考虑电离作用的淤泥异向絮凝三维数值模拟

鉴于试验观察淤泥絮凝结构的技术难度,本文尝试以布朗动力学为基础,采用蒙特卡洛方法动态模拟电离作用下颗粒成长为絮团的过程.为结合实际情况,泥沙颗粒初始位置由颗粒粒径和淤泥密度决定,颗粒初始速度按照相应条件下高斯随机分布给定.边界条件用和实际符合较好的循环边界.在模拟数据分析的基础上,讨论并比较了颗粒粒径和淤泥密度对絮凝时间以及絮团开放程度的影响.另一方面,讨论了电离作用后颗粒电荷量对絮团生长的影响.解释了泥沙颗粒表面电荷密度变化对絮凝过程和絮团结构的影响,模拟结果和实际情况较为一致.     

微尺度稀薄效应下颗粒沉降的格子Boltzmann方法模拟

基于多松弛格子Boltzmann模型,对竖直细长微通道内颗粒自由沉降过程进行模拟,分析气体稀薄效应、初始位置以及颗粒间相互作用对微颗粒沉降特性的影响.研究表明：随Knudsen数增大,微通道内气体稀薄效应增强,颗粒表面气体滑移速度增大,气相流体有效粘度减小,颗粒相同运动状态下受到气体阻力相应减小,颗粒沉降平衡速度明显增大;不同初始位置颗粒沉降过程存在明显差异,初始位置偏离中心线颗粒将发生水平方向位移且呈振荡趋势,最终稳定于中心线平衡位置;在微尺度双颗粒沉降DKT现象过程中,气体稀薄效应影响颗粒运动特性,后颗粒跟随过程明显增长.     

颗粒沉降运动的虚拟区域法直接数值模拟

文采用基于四边形网格的分布式拉格朗日乘子／虚拟区域方法(DLM／FD method)对二维方槽内775个圆形颗粒在流体中的沉降过程进行了直接数值模拟。得到了颗粒流沉降过程中流体和颗粒速度和涡量分布、流场压力分布等流动细节,展示了颗粒在沉降过程中由于相间的相互作用以及颗粒间的作用,使得颗粒流在流场内形成大小不一的旋流区,颗粒回旋着沉降,同时颗粒的尾涡影响附近颗粒的运动．本文的结果说明分布式拉格朗日乘子／虚拟区域方法对模拟存在很多颗粒的悬浮体流动是可行的。     

热对流条件下颗粒沉降的格子Boltzmann方法模拟

采用格子Boltzmann方法模拟了在热对流条件下的颗粒沉降问题, 在研究单颗粒在等温流体、热流体和冷流体中运动的基础上, 进一步模拟了两个不同温度的颗粒在流体中的沉降.结果表明:两等温颗粒的沉降方式与雷诺数Re以及格拉晓夫数Gr密切相关, 而两不同温度的颗粒与两等温颗粒的沉降规律有显著不同.无论初始位置如何, 冷颗粒最终总位于热颗粒下方运动, Re较大时, 发生连续的拖曳、接触现象, 而Re较小时, 冷颗粒会以较大的沉降速度远离热颗粒. 关键词： 格子Boltzmann方法 颗粒沉降 热对流     

溶解椭圆颗粒沉降的介观尺度数值模拟

在任意拉格朗日欧拉(ALE)算法模拟有热对流影响的颗粒两相流动的直接数值模拟基础上, 通过建立颗粒溶解速度和颗粒表面热流密度的关系, 对溶解的椭圆颗粒在垂直管道内牛顿流体中的沉降进行了直接数值模拟. 计算结果表明:与等温惰性椭圆颗粒沉降相比, 流体的对流运动、颗粒质量以及形状的变化等因素使溶解的椭圆颗粒在不同初始角度沉降时, 颗粒沉降动态尾迹、颗粒受力、颗粒沉降速度等都发了较大变化.     

