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1.
On the effect of mid-air collisions on aeolian saltation 总被引：9，自引：0，他引：9
The effect of mid-air collisions on aeolian saltation is investigated using concentration profiles and grain velocities predicted by a numerical saltation model. The probability of a mid-air collision is found to be greater at high wind speeds. It is also found that mid-air collisions tend to reduce the number of grain/bed impacts and thus reduce the number of ejecta near the bed; this, in turn, reduces the intensity of mid-air collisions. It is suggested that this feed-back mechanism significantly influences transport rates at high wind speeds. 相似文献
2.
Observations of collisions of saltating grains with a granular bed from high-speed cine-film 总被引：6，自引：0，他引：6
High-speed photography was used to record saltating sand grains colliding with a horizontal, noncohesive bed of similarly sized grains. Impacting grain/bed interaction is discussed in general. The process, as observed from the films, is then described in terms of the apparent bed contact length (ABCL) and various parameters of the impacting grains and any ejected grains. Examples are given of typical behaviour of bed grains in response to impacting grains of different sizes. Saltating grains that are large in comparison to the bed grains they encounter at collision can churn up the surface layers of soils and sediments, so that previously buried grains become available for entrainment. This process is discussed in relation to the potential release of dust particles into the airflow. 相似文献
3.
A 2D depth-averaged model for hydrodynamic sediment transport and river morphological adjustment was established. The sediment transport submodel takes into account the influence of non-uniform sediment with bed surface armoring and considers the impact of secondary flow in the direction of bed-load transport and transverse slope of the river bed. The bank erosion submodel incorporates a simple simulation method for updating bank geometry during either degradational or aggradational bed evolution. Comparison of the results obtained by the extended model with experimental and field data, and numerical predictions validate that the proposed model can simulate grain sorting in river bends and duplicate the characteristics of meandering river and its development. The results illustrate that by using its control factors, the improved numerical model can be applied to simulate channel evolution under different scenarios and improve understanding of patterning processes. 相似文献
4.
建立了波浪、底部离岸流、泥沙运动和沙滩剖面演变耦合数学模型,模拟了实验室中波浪作用下沙坝剖面的形成过程,讨论了各个物理参数对剖面上的水动力和地形变化的影响。通过参数率定,较好地复演了不规则波作用下沙滩地形由均匀斜坡向沙坝剖面的演变,波高、底部离岸流、输沙率和剖面演变的计算结果与实测数据吻合良好。采用数值实验和误差分析方法,讨论了破碎波水滚倾角、泥沙扩散系数和床面休止角等物理参数对计算结果的影响。研究表明,所建立的模型能较好地描述波浪作用下沙坝剖面形成过程中的重要物理机制。 相似文献
5.
In settings where the transport of sand is partially or fully supply limited, changes in the upstream supply of sand are coupled to changes in the grain size of sand on the bed. In this manner, the transport of sand under the supply-limited case is ‘grain-size regulated’. Since the closure of Glen Canyon Dam in 1963, the downstream reach of the Colorado River in Marble and Grand Canyons has exhibited evidence of sand-supply limitation. Sand transport in the river is now approximately equally regulated by changes in the discharge of water and changes in the grain sizes of sand on the channel bed and eddy sandbars. Previous work has shown that changes in the grain size of sand on the bed of the channel (driven by changes in the upstream supply of sand owing to both tributary floods and high dam releases) are important in regulating sand transport over timescales of days to months. In this study, suspended-sand data are analysed in conjunction with bed grain-size data to determine whether changes in the grain size of sand on the bed of the channel or changes in the grain size of sand on the surface of eddy sandbars have been more important in regulating sand transport in the post-dam Colorado River over longer, multi-year timescales. The results of this study show that this combined theory- and field-based approach can be used to deduce which environments in a complicated setting are the most important environments for regulating sediment transport. In the case of the regulated Colorado River in Marble and Upper Grand Canyons, suspended-sand transport has been regulated mostly by changes in the surface grain size of eddy sandbars. 相似文献
6.
