This report is submitted as one of the requirements for title Ph.D. (degree at the University of Twente and the International Institute for Infrastructural Hydraulic and Environmental Engineering (IHE). The Ph.D. study was carried out at the International Centre for Computational Hydrodynamics (ICCH), at DHI Water and Environment (former Danish Hydraulic Institute), Copenhagen, Denmark. The study was funded by the Danish National Research Foundation, their support is greatly appreciated. The scope of the research has been to perform a numerical investigation of sediment transport and turbulence under waves and currents. New boundary conditions have been investigated to account for the movement of the sediment particles influenced by the turbulence of the flow and their own settling velocity. In order to analyse and verify the behaviour of the proposed formulations the model results were compared to sediment measurements. The results showed good agreement and demonstrated a second peak at flow reversal. The present simulations suggest that at the particular moment of flow reversal, when the flow velocity approaches zero, there is a sudden increase in the turbulent conditions of the flow. This phenomenon has been observed in oscillatory flows and also for combined waves and currents. Comparisons were also pursued for waves and currents with a two-equation turbulence model and the faster and simple momentum integral method where the results showed good agreement. These results show the applicability of this type of models for common engineering practice applications. However. the k - e model showed that the value of the turbulent eddy viscosity peaks a flow reversal. This phenomenon has been observed in a series of direct numerical simulations. Finally it was investigated sediment transport under high-shear conditions. Not only by considering the system as a mixture of sediments but also by investigating the consequences of considering this system as composed of two components, representing a fluid phase and a solid particle phase, where the dynamics of the system are followed through the solution of a pair of momentum equations, with one such equation for each phase and related to each other through certain flow- and sediment-load-defined interacting forces.
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