Vertical Diffusion Coefficient with Stratification Effect for Silty Sediment Suspension Under Waves

To analyze previous experimental data of suspended sediment concentration for silty sediment with different sediment particle sizes due to waves, a new stratification correction coefficient is presented. The suspended sediment concentration gradient and sediment particle diameter are selected as parameters. Furthermore, a diffusion coefficient model with a stratification effect over the whole water depth for silty sediment suspension under waves is developed. The comparison between the suspended sediment concentration calculated by the presented model and several groups of experimental data shows that the model can reasonably reflect the vertical distribution of silty sediment suspension.The stratification effect calculated by the present model decreases with an increase in the sediment particle diameter,which indicates that the model can be extended to describe the suspended sediment concentration of fine to medium sand when the near-bottom sediment concentration is not very high. Although the original model needs to be iteratively solved, the approximate method without iteration is recommended for applications when the near bottom sediment concentration is between 10 and 20 kg/m~3 due to the small difference between the non-iterative and iterative solution for near bed layer suspended sediment concentration, which plays a major role in sediment transport.

Numerical Simulation of Breaking Wave Generated Sediment Suspension and Transport Process Based on CLSVOF Algorithm

The sediment suspension and transport process under complex breaking wave situation is investigated using large eddy simulation (abbreviated as LES hereafter) method. The coupled level set (LS) and volume of fluid (VOF) method is used to accurately capture the evolution of air-water interface. The wall effect at the bottom is modeled based on the wave friction term while the complicate bottom boundary condition for sediment is tackled using Chou and Fringer's sediment erosion and deposition flux method. A simulation is carried out to study the sediment suspension and transport process under periodic plunging breaking waves. The comparison between the results by CLSVOF method and those obtained by the LS method is given. It shows that the latter performs as well as the CLSVOF method in the pre-breaking weak-surface deformation situation. However, a serious mass conservation problem in the later stages of wave breaking makes it inappropriate for this study by use of the LS method and thus the CLSVOF method is suggested. The flow field and the distribution of suspended sediment concentration are then analyzed in detail. At the early stage of breaking, the sediment is mainly concentrated near the bottom area. During the wave breaking process, when the entrapped large-scale air bubble travels downward to approach the bottom, strong shear is induced and the sediment is highly entrained.

Modeling of suspended sediment transport with wave-induced longshore current in Huanghe (Yellow) River Delta

A three-dimensional suspended sediment model （SED） developed by the present authors is coupled with the combinatorial model of COHERENS （Luyten et al., 1999） （the three-dimensional coupled hydrodynamical-ecological model for Regional and Shelf Seas） and SWAN （Holthuijsen et al., 2004） （the third generation wave model）. SWAN is regarded as a subroutine of COHERENS and gets time- and space-varying current velocity and surface elevation from COHERENS. COHERENS gets time- and space- varying wave relevant parameters provided by SWAN. Effects of wave on current are applied in bottom shear stress, wave-induced depth-dependent radiation stress and surface drag coefficient calculation. At the same time, the damping function of suspended sediment on turbulence is introduced into COHERENS. So the sediment model SED has feedback on circulation model COHERENS. The SED obtains current associated parameters from COHERENS. Then a coupled hydrodynamic-sediment model COHERENS-SED being able to account for interaction between wave and current is obtained. COHERENS-SED is adopted to simulate three-dimensional suspended sediment transport in the Huanghe River delta. In terms of simulation results, there is obvious difference between top and bottom layer of wave-induced longshore current. The values of time series of sediment concentration gotten by COHERENS-SED have, generally, an accepted agreement extent with measurement. Significant wave heights and wave periods obtained by COHERENS-SED show that wave simulation case with currentts effect can give better agreement extent with measurement than case without current＇s effect. In the meantime, suspended sediment concentration distributing rule obtained by COHERENS-SED is similar to former researches and measurement.

