Three-Dimensional Mesh Morphing Methodology for Scouring Around Bridge Piers Based on Computational Fluid Dynamic Solution

Flow scour is the engineering term used to describe the erosion of a sediment bed due to fluid flow. Local scour occurs around objects placed in the path of flow, such as bridge piers and abutments. Severe damage or even failure of structures may occur if the amount of scour is too great. Due to the complexity of the fluid/structure interactions and cost of experiments, Computation Fluid Dynamics (CFD) methods are under development to predict the shape and depth of a scour hole. This study extends a previous 3-D iterative methodology, with several improvements to the scouring physics models, implemented in the commercial CFD software STAR-CCM+ to predict the scour hole formation around circular bridge piers. These improvements are inclusion of a variable critical shear stress (VCSS) for the initiation of motion of bed sediment, scouring normal to the sediment bed, and a sand slide model. Reynolds Averaged Navier-Stokes (RANS) equations and a k-ε turbulence model are used to resolve the flow field. The methodology uses a single phase implicit unsteady approach to obtain sediment bed shear stress values. Two moving boundary relations are employed to model the erosion and sand slide physics. One for the erosion rate is based upon an empirical correlation for critical shear stress combined with a sediment entrainment function of Van Rijn, and the other uses the slope of the sediment bed, to iteratively displace the sediment bed in a way that decreases slope as long as it exceeds the angle of repose of the sediment. This is accomplished by a user defined function to move the sediment bed at each time step and the mesh morphing procedure built into STAR-CCM+ to solve fluid-structure interaction problems to stretch the existing mesh to maintain cell quality throughout the flow domain as the bed is displaced. Simulation results have been compared to experimental data found in literature. It was found that simulations over predict the maximum scour depth by up to 35%, but show a large improvement in capturing the overall shape of the scour hole in comparison to models that do not include a sand slide model.