distribution function, whereas the horizontal, vertical, and resultant liftoff velocities follow a Gamma distribution function. We also demonstrated that it is feasible to colligate initial liftoff velocities of saltating sand grains obtained under different frictional wind velocities by different scholars in wind tunnel experiments and comprehensively analyze their distributions. Therefore the distribution functions of initial liftoff velocities of saltating sand grains presented in this paper do a good job of reflecting the underlying physics.

Tracking of saltating sand trajectories over a flat surface embedded in an atmospheric boundary layer

Saltation is a major mechanism for the transport of soil particles. In the present study, we carried out wind tunnel tests to examine the saltating trajectories of two types of natural sand collected from a beach (diameter, d = 300–500 μm and 200–300 μm respectively) as well as sand from the Taklimakan desert ( d = 100–125 μm) in an atmospheric boundary layer. Consecutive images of saltating particles were recorded using a high-speed digital camera at a rate of 2000 fps with a spatial resolution of 1024 × 1024 pixels. The high temporal resolution of the acquired images enabled us to study the particle motion very close to the surface. The saltating particle trajectories were reconstructed from consecutive images, and the physical quantities characterizing the initial and final stages of the particle flight in the windward direction at friction velocities of about 10%–25% above the threshold friction velocity ( u ⁎ / u ⁎ t = 1.11–1.26) were analyzed statistically. In addition, the transverse deviation of the saltating particles from the main streamwise direction was evaluated. The results shed new light on the complicated motions involved in sand saltation and should prove useful in the evaluation and formulation of theoretical models.

Dune migration and slip face advancement in the Rabe Crater dune field, Mars

Eight overlapping images of a dune slip face in Rabe Crater (35°E, 44°S) from the Mars Global Surveyor Mars Orbiter Camera show changes interpreted to be multiple grainflow events that would indicate present‐day sand saltation and dune migration. New occurrences of these features appear sporadically throughout late southern summer and early fall, and then no further changes occur throughout winter. By the following summer the pattern of old streaks had been almost completely covered by new dark streaks. Assuming that this activity is typical from year to year, migration rates are estimated to be on the order of 1–2 cm per martian year, produced by south to southeasterly winds that blow mostly during the southern spring and early summer. This slow migration rate is consistent with a present‐day sediment state that is either transport or availability limited.

Particle dynamics method simulations of stochastic collisions of sandy grain bed with mixed size in aeolian sand saltation

This paper presents a statistical approach to predict the behavior of liftoff grains on the basis of simulation of random collisions between incident grains and the mixed sand bed by means of the particle dynamics method (PDM). In the simulation a two‐dimensional sand bed of grain‐grain contact between the mixed size grains, which are simplified by circular disks, is stochastically generated by a random number generator and the “particle cooling” technique. After the diameter distribution of incident grains is divided into n subregions of representative diameters, first, the collisions are simulated for each representative diameter of incident grains with an incident velocity impacting the mixed sand bed, for which the information about liftoff grains may be gained at the end of collisions. After that a statistic approach is proposed to get the average values of velocity and number of the ejecting and rebounding grains as well as the probability density function (pdf) of liftoff velocity in terms of magnitude and angle. In order to guarantee steady state of the statistical quantities, 110 samples of the random collisions of each representative diameter of grains with an incident velocity but different (or random) contact position are taken in a case study in which the average diameter of the grains is the same as that of the uniform sand grains employed by Anderson and Haff (1991). The numerical results show that there are notable differences in the statistical quantities between them, and some of the results obtained here are in good agreement with the measurement data of multiple grain size. Finally, the pdf of initial velocity in terms of both magnitude and angle is derived on the basis of the simulation results.

