A numerical and experimental study of differential settling in cohesive sediments.

摘要

Accurate sedimentation modeling has important applications in a number of fields. This includes modeling the accumulation of sediment in harbor channels and modeling the sedimentation and accumulation of particles in retention ponds in the still waters behind dams in order to calculate the trap efficiency. By accurately modeling sedimentation, engineering failures such as the 2008 TVA spill at the Kingston Fossil Plant could possibly be avoided.;Three different numerical models comparing the settling rates of individual particles in quiescent waters were created to compare the change of settling velocity due to particle size distributions, change of local concentrations, and flocculation due to inter-particle collisions. In the models the change of local concentrations at specified heights over time was recorded to create concentration profiles.;The results of the models were compared to four small scale laboratory experiments using light attenuation to measure the change of local concentrations due to the settling of clay particles that were passed through a number 200 sieve. The goal of this work was to see if the three numerical models could be used in larger scale sedimentation modeling programs to more accurately represent the accumulation of sediment in still waters through the use of a Lagrangian approach and by using local concentrations and inter-particle collisions to calculate individual particle velocities.;The model data was non-dimensionalized by dividing the local concentrations with an initial concentration and the height of the control volume by a mean particle diameter. This technique of using non-dimensionalized data was able to scale all of the concentration profiles into similar ranges allowing for a more comprehensive quantitative analysis of the effect of specified parameters, such as the effect of increasing settling velocity due to increased standard deviation of the particle diameters, for each of the different numerical models and a thorough qualitative comparison with the four experiments conducted. The comparison of the concentration and collision model results with observations from the two experiments that had low initial concentrations showed satisfactory agreement; however, the two experiments with higher initial concentrations showed signs of entrainment due to a thermal gradient phenomenon which slightly altered the data and concentration profiles.