Numerical simulation of sediment transport in coastal waves and wave-induced currents

Prediction of coastal sediment transport is of particularly importance for analyzing coast erosion accurately and solving the corresponding coast protection engineering problems.The present study provided a numerical scheme for sediment transport in coastal waves and wave-induced currents.In the scheme,the sand transport model was implemented with wave refraction-diffraction model and near-shore current model.Coastal water wave was simulated by using the parabolic mild-slope equation in which wave refraction,diffraction and breaking effects are considered.Wave-induced current was simulated by using the nonlinear shallow water equations in which wave provides radiation stresses for driving current.Then,sediment transport in waves and wave-induced currents was simulated by using the two-dimensional suspended sediment transport equations for suspended sediment and the bed-load transport equation for bed load.The numerical scheme was validated by experiment results from the Large-scale Sediment Transport Facility at the US Army Corps of Engineer Research and Development Center in Vicksburg.The numerical results showed that the present scheme is an effective tool for modeling coastal sediment transport in waves and near-shore currents.

Settlement prediction model of slurry suspension based on sedimentation rate attenuation

This paper introduces a slurry suspension settlement prediction model for cohesive sediment in a still water environment. With no sediment input and a still water environment condition, control forces between settling particles are significantly different in the process of sedimentation rate attenuation, and the settlement process includes the free sedimentation stage, the log-linear attenuation stage, and the stable consolidation stage according to sedimentation rate attenuation. Settlement equations for sedimentation height and time were established based on sedimentation rate attenuation properties of different sedimentation stages. Finally, a slurry suspension settlement prediction model based on slurry parameters was set up with a foundation being that the model parameters were determined by the basic parameters of slurry. The results of the settlement prediction model show good agreement with those of the settlement column experiment and reflect the main characteristics of cohesive sediment. The model can be applied to the prediction of cohesive soil settlement in still water environments.

CHARACTERS OF SEDIMENTATION OF SUSPENSION FLOCCULATED OR COAGULATED

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Plotting the Clarifying Curves and Determination of the Specific Amount of Settled Material during the Initial Period to Sedimentation in Stationary Column of Aqueous Suspensions of Solids

The present paper presents an algorithm and a program for plotting the clarifying curves and for determination of the specific amount of settled material during the initial period based on data obtained from sedimentation experiments in stationary column of aqueous suspensions of solids particles with different concentrations. The algorithm and program have been developed for an interactive, rapid and convenient processing of the data obtained from sedimentation experiments in stationary column of aqueous suspensions of solids particles and allow obtaining with accuracy of precise and expressive graphs which characterize the behaviour of aqueous suspensions to sedimentation in stationary column. Sedimentation study of different aqueous suspensions of solids particles in stationary column is of great importance for experimentally determining the important parameters required to design and exploitation clarifiers and sludge thickeners from waste water treatment plants.

Flow Behavior of Clay-Silt to Sand-Silt Water-Rich Suspensions at Low to High Shear Rates: Implications for Slurries, Transitional Flows, and Submarine Debris-Flows

Water-rich clay to sand suspensions show a shear rate dependent flow behavior and knowledge of the appropriate rheological model is relevant for sedimentological, industrial and hydraulic studies. We present experimental rheological measurements of water-rich(40 to 60 wt%) clay to silt(population A) and silt to sand(population B) suspensions mixed in different proportions. The data evidence a shear rate dependent shear thinning-shear thickening transition. At lower shear rates, the suspensions organize in chains of particles, whereas at higher shear rates, these chains disrupt so increasing the viscosity. The viscosity, consistency and yield stress decrease as the A+B fraction decreases as the content of B particles increases. This behavior reflects the competing effects of the lubrication and frictional processes as a function of particle size and water content. Transitional flows form by the incorporation of small amounts of the finer fraction while ‘oceanic floods’ form at the estuary of rivers and the submarine debris-flows increase their velocity by incorporating water. The critical Reynolds number of the studied suspensions is ～2000±100 suggesting that the grainsize plays a major role in the laminar to turbulent transition. Our results have implications for the modeling of sediment flows and the hazard related to floods.

