Mixed Sand Bed Research Articles

Blowing sand flow profiles with different particle sizes and blowing sand load characteristics on low-rise buildings

To obtain blowing sand flow profiles with different particle sizes and then assess blowing sand loads on buildings, we simulated blowing sand climates with three types of particle sizes respectively and performed the overall force tests for low-rise buildings by a blowing sand tunnel. We found that all particle concentration profiles reveal an exponential decline. The particle concentrations with coarse sand bed, fine sand bed and mixed sand bed increase successively with increasing experimental airflow velocity, and their ability to weaken airflow velocities and enhance turbulence characteristics also increase correspondingly. Wind loads in blowing sand climates are smaller than those in wind fields free from impurities, and wind loads in coarse particle, fine particle and mixed particle flow fields decrease successively, but impact loads of particles and the ratio of blowing sand loads to wind loads increase successively. Compared with airflow free from impurities, the average shear coefficients for structures in coarse particle flow, fine particle flow and mixed particle flow increase by 3%∼5%, 7%∼10%, and 11%∼14%. Blowing sand flows could increase pulsating shear coefficients for structures when the experimental airflow velocity is below 16.68 m/s, but blowing sand flows reduce pulsating shear coefficients for structures when the experimental airflow velocity exceeds 16.68 m/s. To ensure structural safety, the blowing sand resistance design for structures should consider both impact loads and wind loads.

Grain size ratio may be crucial for the fluid initiation of martian sand grains

The active Aeolian sand movement on the Martian surface has been puzzling scientists for several decades because the existing theories do not support the initiation of sand grains under the current Martian environment. In this study, I theoretically calculated the fluid threshold (u⁎ft) for the initiation of sand grain movement by considering the gravitational force, the drag force, the lift force, and the cohesive force. For a fixed grain diameter, u⁎ft calculated by considering the wind turbulence is generally less than the values obtained from well-known initiation threshold expressions. It also reveals that wind turbulence has a significant impact on u⁎ft of grains with diameters of greater than 50 μm on Earth and for those with diameters of greater than 500 μm on Mars. The grain size ratio (the ratio of a grain diameter on the bed surface versus the averaged diameter of a mixed sand bed) is a key parameter in determining u⁎ft of mixed sand beds. On Mars, the lowest u⁎ft, which occurs for mixed sand beds with average grain diameters of 50–150 μm, could be 0.6–0.7 m/s when the optimal grain size ratio is determined from the given grain size ratio range. Thus, the results of this study not only quantitatively agree with recent findings from wind tunnel experiments but also provide a theoretical explanation for the observations of active sand movement on Mars.

The saltations of different sized particles in aeolian sand transport

Wind tunnel experiments were performed in order to investigate the effect of mixing on the aeolian transport of sands with different grain sizes. Two types of sand with different grain size distributions and an equal-mass binary mixture of these sands were used. Comparing the gradients of their measured mass flux profiles and some published profiles for mixed sand transport with those for nearly uniform sand transport, it was found that for both of these types of sand bed, the negative gradient of the mass flux profile on a log-linear plot varies with the mass averaged grain size of the sand bed according to the same power law. Hence it can be deduced that, during the aeolian transport of mixed sand beds, the mean vertical ejection speeds of different sized grains are nearly identical to that for the transport of monospecific-sized sand with the same mass averaged grain size. Theoretical analysis was undertaken to explore the characteristics of ejections of different sized grains during the aeolian transport of sand with mixed-size grains. It is proposed that the mean ejection angles and the mean ejection speeds for sand grains of different sizes are nearly identical and are equivalent to that for monospecific-sized sand with the same mass averaged grain size. It was also evident that the ratio between the transport rate of each grain size group expressed as the fraction in the whole transport rate and its mass fraction in the mixed-size sand is a combined consequence of both the wind effect on the mean saltation distance of different sized grains and the effect of the mass fraction of each grain size group in the original mixture on its ejection potential responding to an impact of a saltating sand grain.

Statistical behaviors of different‐sized grains lifting off in stochastic collisions between mixed sand grains and the bed in aeolian saltation

Multiple‐size splash models are derived from the simulation results of mixed grain‐bed impact process of windblown sand flow based on the Particles Dynamics Method (PDM) and parallel algorithm. Unlike previous studies, a probability density distribution of sand diameter is considered in the present study, in which a two‐dimensional mixed sand bed is generated by a random method. After the diameter distribution of incident grains is divided into n subregions of representative diameters, first, the collisions are simulated out for each representative diameter of incident grains with an incident velocity impacting the mixed sand bed, to which the information of grains experiencing saltation liftoff may be gained at the end of collisions. After that, a statistical approach is proposed to obtain the average values of velocity and number of the ejecting and rebounding grains as well as the probability density function (PDF) of ejection particle sizes. The results confirm earlier findings that the ratio between outgoing and incoming speed remains about 60% and the ejection angles are typically between 60–80°. However, other properties of grains experiencing saltation liftoff depend not only upon incident velocity as previously argued, but also much upon grain size of incident grains. We found that rebound angle decreases and death rate of incident particles increases with incident grain size exponentially, and ejection speed and number increase logarithmically with both speed and diameter of incident grains. In addition, the PDF of initial diameter is also presented for the first time. These results well agree with the multiple‐size measurement data.

The harmonious character in equilibrium aeolian transport on mixed sand bed

Wind tunnel experiments were carried out to measure the spatial distribution in equilibrium transport of four types of mixed sand. The flux profiles of each grain size group were calculated. It is found that the vertical distribution of mean grain size has a close relation with the grain size distribution in the sand bed. In a log-linear plot, the flux profiles of main grain size groups are all straight lines and their slopes are nearly equal. It is also found that the ratio of transport rate of each size group to the whole transport rate is directly proportional to the mass ratio of each size group in the sand bed and the proportion value is only dependent on the grain size. This harmonious law is applicable to all four types of mixed sand used in the experiment.

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.

Initial motion and pivoting characteristics of sand particles in uniform and heterogeneous beds: experiments and modelling

Experiments are described in which the threshold conditions for sediment entrainment are measured for uniform and mixed sand beds beneath both steady and combined steady/oscillatory flows. Derived critical shear stresses are compared with the mixed bed entrainment model of Wiberg & Smith (1987). As predicted by the model, coarser grains within a sand mixture are entrained at lower bed shear stresses than progressively finer grains. Entrainment occurs generally at lower shear stresses than predicted by the model, especially under unidirectional flows. This may be the result of grains resting in unusually unstable positions during the experiments because the beds are ‘unworked’ at the start of the experiments.The model of Wiberg and Smith predicts threshold conditions more accurately for the mixed beds if the bed pivoting angle is correctly defined. The pivoting angles of the beds used here are measured using a new technique designed specifically for comparison with the threshold data. The measured angles repeat the finding that the coarse grains are more mobile than the finer fractions of a mixture. The results are poorly described by the pivoting angle model presented by Wiberg & Smith (1987) and are better represented by a model of the form Φ = αDγ(Di/D50)β (after Li & Komar, 1986), where α, γ and β are empirical constants. The threshold model is found to be more effective using the improved pivoting relationship.The entrainment of grains is found to be easier beneath unidirectional flows than combined flows, in accordance with previous authors’ findings. A suggestion that this result is caused by a change in the erosion mechanism beneath wave flows is made. Wave boundary layers may act as an extended laminar sublayer over bed grains and reduce the erosive efficiency of the overlying current flow. The results of the experiment have implications for the natural sorting mechanisms of sediment beds being deposited in near‐threshold flows.