阶梯-深潭系统是山区河流广泛分布的控制性河床结构,泥沙输移过程中大颗粒碰撞阶梯关键石块,使其发生位移,强烈影响阶梯-深潭的稳定性。以单个阶梯-深潭的关键石块为研究对象,重点考虑碰撞对阶梯-深潭的影响,量化来沙中大颗粒碰撞作用并改进稳定性理论模型,利用新模型分析阶梯-深潭的临界条件和破坏机制。来沙颗粒对关键石块的碰撞作用受自身粒径、运动速度和阶梯下游冲刷程度影响且皆为正相关关系。颗粒撞击减小阶梯失稳临界流量,且参与碰撞的石块粒径越大,减小作用越明显。当η> 0.55时（η=D1/D,D1为碰撞石块粒径,D为关键石块粒径）,临界流量下降幅度达到50%以上,表明来沙中卵石漂石对阶梯-深潭稳定性发挥主要影响。山区河流发生低频率洪水或滑坡泥石流,向下游河道输运大粒径石块并与阶梯碰撞,显著增大转动合力矩并降低失稳临界流量,使得单个阶梯-深潭更易达到临界条件发生破坏。 相似文献
7.
采用图像识别与推移质动态监测技术,开展基于双峰型非均匀推移质的系列水槽试验.通过引入反映床面粗糙度、粘性底层特性与颗粒非均匀度η(粗细比)的综合水流强度函数Ψb、特征弗劳德数Frb,系统研究了不同水流强度与床沙组成条件下的推移质输移特性以及颗粒非均匀度对输沙率的影响.通过对关键因子的辨识与量纲分析,提出了双峰型非均匀推移质输移模式,建立了基于近壁特征因子的水流强度Ψb与非均匀推移质输移强度Φ'的函数关系.对双峰型底沙输移机理的分析表明,非均匀沙的组成特征使得η成为影响Φ'的重要参量;正是细粒对粗粒的解怙作用对粗沙运动产生重要影响,使推移质输移率与颗粒非均匀度间呈现驼峰关系,峰值对应的粗细比ηc约为3∶7. 相似文献
8.
Grain size distribution in suspension from bed materials 总被引：1，自引：0，他引：1
B. S. MAZUMDER 《Sedimentology》1994,41(2):271-277
Experimental results show that the grain size distribution of suspended material is related to flow parameters and grain size distribution in the bed. A theoretical model has been developed to compute the suspension grain size distribution on the basis of diffusion equations, taking into account the effect of hindered settling due to the increased concentration in suspension. Fluid velocity closest to the bed is estimated by using the concept of migration velocities of particles in the bed layer. Comparisons of data computed by the proposed method and data from actual observations show generally good agreement. 相似文献
9.
为提高推移质试验的观测精度与效率,基于水下摄影和粒子跟踪（UP/PTV）技术,实现明槽流床面泥沙运动状态的精细试验观测。通过剔除床面颗粒震颤干扰、设置颗粒临界运动阈值和多重滤波筛选程序,提高采集样本数据的有效性。利用多组低强度推移质试验,提取床面颗粒运动轨迹、速度、单步时长等数据,并进行粒子运动的Lagrange过程分析和概率密度分布（PDF）研究。研究表明:粒间碰撞和近底紊流扫荡的影响使粒子速度在单步步长内呈现先急剧增加再缓慢衰减的变化特征;速度PDF曲线显示细尾Gamma函数特性,同时受粒子震颤效应影响,加速度PDF曲线则具有拉普拉斯分布特征;粒子速度与摩阻流速之间关系密切,保持3.4~3.5的比值;粒子单步时长与步长的联合分布呈幂函数变化规律,拟合曲线指数一般为1.25~1.3。幂律指数大小受推移质输沙强度与床面粒子异质性的影响。 相似文献
10.
An experimental study of multiple grain-size ejecta produced by collisions of saltating grains with a flat bed 总被引：11，自引：0，他引：11
Collision data are presented from coloured high-speed films of three size fractions of sand grains saltating over a bed of the total grain population. Each fraction was colour tagged and the proportion of each size ejected by grains colliding with the surface was recorded on a number of films taken as the bed was progressively eroded. The results confirm earlier findings that V3/V1?0.5–0.6, Vn/V1?.08 and the rebound angle increases with decreasing grain size. Ejected grains are examined in relation to their size, the impactor size, ejection speed and angle and the number of ejecta per collision. In addition, changes in grain parameters are observed with time. For fine impactors, ejection speeds generally increase with a decrease in ejecta size, but the fine fraction does not follow this trend for the coarse and medium impactors. Ejection angles are typically between 40° and 60°, with coarse grains having shallower mean angles than fine ejecta. The number of ejections per collision increases with a decrease in particle size for each impactor size. The general tendency for coarse particles to be ejected at lower speeds and shallower angles than fine particles will lead to sorting of the grain sizes. There is poor correlation between the forward momentum loss of the saltating grams at collision and both the forward momentum of the ejected grains and the number of ejected grains. Much of the forward momentum of the saltating grains is transfered to creeping grains. The composition and geometry of the bed are considered to be important factors in the evolution of the saltation cloud. 相似文献
11.