悬浮区对竖流沉淀效果影响研究

竖流沉淀依靠絮体自然沉淀和悬浮区拥挤沉淀实现固液分离,适度絮凝能充分发挥二者的协同作用,降低药剂投加量,提高沉淀效果。为了明确悬浮区的作用,开展了连续流竖流沉淀试验,考察测试不同工况下絮体的形态、悬浮区状态、沉淀效果等变化规律,重点分析了轻度絮凝、中度絮凝和重度絮凝3个典型的工况。结果表明:轻度絮凝的絮体自然沉降效能和悬浮区拦截效果均不佳;重度絮凝虽然絮体自然沉降效果好,但是悬浮区拦截效果较差;只有中度絮凝可以较好地平衡絮体自然沉降和悬浮区拦截的协同作用,沉淀效果最好,对应的絮体分型维数为1.83,密度为1.391 g/cm^(3),悬浮区高度为2.2 m左右。

Field Observation and Analysis of Wave-Current-Sediment Movement in Caofeidian Sea Area in the Bohai Bay, China

In order to study the mechanism of flow-sediment movement, it is essential to obtain measured data of water hydrodynamic and sediment concentration process with high spatial and temporal resolution in the bottom boundary layer （BBL）. Field observations were carried out in the northwest Caofeidian sea area in the Bohai Bay. Near 2 m isobath （under the lowest tidal level）, a tripod system was installed with AWAC （Acoustic Wave And Current）, ADCP （Acoustic Doppler Current Profilers）, OBS-3A （Optical Backscatter Point Sensor）, ADV （Acoustic Doppler Velocimeters）, etc. The accurate measurement of the bottom boundary layer during a single tidal period was carried out, together with a long-term sediment concentration measurement under different hydrological conditions. All the measured data were used to analyze the characteristics of wave-current-sediment movement and the BBL. Analysis was performed on flow structure, shear stress, roughness, eddy viscosity and other parameters of the BBL. Two major findings were made. Firstly, from the measured data, the three-layer distribution model of the velocity profiles and eddy viscosities in the wave-current BBL are proposed in the observed sea area; secondly, the sediment movement is related closely to wind-waves in the muddy coast area where sediment is clayey silt： 1） The observed suspended sediment concentration under light wind conditions is very low, with the peak value generally smaller than 0.1 kg/m^3 and the average value being 0.03 kg/m^3; 2） The sediment concentration increases continuously under the gales over 6-7 in Beaufort scale, under a sustained wind action. The measured peak sediment concentration at 0.4 m above the seabed is 0.15-0.32 kg/m^3, and the average sediment concentration during wind-wave action is 0.08-0.18 kg/m^3, which is about 3-6 times the value under light wind conditions. The critical wave height signaling remarkable changes of sediment concentration is 0.5 m. The results show that the suspended load sediment concentration is mainly influenced by wave-induced sediment suspension.

低压浑水管道输沙规律研究及水流挟沙力计算

【目的】确定输水工程适宜运行的水力条件。【方法】以巴音沟河流域输水工程为研究对象,通过数值模拟的方法研究不同泥沙质量浓度、压力条件下浑水管道输沙规律及水流挟沙力特性。【结果】(1)管道断面垂向泥沙质量浓度分布受压力、体积含沙量及管径等因素的影响;当压力及管径较小时,泥沙淤积厚度为6.3~8.0 mm,压力及管径较大时,在Y=-0.07区域以下淤积较多,淤积厚度达到8.6~9.4 mm;(2)同一管径及流速下,随着体积含沙量的增大,该区域流速梯度增大,最大增幅为14%;(3)随着水流速度增大,水流挟沙力增大,悬移质质量浓度也越大,但是悬浮指标逐渐变小。【结论】在低压浑水管道中,管径越大、泥沙质量浓度越大、压力越小,水流挟沙力越小,泥沙越容易淤积。

基于多分散非球颗粒CFD-DEM的沉积物排放扩散特性研究

为从颗粒及流场尺度探究沉积物颗粒在水流作用下的悬浮与扩散机制,在离散元中引入高斯曲率模型表征非球颗粒非线性接触特性,并改进非球颗粒的拖曳力模型及孔隙模型,建立了多分散非球颗粒CFD-DEM流固耦合数值方法;通过与物理模型试验对比,验证了该方法的有效性与优越性;随后对沉积物在水流作用下的悬浮扩散过程进行模拟。研究结果表明,沉积物沉降率与颗粒入流量成正相关,与流速成负相关;而逸出率与沉降率呈相反趋势;在高初始排放量下,颗粒团簇效应促进流体动能耗散,颗粒所受重力作用超过惯性力作用,颗粒快速沉降;在低初始排放量下,较高流体动能促进颗粒水平向输运;在流速较小情况下,颗粒形状对沉积物的空间分布影响不可忽略。

