Model studies for flocculation of sand-clay mixtures
Model studies for flocculation of sand-clay mixtures
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DOI：
10.1016/j.coastaleng.2017.11.006
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发表时间：
2018-02
影响因子：
4.4
通讯作者：
A. Cuthbertson;Farzin Samsami;P. Dong
中科院分区：
文献类型：
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作者：
A. Cuthbertson;Farzin Samsami;P. Dong
结果是从一个相结合的实验和数值研究，旨在比较纯粘性（粘土）和混合（砂粘土）沉积物悬浮液的絮凝行为等效控制水动力条件下。实验在网格搅拌沉降柱中进行，并着重于测量网格产生的剪切速率和局部悬浮泥沙浓度对柱中产生的微絮凝体和大絮凝体尺寸分布的时间演变的参数影响，以及代表性的最大和均方根絮凝体尺寸。结果表明，对于高岭土粘土悬浮液在低-中等剪切速率下，初始聚集率和峰值或准平衡絮凝体尺寸达到的粘土输入浓度的增加，后者的效果，由于这些运行中产生的大絮凝体的比例较大。相比之下，在高剪切速率下，高岭土悬浮液的代表性絮凝物尺寸在实验持续时间内基本保持不变，粘土输入或原位浓度的影响很小，并且所得絮凝物尺寸分布中不存在大絮凝物。细砂部分的高岭土悬浮液中的添加被示出为降低初始聚集率和在柱中获得的代表性絮凝物尺寸，用于在低-中等剪切速率下运行，同时在高剪切速率下对砂-粘土混合物的絮凝行为具有可忽略的影响。这些结果表明，砂馏分抑制絮凝在较低的剪切速率，由于一个额外的絮凝体破碎机制，直接砂-粘土相互作用（如颗粒-絮凝体碰撞）。在较高的剪切条件下，这些分数间（砂-粘土）的相互作用的重要性减少，在剪切诱导的絮凝体破碎相比。一个一维垂直（1DV）模型，结合人口平衡方程（PBE），其中包括这些多分数（砂-粘土）的碰撞的新的代表性被施加到模拟高岭土和砂-粘土沉降柱试验。在一般情况下，1DV PBE模型的预测提供了良好的协议与测得的原位浓度和准平衡絮凝体的大小达到，但在初始聚集阶段，由于在1DV模型域的上边界条件的不确定性预测絮凝体的大小。此外，1DV PBE模型预测的经验絮凝体破碎率与剪切诱导的絮凝体破碎和多分数（砂-粘土）碰撞的依赖值得进一步关注，以更好地定义这些过程的微尺度动态，以改善其在PBE模型中的代表性。预计这种多组分方法代表了模拟自然沉积环境中絮凝过程的改进基础，例如河口和潮汐入口，其中床沉积物通常由相互作用的粘性（即泥浆）和非粘性（即淤泥，砂）组分组成。
Results are presented from a combined experimental and numerical study aimed at comparing the flocculation behaviour of purely-cohesive (clay) and mixed (sand-clay) sediment suspensions under equivalent controlled hydrodynamic conditions. The experiments were conducted in a grid-stirred settling column and focussed on measuring the parametric influences of grid-generated shear rate and local suspended sediment concentrations on the time-evolution of the micro- and macrofloc size distributions generated in the column, as well as representative maximal and root-mean-square floc sizes. The results indicate that for kaolin clay suspensions under low-medium shear rates, initial aggregation rates and the peak or quasi-equilibrium floc sizes attained increase with the clay input concentration; this latter effect due to the larger proportion of macroflocs generated within these runs. By contrast, under high shear rates, representative floc sizes for kaolin clay suspensions remain largely unchanged over the experimental duration, with little influence from clay input or in-situ concentrations, and no macroflocs present in the resulting floc size distributions. The addition of the fine sand fraction to the kaolin clay suspensions is shown to reduce both initial aggregation rates and the representative floc sizes attained in the column for runs under low-medium shear rates, whilst having negligible effect on the flocculation behaviour for the sand-clay mixtures under high shear rates. These results suggest that the sand fraction inhibits flocculation at lower shear rates due to an additional floc break-up mechanism resulting from direct sand-clay interactions (e.g. particle-floc collisions). The importance of these inter-fractional (sand-clay) interactions diminishes, in comparison to shear-induced floc break-up, under higher shear conditions. A one dimensional vertical (1DV) model incorporating a population balance equation (PBE) that includes new representation of these multi-fractional (sand-clay) collisions is applied to simulate the kaolin clay and sand-clay settling column tests. In general, the 1DV PBE model predictions provide good agreement with the measured in-situ concentrations and quasi-equilibrium floc sizes attained, but under-predict floc sizes during the initial aggregation phase due to uncertainty with the upper boundary condition in the 1DV model domain. Furthermore, the reliance of the 1DV PBE model predictions on empirical floc break-up rates associated with shear-induced floc fragmentation and multi-fractional (sand-clay) collisions warrants further attention to better define the microscale dynamics of these processes for their improved representation in the PBE model. It is anticipated that this multi-fractional approach represents an improved basis for modelling flocculation processes within natural sedimentary environments, such as estuaries and tidal inlets, where bed sediments often consist of interacting cohesive (i.e. muds) and non-cohesive (i.e. silts, sands) fractions.