Discrete element method (DEM) has become a preeminent numerical tool for investigating the mechanical behavior of granular soils. However, traditional DEM uses sphere clusters to approximate realistic particles, which is computationally demanding when simulating many particles. This paper demonstrates the potential of using a physics engine technique to simulate realistic particles. The physics engines are originally developed for video games for simulating physical and mechanical processes that occur in the real world to produce realistic game experiences. The simulation accuracy and efficiency of physics engines have been significantly improved in the last two decades allowing them to be used as a scientific tool in many disciplines. This paper introduces modeling methodologies of physics engine including realistic particle representation and the contact model. Then, oedometer tests are simulated using realistic particles scanned by X-ray computed tomography (X-ray CT). The simulation results agree well with experimental results. This paper demonstrates that physics engines can output contact parameters for geotechnical analysis and force chains for visualization. 相似文献
12.
In the classical view of fine sediment transport and deposition in streams, particles are expected to be removed from flowing water simply by direct sedimentation onto the streambed. However, recent research has demonstrated that fine sediments can propagate into pore spaces in the streambed due to hyporheic exchange and be removed by a combination of physical and chemical processes. This behaviour can significantly alter fine sediment size distributions during in-stream sediment transport because the physical transport of fine particles and their attachment to bed sediment grains are both a function of the particle size. Herein, we present model simulations for deposition of suspended sediments with a bimodal size distribution. We also applied this approach to analyse the results of laboratory flume observations of suspended sediment deposition. Results from model simulations and flume experiments clearly show that the rate of particle deposition increases with increasing particle size. Thus, the larger particles are preferentially removed from mixtures and there is a fining of the mixed suspensions over time. Both particle deposition mechanisms, i.e. particle sedimentation and filtration, contribute to the fining of the mixed fine particle suspensions over time, and their effects are clearly demonstrated using the fundamental process-based model. These results clearly demonstrate the effects of stream-subsurface exchange on the temporal evolution of the suspended fine sediment size distribution in downstream transport. 相似文献
13.
Direct numerical simulation of bedload transport using a local, dynamic boundary condition 总被引：3，自引：0，他引：3
ABSTRACT Temporally and spatially averaged models of bedload transport are inadequate to describe the highly variable nature of particle motion at low transport stages. The primary sources of this variability are the resisting forces to downstream motion resulting from the geometrical relation (pocket friction angle) of a bed grain to the grains that it rests upon, variability of the near‐bed turbulent velocity field and the local modification of this velocity field by upstream, protruding grains. A model of bedload transport is presented that captures these sources of variability by directly integrating the equations of motion of each particle of a simulated mixed grain‐size sediment bed. Experimental data from the velocity field downstream and below the tops of upstream, protruding grains are presented. From these data, an empirical relation for the velocity modification resulting from upstream grains is provided to the bedload model. The temporal variability of near‐bed turbulence is provided by a measured near‐bed time series of velocity over a gravel bed. The distribution of pocket friction angles results as a consequence of directly calculating the initiation and cessation of motion of each particle as a result of the combination of fluid forcing and interaction with other particles. Calculations of bedload flux in a uniform boundary and simulated pocket friction angles agree favourably with previous studies. 相似文献
14.
The Udo tuff cone, Cheju Island, South Korea: transformation of pyroclastic fall into debris fall and grain flow on a steep volcanic cone slope 总被引：1，自引：0，他引：1
The Udo tuff cone of Cheju Island, South Korea, is a middle Pleistocene basalt tuff cone that has formed by early Surtseyan-type eruptions and later drier hydroclastic eruptions. The tuff cone comprises steep (20–30°) and planar beds of lapillistone, lapilli tuff and tuff that can be grouped into seven sedimentary facies (A-G). Facies A and B comprise continuous to lenticular layers of grain-supported and openwork lapillistone that are inversely graded and coarsen downslope. They suggest emplacement by grain flows that are maintained by gravity-induced stress and grain collisions. Facies C includes poorly sorted, crudely bedded and locally inversely graded lapilli tuff, also suggestive of rapid deposition from highly concentrated grain flows. Facies D includes thinly stratified and mantle-bedded tuff that was probably deposited by fallout of wind-borne ash. Other facies include massive lapilli tuff (Facies E), chaotic lapilli tuff (Facies F) and cross-bedded tuffaceous sandstone (Facies G) that were deposited by resedimentation processes such as debris flow, slide/slump and stream flow, respectively. The grain flows that produced Facies A, B and C are interpreted to have originated from falling pyroclasts, which initially generated highly dispersed, saltating avalanches, in which momentum was transferred by the particles themselves. This transport mechanism is similar to that of debris fall. As the slope gradient was too low to maintain a highly dispersed flow, the debris fall decelerated and contracted due to a decrease in dispersive pressure. The mode of momentum transfer changed to one of collision because contraction of the debris fall resulted in an increase in particle concentration. This transport mechanism is similar to that of common grain flows. Grain segregation occurred in several ways. Initial segregation of ash from lapilli occurred due to their differing terminal fall velocities, and their contrasting degrees of sliding friction with the bed. Percolation of ash into interstices of lapilli during flow (kinematic sieving) augmented further segregation of ash from lapilli. The latter process, along with a dispersive pressure effect, gave rise to vertical inverse size grading. Downdip inverse grading was produced by particle overpassing. 相似文献
15.
16.
Bubble–particle encounter during flotation is governed by liquid flow relative to the rising bubble, which is a function of the adsorbed frothers, collectors, and other surfactants and surface contaminants. Due to surface contamination, the bubble surface in flotation has been considered as immobile (rigid). However, surface contamination can be swept to the backside of the rising bubble due to the relative liquid flow, leaving the front surface of the rising bubble mobile with a non-zero tangential component of the liquid velocity. The bubble with a mobile surface was considered by Sutherland who applied the potential flow condition and analyzed the bubble–particle encounter using a simplified particle motion equation without inertia. The Sutherland model was found to over-predict the encounter efficiency and has been improved by incorporating inertial forces which are amplified at the mobile surface with a non-zero tangential velocity component of the liquid phase. An analytical solution was obtained for the encounter efficiency using approximate equations and is called the Generalized Sutherland Equation (GSE). In this paper, the bubble–particle encounter interaction with the potential flow condition has been analyzed by solving the full motion equation for the particle employing a numerical computational approach. The GSE model was compared with the exact numerical results for the encounter efficiency. The comparison only shows good agreement between the GSE prediction and the numerical data for ultrafine particles (< 10 μm in diameter), the inertial forces of which are vanishingly small. For non-ultrafine particles, a significant deviation of the GSE model from the numerical data has been observed. Details of the numerical methodology and solutions for the (collision) angle of tangency and encounter efficiency are described. 相似文献
17.
S. A. Junejo Qi You Zhou M. A. Talpur Dongqing Wang Jian-long Yang 《Geochemistry International》2014,52(9):794-804
Accurate prediction of solute transport processes in surface water and its underlying bed is an important task not only for proper management of the surface water but also for pollution control in these water bodies. Key issue in this task is an estimation of parameters as diffusion coefficient and velocity for solute transport both in water body and in the underlying bed. This estimation would greatly help us to understand the deposition and release mechanism of solute across the water-bed interface. In this study, a column experiment was conducted in laboratory to estimate the velocity and diffusion coefficient of sodium chloride (NaCl) in water body and underlying sand layer (bed). The column used with a diameter of 30 cm and a height of 100 cm, was filled with sand at the lower half part and water at the upper half part. Total 64 stainless steel electrodes were installed on its surface around. The sodium chloride solution was injected from the top of the column, and electrical resistance between electrodes was monitored for 71 h. Then the dimensionless resistance breakthrough curve was fitted with one dimensional analytic solution for solute transport and the related diffusion coefficient and velocity parameters were estimated. The results show that the NaCl transport velocity was high in the water body but extremely low in the underlying sand layer (bed). The diffusion coefficient estimated in sand layer coincides with those reported well. This indicates that the electrical resistance based solute transport parameter estimation method is not only effective but also has an advantage of multipoints monitoring. This is useful both in mapping solute transport parameter for solute transport process analysis and in providing parameter input for solute transport numerical modeling. 相似文献
18.