溶解与热对流对固体颗粒运动影响的直接数值模拟

对牛顿流体内溶解与热对流对单颗粒在垂直管道中的沉降运动进行了直接数值模拟.流体运动由守恒方程计算,密度和黏性的变化考虑流场温度变化的影响,通过积分黏性应力和压力获得颗粒的受力跟踪颗粒运动,溶解引起的相变及其形状的变化由溶解潜热、溶解质量与分散相边界处的温度梯度的关系建立的方程决定.通过颗粒和流体间相互的作用力和力矩及边界条件的施加实现相间耦合.分别模拟了颗粒在等温流体、热流体、冷流体及颗粒溶解四种情况下的沉降过程.结果表明,在一定雷诺数内,热对流产生的颗粒尾迹处涡的脱落以及溶解引起的颗粒表面形态的变化引起了颗粒的横向摆动,并使颗粒沉降速度发生了变化. 关键词： 溶解 热对流 颗粒两相流 直接数值模拟     

基于PNK理论的颗粒群Boycott效应研究

科学家Boycott发现草酸化或去纤维化血球沉降在倾斜容器中比在垂直容器中更快,这就是所谓的Boycott效应.本文衍生探究不溶固体颗粒在装有蒸馏水倾斜容器中的沉降是否也会产生Boycott效应以及相关参数影响.利用PNK理论推导出颗粒沉降速率关系式,研究其中影响体系的3个变量,分别为竖直沉降速率、容器底部直径、容器倾斜角度.同时,通过实验探究改变单一变量对沉降速率的影响,与理论计算结果基本吻合.结果表明,不溶固体颗粒也会产生Boycott效应;随着容器倾斜角度增大,颗粒在其中沉降速率先加快后减慢;随着容器底部直径增大,沉降速率与之正相关;随着颗粒垂直沉降速率增大,倾斜沉降速率也与之呈正相关.     

液固两相流中流体旋涡对固体粒子运动影响的数值研究

基于离散涡方法求得的非定常、不稳定流场和颗粒的Lagrangian运动方程,数值计算了四种St数下的泥沙粒子在圆柱绕流场中的运动。计算结果证明了颗粒运动与流体旋涡存在着明确的相关结构：在钝体（圆柱）表面附着泥沙,当流体流过钝体时产生具有剧烈分离的不稳定流动,带动钝体表面泥沙起浮。对于小St数（0．15～0．59）与中等St数（1．33～2．36）的泥沙颗粒被流体旋涡所带起,井被卷入流体旋涡结构内,被卷入流体旋涡结构内的泥沙颗粒在运动过程中始终分布于旋涡区,即在旋涡区聚集。     

热对流条件下双颗粒沉降的直接数值模拟

在任意拉格朗日-欧拉(ALE)算法模拟等温惰性颗粒两相流的基础上,增加对能量方程的联立求解,对热对流条件下双颗粒的沉降进行了直接数值模拟.结果表明:双颗粒在等温流体、热流体和冷流体中运动形式是不同的,造成不同的主要因素是热对流引起了颗粒不同的动态尾迹,在热流体中颗粒尾部形成了涡的脱落,在冷流体中颗粒尾部形成了羽流.     

直接模拟中不同边界条件的实施及对沉降规律的影响

在牛顿流体中, 对颗粒在4种不同边界的垂直通道中的沉降运动进行了直接数值模拟. 计算结果表明:通过计算区域随颗粒运动而移动构建的无限长通道能准确模拟颗粒自由下落到稳定沉降的发展过程; 周期性边界条件由于流场变化, 对颗粒沉降产生了影响, 不能模拟颗粒的自由沉降过程; 底部封闭边界适合模拟封闭容器内颗粒与固壁的相互作用过程, 若颗粒达到稳定沉降, 也能模拟无限长通道内的沉降过程; 流化边界适合模拟流化床内气固两相流动. 计算结果有助于更好地理解和使用不同边界条件. 关键词： 直接数值模拟 边界条件 沉降 任意拉格朗日-欧拉方法     