Thermal properties of sand from Thermal Emission Spectrometer (TES) and Thermal Emission Imaging System (THEMIS): Spatial variations within the Proctor Crater dune field on Mars

Thermal inertia, a parameter calculated from surface temperatures obtained from spacecraft, has long been used to quantify the amount of loose, fine‐grained material on the Martian surface. With little “ground truth” available, studies often refer to Martian dune fields to calibrate thermal inertias. The well‐understood physical properties of dune sand make it an ideal basis for comparison to more complex surfaces. However, higher‐resolution data sets available from the TES (Thermal Emission Spectrometer onboard Mars Global Surveyor) and THEMIS (Thermal Emission Imaging System onboard Mars Odyssey) show spatial variations in the thermal properties within dune fields, calling into question their effectiveness as controls for thermal inertia studies. In order to explain these variations, we apply a thermal model developed for TES data to a commonly investigated dune field in Noachis Terra, that on the floor of Proctor Crater. We show that in this dune field, the thermal variations on the scale of 30 J m−2 s−0.5 K−1 are present and correlate spatially with aeolian features in the dune field. These variations correspond to three types of surfaces observed in the Mars Orbital Camera Narrow Angle (MOC NA) images: (1) dune sand, (2) interdunes exposing the surface underlying the dune field, and (3) sand‐covered interdunes, or dune troughs. Both the interdunes and the dune troughs have cooler nighttime temperatures than the dune sand, corresponding to lower thermal inertia values. The dune troughs may be sand‐covered areas with either minor amounts of dust accumulation or a mean sand grain size lower than that of dune sand. Because fine sand grains tend to preferentially accumulate on dune crests rather than in dune troughs, the second hypothesis is considered less likely than the first. This has implications for the recent sedimentary history of the dune field: Dust accumulation in dune troughs may imply that sand saltation is not prevalent enough to scour away all of the dust settling out from suspension in the atmosphere; however, it is prevalent enough to keep the dunes crests themselves clear of dust.

Eulerian–Lagrangian simulation of aeolian sand transport

A numerical investigation of aeolian sand transport is performed with an Eulerian–Lagrangian model. In this model, the gas phase is described by the volume-averaged Navier-Stokes equations of two-phase flow. The particle motion is obtained by solving Newton's second law of motion taking into account the inter-particle collisions, where a soft sphere model is used to describe inter-particle collisions. The dynamic process of aeolian sand transport is simulated. The simulation results show that the variation of mean horizontal velocity of the particles with height can be expressed by a logarithmical function or a power function at h > 0.02 m, and the power function can be described below 0.02 m. The sand mass flux decreases exponentially with height for h > 0.02 m, but there is a deviation from the exponential decay due to the creep grains in the near-bed region. It is also shown that the inter-particle collisions play an important role in sand saltation. Therefore the present numerical model is capable of being applied to the study of windblown sand movement.

The effect of electrostatic force on the evolution of sand saltation cloud

In a wind-blown sand layer, it has been found that wind transport of particles is always associated with separation of electric charge. This electrification in turn produces some electrostatic forces in addition to the gravitational and fluid friction forces that affect the movement of saltating sand particles, further, the wind-blown sand saltation. To evaluate this effect quantitatively, this paper presents a simulation of evolution of wind-blown sand grains after the electrostatic forces exerted on the grains are taken into account in the wind feedback mechanism of wind-blown saltation. That is, the coupling interaction between the wind flow and the saltating sand particles is employed in the simulation to the non-stationary wind and sand flows when considering fluid drag, gravitation, and a kind of electrostatic force generated from a distribution of electric field changing with time in the evolution process of the sand saltation. On the basis of the proposed simulation model, a numerical program is given to perform the simulation of this dynamic process and some characteristic quantities, e.g., duration of the system to reach the steady state, and curves of the saltating grain number, grain transport rate, mass-flux profile, and wind profile varying with time during the non-stationary evolution are displayed. The obtained numerical results exhibit that the electrostatic force is closely related to the average charge-to-mass ratio of sand particles and has obvious influence on these characteristic quantities. The obtained results also show that the duration of the system to reach the steady state, the sand transport rate and the mass flux profile coincide well with experimental results by Shao and Raupach (1992) when the average charge-to-mass ratio of sand particles is 60 microC/kg for the sand particles with average diameter of 0.25 mm. When the average charge-to-mass ratios of sand particles are taken as some other certain values, the calculation results still show that the mass flux profiles are well in agreement with the experimental data by Rasmussen and Mikkelsen (1998) for another category of sand particles, which tell us that the electrostatic force is one of main factors that have to be considered in the research of mechanism of wind-blown sand saltation.

