Civil Engineering

 

Date of this Version

Summer 7-19-2011

Comments

A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Engineering, Under the Supervision of Professor Junke Guo. Lincoln, Nebraska: July, 2011

Copyright 2011 Zhaoding Xie

Abstract

Research of river bed erosion is an important part of designing bridges and transportation networks since the stability and hydrodynamic response of bridges critically depends on river bed scour. The engineering designers traditionally focus on the local scour around hydraulic structures, such as piers and abutments, in open channel flow. The pressurized scour under a submerged bridge deck is seldom researched.

In this study, the commercial CFD codes, FLOW3D and STARCCM+, are adopted to simulate pressure scour under a bridge deck with six girders. Several limitations for FLOW3D are found during its application of sediment transport model: a) the scour model is sensitive to the vertical cell size on the sediment interface, the scour holes impossibly form for the large cell size; b) large sediment diameters fail the drifting model comprised of the scour model; c) the bed load model cannot be available to

simulate the saltation load; d) large computational times are required to obtain the scour results. In STARCCM+, the morphing vertices model on the wall boundary can effectively mock the deformation of river bed dependent on the entrainment rate for sediment transport model that we adopt. Guo’s empirical formulas for pressurized scour profile and time dependent scour depth are incorporated into STARCCM+ as the model of morphing mesh. The recession rate is obtained as the function of the maximum bed shear stress by fitting the numerical results.

A theoretical model for analyzing the sediment bed load with arbitrary bed slopes is developed to calculate the erosion profile on the sediment bed in flow condition. It found that the entrainment rate of sediment particles is also dependent on the changing rate of bed load layer thickness and mixture density from the continuity equation compared to Exner equation. Further, the two dimensional solution shows that the additional shear stress due to the longitudinal slope has an important influence on the bed shear. The modified pressure drop formula based on Ergun equation is developed to compute the gradient of pressure drop for a fluidized bed as well.

The results of this research provide an effective approach to analyze the scour profile with the combination of theoretical and numerical computation.

Advisor: Junke Guo

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