Entrainment of Sediment by Unsteady Turbulent Flows

198 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1999.With the help of a special purpose facility, an extensive set of experimental data was gathered that demonstrates the response of sediment to unsteady flows. The data that were gathered include temporal distributions of sediment concentration, shear stress, and velocity for two types of flow conditions: tests where the flow was increased from rest to a peak velocity and was then held constant, and tests where the flow was accelerated from rest to a peak velocity and was then decelerated back to a no flow condition. An acoustic profiler was used to measure suspended sediment concentrations of two sizes of non-cohesive sediment, a coarse sand and a fine sand. Existing entrainment relations were verified with data that were gathered in steady flow conditions, and it was shown that the Garcia-Parker relation also works well for moderately unsteady flows. For unsteady flows a time lag is observed between the shear velocity and the entrainment. The time lag is larger for flows with lower Reynolds numbers since turbulence plays an important role in the entrainment process. Experiments conducted with fine sediment had lower Reynolds numbers than experiments conducted with coarse sediment, and a larger time lag was observed between shear velocity and entrainment for the fine sediment. Consequently, existing entrainment relations did not need to be corrected when they were used to predict entrainment of coarse sand in unsteady flows, even for the highest accelerations observed. However, corrections were necessary for predicting entrainment in all of the unsteady flows that had fine sediment. Two methods of correcting the Garcia-Parker relation for use with unsteady flows were explored. The first method requires the calculation of time lag between entrainment and shear stress. The time lag is then used to estimate the current entrainment. The second method correlates entrainment to the history of the shear stress. Both methods revert to the Garcia-Parker relation when the flow unsteadiness is removed. Calculations using existing and modified entrainment relations showed that for many unsteady flows, existing entrainment relations perform adequately, even if the sediment is fine.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD