Simulation of flow around a circular pier on a scoured bed using CFD modelling software

Abstract

Bridges are an important part of a transportation network. Piers on which the superstructure of the bridge is provided are important components for the stability and safety of the bridge. There are many cases of failure of bridges due to scouring at their foundations. The scour around the piers is caused due to changes in flow patterns around piers due to the three-dimensional separation of the boundary layer, which has a large magnitude of turbulence and vortices around the piers. For designing pier foundations, it is very essential to estimate the correct depth of scour below river bed level since it affects the depth of the foundation and ultimately the cost of the bridge. The present study is directly relevant to understanding erosion mechanisms around a circular bridge pier model constructed in a river bend. In this research, a Unsteady Reynolds Averaged Navier-Stokes (URANS) approach is applied, to enable the simulation of scouring around the bridge pier model. This methodology allows an approach that numerically describes a complex three-dimensional (3D) scour behavior around bridge piers by coupling the changes in topography with the local detailed hydrodynamics. By using a commercial Computational Fluid Dynamics (CFD) tool, ANSYS 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 RANS approach. The projected results have been compared with those obtained from URANS models (i.e., k-e standard and k- SST (Shear Stress Transport), and with the experimental data, in order to verify the reliability and feasibility of the URANS. The coupling between RANS and the dynamic mesh technique has been undertaken to study the coherent structures around a circular bridge pier model in the initial stage of scour evolution. The approach produces a good prediction of the qualitative and quantitative characteristics observed by the experiment of Dargahi (1989). Furthermore, studying the transient behavior of the horseshoe vortex system upstream of the circular pier model, during the scour process evolution shows that the main vortex has the bimodal oscillation phenomenon, which increases the ability of these vortices to remove the sediment around the circular pier model.