On possible release of microbe-containing particulates from a Mars lander spacecraft

Due to possible planet contamination, before Earth-departure, Mars landers and/or rovers are subject to strict requirements on the maximum number of attached spores or particles that carry viable microbes. Estimates of the release rates of these particles on Mars are made considering the three mechanisms of wind shear, collision with suspended dust, and collision with saltating sand particles. The first mechanism is found to apply only to particles of size greater than 10 μ m , the second mechanism has a characteristic particle adhesion half life that is so long as to be of no concern, and the third mechanism is deemed of possible importance, vitally depending on attached particle size and detailed surface characteristics of sand and spacecraft. While not investigated in detail, dust devils are shown to be possible contributors to release of microbe-containing particles.

Splash–Saltation of Sand due to Wind‐Driven Rain

Transport of sediment under wind‐driven rains is generally not accounted for in equations for sediment transport by wind. However, the contribution of this rainsplash–saltation process can be substantial. Wind‐tunnel experiments, in which horizontal fluxes were measured at four heights above a sand surface, were conducted to study sediment transport under wind‐driven rain and rainless wind conditions. It was shown that the horizontal flux could be described by a single exponential equation under both conditions. By integration of the horizontal flux over the height of rainsplash–saltation and saltation, respectively, the sediment transport rate was computed. Hence the obtained data set was used to validate the sediment transport models of Cornelis et al., which were developed from measurements of vertical deposition fluxes under wind‐driven rain and rainless wind conditions. The data followed the models very well, which suggests that they are adequate to predict the transport rate of sediment under wind‐driven rain and rainless wind.

Splash–Saltation of Sand due to Wind-Driven Rain

Splash–Saltation of Sand due to Wind-Driven Rain

Splash–Saltation of Sand due to Wind-Driven Rain

Splash–Saltation of Sand due to Wind-Driven Rain

Splash–Saltation of Sand due to Wind‐Driven Rain

Although transport of sediment under wind‐driven rains is generally not accounted for in equations for sediment transport by wind, the contribution of this rainsplash–saltation process can be substantial. Wind‐tunnel experiments, in which vertical deposition fluxes were measured at 23 distances from a sand tray, were conducted to study sediment transport under wind‐driven rain and rainless wind conditions. It was shown that the vertical deposition flux could be described by a double exponential equation. By integration of the vertical deposition flux over the distance of deposition, the sediment transport rate was computed. A power‐law function including both the normal component of the kinetic energy or momentum of the raindrops and the wind shear velocity was presented. However, including the wind shear velocity in the equation increased the model performance only slightly. When comparing the sediment transport rates as determined under the wind‐driven rain events with those that were observed when rain was absent, it was shown that in the latter case, the transport rate is much higher at high wind shear velocities. However, at low wind shear velocities and moisture conditions where no motion is predicted by aeolian equations, saltation due to rainsplash is likely to occur and can be predicted with the presented model.

Structural variations of sand-bearing airflow over dune at southeastern fringe of Tengger Desert

From the analytical results of observed data on sand transport rates at different positions of dune surface at southeastern fringe of Tengger Desert, it has been found that the vertical distribution of blown sand flux exhibits a noticeable variation when sand grains move upslope and downslope of dune surface under the action of wind force. Within the height of 20 cm above the sand surface, the vertical distribution of mass flux at different positions from toe to dune crest of stoss slope of sand dunes coincides with a single exponentially decaying law; while the vertical distribution of mass flux over the lee slope occurs as two variable zones, with the height of 8–12 cm as the dividing line, the sand transport rate below this height exponentially decreases with increasing height, but above this height decreases in a power function law. On the stoss slope, the relative sand transport rate in the upper layer of sand flow tends to decrease with the increasing wind velocity and the total sand transport rate towards the dune crest due to the shortened trajectory length of saltation sand grains moving upslope. On the lee slope, the increase in lift-off height and trajectory length of saltation sand grains moving downslope leads to the increase in relative sand transport rate in the upper layer of sand flow.

