1 Department of Mechanical Engineering, Technical University of Denmark2 Fluid Mechanics, Coastal and Maritime Engineering, Department of Mechanical Engineering, Technical University of Denmark3 DHI Denmark
Hopper sedimentation is the result of precipitation of typically fine sediment from a homogenous, high-concentration mixture, which is not completely deficient of turbulence. If hopper sedimentation or loading is accomplished through a single-inflow system, or if the irregularity of the inflow concentrations is pronounced or simply terminated, then the hopper mixture will clear. Whereas turbulent mixing is redundant, when the mixture is homogeneous, it may take an active role when the mixture is clearing. The role of turbulence on hopper sedimentation has been the focus of several studies, and a common perception of turbulence (or at least of mixing) is that it delays sedimentation. Existing measurements of sedimentation rates in a closed-flume experiment, engineered to provide input to a hopper sedimentation model, revealed that turbulence in a clearing mixture is not necessarily associated with a delay in sedimentation. The experiment showed that sedimentation was boosted by adding a current to a clearing mixture, which infers that turbulence, under certain conditions, may act as a sedimentation agent on the excess sediment in suspension. Therefore, the interactions between turbulent mixing and settling in high-concentration mixtures were examined theoretically. Analytical solutions for clearance of excess concentrations were derived for the limiting cases of (1) still-water clearance and (2) clearance when the amount of turbulence is abundant. When examining these analytical solutions, a potential for enhanced sedimentation was revealed. It was found that mixing-induced dilution of concentration weakens the hindrance in settling to a degree that enhances sedimentation. The analytical findings prompted a more elaborate analysis of the mechanism using a numerical model, which encompassed time- and depth-varying turbulence. This allowed the experimental setup and the observed settling effects to be simulated. The potential of the enhancing sedimentation mechanism was analyzed under more general conditions with the numerical model.
Journal of Waterway, Port, Coastal, and Ocean Engineering, 2015, Vol 141, Issue 2