Distribution of Time-Averaged Suspended Sediment Concentration Due to Waves

According to the mechanism of sediment suspension under waves, namely, the main reason of sediment suspension changes from the turbulent mixing in the bottom boundary layer to the periodic motion of the water particle near the free water surface, a three-layer model of sediment concentration distribution due to waves is presented along the whole water depth based on the concept of the finite mixing length. 1he determination of the parameters in the model is discussed and an empirical formula is suggested. Comparisons between the calculated results and the measurements indicate that the resuits of the model agree well with the data from both the large and small scale flume experiments.

Analytical solutions for sediment concentration in waves based onlinear diffusivity

Two kinds of analytical solutions are derived through Laplace transform for the equation that governswave-induced suspended sediment concentration with linear sediment diffusivity under two kinds ofbottom boundary conditions,namely the reference concentration(Dirichlet)and pickup function(Nu-mann),based on a variable transformation that is worked out to transform the governing equation intoa modified Bessel equation.The ability of the two analytical solutions to describe the profiles of sus-pended sediment concentration is discussed by comparing with different experimental data.And it isdemonstrated that the two analytical solutions can well describe the process of wave-induced suspendedsediment concentration,including the amplitude and phase and vertical profile of sediment concentra-tion.Furthermore,the solution with boundary condition of pickup function provides better results thanthat of reference concentration in terms of the phase-dependent variation of concentration.

Hydrostatic settling velocity of sediments

The settling of sediments is an accelerative process in which the concentration of the main sediments zone will heavily influence settling velocity,but the explicit relationship between the concentration and settling velocity has not been reported in literature. Here a theoretical function was built for the time dependent concentration and time dependent settling velocity of sediments;then the entire settling process reflecting concentration was shown on the basis of sediments instant\|settlement theory and mathematical method. Agreement of computed results and experimental data was found. Several governing parameters,including particle size,particle density,initial suspended sediments concentration and suspension height,were discussed with a series of calculated velocity curves. The research indicated that ⑴ the presented concentration\|velocity time relationship is rational,⑵ settling process of the sediments group with variation of concentration consists of acceleration stage,uniform motion stage and deceleration stage,and ⑶ particle size,particle density and initial suspended sediments concentration have more influence on the settling velocity than the suspension height and water temperature.

波浪条件下悬移质泥沙分布特性的数值模拟研究

在近海区域,波浪是引起掀沙的常见动力因素,而泥沙的运动又对海岸工程具有较强的参考价值,所以对波浪条件下悬移质泥沙分布特性开展数值模拟具有重要的现实意义。引入三维水动力学代码求解Navier-Stokes(NS)方程,采用流体体积法(VOF)捕捉自由水面运动,在悬沙运动模拟中,求解浓度扩散方程,并利用Van Rijn实验分析得出的经验泥沙扩散系数,较好地解决了泥沙扩散系数的难确定性问题,充分考虑非恒定流条件下对流与扩散的混合模式,实现波浪条件下悬移质泥沙分布特性的数值模拟。

Fully resolved simulations of viscoelastic suspensions by an efficient immersed boundary-lattice Boltzmann method

