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Sediment transport is important for predicting the impact of human intervention on river and coastal systems. In the EU, sediment-related river and coastal maintenance work costs many tens of millions of Euros per year and one of the barriers to reducing these costs is our present inability to accurately model the sediment entrainment transport process. The main deficiency of current sediment transport models is that they do not take account of the turbulent structure of the flow and, more importantly, the interactions between the flow and the sediment. This project is aimed at investigating these influences. We will look at factors effecting the stability of the particles, such as: drag and lift forces, degree of protrusion, blocking by particles upstream, effect of particles downstream on the points of rotation of a moving particle together with how the particles move and then become stationary again.

At the moment, the work carried out is as follows:

(a) We have modified an in-house computational fluid dynamics (CFD) compute program called CgLes so that it can accommodate moving curved surfaces using the immersed boundary (IB) method. A series of verification cases has been performed.

(b) We have also coupled CgLes and an in-house discrete element modelling (DEM) compute program called Y-code and the coupled program can model the particles' movement, collision and also effects on surrounding flow. An extensive series of verification cases has been performed, ranging from 2D laminar flow to 3D turbulent flow around moving objects.

(c) We have performed necessary modifications, optimisation and parallelisation of the coupled code to ensure that it is working at its maximum efficiency.

(d) A direct numerical simulation (DNS) of a fully developed turbulent open channel flow over a water-worked rough bed consisting of densely packed 2-3 layers of spheres have been performed.

(e) After a statistically steady flow had been obtained from (d), the particles were set free and their entrainment, drag and lift forces, and also surrounding turbulence structures were recorded.

The main results achieved so far are two-folds. On one hand, three major contributions which are an iterative IB method, a novel half-distributing forcing strategy and a new wall-layer model for large-eddy simulation have been made to the state-of-the-art of the IB method. A combined compute program which is capable of correct modelling not only the turbulence but also the movement of the particles has been developed. On the other hand, numerical results of sediment entrainment in a turbulent channel flow was presented in figure 1. Three DNSs with different values of the Shields Function S show three distinctly different sediment entrainment patterns. When S=0.045 (upper), just below the entrainment threshold of 0.055 particles are almost stationary. When S=0.065 (middle), bed load is observed. When S=0.05 (below), suspended load is observed.

In accordance with the project plan, analysis work on the achieved sediment entrainment results will be carried out in the return phase. Conclusions about factors that influence the stability of the particles, the maximum moments exerted on the particles as a function of different position parameters together with how the particles move and then become stationary again will be made.

The potential benefits of the results are enormous as it would be possible to investigate the exact particle movement mechanism - something that is very difficult, if not impossible, to adequately measure experimentally. From a geomorphological point of view, the research will be a major contribution to the understanding of sediment dynamics by focusing on the entrainment of individual particles, turbulence around these discrete particles and the role of turbulent eddies. It will lead to improved understanding of bed mobility in rivers with non-cohesive sediment.