The variability of critical shear stress, friction angle, and grain protrusion in water-worked sediments 总被引：3，自引：0，他引：3
JAMES W. KIRCHNER WILLIAM E. DIETRICH† FUJIKO ISEYA‡ HIROSHI IKEDA‡ 《Sedimentology》1990,37(4):647-672
The erodibility of a grain on a rough bed is controlled by, among other factors, its relative projection above the mean bed, its exposure relative to upstream grains, and its friction angle. Here we report direct measurements of friction angles, grain projection and exposure, and small-scale topographic structure on a variety of water-worked mixed-grain sediment surfaces. Using a simple analytical model of the force balance on individual grains, we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography. The friction angle, projection, and exposure of single grain sizes vary widely from point to point within a given bed surface; the variability within a single surface often exceeds the difference between the mean values of disparate surfaces. As a result, the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution, rather than a single value. On a given bed, the crtitical shear stress distributions of different grain sizes have similar lower bounds, but above their lower tails they diverge rapidly, with smaller grains having substantially higher median critical shear stresses. Large numbers of fines, trapp.ed within pockets on the bed or shielded by upstream grains, are effectively lost to the flow. Our calculations suggest that critical shear stress, as conventionally measured, is defined by the most erodible grains, entrained during transient shear stress excursions associated with the turbulent flow; this implies a physical basis for the indeterminacy of initial motion. These observations suggest that transport rate/shear stress relationships may be controlled, in part, by the increasing numbers of grains that become available for entrainment as mean shear stress increases. They also suggest that bed textures and grain size distributions may be controlled, within the constraints of an imposed shear stress and sediment supply regime, by the influence of each size fraction on the erodibility of other grain sizes present on the bed. 相似文献
19.
基于A.Nermoen的物理实验,文章进行了浅层气溢出过程数值模拟,区别于以往常用的欧拉-欧拉方法,文章采用了基于欧拉-拉格朗日方法的新型的离散粒子模拟方法MP-PIC（multiphase particle-in-cell）模型。揭示超压气体释放的机制,以及浅层气溢出过程中速度、压力的变化情况和变化规律。模拟得到的颗粒流化现象以及气泡的产生、运动和溢出现象与试验现象相一致,数模计算结果与实验资料两者相关度为0.94,相关性较好。临界流化速度和压力与土层的高度呈正相关,临界流化压力与实验结果较为接近,但由于马格努斯力（magnus）等作用力未考虑,导致临界流化速度相比于实验偏低。该模型方法能处理任意尺寸分布的颗粒,适用庞大的颗粒量,紧密耦合气固间相互作用,适用性好,准确性高,为数值模拟提供了便利。 相似文献
20.
《Sedimentology》2018,65(2):561-581
Layered deposits of relatively light and heavy minerals can be found in many aquatic environments. Quantification of the physical processes which lead to the fine‐scale layering of these deposits is often limited with flumes or in situ field experiments. Therefore, the following research questions were addressed: (i) how can selective grain entrainment be numerically simulated and quantified; (ii) how does a mixed bed turn into a fully layered bed; and (iii) is there any relation between heavy mineral content and bed stability? Herein, a three‐dimensional numerical model was used as an alternative measure to study the fine‐scale process of density segregation during transport. The three‐dimensional model simulates particle transport in water by combining a turbulence‐resolving large eddy simulation with a discrete element model prescribing the motion of individual grains. The granular bed of 0·004 m in height consisted of 200 000 spherical particles (D50 = 500 μ m). Five suites of experiments were designed in which the concentration ratio of heavy (5000 kg m−3) to light particles (i.e. 2560 kg m−3) was increased from 6%, 15%, 35%, 60% to 80%. All beds were tested for 10 sec at a predefined flow speed of 0·3 m sec−1. Analysis of the particle behaviour in the interior of the beds showed that the lighter particles segregated from the heavy particles with increasing time. The latter accumulated at the bottom of the domain, forming a layer, whereas the lighter particles were transported over the layer forming sweeps. Particles below the heavy particle layer indicated that the layer was able to armour the particles below. Consequentially, enrichment of heavy minerals in a layer is controlled by the segregation of a heavy mineral fraction from the light counterpart, which enhances current understanding of heavy mineral placer formation. 相似文献