A PHASE DIAGRAM FOR TURBULENT, TRANSITIONAL, AND LAMINAR CLAY SUSPENSION FLOWS
A PHASE DIAGRAM FOR TURBULENT, TRANSITIONAL, AND LAMINAR CLAY SUSPENSION FLOWS
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DOI：
10.2110/jsr.2009.025
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发表时间：
2009-03-01
影响因子：
2
通讯作者：
Wang, Mi
中科院分区：
文献类型：
--
作者：
Baas, Jaco H.;Best, James L.;Wang, Mi
提出了含粘土明渠流动态结构的新相图。这些图可用于根据湍流力（由流速和湍流强度的水平分量近似）和内聚力（由悬浮粘土浓度和流变学近似）之间的平衡来区分湍流牛顿流、过渡流和层流非牛顿流行为。使用一系列全面的实验室水槽实验定义了五种不同流动类型的稳定性状况，其中深度平均流动、速度范围为 0.13 m s(-1) 至 1.47 m s(-1)，体积高岭石粘土浓度范围为 0.03% (= 0.8 g L(-1)) 至 16.7% (= 434 g L(-1))。随着粘土浓度的增加，可以区分出五种流动类型：湍流、湍流增强过渡流、下部和上部过渡塞流以及准层流塞流。过渡流的湍流特性比逐渐湍流阻尼的常见概念要复杂得多。湍流增强的过渡流比相似速度的湍流表现出更高的湍流强度，这种增强源自于床层约 0.01 m 内高度湍流的基底内剪切层的发展。在下部过渡活塞流中，基底内部剪切层将水平速度垂直梯度高且湍流强的下部区域与速度梯度较缓和湍流强度较低的活塞流上部区域分开。高湍流剪切层内的开尔文-亥姆霍兹剪切不稳定性表示为下游速度时间序列中明显的二阶振荡。湍流阻尼在上部过渡塞流中占主导地位，因为强内聚力（推测是由高浓度粘土悬浮液的胶凝引起）开始超过湍流力。在准层流塞流中，除了在这些流的底部附近有一些小的残留湍流外，胶凝是普遍存在的，并且湍流被完全抑制。除了极少数例外，所有流动都随着粘土浓度的增加而经历相同的发展阶段，无论它们的速度如何，但是湍流增强、胶凝以及内部剪切层和塞流的发展的阈值浓度与流速成正比。在流速低于 0.5 m s(-1) 时，仅需要低浓度 (< 0.75%) 的高岭石即可引起过渡流动行为，从而可能影响自然沉积环境中许多缓慢移动和减速的粘土流。然而，在流速高于 1 m s(-1) 时，需要至少 6% 的粘土浓度才能使流进入过渡流相，但即使在这些速度下，过渡流相也占流相空间的很大一部分。通过将实验数据转换为无量纲弗劳德数（动量项）和雷诺数（内聚项），表明湍流、过渡流和层流相之间的每个边界都可以通过特定的窄范围雷诺数来描述。在实验期间，粘土颗粒的沉降仅发生在低流速（和低弗劳德数）的塞流中，此时流动缺乏支撑整个粘土悬浮载荷的强度。
New phase diagrams for the dynamic structure of clay-laden open-channel flows are proposed. These diagrams can be used to distinguish between turbulent Newtonian, transitional, and laminar non-Newtonian flow behavior, on the basis of the balance between turbulent forces (approximated by the horizontal components of flow velocity and turbulence intensity) and cohesive forces (approximated by the suspended clay concentration and rheology). Stability regimes for five different flow types are defined using a comprehensive series of laboratory flume experiments at depth-averaged flow, velocities ranging from 0.13 m s(-1) to 1.47 m s(-1), and at volumetric kaolinite clay concentrations ranging from 0.03% (= 0.8 g L(-1)) to 16.7% (= 434 g L(-1)). As clay concentration increases, five flow types can be distinguished: turbulent flow, turbulence-enhanced transitional flow, lower and upper transitional plug flow, and quasi-laminar plug flow.The turbulent properties of transitional flows are shown to be considerably more complex than the common notion of gradual turbulence damping. Turbulence-enhanced transitional flows display higher turbulence intensity than turbulent flows of similar velocity, with such enhancement originating from development of a highly turbulent basal internal shear layer within similar to 0.01 m of the bed. In lower transitional plug flows, the basal internal shear layer separates a lower region of high vertical gradient in horizontal velocity and strong turbulence from an upper region of plug flow with a much gentler velocity gradient and lower turbulence intensity. Kelvin-Helmholtz shear instabilities within the highly turbulent shear layer are expressed as distinct second-scale oscillations in the time series of downstream velocity. Turbulence damping dominates upper transitional plug flows, because strong cohesive forces, inferred to be caused by gelling of the high-concentration clay suspension, start to outbalance turbulent forces. In quasi-laminar plug flows, gelling is pervasive and turbulence is fully suppressed, apart from some minor residual turbulence near the base of these flows.With very few exceptions, all flows pass through the same development stages as clay concentration increases, regardless of their velocity, but the threshold concentrations for turbulence enhancement, gelling, and development of internal shear layers and plug flows are proportional to flow velocity. At flow velocities below similar to 0.5 m s(-1), only low concentrations (< 0.75%) of kaolinite are required to induce transitional flow behavior, thus potentially affecting many slow-moving and decelerating clay flows in natural sedimentary environments. However, at flow velocities above 1 m s(-1), clay concentrations of at least 6% are required in order for flows to enter the transitional flow phase, but even at these velocities the transitional flow phases make up a significant proportion of the flow phase space. By converting the experimental data to nondimensional Froude number (momentum term) and Reynolds number (cohesive term), it is shown that each boundary between the turbulent, transitional, and laminar flow phases can be described by a specific narrow range of Reynolds numbers. Within the duration of the experiments, settling of clay particles occurred only in plug flows of low flow velocity (and low Froude number), when the flows lacked the strength to support the entire clay suspension load.