Numerical modeling of sediment transport near the bed using a two-phase flow approach

The prediction of the motion of sediment particles close to river beds is a notably complex task. Experimental and numerical studies have provided a basic understanding of the processes affecting the bed load; however, there are still several issues that are elusive to the current technology/theory.;Bed load transport can be simulated through three sub-models: (a) a set of equations describing the particle "free" flight, (b) a sub-model to calculate the post-collision particle velocity and rotation, and (c) a mathematical representation of the bed roughness. In this dissertation, a new theoretical/numerical model for bed load motion is presented, including different and novel versions of the above sub-models, in three spatial dimensions (3-D).;The "free" flight sub-model includes the effect of several forces over the particle translation (buoyancy, drag, virtual mass, lift, fluid acceleration, Basset and Magnus forces) and also deals with the particle rotation. A new optimized methodology to compute the Basset force is presented. Important savings in computational time are obtained by using this methodology.;The post-collision velocity and rotation sub-model features the conservation of linear and angular momentum during the rebound, and it enables a straightforward extension to inter-particle collisions. A new 3-D representation of the bed roughness is introduced by using geometric considerations between the moving particle and the bed, and a stochastic model. It is concluded that the Dependent Bed Angle (DBA) sub-model provides the best representation of the bed. The importance of the values of the friction and restitution coefficients is also addressed.;The particle tracking model was coupled with a highly-resolved, 3-D, turbulent flow field, to study the effect of the flow turbulence on the particle motion. The one-way coupling model was validated with experimental observations. Fluid/particle interactions were investigated by defining the particle turbulent intensity and the particle turbulent kinetic energy. To that end, a new filter to separate turbulence effects from the "mean" flow conditions is presented. The particle tracking model was supplemented with a sub-model of inter-particle collision. The effects of particle size, flow velocity, and particle concentration on the particle turbulent intensity and turbulent kinetic energy are elucidated.