New models of turbulent sediment transport

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.