CFD MODELING OF FLOW FIELD AROUND BRIDGE PIERS USING ANSYS FLUENT

Abstract

Piers are the one of the most important parts of a bridge. They are the substructures which support all the weight and vertical load of the bridge and moreover, they resist all kind of forces, be it horizontal or transverse, acting on them. They transfer the live and dead load to the footings or the foundation of the bridge. There are various materials used for their construction these days. They are located in the middle points between the bridge abutments and at the span ends. Inappropriate design of the bridge piers can result in the failure of a bridge. Most of the times, the bridges built on the rivers are damaged and hence, they collapse due to defects in their hydraulic designing. One of the main reason of collapsing of a bridge is its inability to bear the shear stresses, hydraulic pressure and torque imposed by the flowing water on its piers. Other reason is the local scour that occurs around the base of the piers due the flowing water, caused by the shear impact and pressure at the bed of the channel, which creates a cavity at the base, making the piers unstable. The bridges built on water channels should be designed keeping in mind both these factors of collapsing and damage to the bridges. The design codes have not covered this area of hydraulics and water resources in deep and so, the traditional methods often lead to the construction of either under-secure bridges (prone to easy damage) or over-secure bridges (lead to over-costing). For more accurate results, one can design the bridges with the use of high efficiency software. The present study is mainly focussed on the bridge piers in an open channel. In this study, a CFD analysis of the flow field around bridge piers has been done with the help of software, CFD ANSYS FLUENT, version 14.5. Various parameters like pressure acting on upstream face of the piers and on the channel bed surrounding the piers, shear stress acting on the piers and the channel bed surrounding the piers, velocity field around the piers and torque acting on the piers have been studied to predict the extent of damage and local scouring that can happen around the piers for various cases. The prediction of the local scour is then cross-checked by calculating the exact values of local scour for each case by using the software MARYLAND SHA BRIDGE SCOUR PROGRAM based on HEC-18 by FWHA. Various comparisons are done on how the above parameters change with the shape of the piers depth of the flow, velocity of the flow (Froude No.) and width of the piers. The results show that the shape, depth, velocity and width of piers have a noticable effect on all the above parameters. It is noticed that curved shaped piers are a better option than the sharp edged piers in case of pressure, stresses, torque and less local scouring. The shape really influences the flow around a pier. The shear stress on the bed of the channel is a very effective parameter to predict the local scouring. It is noted that the shear stresses, hydraulic pressure, torque and hence, local scouring all increase with the increase in Froude No., depth of flow and increase in the pier width.