Flocculation Properties of Some Cationic Polysaccharides

Flocculation of a clay suspension model using some cationic polysaccharides has been studied by turbidity measurements. The investigated polyelectrolytes contain quaternary ammonium salt groups, N-ethyl-N, N-dimethyl-2-hydroxypropylene ammonium chloride, attached to a dextran backbone. The flocculation efficiency was studied as a function of the polycation charge density and some flocculation parameters (polycation dose, settling time, concentration of both clay suspension and parent solution of polymer). All investigated cationic polysaccharides showed high flocculating efficiency at optimum flocculant doses, which correlated well with the charge density of the polycations: the higher charge density, the lower the amount of dextran derivative was required for reaching the maximum clarity degree (of about 100%). This finding is consistent with the neutralization mechanism for a flocculation process. The residual turbidity values also varied with the settling time and the concentrations of clay suspension and parent solution of polymer. Possible reasons for these dependences are discussed.     

Settling velocity of equiaxed dendrites in a tube

The settling velocity of equiaxed dendrites can cause macrosegregation and influence the structure of the equiaxed zone during the casting solidification process. So an understanding of the settling characteristics is needed to predict the structure and segregation in castings. The settling velocity of NH4Cl equiaxed dendrites of non-spherical geometry was studied experimentally in an NH4Cl-70wt.%H2O solution. A calculation formula was proposed to calculate the settling velocity of sediment equiaxed dendrites in a tube filled with saturated solution at a moderate Reynolds number region. The retardation effects of the wall and morphology of the equiaxed dendrite on the settling velocity were taken into account in the development of the calculation formula, and the correction function B of the drag coefficient with consideration of the retardation effects of the wall and morphology of the equiaxed dendrite on the settling velocity of the equiaxed dendrite was calibrated according to the experimental results. A comparison showed that the formula has a good accordance with the experimental results.     

Variations of δ(15)N-Values and Hydrolyzable Amino Acids in Settling Particles in the Ocean

Abstract The modification of nitrogen isotopic signals during particle sedimentation in the sea is of great interest for the use of sedimentary δ(15)N-values as a paleoceanographic tool. The effect of organic matter degradation on such modification was studied by analyzing nitrogen, hydrolyzable amino acids (THAA) and δ(15)N-values in a suit of marine settling particles collected from the Bay of Bengal, Indian Ocean, by using time-series sediment traps, and in underlying sediments. The flux of settling particles showed temporal variations which are related to the monsoons, the major climatic feature of this marine region. During high flux periods settling particles are enriched in nitrogenous material that is less degraded and exhibit higher δ(15)N-values than particles showing characteristics of degradation. At the sediment surface more than 95% of the settling particulate nitrogen is lost and the δ(15)N-values of the residual sedimentary nitrogen are higher than those of settling particles. The observed increase is interpreted to be due to fractionation during degradation of organic matter.     

沉积层声学特性对浅海低频声场分布的影响

已有对浅海低频水声场的讨论多是关注声能量在水体中的分布特性,对水体下沉积层、基底中低频声传播的同步研究相对较少。本文基于波动方程,在柱坐标系下推导了一种浅海水体/海底统一波导下低频声能流的计算模型,在此基础上结合具体仿真算例与波动理论阐述了不同沉积层声学参数对声场能量分布的影响规律及机理。仿真结果表明,在沉积层纵波声速>水中声速>沉积层横波声速的前提下,沉积层中密度与纵波声速数值越大,声能量越趋于保留在水体中而不向海底泄漏,横波声速的影响正好相反;沉积层厚度增加到一定量后,基底对流体层中声传播的影响可近似忽略不计。      

Variations of δ15N-Values and Hydrolyzable Amino Acids in Settling Particles in the Ocean

