New models of turbulent sediment transport

预测悬浮粒子移动的能力对我们对沉积系统的定量理解以及我们的环境是如何随着工程或自然变化而形成和继续演变的至关重要。此外，可移动颗粒系统在应用中无处不在：来自自然界和工业的例子包括风成、河流和海洋沉积陆地和海洋景观的演变、密集污染物的大气扩散、药品加工和废水处理。在所有这些情况下，湍流运动的存在保持了悬浮液中比环境颗粒更密集的颗粒，从而促进了它们的顺流运输，并且运输的定量预测依赖于这种悬浮液的精确模型。尽管受到了相当多的科学关注，但在20世纪30年代基础研究之后的80年里，我们模拟湍流悬浮的能力几乎没有进步，许多流行的方法，包括在海岸工程等关键应用中使用的方法，都是建立在可能容易出错的动力学和相关泥沙负荷的经验公式之上的。我们要解决的正是模型中的这个缺陷。我们将通过开发一个新的、更深层次的理论框架来理解湍流悬浮液的动力学，这将解决量化湍流悬浮液的基本科学挑战，该框架建立在流体湍流理论和计算以及流动粒子的实验测量的最新进展之上。这些进展突出了当前悬架模型的弱点，并激发了进展的前景和应用的巨大潜力，如果数学基础更加安全的话。因此，我们的目标是开发一种预测悬浮泥沙输运的新范式。我们的假设是，水平剪切流中相对致密的沉积物的稀释悬浮物是由间歇性连贯湍流运动维持的，我们建议调查和量化这一过程。在过去的三十年里，单相流的研究取得了许多重要的进展，研究人员将嵌入湍流中的简单相干流分离出来，作为控制Navier-Stokes方程的精确不变解。在没有悬浮粒子的情况下，这些状态中的一小部分已被证明可以指导甚至高度湍流的演变。此外，与湍流本身不同的是，它们的物理性质现在在一些基本的相互作用过程方面得到了很好的理解。同时，计算能力和技术的进步极大地改善了湍流的数值模拟，而现代实验方法开始在高空间和时间分辨率下测量三维速度和泥沙浓度场，这两者都为本项目提供了肥沃的试验场。这提供了一个及时的机会来研究两相流，确定它们的连贯结构，并确定它们与湍流悬浮物的总体特性以及沉积物运输速率的关系。这种方法对单相剪切流的成功表明，这将导致我们对两相情况的科学理解的一个步骤变化，为改进湍流沉积物输运的预测描述铺平了道路，从而消除了不可靠的经验闭合的需要。

The ability to predict the transport of suspended particles vitally underpins our quantitative understanding of sedimentary systems and the ways in which our environment has been shaped and continues to evolve in response to engineered or natural changes. Moreover, mobile particulate systems are ubiquitous in applications: examples from nature and industry include the evolution of aeolian, fluvial and marine sedimentary land- and sea-scapes, atmospheric dispersion of dense pollutants, pharmaceuticals processing and waste water treatment. In all of these situations, the presence of turbulent fluid motions maintain the denser than ambient particles in suspension, thereby facilitating their streamwise transport, and quantitative prediction of transport relies upon an accurate model of this suspension.Despite considerable scientific attention, our ability to model turbulent suspensions has advanced little in the eight decades following the foundational studies of 1930s and many popular approaches, including those used operationally in critical applications such as coastal engineering, are built upon potentially error-prone empirical formulations of the dynamics and associated sediment loads. It is this deficiency in models that we will address. We will tackle the fundamental scientific challenge of quantifying turbulent suspensions by developing a new, deeper theoretical framework to understand the dynamics of turbulent suspensions, which builds on recent advances in the theory and computation of fluid turbulence and the experimental measurement of flowing particles. These advances highlight the current weaknesses in existing models of suspensions and tantalise with the prospects for progress and the vast potential for applications, if the mathematical foundations were more secure. Our aim, therefore, is to develop a new paradigm for predicting suspended sediment transport.Our hypothesis is that dilute suspensions of relatively dense sediment in horizontal shear flows are maintained by intermittent coherent turbulent motions and we propose to investigate and quantify this process. Advances for single-phase flows made during the past three decades have seen many important results in which researchers isolated simple coherent flows embedded within turbulence, as exact invariant solutions to the governing Navier-Stokes equations. In the absence of suspended particles, a small number of these states have been shown to guide the evolution of even highly turbulent flows. Moreover, unlike turbulence itself, their physics is now well-understood in terms of some basic interacting processes. At the same time, advances in computational power and techniques have vastly improved the numerical simulation of turbulent flows, while modern experimental methods are beginning to measure three-dimensional velocity and sediment concentration fields at high spatial and temporal resolutions, both of which provide a fertile testing ground for this project. This presents a timely opportunity to investigate two-phase flows, identify their coherent structures and determine how they are related to overall properties of a turbulent suspension and thus the rate of sediment transport. The successes of this approach for single-phase shear flows indicate that this would lead to a step change in our scientific understanding of the two-phase case, paving the way for improved predictive descriptions of turbulent sediment transport that obviate the need for unreliable empirical closures.