An efficient immersed boundary-lattice Boltzmann method(IB-LBM)is proposed for fully resolved simulations of suspended solid particles in viscoelastic flows.Stress LBM based on Giesekus and Oldroyd-B constitutive equation are used to model the viscoelastic stress tensor.A boundary thickening-based direct forcing IB method is adopted to solve the particle-fluid interactions with high accuracy for non-slip boundary conditions.A universal law is proposed to determine the diffusivity constant in a viscoelastic LBM model to balance the numerical accuracy and stability over a wide range of computa-tional parameters.An asynchronous calculation strategy is adopted to further improve the computing efficiency.The method was firstly applicated to the simulation of sedimentation of a single particle and a pair of particles after good validations in cases of the flow past a fixed cylinder and particle migration in a Couette flow against FEM and FVM methods.The determination of the asynchronous calculation strategy and the effect of viscoelastic stress distribution on the settling behaviors of one and two particles are revealed.Subsequently,504 particles settling in a closed cavity was simulated and the phenomenon that the viscoelastic stress stabilizing the Rayleigh-Taylor instabilities was observed.At last,simulations of a dense flow involving 11001 particles,the largest number of particles to date,were performed to investigate the instability behavior induced by elastic effect under hydrodynamic interactions in a viscoelastic fluid.The elasticity-induced ordering of the particle structures and fluid bubble structures in this dense flow is revealed for the first time.These simulations demonstrate the capability and prospects of the present method for aid in understanding the complex behaviors of viscoelastic particle suspensions.

引黄灌区干渠水沙运移与防淤研究

为处理山西尊村引黄灌区干渠淤积严重的问题,对一干渠做实地水沙试验,运用垂线悬浮指标、规划求解方法探究渠道中悬移质泥沙分布规律、渠道流量与泥沙淤积关系以及渠道水流挟沙能力,使其能够在一定范围内表达出水流的挟沙状况。并提出了渠道粒径所对应的临界悬浮流量,根据工况与粒径大小调整流量益于防淤;优化了水流挟沙力公式。结果表明,引水流量在4.46~24.52 m^(3)/s时,渠道泥沙含量在0.9~4.0kg/m^(3)之间,泥沙含量与流速大小呈正相关;渠道流量调整至39.6 m^(3)/s以上可使渠道内大粒径泥沙有效悬浮;优化后的挟沙力公式针对干渠泥沙计算值与实测值相关性达0.828,说明此公式能够较高程度地反映当前含沙量,为后期渠道冲淤平衡设计与分析提供支撑。

混凝沉淀工艺去除碳钢鳞皮悬浮液颗粒物实验研究

本研究针对碳钢鳞皮悬浮液中悬浮物含量高、颗粒物粒径小不易沉降,水质排放不达标等问题,采用混凝工艺进行处理。探究了氯化铁(FeCl_(3))和硫酸铁(Fe_(2)(SO_(4))_(3))这两种混凝剂的处理效果以及絮凝剂聚丙烯酰胺(PAM)用量对出水水质的影响。实验结果显示仅使用混凝剂进行处理时FeCl_(3)效果优于Fe_(2)(SO_(4))_(3),FeCl_(3)投加量为20 mg/L时,出水水质就可达到10 NTU以下。絮凝剂PAM最佳投加量为0.5 mg/L,进一步提高絮凝剂投加量会导致处理效果较低,实际工程中推荐采用FeCl_(3)+PAM组合处理,投加量为5 mg/L+0.5 mg/L或10 mg/L+0.5 mg/L,出水水质可以保证符合宝钢一类串接水的回用指标。

EFFECTS OF SUBMERGED MACROPHYTES ON SEDIMENT SUSPENSION AND NH_4-N RELEASE UNDER HYDRODYNAMIC CONDITIONS

Aquatic vegetation affects sediment suspension and nutrient release by changing the flow structure. Experiments on the influence of submerged vegetation on flow structure, sediment suspension, and NH4-N release were carried in a flume with natural submerged vegetation. Turbulence characteristics in the vegetation section were measured using a three-dimensional acoustic Doppler velocimeter. The effects of submerged vegetation on bed shear stress （ τb ）, sediment suspension, and NH4-N release were analyzed. Results show that with vegetation, bed shear stress is reduced by about 20% - 80%, which, in turn, reduces sediment suspension. The impact of submerged vegetation on sediment suspension and NH4-N release should be considered along with flow intensity. When the flow Reynolds number is relatively small, the submerged vegetation is quite capable of inhibiting sediment suspension and reducing NH4-N release, but when the Reynolds number reaches a certain value, the presence of aquatic plants exacerbates sediment suspension and promotes NH4-N release. Results also reveal that a highly significant positive correlation exists between NH4-N concentration and water turbidity in both vegetated and non-vegetated channels.