Abstract

The modification of nitrogen isotopic signals during particle sedimentation in the sea is of great interest for the use of sedimentary δ¹⁵N-values as a paleoceanographic tool. The effect of organic matter degradation on such modification was studied by analyzing nitrogen, hydrolyzable amino acids (THAA) and δ¹⁵N-values in a suit of marine settling particles collected from the Bay of Bengal, Indian Ocean, by using time-series sediment traps, and in underlying sediments. The flux of settling particles showed temporal variations which are related to the monsoons, the major climatic feature of this marine region. During high flux periods settling particles are enriched in nitrogenous material that is less degraded and exhibit higher δ¹⁵N-values than particles showing characteristics of degradation. At the sediment surface more than 95% of the settling particulate nitrogen is lost and the δ¹⁵N-values of the residual sedimentary nitrogen are higher than those of settling particles. The observed increase is interpreted to be due to fractionation during degradation of organic matter.     

Effects of sediment acoustic characteristics on low-frequency acoustic field distribution in shallow water

Most of the published literatures on low-frequency underwater sound propagation are focused on the sound propagation features in the water column,while studies on sound propagation features in the sediment layer or the semi-infinite basement are rare.In this paper,based on the wave equation,a computational model for sound energy flux in the sediment layer and the basement as well as in the water column is proposed under a cylindrical coordinate system.On this basis,the effects of various sediment acoustic parameters on the sound energy distribution and the corresponding mechanisms are elaborated through numerical examples and acoustic theory.Simulation results reveal that,in a situation where sediment P-wave speed>water sound speed> sediment S-wave speed,the greater the values of density and P-wave speed in sediment,the more likely it is that the sound energy remains in the water column without leaking to the sea floor.Conversely,the influence of the variation of S-wave speed is reversed.Basement influence on the sound propagation in the fluid layer is approximately negligible if the sediment layer is sufficiently thick.     

MRI Comparative Study of Container Geometry Impact on the PMMA Spheres Sedimentation

Nuclear magnetic resonance (NMR) imaging results are presented for the comparative study of sedimentation of the polymethylmethacrylate colloidal suspensions with spherical particle diameters of 475, 350 and 255 nm. The time evolution of the particle volume fraction in the sedimenting system, velocity of the fluid/suspension interface, interface broadening, and sediment growth velocity are measured against the system concentration as well as the container geometry. Using the experimental data the hindered settling function is evaluated. The influence of the container geometry on the parameters defining the hindered settling function in different approaches is evaluated for the first time. The limiting value of the particle diameter is estimated at which the container geometry has no impact any longer. This effect can be explained by the microscale structure persisting at low Peclet numbers. In addition, the influence of the container geometry on interface broadening and sediment formation for different particle diameters and volume fractions is studied. Spontaneous sediment packing induced by a modified container geometry has been found. NMR imaging has proven to be a highly efficient research tool for studying sedimentation at low Peclet numbers.     

An Extended Entropic Model for Cohesive Sediment Flocculation in a Piecewise Varied Shear Environment

In this study, an extended model for describing the temporal evolution of a characteristic floc size of cohesive sediment particles when the flocculation system is subject to a piecewise varied turbulent shear rate was derived by the probability methods based on the Shannon entropy theory following Zhu (2018). This model only contained three important parameters: initial and steady-state values of floc size, and a parameter characterizing the maximum capacity for floc size increase (or decay), and it can be adopted to capture well a monotonic pattern in which floc size increases (or decays) with flocculation time. Comparison with 13 literature experimental data sets regarding floc size variation to a varied shear rate showed the validity of the entropic model with a high correlation coefficient and few errors. Furthermore, for the case of tapered shear flocculation, it was found that there was a power decay of the capacity parameter with the shear rate, which is similar to the dependence of the steady-state floc size on the shear rate. The entropic model was further parameterized by introducing these two empirical relations into it, and the finally obtained model was found to be more sensitive to two empirical coefficients that have been incorporated into the capacity parameter than those in the steady-state floc size. The proposed entropic model could have the potential, as an addition to existing flocculation models, to be coupled into present mature hydrodynamic models to model the cohesive sediment transport in estuarine and coastal regions.