Archived Computational Fluid Dynamics Student Thesis Abstract

DEVELOPMENT OF A THREE-DIMENSIONAL SCOURING METHODOLOGY AND ITS IMPLEMENTATION IN A COMMERCIAL CFD CODE FOR OPEN CHANNEL FLOW OVER A FLOODED BRIDGE DECK

The Computational Fluid Dynamics staff at TRACC is supporting three students from Northern Illinois University who are working for a Masters degree. The CFD staff is directing the thesis research and working with them on three projects: (1) a three-dimensional scour computation methodology for pressure flow scour under flooded bridge decks (2) three-dimensional simulation of flow through culverts with computation of losses under different culvert flow heights (3) use of mesh morphing in STAR-CCM+ to compute scour around a cylindrical bridge pier. In June, 2010, Bhaskar Rao Tulimilli successfully defended his thesis and has been awarded a Master of Science degree.

Abstract
Design and construction of bridges need re-evaluation as it has raised considerable safety concerns in recent times. One of the major reasons for the failure of the bridges is the scour-hole formation. Under flooding conditions, bridges may be subjected to failure risk due to the accelerated scouring of the river bed at bridge support structures. To understand and evaluate the scouring process, physical modeling is the best approach and therefore experiments are generally conducted by reducing the scale of setup. However, it requires a considerably long time and is very costly. An alternate way to evaluate and understand the process of scour is to perform simulations under experimental conditions. Therefore Computational Fluid Dynamics (CFD) based simulation model can be used as an analysis tool for evaluating scour and failure risk of existing bridges and in the design of new bridges.

This study extends the previous 2-D iterative scouring procedure with an objective to develop an automated iterative methodology for simulating a 3-D computational domain with inundated bridge deck to predict the final shape and size of scour-hole. A single-phase quasi-steady approach was used to obtain the bed shear stresses and the moving boundary formulation was based on an empirical correlation for critical shear stress to iteratively deform the bed toward equilibrium scour conditions. The model solves the flow field using Reynolds Averaged Navier-Stokes (RANS) equations, and high Reynolds number k–e turbulence model in commercial CFD software. BASH scripting was used to integrate STAR-CCM+ and STAR-CD commercial CFD software along with Python script in UNIX environment. Simulations were performed for different inundation ratios and for mean sand diameters of 1 mm and 2 mm. The model was validated by comparing results with experimental data and it was found that the modeled equilibrium scour shape and size agrees reasonably well with experiments for fully submerged cases where the bridge deck is far away from interface compared to the cases where the bridge is close to interface and/or partially submerged. This 3-D CFD scour computation procedure developed to use well-benchmarked commercial CFD software provides a basis for adding additional physics models while also providing a tool that can be used immediately by engineers engaged in scour risk analysis and assessment.