Splash-saltation trajectories of soil particles under wind-driven rain

It is usually recognized that relatively large amounts of soil particles cannot be transported by raindrop splashes under windless rain. However, the splash-saltation process can cause net transportation in the prevailing wind direction since variations in splash-saltation trajectory due to the wind are expected in wind-driven rain. Therefore, determining the combined effect of rain and wind on the process should enable improvement of the estimation of erosion for any given prediction technique. This paper presents experimental data on the effects of slope aspect, slope gradient, and horizontal wind velocity on the splash-saltation trajectories of soil particles under wind-driven rain. In a wind tunnel facility equipped with a rainfall simulator, the rains driven by horizontal wind velocities of 6, 10, and 14 m s −1 were allowed to impact three agricultural soils packed into 20×55 cm soil pans placed at both windward and leeward slopes of 7%, 15%, and 20%. Splash-saltation trajectories were measured by trapping the splashed particles at distances downwind on a 7-m uniform slope segment in the upslope and downslope directions, respectively, for windward and leeward slopes. Exponential decay curves were fitted for the mass distribution of splash-saltation sediment as a function of travel distance, and the average splash-saltation trajectory was derived from the average value of the fitted functions. The results demonstrated that the average trajectory of a raindrop-induced and wind-driven soil particle was substantially affected by the wind shear velocity, and it had the greatest correlation ( r=0.96 for all data) with the shear velocity; however, neither slope aspect nor slope gradient significantly predicted the splash-saltation trajectory. More significantly, a statistical analysis conducted with nonlinear regression model of C 1( u * 2/ g) showed that average trajectory of splash saltation was approximately three times greater than that of typical saltating sand grain.

A Theoretical Study of the Distribution of the Initial Velocity of Saltating Sand Particles by Collision

A Theoretical Study of the Distribution of the Initial Velocity of Saltating Sand Particles by Collision

Laboratory measurement of electrification of wind‐blown sands and simulation of its effect on sand saltation movement

This article presents a measurement of electrification generated by wind‐blown sands in a field wind tunnel and a numerical methodology to simulate the effect of electrification on the sand saltation movement after the mutual couple interaction between the sand movement and the wind flow is taken into account. The measured data of electric charge on the “uniform” sands in the wind‐tunnel tests show that the sign of electric charge, either negative or positive, is mainly dependent on the diameter size of sand particles, i.e., negative charge is gained when the diameter is smaller than 250 μm and positive charge is obtained if the diameter is larger than 500 μm, and that for both “uniform” and mixed sands, the average charge‐to‐mass ratio decreases with increasing the wind velocity, and increases with height from sand bed. Meanwhile, the measurement of electric field in wind‐sand cloud related to the electric charge displays that the magnitude of electric field increases generally as the wind velocity and the height increase, and the direction of the field is always upwardly vertical to the Earth's surface, which is opposite to that of the fair‐weather field. In order to exhibit the effect of electrification on sand saltation movement, a theoretical model by considering the mutual coupling interaction between wind flow and sand movement is proposed after the electric force exerted on the moving sands is considered. Through solving the nonlinear coupling dynamic equations by a proposed program, the effect of electrification on sand saltation motion, e.g., trajectory, is discussed quantitatively. After that, its effect on wind‐sand transport flux, sand ejecta flux, and wind profile is also displayed. The results show that the prediction for the Bagnold's kink is good agreement with the measurement in literature.