Detailed modelling of fluid-particle interaction in sediment transport with applications in rivers

Flood risks, channel and bank erosions are directly related to the sediment transport discharge, its understanding and control. Moreover the prediction of sediment entrainment, transport and deposition, predicting the river bed-form (e.g. ripples and dunes) changes is an important research field due to its substantial practical worth. The prediction process of sediment transport over bed-forms in open-channel flow is strongly affected by the complex turbulence structures. Witnessing effects of small and large turbulent scales on particles while considering inter-particle collisions remain challengeable. On the other hand it is clear that, not only the movement of sediments at river beds is influenced by turbulent flows the but also on most cases the solid particles have a direct impact on the flow regime. One of the tasks remain in this regard is to measure the aforementioned effects, on a very small scales where the momentum exchange at the particulate scales occurs. In order to study such challenges in a more faithful approach, four-way coupling through open source code of CFD-DEM (a coupling code between Computational Fluid dynamics (CFD) and Discrete Element Method (DEM)), is demonstrated in this research for bed-load sediment transport on a particulate scale. Understanding the fluid-particle interaction for application in rivers where the presence of micro and macro turbulent structures in the fluid plays a significant role, have been the focus of this study. Furthermore this thesis is furnished by conducting numerical and experimental investigations to obtain better understanding of turbulent flows in geometries similar to river bed-forms, e.g. dune-form and bar-form. This research demonstrates that complexity of particle-laden turbulent flows is a result of particle-fluid, fluid-particle, particle-particle and particle-structures that takes place close to bed. Turbulence and near-bed flow velocity along with its irregular risings and fallings have a direct impact on the sediment particles motion. By utilising Large Eddy Simulation (LES) turbulent modelling, turbulent scales are captured. Moreover inter-particle collision of sediments has been highlighted by the means of four-way coupling. Consequently the effect of fluid on the particles and vice versa is demonstrated. It is revealed that the presence of sediment particles in turbulent flows affect the fluid motion along with its accompanying turbulent activities. Particles are lifted as a result of applied forces from eddies and significant influence is therefore captured on the moving particles that are in the vicinity of eddies. The effects that sediments apply on the turbulent structures in the flow have also been captured due to momentum exchange between particle and fluid phase. This has been shown by the means of fluctuation variations at the location of interacting particles.