Scour around Complex Shape Bridge Piers in a River

The research undertaken develops a numerical modelling approach to investigate the interaction between the turbulent coherent structures and the sediment transport that occur around bridge piers. The study is directly relevant to understanding of erosion mechanisms around both circular and non-circular bridge piers constructed in a river bend. In this research the Large Eddy Simulation (LES) approach is adopted, alongside a dynamic meshing technique to enable the simulation of scouring around the bridge pier. This methodology allows an approach that numerically describes a complex three dimensional (3D) scour behaviour around bridge piers through coupling the changes in topography with the local detailed hydrodynamics. By using of a commercial ‘Computational Fluid Dynamics’ (CFD) code and User-Defined Function (UDF) written in C++, the bed shear stress can be computed from the LES predictions model at each time step and stored in order to calculate scour. The dynamic mesh updating technique is implemented to move the bed and assures a practical and accurate scour simulation by individually updating the domain’s riverbed at each time step. Simulation, in accordance with the experimental configuration of Dargahi (1989) for the turbulent channel flow past a circular cylinder, mounted in a flatbed, has been undertaken using the LES approach. The predicted results are compared with those obtained from URANS models (i.e., k-ɛ standard and k-ω SST), and with the experimental data, in order to verify the reliability and feasibility of the LES. The coupling between LES and the dynamic mesh technique has been undertaken to study the coherent structures around a circular bridge pier in the initial stage of scour evolution. The approach produces a good prediction of the qualitative and quantitative characteristics observed by Dargahi (1990). Furthermore by studying the transient behaviour for the horseshoe vortex system upstream of the cylinder, during the scour process evolution using LES shows that the main vortex has the bimodal oscillation phenomenon, which increases the ability of these vortices to remove the sediment around the cylinder. The current investigation consider the most relevant range of angles of attack (α=0^°to 60^°) in design of the oblong pier that is one of the most common shapes of the piers. Predicted results demonstrates a very significant effect of the angle of attack on the complexity of the flow, and thus on the scour pattern and the maximum scour depth around the bridge pier. Finally a modified equation is proposed to compute the angle of attack correction factor in case of oblong bridge piers. This equation adopts previous work to take into account the results from LES predictions that have been shown to more reliably predict the complex flow behaviour important in determining scour rates. The hypothesis suggested in this study was based on LES, for calculating the angle of attack correction factor for oblong bridge with aspect ratio L/b= 3.0. It’s more accurate than the previous equation, which is depended on the URANS model, but to make this hypothesis more general and applicable in the engineering application its need to be tested against a range of L/b.