How do changes in suspended sediment concentration alone influence the size of mud flocs under steady turbulent shearing?
How do changes in suspended sediment concentration alone influence the size of mud flocs under steady turbulent shearing?
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DOI：
10.1016/j.csr.2018.02.008
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发表时间：
2018-04-15
影响因子：
2.3
通讯作者：
Strom, Kyle
中科院分区：
文献类型：
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作者：
Duc Tran;Kuprenas, Rachel;Strom, Kyle
在农业河流、大型沿海河流、河口、河流羽流和浊度流等环境的输沙模拟中，絮凝作用下泥沙大小和沉降速度的模拟对于准确预测泥沙运动和沉积至关重要。然而，收集准确、高分辨率的泥浆絮凝体数据是困难的。因此，考虑絮凝作用对泥浆沉降速度影响的模型是基于稀疏数据的，这些数据往往与絮凝作用的基本影响因素（如悬浮泥沙浓度）存在不一致的关系。本研究考察了悬浮泥沙浓度对湍流悬浮液中絮体大小种群的影响。具体而言，本文研究了：(1)在稳定浓度和湍流剪切条件下，平衡絮体大小与悬沙浓度之间的关系；(2)湍流剪切一定时，成熟絮凝体对浓度非定常下降的适应速度。采用两组实验对目标过程进行了研究。所有的工作都是在实验室的混合罐中进行的，使用了一个絮体摄像机和一种新开发的图像采集方法。新方法允许在浓度为15至400 mg/L的粘土湍流悬浮液中直接成像和确定絮凝体的大小，因此无需将样品转移到另一个沉降柱或成像槽。两组实验的主要结论是：(1)在C = 50 ~ 400 mg/L范围内，含能湍流悬浮液的平衡絮体粒径与浓度呈线性正相关，但斜率小于先前低能环境数据和模型的预期；(2) G = 50 s湍流剪切时，絮凝体粒径迅速减小（有1 ~ 15 min的时滞），浓度随时间变化减小（-1）。总的来说，数据表明，平衡絮团大小是浓度的正函数，但增加的速度比预期的要弱。数据还表明，如果感兴趣的时间步长在10分钟或更大的数量级上，用简单的平衡模型近似某些泥浆的大小或沉降速度可能是合适的。数据还显示了校准历史泥浆沉降速度方程对于准确预测的重要性。
Modeling the size and settling velocity of sediment under the influence of flocculation is crucial for the accurate prediction of mud movement and deposition in sediment transport modeling of environments such as agricultural streams, large coastal rivers, estuaries, river plumes, and turbidity currents. Yet, collecting accurate and high resolution data on mud flocs is difficult. As a result, models that account for the influence of flocculation on mud settling velocity are based on sparse data that often present non-congruent relationship in floc properties with basic influencers of flocculations such as suspended sediment concentration. This study examines the influence of suspended sediment concentration on floc size populations within a turbulent suspension. Specifically, the work investigates: (1) the relationship between the equilibrium floc size and suspended sediment concentration under conditions of steady concentration and turbulent shearing; and (2) the speed at which mature flocs adapt to an unsteady drop in the concentration when turbulent shear is constant. Two sets of experiments were used to investigate the target processes. All work was conducted in laboratory mixing tanks using a floc camera and a newly developed image acquisition method. The new method allows for direct imaging and sizing of flocs within turbulent suspensions of clay in concentrations ranging from 15 to 400 mg/L, so that no transfer of the sample to another settling column or imaging tank is needed. The primary conclusions from the two sets of experiments are: (1) that the equilibrium floc size in an energetic turbulent suspension is linearly and positively related to concentration over the range of C = 50 to 400 mg/L, yet with a smaller-than-expected slope based on previous data and models from low-energy environments; and (2) that floc sizes decrease quickly (with a time lag on the order of 1-15 min) to time-varying decreases in concentration at turbulent shearing of G = 50 s(-1). Overall the data illustrate that equilibrium floc size is a positive function of concentration, but that the rate of increase is weaker than expected. The data also suggest that approximating the size or settling velocity of some muds with a simple equilibrium model might be appropriate if the time steps of interest are on the order of 10 min or larger. The data also shows the importance of calibrating historic mud settling velocity equations for accurate predictions.